1,721,082 research outputs found
Selbstverwaltung in der Fischerei auf Nordseegarnelen (Crangon crangon): Potenziale, Wahrnehmungen und Herausforderungen
Get PDFEffective fisheries management increasingly relies on science-based harvest strategies that prioritize long-term ecosystem and stock sustainability over short-term yields. For short-lived and data-limited species like the North Sea Brown Shrimp (Crangon crangon), the use of traditional stock assessments is hampered by high natural mortality, rapid population turnover, and incomplete or inconsistent data on fishing effort. At the core of the harvest strategies for such species are thus harvest control rules (HCRs)—predefined protocols that regulate fishing opportunities based on real-time or near-real-time indicators of stock status.
Despite being one of Europe’s most valuable fisheries, the North Sea Brown Shrimp has lacked regular stock assessment and population-based management measures, relying instead on technical measures. However, advances in growth rate modelling and integrated international datasets have revealed evidence of growth overfishing, highlighting the need for formal management. In response, a collaborative management framework was established, culminating in Marine Stewardship Council (MSC) certification of the main fleets and the adoption of a self-regulatory management plan based on Landings per Unit Effort (LPUE) thresholds, effort restrictions and gear modifications to reduce growth overfishing and delimit the risk of potential recruitment overfishing.
This dissertation provides a comprehensive analysis of the recent dynamics in the Brown shrimp stock, the effectiveness of current management approaches, and the challenges faced in implementing effective and accepted management measures.
In Chapter I, monthly VMS and logbook data of the North Sea Brown shrimp fleets of Denmark, Netherlands and Germany from 2009 to 2018 were analysed for trends and regional patterns in five subareas. Effort increased by 12% while landings decreased by 9% from the first five to the second five years of the time series. All areas showed a significant decreasing trend of LPUE in the fishery of the first quarter of the year from 2009 to 2018. Fishing effort in Dutch and East Frisian waters during winter was negatively correlated with LPUE in the same and adjacent areas in March – April. Furthermore, highly significant negative correlations were found between fishing effort in January and February in Dutch waters and LPUE in July and August in areas further north, explaining up to 86% of the variance. Together our results support the hypothesis that early recruitment in the Northern areas partially depends on a new cohort coming from the south and that reduced recruitment in Northern areas may be a consequence of previous local depletion of egg-bearing females further south. Egg bearing shrimp appear to concentrate in southern areas in January and February and migrate to adjacent northern areas for larval release in March and April. The findings demonstrate that neither environmental drivers nor predation pressure alone can fully explain the recent decline in stock abundance. Instead, fishing pressure—especially during critical winter months when the stock appears most vulnerable—emerges as a key factor influencing population trends. To prevent economic and ecological consequences for the shrimp stock and the fishery, transboundary management measures need to be considered and implemented. Further investigations of migration and drift patterns of brown shrimp are recommended.
In Chapter II, the effectiveness of the current HCR is quantified and alternative, more stringent HCR designs are tested under two plausible scenarios: a general reduction in stock abundance and a recruitment failure event, both representing common risks for short-lived stocks. A comprehensive shrimp population simulation model, parameterized with recent fisheries and biological data from the German Exclusive Economic Zone, replicates the seasonal patterns of growth, mortality, and fishery removals. The performance of HCR designs was evaluated in terms of their effects on shrimp biomass, egg production, landings, and total fishing effort. Results demonstrate that all HCR variants yield modest improvements in LPUE, biomass, and egg production relative to unmanaged scenarios, while annual landings remain largely unchanged. Importantly, the magnitude and speed of stock recovery depend considerably on the stringency of the effort reduction. The currently implemented approach, which employs stepwise weekly effort limits when LPUE reference points are breached, achieves only moderate effort reductions (averaging 12–19%), with limited capacity to rebuild biomass after major declines. In contrast, simulated approaches involving greater reductions in weekly fishing time or short complete fishery closures (up to 99% effort cut for two weeks) trigger much faster LPUE recovery and higher increases in spawning biomass and egg production, especially following recruitment failure scenarios. However, such drastic measures could pose operational difficulties and may be less acceptable to stakeholders due to increased variability in landings and disruption to fleet activities.
The current harvest control rule, based on empirical LPUE thresholds, provides a necessary foundation, but simulation results indicate that this mechanism may be insufficiently responsive under serious stock declines. More drastic, short-term reductions in effort, informed by timely and accurate abundance indices, are likely needed to prevent stock and fishery collapse, and could even result in increased future yields without compromising market supply.
In Chapter III, the self-management regime of the North Sea Brown Shrimp fishery was analyzed, drawing on Ostrom’s design principles for robust self-governance of common-pool resources. The study employs a quantitative survey of shrimp fishers, complemented by qualitative insights, to examine user perceptions of the self-governance system. Fishermen’s attitudes toward management efficacy, enforcement, participation, and adaptation to local conditions were assessed across three clusters of fishers.
Results indicate limited perceived need for management among fishers, with only a minority recognizing stock decline despite scientific evidence of increased fishing mortality and growth overfishing. While the fishery’s biological and institutional characteristics—restricted user group, monopolistic market structure, and homogeneity in economic interests—are conducive to self-governance, significant challenges remain. Notably, trust is lacking in the fairness and effectiveness of monitoring and enforcement, particularly across national producer organizations (POs). Many fishers express skepticism about the impact of measures such as increased mesh size, viewing technical adjustments as easily circumvented. Effort reduction is more widely accepted, especially during periods of low stock, but regionalization of management is resoundingly rejected in favor of equal treatment for all fleet segments.
Fishermen report feeling insufficiently involved in the development and ongoing adaptation of management measures, leading to perceptions of remote, top-down decision-making—potentially undermining legitimacy and compliance. Nevertheless, there is strong interest, especially among less mobile, home-port-based fishers, for increased participatory roles in management design and rule-making.
The North Sea Brown Shrimp fishery illustrates both the potential and limitations of self-management frameworks. While user-led regulation can stabilize and add value to the fishery, persistent gaps exist between scientific advice, institutional frameworks, and user perceptions. Addressing these gaps will require enhanced communication of scientific findings, fostering trust among user groups and POs, broader stakeholder engagement, and greater transparency in enforcement
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Wachstum der Nordseegarnele Crangon crangon (Caridea, L. 1758). Verbesserung der Wachstumsparametrisierung in zukünftigen Lebenszyklusmodellen
No full textThe common brown shrimp (Crangon crangon) is a species of high economical and ecological value in the southern North Sea ecosystem. Its presence in almost all areas, not only makes C. crangon a key species by being both predator and prey for a wide variety of other ecosystem inhabitants, but also forms the basis of a 500-vessel strong fishery that lands up to 40,000 tons annually, generating revenues of around 120 million euros. Due to the lack of moult enduring hard structures, a year-round lasting spawning period and the short-lived nature of the species, age determination in the common brown shrimp is nearly impossible. Hence, common-, age-based management approaches, such as the maximum sustainable yield (MSY) were found not applicable for the common brown shrimp stock. Aiming at the certification of the fishery by the marine stewardship council (MSC), in 2016 a self-management plan was introduced, in which measures alternative to a quota were set. The potential benefits and effects of these measures were calculated using a species-specific yield per recruit model, parameterized with growth data from decades of growth research on the species. The results of this model and the conclusions drawn for the management are highly influenced by the given growth input. Although the species, due to its extensive use, has been the subject of growth research since the late 1940th, to date questions remain about the factors influencing the growth in common brown shrimp. The aim of this thesis is to address some of these questions, and to further improve the parameterization in future growth calculations.
Crucial for the survival and growth of common brown shrimp is known to be food availability. At the same time, parts of the common brown shrimp population are-, depending on season, known to be exposed to prolonged periods of starvation. Despite their omnivorous feeding
behaviour and the otherwise very productive ecosystem, in winter up to 80 percent of the population can be in a starving condition. The response of C. crangon to starvation has studied previously, but no study has been conducted in which the animals were starved for long periods and then fed again. Manuscript 1 addresses the influence of prolonged food deprivation followed by re-feeding on the growth of the common brown shrimp. In laboratory experiments, animals were deprived of food for different periods of time and were re-fed thereafter. Subsequently, the moulting interval and the growth increment, determined individually, were compared with a control group that was fed ad libitum for the entire trial period. Food deprivation significantly increased the time between two successive moults and reduced the growth increment into a negative range. Shrinkage had previously been observed in the common brown shrimp occasionally but was first observed systematically in manuscript 1. In addition to the prolongation of the moulting interval during food deprivation, which was previously considered to mainly be a function of temperature and size, shrinkage was identified as a physiological necessity to compensate for the dry mass lost during the starvation period. The results from manuscript 1, combined with the observations of other authors that especially in winter up to 80 % of the population show a condition that indicate prolonged starvation, could be used to adjust growth predictions of the stock in winter. Since egg deposition and egg laying are linked to a moult event, the delayed moult due to starvation could affect the reproductive cycle of starving females. Reduced growth during starvation, and especially shrinkage could reduce catch quantities in spring fisheries, after winter with food scarcity.
In addition to gaining a better understanding of the influence of starvation and re-feeding on growth, in Manuscript 2 the effects of in field density, a factor that could potentially trigger starvation in the shrimp stock, was investigated. Conserving the stock due to management measures, especially when reducing effort or increasing mesh size, leads to a local increase of individuals and hence density. Concerns towards density dependent growth counteracting potential benefits of a newly implemented measures, showed the need of further investigations into shrimp growth at different densities. In Manuscript 2, three different approaches, in which different proxies for growth were used, to compare growth potential in common brown shrimp caught at different densities. Besides laboratory growth observations and dry weight conditions from field samples the relative abundance of two successive length classes in samples from the demersal young fish survey (DYFS) were used to detect a potential decrease in growth, with increasing density. Growth proxies at different densities were compared within a short period of time (30-50 days), within month of one summer and between the same months in different years. In none of the three approaches indications for a potential growth limitation with increasing density was found. In summer, when the expected effects of a conserving measure are greatest, no signs for poorer growth at higher densities was found. On the contrary, animals sampled at the highest observed density usually showed the best growth potential. This led to the conclusion that some of the main factors determining population size are bottom up driven. Hence, do good conditions lead to high densities, rather than high densities tend to worsen the shrimps’ conditions. The fact that no deterioration in growth performance was observed at higher densities in any of the approaches, as well as the fact that the density in the field fluctuates far more naturally, than it would with a sparing measure, led to the conclusion that concerns about loss of profit with density limitation are unjustified.
While density did not serve as an explanation for any of the observed growth differences between different growth trials, the seasonal origin of the sampled individuals was found valid to do so. During the analysis of the laboratory experiments in the context of density limitation of growth, there was a striking pattern towards particularly well growing animals in different length classes, recognisable through the season. In 2011, Hufnagl and Temming published laboratory experiments that pointed to a so-called cohort effect. Animals of the same length grew very differently, which was attributed to the fact that they were caught five weeks apart, and belonged to two different cohorts. Since different growth rates within the season would have an impact on the calculations to determine stock development, the focus of Manuscript 3 was on the investigation of the so called “Cohort Effect”. Growth potential, both as a direct measure from laboratory experiments and derived from a set of dry weight condition data was compared for individuals of the common brown shrimp (Size 20-70 mm), and during the season of different years between 2006 and 2021. Both the growth experiments as well as the analysis of dry weight data, pointed at a specific cohort being consistently responsible for the highest growth observations throughout the season. Larger increments as well as shorter moulting intervals at the same experimental conditions, were found in individuals that were supposedly hatched from winter eggs. The cohort effect was observed to be stronger in some years of the time series than in others. Especially in 2019, which was also an exceptional year from the fishery point of view, no growth differences were observed within the season. The enormous shrimp stock in 2018, as well as a relatively cold winter in 2018/19, were identified as potential reasons for the absence of the cohort effect in 2019. Implications for growth calculations were highlighted, and the protection of specifically the well-growing cohort was identified as a possible management objective.
In Manuscript 4 the development of a method to determine moult interval in common brown shrimp, from frozen field samples, was intended to allow in situ growth rate without the necessity of costly and elaborate growth experiments. Besides growth increment, the actual increase in length, moulting interval determines the frequency at which the length increase occurs. Hence knowing the period between two successive moults, is essential for growth rate calculations. Since the determination of moulting interval requires time consuming and elaborate laboratory experiments, and the results are error prone to the housing condition, the need for a method of in situ moulting interval determination from field samples is high. This is also the reason, why a large part of the observed moulting intervals was not measured individually but calculated on the basis of animals moulting within a short period of time. Based on this principle, the moult interval for a given length class should be calculated using recently moulted animals within a field sample. To identify the freshly skinned animals, a device was developed that could empirically determine the carapace hardness using a method for hardness determination known from materials science. By validating the device with moulting observed in the laboratory, a method was developed based on which it is possible to identify a recent skinning by means of carapace hardness and dry weight
Effekte auf und durch das Nahrungsnetz in den auf maximalen Dauerertrag ausgerichteten Fischereien der südlichen Nordsee
No full textA main objective of fisheries management for the southern part of the North Sea under the European Commission’s Common Fisheries Policy is the achievement of maximum sustainable yields (MSY) from each stock. However, the stocks are not exploited in isolation, but are linked through biological and technical interactions, and the food-web and ecosystem they are part of. This PhD thesis describes the functioning of the southern North Sea’s food-web. It evaluates if, given this functioning, optimum yields of plaice, sole, cod, and brown shrimp can be achieved simultaneously. It also tests if such optimum solutions can be aligned with proxies of good environmental status. Potential changes in the catchabilities of sole and plaice, and their impacts upon MSY fishing are addressed. With the southern North Sea being subject to important ecosystem changes, such as increases in marine mammals and decreased nutrient loads through de-eutrophication measures, this thesis also investigates these processes’ consequences for fishing yields and strategies.
Main tool of this thesis is the parametrization and use of a food-web model of ICES areas IVb and c (Figure I on page VII), using the Ecopath with Ecosim approach and software. The time-static Ecopath model represents the structure and functioning of the southern North, which turned out to be a highly connected, mature food-web (Chapter 1). In Chapter 2, the time-dynamic Ecosim model is subjected to a range of different fishing effort regimes to seek solutions leading to concurrent optimum yields of plaice, sole, cod and brown shrimps; and to test for their compliance with proxies of good environmental status. Its results show that direct and indirect trophic interactions between the four species considerably impair these endeavours. Running empirical data analyses, drivers of changes in the catchabilities of sole and plaice were identified, and their consequences upon MSY fishing and bycatch examined using Ecosim in Chapter 3. The results indicate density-dependent changes in catchabilities of sole and plaice. Higher efforts are thus needed to obtain their MSYs in the model, leading to negative effects on bycatch and other species. Chapter 4 uses predictions made by population and ecosystem models to implement likely developments of marine mammal populations and primary productivity in the Ecosim model to explore their effects on MSYs and associated fishing strategies. Its results indicate that, while reduced system productivity severely affects fishing yields and effort strategies, marine mammals are less of a concern, but for cod fisheries.
The studies compiled in this PhD thesis present the development and use of the first model to holistically address trophic and technical interactions between the fisheries for flatfish, cod and brown shrimps in an ecosystem context in the southern North Sea. They stress the importance of considering multispecies interactions and the role of bottom-up control for MSY-infused fisheries management.Eine der Säulen der Gemeinsamen Fischereipolitik der Europäischen Union ist das Streben danach, allen befischten Beständen die größtmögliche Produktivität abzuschöpfen, sie also nach dem Prinzip des maximalen Dauerertrags (engl.: Maximum Sustainable Yield, MSY) zu befischen. Jedoch kann ein einzelner Bestand nie als abgeschlossenes System betrachtet werden, ist er doch durch biologische und fischereitechnische Interaktionen und durch das Nahrungsnetz mit anderen Beständen und Arten verwoben. Diese Dissertation quantifiziert die Funktionsweise des Nahrungsnetzes der südlichen Nordsee. Darauf aufbauend befasst sie sich mit der Frage, ob maximale Dauererträge von Schollen, Seezungen, Kabeljau und Nordseegarnelen (oder Nordsee-Krabben) simultan zu vereinbaren sind und wie derartige potentielle, mehrere Arten umfassende Optimal-Lösungen sich gegenüber Indikatoren eines guten Umweltzustandes verhalten. Ändert sich die fischereiliche Fängigkeit von Scholle und Seezunge, so kann sich das auf ein auf MSY ausgerichtetes Fischereimanagement auswirken. Auch diesem Punkt geht diese Dissertation nach. Zudem war und ist die südliche Nordsee im Prozess tiefgreifender Änderungen der Meeresumwelt. So haben etwa die Populationen von Meeressäugern nach Tiefständen in den 70ern beachtlich zugenommen. Am anderen Ende der Nahrungkette wurden seit den 80ern Maßnahmen zur Verringerung der Einfuhr von Nährstoffen durch Abwässer und Landwirtschaft ins Meer umgesetzt. Die vorliegende Dissertation widmet sich daher auch der Frage, wie sich diese Umwälzungen auf die Fischereien auswirken.
Das hauptsächliche Werkzeug dieses Promotionsvorhabens ist ein mathematisches Modell, welches das Nahrungsnetz im Managementbereich IVb und c des Internationaler Rates für Meeresforschung, ICES, darstellt (Figure I auf Seite VII). Parametrisiert wurde das Modell gemäß dem Ecopath with Ecosim Ansatz unter Verwendung der dazugehörigen Software. Das fertige Ecopath-Modell präsentiert eine Momentaufnahme von Aufbau und Funktionsweise der südlichen Nordsee, welches sich als ein hoch vernetztes und komplexes Nahrungsnetz erweist (Kapitel 1). Im zweiten Kapitel wurde im zeitlich aufgelösten Ecosim-Modell das simulierte Ökosystem einer Reihe verschiedener Szenarien gemischter Fischerei-Aufwände unterworfen um zu sehen, ob und welche Aufwandskombinationen der verschieden Flotten zum gleichzeitigen Erreichen maximaler Dauererträge von Scholle, Seezunge, Kabeljau und Nordseegarnele führen. Es zeigte sich, dass direkte und indirekte Verknüpfungen dieser vier Zielarten durch das Nahrungsnetz dieses Unterfangen behindern. Selbiges gilt für die Vereinbarkeit des maximalen simultanen Dauerertrags der vier Arten mit Indikatoren eines guten Umweltzustandes. In einer Analyse empirischer Daten zeigt sich im dritten Kapitel, dass die Fängigkeiten von Scholle und Seezunge durch Baumkurren in der Vergangenheit bei sinkenden Bestandsdichten zunahm. Implementiert man dies in dem Modell des Nahrungs- und Flottennetzes, so führen nun höhere Fischereiaufwände zu MSY als dies in einem Modell ohne dichteäbhängige Fängigkeiten der Fall wäre. Dies wirkt sich allergrößtenteils negativ auf Beifangarten und andere Komponenten des Nahrungsnetzes aus. Das vierte Kapitel dieser Dissertation stützt sich auf die Ergebnisse externer Populations- und Ökosystemmodelle, um die wahrscheinlichen Entwicklungen von Meeressäugern und der Produktivität des Phytoplanktons im Nahrungsnetz-Modell zu implementieren. Im so modifizierten Modell werden nun Fischerei-Strategien gesucht, welche zu maximalen kombinierten Dauererträgen von Scholle, Seezunge, Kabeljau und Nordseegarnele führen. Vergleicht man diese Strategien zwischen den verschiedenen Szenarien – Status quo, mit vermehrten Meeressäugern und mit verringerter Primärproduktion durch regulierten Nährstoffeinfluss – so zeigt sich ein drastischer Effekt reduzierter Algen-Produktivität, während mehr Robben und Schweinswale vor allem die Kabejau-Fischerei betreffen, sonst aber eher geringe Konsequenzen für die Fischereien verursachen.
Die in dieser Dissertation zusammengefassten Studien beschreiben die Entwicklung und Anwendung des ersten mathematischen Modells, welches die fischereitechnischen und Nahrungsbeziehungen zwischen den verschiedenen Fischereien auf Plattfische, Kabeljau und Nordseegarnele in der südlichen Nordsee im Rahmen eines Ökosystemansatzes in der Fischerei holistisch erfasst. Die Studien belegen, wie wichtig es ist, Mehrarten- und Beute-gesteuerte Interaktionen zu berücksichtigen, wenn maximale Dauererträge angestrebt werden
Effects of salinity on reproduction - and behaviour of round goby Neogobius melanostomus in the Baltic Sea
No full textOver the last centuries, the conditions and inhabitants in many ecosystems changed. This applies likewise to the relatively young Baltic Sea. It is a not fully occupied ecosystem and therefore vulnerable to disturbance by newly introduced species. One of the most successful invaders of the Baltic Sea in the last decades is the round goby Neogobius melanostomus. This bottom-living gobiid species was first observed in the early nineties in Puck Bay Poland and subsequently spread over wide areas within the Baltic Sea. Due to the changing salinity conditions from nearly freshwater in the Bothnian Sea up to nearly full marine in the Kattegat, the Baltic Sea is not uniquely well suited for round goby. For adult round gobies, some studies on salinity tolerance exist. But for a full understanding and forecast of further dispersal also the ability to reproduce in higher salinities needs to be considered. The knowledge in this field is still very limited and will hence be a focus of this thesis. Moreover it is known that there is a difference in the salinity tolerance of adult gobies depending on whether they originate from fresh water or salt water, which was the motivation to conduct a salinity preference experiment with a comparison of round goby from the brackish and freshwater origin.
The first question in this context is about the ability of round goby to bread eggs in different salinities. The ability of round goby eggs to develop in different combinations of salinity and temperature typical for the Baltic Sea, was investigated in a full factorial design. Temperature mainly affected the development time, whereas hatching success was strongly influenced by the salinity. Hatching success was generally low at 10°C but best in 5 PSU and decreasing with rising salinity. At 5 and 10 PSU hatching success declined with increasing temperature from 15°C to 20°C while at 15 and 20 PSU hatching success remained stable with high variability between clutches. The highest hatching success overall was found 5 and 10 PSU at 15°C.
To further clarify which life stage of limits mostly the spread of round goby into areas with higher salinities an experimental approach was used to determine post-hatch growth and mortality rates. An aquarium experiment was condicted to disentangle the salinity influence on length growth, weight gain, and survival of round goby larvae. Larval hatching success was very low at 25 PSU and larvae hatched at 25 PSU did not survive longer than one day. Whereas larvae that were older than 20 days at exposure (hatched at 15 PSU) survived salinities of 29PSU for 10 days. The length growth of larvae was not significantly affected by salinity, in contrast to the weight at length that was highest at 10 PSU. Moreover we conducted first tests of spAARS measurements as a possible proxy of growth, which is till today mainly used in zooplankton. This method did not deliver the expected results, and no clear correlation between spAARS and growth could be established
Finally, the question of salinity preferences of adult round goby from freshwater and brackish water populations was addressed with a shuttle box experiment. The challenge was to provide a suitable experiment setup that worked for the specific behaviour of this species. Used was the most suitable setup, a shuttle box, in which round gobies from freshwater and a brackish water habitat were exposed to the choice of salinity. Slight differences in salinity preference between the different origins (brackish and freshwater) were found. Both preferred salinities below 20 PSU. Fish from freshwater showed a tendency to lower salinities (below 15 PSU) in comparison to those from brackish water. Moreover, while 1/3 of brackishwater fishes stopped shuttling in high salinities 2/3 of the freshwater fishes did. This stopping of the shuttle behaviour indicates a higher physiological stress for the freshwater fish
Evaluierung der räumlich-zeitlichen Einflüsse von umweltlichen, politischen und ökonomischen Änderungen auf die Nordost-Atlantik Makrelen- und Heringspopulationen und Fischereien
No full textThe future availability of fisheries resources is highly dependent on international coordination of management decisions, environmental conditions as well as fleet behaviour. Managing straddling and transboundary stocks (i.e. crossing Exclusive Economic Zone boundaries into adjacent international waters) is currently especially difficult due to many changes. Two such stocks, that are highly valuable for European fleets and provide many job opportunities, are the Northeast Atlantic (NEA) mackerel (Scomber scombrus) and North Sea autumn spawning (NSAS) herring (Clupea harengus) stocks. Current conflicts resulting from lack of management coordination or changes in environmental conditions seem, however, to threaten the status of those two stocks. These, in turn, can massively impair the livelihood of European fleets and corresponding employees targeting NEA mackerel and NSAS herring. It is therefore important to evaluate the effects of new management measures and other major impact factors on both the resource availability as well as the fleet behaviour as they are interdependent. Dynamic bio-economic models are now more commonly used as tools for fisheries management. They incorporate anthropogenic as well as natural processes to generate a better understanding of feedback mechanisms between the two systems. This thesis addresses the need to evaluate the impacts of current environmental, economic and political issues on the highly valuable NEA mackerel and NSAS herring stocks and the corresponding fisheries by applying and further developing the FishRent model. It is an age-structured simulation and optimization model that incorporates detailed stock and fleet dynamics on a short- to mid-term time-frame. By identifying the optimal effort allocation under a set of constraints, it can determine the equilibrium state that optimises a certain target variable, i.e. net profit.
In the first chapter, the structure of eight pelagic fleets was investigated in order to understand the underlying data and illustrate possible differences. Furthermore, an existing version of FishRent was adapted from a demersal fishery to the pelagic and impacts of external factors (i.e. changes in recruitment, fish and fuel prices and an adaptation of the quota repartition key) on fleet net profit and stock biomass availability were determined on a temporal scale. In all scenarios, the Irish and German fleets were most vulnerable due to being very close to the economic break-even point. The implementation of a new quota repartition key according to biomass distribution of NEA mackerel and NSAS herring had the largest effect on all fleets, leading to losses of the German, Dutch and Danish fleets. The UK and Irish fleets, on the other hand, more than doubled their profit within a year. Alterations in fish and fuel prices had the second largest impacts on the eight fleets and the UK, Icelandic and large-scale Irish fleets had to disinvest nearly half of their fleet due to reporting losses. Reduced recruitment and a continued NSAS herring recruitment failure had the least influence on fleet profit compared to the other scenarios tested within this Chapter
Temperaturpräferenz und kritische untere Temperaturtoleranzgrenzen der Nordseegarnele (Crangon crangon, L.)
No full textThe common brown shrimp (Crangon crangon, L.) is a central component of the Wadden Sea ecosystem and represents one of the most valuable fisheries resources in the North Sea. As for all ectothermic organisms, temperature is considered as a central environmental factor for the common brown shrimp. During the annual cycle, brown shrimp are confronted with a wide range of environmental temperatures with thermal extremes during summer and winter. Still, our understanding of the brown shrimp’s thermal biology as well as the behavioral component of thermal selection in this species is limited. This is the more striking as contemporary shifts in the ocean’s water temperatures as well as future scenarios about climate change raise the question how aquatic ectothermic species like the common brown shrimp will respond to these changes. By investigating the mechanisms of temperature selection and thermal preference behavior as well as the brown shrimp’s thermal capacities towards low temperature, the present thesis aims to contribute to a better understanding of the brown shrimp’s thermal biology.
Chapter I describes the design, construction and evaluation of an annular chamber system for thermal preference experiments on aquatic ectotherms. This system was designed to investigate the mechanisms of temperature selection and thermal preference behavior in the common brown shrimp. Chapter I also introduces a computational approach for automated recording and data analysis of thermal preference experiments in annular chamber systems. By means of this program, an in silico comparison of different thermal gradient representations and temperature assignment procedures was conducted, evaluating the effect of spatial resolution on thermal preference estimates. The results of this chapter revealed annular chamber systems to represent a powerful tool for determining thermal preferenda of aquatic ectotherms. Automated data recording and analysis simplified the implementation of thermal preference experiments considerably and allowed for highly resolved thermal preference data for even prolonged experimental trials.
Chapter II aims to establish a methodological framework for thermal preference experiments on the common brown shrimp. This chapter therefore investigates and compares the short- and long-term thermoregulatory behavior of adult common brown shrimp by means of acute and gravitational thermal preference experiments using the annular chamber system. For the acute approach, brown shrimp were acclimated to 5 temperatures between 9°C and 19°C and for the gravitational approach to 3 temperatures within the same range. Following acclimation, thermoregulatory and thermal preference behavior of brown shrimp were studied for 2 h (acute) and up to 48 h (gravitational). The results of these experiments revealed brown shrimp to be thermosensitive and perform behavioral thermoregulation. Thus, temperature can be considered as a directive factor in the behavior of this species. Acute preferenda were found to be highly affected by prior thermal acclimation resulting in a final thermal preferendum of 15.9°C. In contrast gravitational preferenda were unaffected by the prior thermal history and brown shrimp selected a uniform thermal preferendum when being exposed to the thermal gradient for at least 20-24h. Compared to the acute preferendum, gravitational temperature preferenda were lower and ranged between 13.5-15.0°C after 24 h and 12.0-14.9°C after 48 h, respectively. No significant difference between the 24 h and 48 h thermal preferendum was detected. Based on the results of this chapter, gravitational thermal preferenda of the common brown shrimp can be obtained after 20 h of gradient exposure, representing a shorter period as reported for other aquatic ectothermic species before.
Chapter III uses the annular chamber system from Chapter I to investigate the seasonal thermal preferenda of the common brown shrimp during a 14 month period. Gravitational thermal preferenda were determined for juvenile and adult, male and female brown shrimp of different size groups, sampled at several localities in the German Wadden Sea. These experiments should reveal whether brown shrimp show distinct seasonal thermal preferenda and whether the final thermal preferendum paradigm holds for the common brown shrimp. The results of these experiments revealed a huge variability in thermal selection of the common brown shrimp. Thermal preference of brown shrimp differed throughout the seasonal cycle with low preferenda during winter and high preferenda during late summer. However, thermal preferenda also differed between the cold winter 2010/11 and the less cold winter 2011/12. Statistical analysis by means of a generalized additive model (GAM) revealed seasonality as well as body size as main determinants of thermal selection. Moreover, the temporal development of thermal preference over time also suggested an effect of cohort on thermal selection. Consequently, brown shrimp did not share a common thermal preferendum and thus thermal selection of the common brown shrimp does not comply with the final thermal preferendum paradigm, representing the first evidence for a marine, invertebrate ectotherm.
In Chapter IV, the lower thermal capacity limits of the common brown shrimp were analyzed by means of laboratory experiments determining the critical thermal minima (CTmin) as well as the critical lethal minima (CLmin). CTmin were analyzed in animals acclimated to 4.0, 9.0 and 14.0°C using a cooling rate of -0.2°Cmin-1 whereas CLmin were determined at a cooling rate of -1.0°C day-1 without prior acclimation. Both types of critical lower thermal limits were obtained for brown shrimp of different body size, gender and maturation state. CTmin were significantly affected by acclimation temperature and a positive correlation of acclimation temperature and lower thermal tolerance was identified. Depending on the acclimation temperature, CTmin in brown shrimp varied from -1.4°C to 2.5°C. In contrast to acclimation temperature, thermal tolerance just varied slightly with gender and no effect of size or maturation state was identified. As brown shrimp even tolerated the lowest temperature that could be established in the setup, the CLmin could not be determined. In the CLmin experiments, however, a negative relationship of temperature and reactivity within the range of 7°C and 1°C was observed. When triggered with a single electrical pulse, the number of flicks first increased as temperature decreased. This relationship broke down between 1°C and 0°C where an abrupt drop in reactivity of the shrimp became apparent. The results of this chapter demonstrated a high potential of adaptation towards a wide range of temperatures in brown shrimp including even subzero temperatures. However, low responsiveness at temperatures approaching 0°C also reveals that brown shrimp are adversely affected by low temperatures. In addition, low responsiveness of brown shrimp at low temperatures is of high relevance for scientific surveys, as parts of these surveys are being conducted during the cold winter season.Die Nordseegarnele (Crangon crangon, L.) stellt eine Schlüsselart für das Ökosystem des Wattenmeeres dar und ist gleichzeitig eine der wertvollsten fischereilich genutzten Ressourcen in der Nordsee. Wie für alle ectothermen Organismen so ist auch für die Nordseegarnele Temperatur ein Umweltparameter von zentraler Bedeutung. Im Jahresverlauf ist die Nordseegarnele großen Temperaturunterschieden mit Extremtemperaturen während des Sommers sowie Winters ausgesetzt. Trotz der großen Bedeutung von Temperatur ist deren Einfluss auf viele Aspekte der Biologie der Nordseegarnele bisher noch unzureichend beschrieben. Dies gilt vor allem für das Temperaturwahl- und Temperaturpräferenzverhalten von Nordseegarnelen. Aufgrund gegenwärtig festgestellter sowie prognostizierter Veränderungen in den Temperaturen der Weltmeere ist ein genaues Verständnis dieser Zusammenhänge allerdings von großer Wichtigkeit, um abschätzen zu können, wie wechselwarme Organismen wie die Nordseegarnele auf diese Änderungen reagieren werden. Diese Arbeit versucht deshalb mittels der Untersuchung des Temperaturwahlverhaltens sowie der Fähigkeit niedrige Temperaturen zu überdauern einen Beitrag zu einem besseren Verständnis hinsichtlich der Bedeutung von Temperatur auf die Nordseegarnele zu liefern.
In Kapitel I dieser Arbeit wird die Entwicklung, Konstruktion und Evaluierung einer ringförmigen Temperaturpräferenzorgel beschrieben, mittels derer das Temperaturwahl- sowie Temperaturpräferenzverhaltens bei Nordseegarnelen untersucht werden soll. In diesem Kapitel wird zudem ein computergesteuertes Verfahren zur automatischen Aufzeichnung und Auswertung von Temperaturpräferenzversuchen in ringförmigen Temperaturpräferenzorgeln vorgestellt. Mit Hilfe dieses computergesteuerten Verfahrens wurde zudem ein Simulationsansatz unternommen, um den Zusammenhang zwischen der Präzision in der Auflösung des Temperaturgradienten und Temperaturzuweisung auf die ermittelten Präferenda zu analysieren. Anhand der Ergebnisse dieses Kapitels konnte gezeigt werden, dass ringförmige Temperaturpräferenzorgeln ein vorteilhaftes System zur experimentellen Bestimmung der Temperaturpräferenz darstellen. Mittels des computer-gesteuerten Verfahrens zur automatischen Aufzeichnung und Auswertung konnte die Durchführung von Experimenten zur Temperaturpräferenzbestimmung erheblich erleichtert werden. Zudem wird die Erhebung hochaufgelöster Daten, auch während langandauernder Versuchsansätze ermöglicht.
In Kapitel II wird ein methodischer Ansatz zur Bestimmung der Temperaturpräferenz bei Nordseegarnelen vorgestellt. Hierfür werden das kurz- und langfristige Temperatur-wahlverhalten adulter Nordseegarnelen mittels der akuten und gravitationellen Methode zur Präferenzbestimmung in der ringförmigen Temperaturorgel untersucht. Für den Kurzzeitansatz wurden Nordseegarnelen bei 5 unterschiedlichen Temperaturen zwischen 9°C und 19°C akklimiert. Für den gravitationellen Ansatz wurden die Tiere bei 3 Temperaturen innerhalb des gleichen Bereichs akklimiert. Hiernach wurden die Tiere dem Temperaturgradienten in der Temperaturorgel für 2 Stunden (akut) und bis zu 48 Stunden (gravitationell) ausgesetzt und die Temperaturpräferenz bestimmt. In beiden Ansätzen konnte gezeigt werden, dass Nordseegarnelen temperaturempfindlich sind und ihre Körpertemperatur mittels des Verhaltens regulieren. Hieraus kann man ableiten, dass Temperatur für die Nordseegarnele einen richtungsweisenden Umweltfaktor darstellt und das Verhalten beeinflussen kann. Die akuten Temperaturpräferenda waren hierbei hochgradig von der Akklimationstemperatur abhängig und resultierten in einer finalen thermischen Temperaturpräferenz von 15.9°C. Im Gegensatz dazu war die gravitationelle Temperatur-wahl von der vorherigen Akklimationstemperatur unbeeinflusst und Nordseegarnelen selektierten einheitliche Temperaturen nachdem sie dem Temperaturgradienten für mindestens 20-24 h ausgesetzt waren. Im gravitationellen Versuchsansatz selektierten die Nordseegarnelen niedrigere Temperaturen als in den Versuchen zur akuten Präferenz. Nach 24 h selektierten die Nordseegarnelen Temperaturen von 13.5-15.0°C und nach 48 Stunden 12.0-14.9°C. Die unterschiedlichen Temperaturpräferenzen unterschieden sich jedoch nicht statistisch signifikant voneinander. Ausgehend von diesen Ergebnissen kann die gravitationelle Temperaturpräferenz, welche von vorherigen Temperatureffekten unbeeinflusst ist, frühestens 20 Stunden nach Einbringen in einen Temperaturgradienten bestimmt werden. Dieser Zeitraum ist kürzer als er für andere aquatische ectotherme Organismen angeben wurde.
In Kapitel III wurde das Temperaturwahlverhalten von Nordseegarnelen im Jahresverlauf untersucht. Hierfür wurde die gravitationelle Temperaturpräferenz juveniler und adulter Garnelen beider Geschlechter sowie unterschiedlicher Körpergrößen, welche aus verschiedenen Bereichen des deutschen Wattenmeeres stammten, über eine Versuchsdauer von 14 Monaten in der ringförmigen Temperaturorgel bestimmt. Neben der Erhebung der saisonalen Temperaturpräferenz sollte mit diesen Versuchen zudem ermittelt werden, ob das „final thermal preferendum paradigm“ für Nordseegarnelen gültig ist. Die Ergebnisse dieser Experimente zeigten, dass das Temperaturwahlverhalten von Nordseegarnelen hochgradig variabel und im Jahresverlauf erheblichen Änderungen unterworfen ist. Nordseegarnelen präferierten im Winter tiefe und im Spätsommer hohe Temperaturen. Die Präferenzen unterschieden sich allerdings auch zwischen dem kalten Winter 2010/11 und dem milden Winter 2011/12. Eine statistische Analyse mittels einer nichtparametrischen Regression (generalized additive model, GAM) erbrachte, dass dies vor allem auf saisonale sowie auf Unterschiede zwischen den Größenklassen zurückzuführen ist. Die zeitliche Entwicklung der Temperaturpräferenz lässt zudem darauf schließen, dass auch die Kohortenzugehörigkeit Einfluss auf die Temperaturwahl hat. Hierdurch ergab sich, dass Nordseegarnelen keine einheitliche finale thermische Präferenztemperatur selektieren. Anhand der Versuche an der Nordseegarnele konnte das final thermal preferendum paradigm somit zum ersten Mal für einen marinen, evertebraten Organismus widerlegt werden.
In Kapitel IV wurde die Toleranz von Nordseegarnelen gegenüber tiefen Temperaturen mittels Kurz- und Langezeitversuchen zur Bestimmung der kritischen thermischen Minima (CTmin) und der kritischen lethalen Minima (CLmin) untersucht. Zur Bestimmung der CTmin wurden Nordseegarnelen bei 4.0, 9.0 und 14.0°C akklimiert. Die CTmin wurden bei einer Kühlungsrate von -0.2°C pro Tag ermittelt. Die Bestimmung der CLmin hingegen erfolgte bei einer Kühlungsrate von -1.0°C pro Tag und ohne vorherige Akklimierung. Beide Ansätze wurden für Tiere von unterschiedlicher Größe, Geschlecht und Geschlechtsreife durchgeführt. Die CTmin waren statistisch signifikant von der Akklimationstemperatur beeinflusst und variierten zwischen -1.4°C und 2.5°C. Das Geschlecht der Nordseegarnelen beeinflusste die CTmin dagegen nur geringfügig. Körpergröße und Geschlechtsreife hatten keinen nachweisbaren Einfluss. Da die Garnelen die niedrigste Temperatur, die in der Versuchsanlage erzeugt werden konnte zu überdauern vermochten, konnte das CLmin nicht bestimmt werden. In den Versuchen zur Bestimmung der CLmin konnte allerdings ein negativer Zusammenhang zwischen der Wassertemperatur und dem Reaktionsvermögen im Bereich von 7°C bis 1°C festgestellt werden. Dieser Zusammenhang brach jedoch unterhalb von 1°C abrupt ab und die Nordseegarnelen zeigten ein stark verringertes Reaktionsvermögen zwischen 1°C und 0°C. Temperaturen unterhalb von 1°C können für Nordseegarnelen deshalb als kritisch angesehen werden. Die Ergebnisse dieses Kapitels bestätigten das große Anpassungsvermögen von Nordseegarnelen gegenüber eines weiten Temperaturbereichs. Zudem wurde gezeigt, dass Nordseegarnelen Minustemperaturen überdauern können. Der negative Zusammenhang von Temperatur und Reaktionsvermögen zeigt jedoch auch, dass Temperaturen unter 0°C einen negativen Einfluss auf Nordseegarnelen haben. Die Erkenntnis des stark verringerten Reaktionsvermögens bei niedrigen Temperaturen ist von hoher Relevanz für wissenschaftliche Feldbeprobungen, die standardisiert auch im Winter durchgeführt werden
Steuerungsfaktoren der Alters-Längen-Beziehung : Einfluss von Geschlecht, Nahrungsverfügbarkeit, Temperatur und Fischerei auf die Wachstumsleistung von Wittling Merlangius merlangus und Kabeljau Gadus morhua in der Nordsee
No full textStoffwechselraten und Fressverhalten der Sprotte, Sprattus sprattus L.
No full textThe aim of the present thesis was to investigate physiological and behavioural attributes relevant for the ongoing development of bioenergetics growth and population models for Baltic Sea sprat. As a central component of a bioenergetics model for sprat, the effects of temperature and body mass on metabolic rates were investigated by measuring oxygen consumption rates of sprat over nine different temperatures between 9 and 21°C. Standard metabolic rates (Rs) were on average 12% lower than routine metabolic rates (RR) and still influenced by continuous swimming activity. Metabolic rates increased exponentially with temperature similarly as reported for other clupeoids, but the metabolic scaling exponent was higher (b = 1.073) than typically found exponents of b ~ 0.8. This is most likely a consequence of the permanently elevated activity level in sprat. The high permanent swimming activities and the little difference between RS and RR indicated that the concept of standard metabolism may not be meaningful in schooling planktivorous fish. Since swimming has in general a great impact on fish metabolism, the costs for spontaneous swimming patterns in sprat were determined from respirometry experiments with simultaneous image recordings of fish. Therefore only measurements obtained in the acclimation periods were used, because here swimming activities and metabolic rates were more variable than during later routine phases. An automated fish-tracking program was applied in order to detect different movement patterns like mean swimming speeds and turning rates from simultaneous image recordings. Spontaneous swimming was dominated by low swimming speeds and frequent sharp turns. Temperature had no direct effect on the performed swimming activity, but MO2 showed a greater increase with swimming speed at higher temperatures. The final model for the energy costs of spontaneous activity can be used to approximate for swimming costs when the frequency of such swimming patterns is known. So far no information on feeding rates in relation to prey densities is available for sprat, although this is essential for the evaluation of maximum consumption rates. Sprat and herring are directly competing for the same food resources, especially during the juvenile phase when both clupeids school together in shallow coastal regions. Thus, laboratory feeding experiments were conducted in order to investigate the relationship between particulate-feeding rates and prey concentrations of juvenile sprat and herring. Two different single prey types were used, which differed in their escape response to predator presence. While herring were occasionally filter-feeding, sprat were strictly sticking to particulate feeding mode. Both fishes showed a type-II functional response with lower feeding rates when fish preyed on A. tonsa compared to experiments with Artemia. During feeding on copepods, both fishes showed an S-shaped deformation of their body before the biting attack, which is related to the escape response of A. tonsa. Functional response parameters were not significantly different between sprat and herring, despite at copepods concentrations below ~40 l-1, where herring could achieve higher biting rates than sprat. The parameters of the functional response curves can improve fish bioenergetics models implemented into end-to-end ecosystem models. However, since a realistic model framework must include estimates of swimming patterns in relation to prey densities, the above mentioned feeding experiments were also used to investigate foraging related swimming patterns in sprat and herring. Therefore, swimming patterns and behavioural components were analysed from digital images. Both fish species increased their horizontal swimming activity with prey concentrations. Overall were the maximum values of all horizontal swimming patterns significantly lower when fish were feeding on copepods instead of Artemia. Transferred into energy costs associated with feeding, the present results showed that herring had a clear energetic advantage above sprat. Vertical swimming speeds had a great impact on the total swimming speed at concentrations >20 l-1, but were less pronounced at lower prey concentrations. At high prey concentrations vertical swimming speeds were up to 1.6-times higher than horizontal speeds. Transferred into energy costs this indicates that the high vertical speeds are energetically efficient only at prey concentrations above ~14 l-1, which is in accordance with the observed behaviour of sprat and herring. The knowledge and data obtained by the present study can serve as basis for the development of bioenergetics budgets for sprat considering (I) body mass and temperature effects on metabolic rates, (II) the influence of activity on metabolism in spontaneously active sprat, (III) feeding rates in relation to prey concentrations and types and (IV) foraging related swimming patterns in relation to prey concentrations.Das Ziel der vorliegenden Arbeit war es, physiologische Prozesse und Verhaltensmuster zu untersuchen, welche für die laufende Entwicklung von bioenergetischen Wachstums- und Populationsmodellen für Ostseesprotten von Relevanz sind. Als zentrale Komponente eines bioenergetischen Modells wurde der Einfluss von Temperatur und Körpergröße auf Stoffwechselraten bestimmt. Dafür wurde der Sauerstoffverbrauch von Sprotten bei neun verschiedenen Temperaturen zwischen 9 und 21°C gemessen. Der Standardstoffwechsel (RS) war dabei im Mittel nur 12% niedriger als der Routinestoffwechsel (RR) und immer noch von andauernden Schwimmaktivitäten der Fische beeinflusst. Die Stoffwechselraten stiegen exponentiell mit der Temperatur in ähnlicher Weise wie es bereits für andere Clupeoide bekannt ist. Allerdings ergab sich ein höher Exponent für die Größenabhängigkeit des Stoffwechsels (b = 1.073) als der typischer Weise verwendete Exponent von b ~ 0.8. Dies ist wahrscheinlich durch die permanente Schwimmaktivität der Sprotten begründet. Die hohen andauernden Schwimmaktivitäten und der geringe Unterschied zwischen Standard-und Routinestoffwechsel weisen darauf hin, dass das theoretische Konzept einer Standardstoffwechselrate unter Ausschluss jeglicher Spontanaktivität für pelagische Schwarmfische nicht sinnvoll ist. Da das Schwimmen generell einen großen Einfluss auf die Stoffwechselrate bei Fischen hat, wurden die energetischen Kosten spontaner Schwimmaktivitäten anhand einiger Respirometrie-Experimente bestimmt, bei denen eine zeitgleiche Bildaufnahme der Fische vorlag. Dafür wurden nur Messungen aus der Akklimatisierungsphase der Versuche verwendet, da hier die Schwimmaktivitäten und Sauerstoffverbrauchsraten variabler waren als in den späteren Routinephasen. Ein automatisches Bildanalyseprogramm zur Fischverfolgung wurde verwendet, um anhand der aufgenommenen Bilder verschiedene Bewegungsmuster zu bestimmen. Die Spontanaktivität war dominiert durch niedrige Schwimmgeschwindigkeiten und häufige, scharfe Körperdrehungen. Die Temperatur hatte keinen direkten Einfluss auf die Schwimmaktivitäten, jedoch stieg der Sauerstoffverbrauch bei höheren Temperaturen steiler mit der Schwimmgeschwindigkeit an als bei niedrigeren Temperaturen. Das finale Modell für die energetischen Kosten spontaner Aktivität kann angewendet werden um die Schwimmkosten im Feld abzuschätzen, wenn die Häufigkeit dieser Bewegungsmuster bekannt ist. Bislang liegen keine Informationen zur Abhängigkeit der Fressrate von der Beutekonzentration bei Sprotten vor, obwohl diese Information essentiell zur Beurteilung der maximalen Nahrungsaufnahme ist. Sprotten und Heringe konkurrieren um dieselben Nahrungsressourcen, vor allem während der juvenilen Lebensphase. Um die Abhängigkeit der Fressrate von der Beutekonzentration bei juvenilen Sprotten und Heringen zu bestimmen, wurden daher umfangreiche Laborversuche durchgeführt. Dabei wurden zwei verschiedene Beutearten verwendet, welche sich durch ihr Fluchtverhalten unterscheiden. Während Heringe gelegentlich zum Filtrieren übergingen, blieben Sprotten strikt bei der partikulären Nahrungsaufnahme. Beide Fischarten zeigten eine type-II-functional response mit signifikant niedrigeren Fressraten bei Versuchen mit A. tonsa im Vergleich zu Experimenten mit Artemien. Beim Fressen von Copepoden zeigten beide Fischarten eine S-förmige Körperverbiegung unmittelbar vor dem Schnappen, was durch das Fluchtverhalten von A. tonsa verursacht wurde. Die Parameter der functional response-Kurven waren nicht signifikant verschieden zwischen Sprotten und Heringen, außer bei Konzentrationen unter 40 Copepoden l-1, wo Heringe höhere Schnappraten erreichen konnten. Die Parameter der functional response-Kurven können bioenergetische Modelle verbessern, welche in umfassende Ökosystemmodelle eingebunden sind. Da aber eine realistische Modelstruktur auch Abschätzungen der Schwimmaktivitäten bei verschiedenen Beutedichten enthalten muss, wurden die Fress-Experimente auch dazu verwendet, die fressbedingten Bewegungsmuster bei Sprotten und Heringen zu untersuchen. Dafür wurden die Schwimmbewegungen und das Verhaltensmuster anhand digitaler Bilder bestimmt. Beide Fischarten steigerten die horizontale Schwimmaktivität mit der Beutekonzentration. Insgesamt ergaben sich für alle horizontalen Schwimm-Muster signifikant niedrigere Maximalwerte wenn die Fische Copepoden gefressen haben anstelle von Artemien. Übertragen in Energiekosten durch das Fressen, deuten die Ergebnisse dieser Studie darauf hin, dass Heringe einen klaren energetischen Vorteil gegenüber Sprotten haben. Die vertikalen Schwimmgeschwindigkeiten hatten einen großen Einfluss auf die Gesamtschwimmgeschwindigkeit bei Konzentrationen >20 l-1, waren aber weniger stark ausgeprägt bei niedrigeren Konzentrationen. Bei hohen Beutekonzentrationen (>40 l-1) waren die vertikalen Schwimmgeschwindigkeiten bis zu 1.6-mal höher als die horizontalen Geschwindigkeiten. In Energiekosten übertragen würde dies bedeuten, dass sich die hohen vertikalen Geschwindigkeiten erst ab einer Beutekonzentration von etwa 14 l-1 lohnen, was mit dem beobachteten Verhalten von Sprotten und Heringen übereinstimmt. Die Ergebnisse der vorliegenden Arbeit können als Basis für die Weiterentwicklung eines bioenergetischen Modells für Sprotten dienen und liefern Informationen zum (I) Einfluss von Körpergröße und Temperatur auf Stoffwechselraten, (II) zu den energetischen Kosten spontaner Schwimmbewegungen, (III) zur Fressrate in Abhängigkeit von Beutekonzentration und -art bei Sprotten und Heringen gleicher Größe, sowie (IV) zu fressabhängigen Bewegungsmustern
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