1,319 research outputs found
Estimating the distribution of demand for Antarctic krill (Euphauisa superba) from land-based predators at South Georgia
No full textSouth Georgia is renowned for the abundance of Antarctic krill (Euphausia superba) and a range of krill predators. Variability in krill availability at a range of scales, and the consequences of this for predator-prey interactions, mean that quantifying the spatially explicit demand for krill by those predators is essential to understanding the mechanisms underlying ecosystem changes in the region. In this thesis demand within a distinct study box to the northwest of the island has been assessed. The thesis has three sections; (1) the number of predators; (2) the distribution of predators; and (3) the demand for krill by those predators.
(1) Predator densities with confidence intervals were determined from appropriately designed shipboard transect survey; counts of Antarctic fur seals (Arctocephalus gazella), macaroni penguins (Eudyptes chrysolophus), gentoo penguins (Pygoscelis papua), and Antarctic prions (Pachyptila desolata) were adjusted for sea state, distance from observer and dive behaviour. Providing the first at-sea predator density estimates for the region.
(2) A comparison of the distribution of female Antarctic fur seals engaged in pup-rearing (using satellite telemetry) and the whole population that were not restricted to a single part of the population (from shipboard transect survey) was undertaken. Using two general additive models based on the relationship between seal distribution (one derived from transect and the other from telemetry) and the physical environment indicated that the spatial distribution of lactating females is representative of the general population.
(3) Using the derived predator density, the local krill demand estimate was 2581 tonnes krill per day, a consumption rate of 0.45% per day of the concurrently estimated krill biomass (using shipboard acoustics). Antarctic fur seals accounted for 75% of this demand. This level of demand was less than the increase in biomass resulting from krill growth. However, based on the length-specific demand, determined from concurrent predator diet samples demand exceeded growth for krill >48mm
Acoustic and ecological investigations into predator-prey interactions between Antarctic krill (Euphausia superba) and seal and bird predators
No full text1. Antarctic krill (Euphausia superba) form aggregations known as swarms that vary greatly in size and density. Six acoustic surveys were conducted as part of multidisciplinary studies at two study sites, the western and eastern core boxes (WCB and ECB), during the 1997, 1998 and 1999 austral summers, at South Georgia. A quantitative, automated, image processing algorithm was used to identify swarms, and calculate swarm descriptors, or metrics. In contrast to acoustic surveys of aggregations of other pelagic species, a strong correlation (r = 0.88, p = 0.02, 95% C.I.= 0.24 to 0.99) between the number of krill swarms and the mean areal krill density [rho.hat] was found. Multivariate analysis was used to partition swarms into three types, based on contrasting morphological and internal krill density parameters. Swarm types were distributed differently between inter-surveys and between on and off-shelf regions.
This swarm type variation has implications for krill predators, by causing spatial heterogeneity in swarm detectability, suggesting that for optimal foraging to occur, predators must engage in some sort of adaptive foraging strategy.
2. Krill predator-prey interactions were found to occur at multiple spatial and temporal scales, in a nested, or hierarchical structure. At the largest inter-survey scale, an index of variability, I, was developed to compare variation in survey-scale predator sightings, sea temperature and [rho.hat]. Using I and a two-way ANOVA, core box, rather than year, was found to be a more important factor in determining species distribution. The absence of Blue-petrels (Halobaena caerulea) and the elevated number of Antarctic fur seals (Arctocephalus gazella) suggest that 1998 was a characterised by colder than average water surrounding South Georgia, and a high [rho.hat] in the ECB. At the smaller, intra-survey scales (<80 km, <5 day), the characteristic scale (distances in which
predator group size, or krill density were similar, L_s) were determined. For krill and predators L_s varied by survey and the L_s of krill also varied by depth within a survey. Overlap in L_s were stronger between predator species than between a predator species and krill, indicating predators were taking foraging cues from the activity of predators, rather than from the underlying krill distribution. No relationship was found between swarm characteristics and predator activity, suggesting either there is no relationship between krill swarms and predators, or that the predator and acoustic observation techniques may not be appropriate to detect such a relationship.
3. To overcome the 2-D sampling limitations of conventional echosounders, a multibeam echosounder (MBE) observed entire swarms in three-dimensions. Swarms found in the nearshore environment of Livingston Island situated in the South Shetland Islands, exhibited only a narrow range of surface area to volume ratios or roughnesses (R = 3.3, CV = 0.23), suggesting that krill adopt a consistent group behaviour to maintain swarm shape. Generalized additive models (GAM) suggested that the presence of air-breathing predators influenced the shape of a krill swarm (R decreased in the presence of predators: the swarm became more spherical). A 2D distance sampling framework was used to estimate the abundance, N, and associated variance of krill swarms. This technique took into account angular and range detectability (half-normal, [sigma_r.hat] = 365.00 m, CV = 0.16) and determined the vertical distribution of krill swarms to be best approximated by a beta-distribution ([alpha.hat] = 2.62, [CV.hat] = 0.19; [beta.hat] = 2.41, [CV.hat] = 0.15), giving the abundance of swarms in survey region as [N.hat] = 5,062 ([CV.hat] = 0.35). This research represents a substantial contribution to developing estimation of pelagic biomass using MBEs.
4. When using a single- or split-beam missing pings occur when the transmit or receive cycles are interrupted, often by aeration of the water column, under the echosounder transducer during rough weather. A thin-plate regression spline based approach was used to model the missing krill data, with knots chosen using a branch and bound algorithm. This method performs well for acoustic observations of krill swarms where data are tightly clustered and change rapidly. For these data the technique outperformed the standard MGCV GAM, and the technique is applicable for estimating acoustically derived biomass from line transect surveys
The application of optical coherence tomography to image subsurface tissue structure of Antarctic krill Euphausia superba
No full textMany small open ocean animals, such as Antarctic krill, are an important part of marine ecosystems. To discover what will happen to animals such as krill in a changing ocean, experiments are run in aquaria where conditions can be controlled to simulate water characteristics predicted to occur in the future. The response of individual animals to changing water conditions can be hard to observe, and with current observation techniques it is very difficult to follow the progress of an individual animal through its life. Optical coherence tomography (OCT) is an optical imaging technique that allows images at high resolution to be obtained from depths up to a few millimeters inside biological specimens. It is compatible with in vivo imaging and can be used repeatedly on the same specimens. In this work, we show how OCT may be applied to post mortem krill samples and how important physiological data such as shell thickness and estimates of organ volume can be obtained. Using OCT we find an average value for the thickness of krill exoskeleton to be (30±4) µm along a 1 cm length of the animal body. We also show that the technique may be used to provide detailed imagery of the internal structure of a pleopod joint and provide an estimate for the heart volume of (0.73±0.03) mm3
Seabed foraging by Antarctic krill: Implications for stock assessment, bentho-pelagic coupling, and the vertical transfer of iron
No full textA compilation of more than 30 studies shows that adult Antarctic krill (Euphausia superba) may frequent benthic habitats year-round, in shelf as well as oceanic waters and throughout their circumpolar range. Net and acoustic data from the Scotia Sea show that in summer 2-20% of the population reside at depths between 200 and 2000 m, and that large aggregations can form above the seabed. Local differences in the vertical distribution of krill indicate that reduced feeding success in surface waters, either due to predator encounter or food shortage, might initiate such deep migrations and results in benthic feeding. Fatty acid and microscopic analyses of stomach content confirm two different foraging habitats for Antarctic krill: the upper ocean, where fresh phytoplankton is the main food source, and deeper water or the seabed, where detritus and copepods are consumed. Krill caught in upper waters retain signals of benthic feeding, suggesting frequent and dynamic exchange between surface and seabed. Krill contained up to 260 nmol iron per stomach when returning from seabed feeding. About 5% of this iron is labile, i.e., potentially available to phytoplankton. Due to their large biomass, frequent benthic feeding, and acidic digestion of particulate iron, krill might facilitate an input of new iron to Southern Ocean surface waters. Deep migrations and foraging at the seabed are significant parts of krill ecology, and the vertical fluxes involved in this behavior are important for the coupling of benthic and pelagic food webs and their elemental repositories
Ecological investigations of euphausiids at high latitudes
No full text1. Euphausiids are an important component of high latitude pelagic ecosystems, but there is a paucity of information on their distribution, abundance and population processes on within-year time scales. This thesis encompasses new research into the euphausiid-ocean component of two important high latitude ecosystems (South Georgia and the Irminger Sea) on sub-annual time scales.
2. A new method for measuring abundance of Antarctic krill (Euphausia superba) continuously at South Georgia (Southern Ocean) was devised using upward-looking acoustic devices deployed on moorings. These novel platforms provide a new window of observations onto marine systems not open from conventional research vessels. At South Georgia, the moorings provided data at a high temporal resolution giving completely new insight to the function of the coupled biological-physical marine ecosystem. The use of moorings may aid ecosystem-based management at South Georgia and elsewhere.
3. Analysis of mooring data collected between October 2002 and December 2005 indicated a regular annual cycle in krill density: high in summer and low in winter. Mooring estimates of krill density were not statistically different from estimates derived from standard ship-based surveys in adjacent time periods suggesting that the mooring point estimates had relevance in a wider spatial context (c. 100 x 100 km). The results indicated that because of the sharp peaks in the biomass cycle, the exact timing of repeated ship-based acoustic surveys might be critical. Surveys that differ in their timing by only a few weeks might exhibit quite different estimates of biomass because they fall at different points of the cycle. Additionally, within this intra-annual framework, annual ship-based surveys may be able to detect differences between high and low krill years only if they differ by densities of c. 35 g per square m. in summer and c. 20 g per square m. in winter.
4. The mechanisms driving intra-annual variability in Antarctic krill density at South Georgia are likely to be complex. Analysis of mooring data revealed a possible association between high krill density and low water temperatures (at 200 m) at South Georgia. There was evidence that seasonal variation in krill density off-shelf was linked to seasonal variation in current velocity: marked increases in velocity at the end of summer coincided with marked decreases in krill density and abrupt changes in water temperature characteristic of the Sub-Antarctic Current Front (SACCF). Oceanographic data indicated that the SACCF might have impinged in proximity to the moorings during the winter season. However, krill densities were low during these periods and analyses suggest that seasonal variations in krill density were not driven by seasonal oscillations in the position of the SACCF. The data are not consistent with a pattern of seasonal growth, production and mortality of a resident krill population at South Georgia, but are consistent with the notion of large influxes of krill in early summer, and of a predator-driven reduction at between mid- and late-summer.
5. The seasonal distribution, abundance and growth of key euphauiids in the Irminger Sea, North Atlantic was quantified using the first net haul data from the region since the 1930s. Results show a high degree of spatial heterogeneity in the seasonal distribution of euphausiid abundance/biomass during 2001-2002. There was evidence to suggest regional variation in growth and population processes of Meganyctiphanes norvegica and Thysanoessa longicaudata, and this corresponded broadly to distinct physical zones in the Irminger Sea. There were, however, no significant links between growth and temperature and chlorophyll a concentration. This was attributed to high spatial and temporal variability in biological and physical sampling. These data are a prerequisite for understanding ecosystems dynamics in the North Atlantic, and are important for robust ecosystem-based management strategies.
6. Controls on euphausiid spatial heterogeneity at high latitudes are likely to be complex. Important factors include horizontal advection, temperature, resource availability and behavioural mechanisms. Short-term (intra-annual), small-scale (basin-scale) data are fundamental to understanding variability in euphausiid abundance and distribution on broader spatial and temporal scales in these ecosystems
David G. Krill
No full textMarquette University alumnus David G. Krill '64, who received the College of Business Administration's Alumni Service Award in 1997
Multi-species models of Antarctic krill predators : do competitive effects influence estimates of pre-exploitation whale abundance and recovery?
No full textIncludes abstract.
Includes bibliographical references (leaves 92-111).Many species of baleen whales and seals in the Southern Hemisphere were subject to intensive overexploitation by commercial harvesting in the last two centuries, and many populations were reduced to very low levels. Krill is the dominant prey item of these species. Harvesting (to near extinction) of the large baleen whales (blue, humpback and fin whales) from the start of the 20th century led to a likely increase in the availability of krill to other krill predators such as the Antarctic minke whales and crabeater seals. This phenomenon is referred to as the “krill surplus” hypothesis and has been a central hypothesis of Antarctic ecosystem studies. This thesis aims to better understand species interactions in the Antarctic through developing and extending multispecies models of the system. The study considered only Region A (IWC Management Areas II, III and IV, 60°W to 130°E) because the numbers of baleen whales harvested in Atlantic/Indian Oceans were far greater than in other Oceans, so that the impacts on the dynamics of these species are likely greater. The simple models of competition between blue and fin whales developed give qualitatively similar results to the Mori-Butterworth Antarctic ecosystem model of an initial number of fin whales before exploitation began that is much lower than single species models suggest. However, there are important features of blue and fin whale CPUE data off Durban over the middle decades of the last century that are not reflected by the model results, and a number of possible reasons for this are advanced. In particular, the introduction of competition in the models predicts a steady fin whale population until 1950, but cannot reproduce the feature in the CPUE data of an increase from the 1920’s to 1950’s. The study then extends the Mori-Butterworth Antarctic ecosystem model by adding squid, which has fast dynamics compared to whales and seals. The model estimates population trends in terms of numbers or biomass. This study indicates that results are particularly sensitive to the density dependence assumed for natural mortality and/or birth rate. The results highlight that the squid biomass trajectory is relatively insensitive to initial squid abundance but depends strongly on the density dependence assumed for squid. Generally, the estimated historical trajectories suggest that the inclusion of squid in the model hardly impacts the maxima reached by other species that benefited from the krill surplus. The model predicts that squid started to increase at about the same time (1920) that the reduction of large baleen whales (blue, humpback and fin whales) commenced under heavy harvesting. This suggests that species with fast dynamics such as squid were possibly the first to benefit from krill surplus, even before minke whales and crabeater seals, which started to increase only about a decade later. The study provides a potential framework for understanding the interplay between species with slow and fast dynamics
Investigations into the use of quantified Bayesian Maximum Entropy methods to generate improved distribution maps and biomass estimates from fisheries acoustic survey data
No full textThis thesis describes elements of the assessment and application of Bayesian Maximum Entropy (MaxEnt) image reconstruction techniques for the analysis of fisheries acoustic survey data. The objective is to investigate the utility of this approach in mapping density distributions and estimating biomass. The MaxEnt image reconstruction method derives originally from the field of astrophysics, and this thesis represents an attempt to apply the principles of MaxEnt to the field of ocean ecology. Essentially, what is required is to generate maps of the density distribution of pelagic species (species living in the water column) from extremely limited and sometimes skewed line-transect acoustic survey data. Techniques used presently are largely unsatisfactory for a variety of reasons, and are often inapplicable for data from surveys that do not follow a particular design strategy. This thesis investigates the usefulness of the MaxEnt technique in overcoming some of the difficulties of acoustic survey analysis. A study is made into the possibility of objectively testing whether these techniques offer improvements in accuracy over existing techniques, by attempting to reconstruct simulated data from a virtual survey. I find that plausible reconstructions are possible, and that statistical comparisons indicate these reconstructions are accurate. The technique is also applied quantitatively to
real-world survey data, offering new insights into the abundance of Antarctic krill (Euphausia superba) in the Scotia Sea - raising abundance estimates from 109 million tonnes to 208 million tonnes - and into the relative abundance of fish and jellyfish in the Namibian Benguela, where it is shown that the biomass of jellyfish (12.2 million tonnes) now exceeds that of fish (3.6 million tonnes)
Influence of individual state on swimming capacity and behaviour of Antarctic krill Euphausia superba
No full textSwarms of Antarctic krill are frequently biased towards certain body sizes, sexes, maturities and physiological states. However, the mechanisms causing such biases remain unclear, with some speculating on differential responses to chemical or visual cues. This study examines the influence of individual state on swimming behaviour and performance, which, in turn, may cause biases in swarm composition. The frequency and strength of pleopod beats as well as the overall swimming capacity of Antarctic krill Euphausia superba were measured using a tethering technique. Individual state was considered in terms of body size, sex, maturity, moult stage and stomach fullness. Significant differences in the rate and strength of pleopod strokes as well as in the overall swimming capacity were found between individuals in different states. Body size had one of the strongest influences, with beat rate decreasing and beat strength increasing with increasing size. The sex and maturity of an individual altered this pattern, with adult females having faster but weaker pleopod beats than males of equivalent size. The difference in swimming style of females may impose a different lifestyle to that of males. Moult stage also had a significant influence on swimming, with moulting/post-moult individuals having significantly lower swimming capacities, making them unlikely to keep pace with intermoult counterparts. Both the rate and the strength of the pleopod beat were significantly lower in satiated compared to unsatiated individuals, which may contribute to the dispersion of swarms during feeding
Images as proximity sensors : the incidence of conspecific foraging in Antarctic fur seals
No full textBackground: Although there have been recent advances in the development of animal-attached ‘proximity’ tags to remotely record the interactions of multiple individuals, the efficacy of these devices depends on the instrumentation of sufficient animals that subsequently have spatial interactions. Among densely colonial mammals such as fur seals, this remains logistically difficult, and interactions between animals during foraging have not previously been recorded. Results: We collected data on conspecific interactions during diving at sea using still image and video cameras deployed on 23 Antarctic fur seals. Animals carried cameras for a total of 152 days, collecting 38,098 images and 369 movies (total time 7.35 h). Other fur seals were detected in 74% of deployments, with a maximum of five seals seen at one time (n = 122 images, 28 videos). No predators other than conspecifics were observed. Detection was primarily limited by light conditions, since conspecifics were usually further from each other than the 1-m range illuminated by camera flash under low light levels. Other seals were recorded at a range of depths (average 27 ± 14.3 m, max 66 m). In terms of bouts of dives, still images of other seals were recorded in 5 single dives (of 330) and 28 bouts of dives <2 min apart (of 187). Linear mixed models suggested a relationship between conspecific observations per dive and the number of krill images recorded per dive. Using light conditions as a proxy for detectability, other seals were more likely to be observed at the bottom of dives than during descent or ascent. Seals were also more likely to be closer to each other and oriented perpendicular to each other at the bottom of dives, and in the same direction as each other during ascent. Conclusions: These results are contrary to animal-attached camera observations of penguin foraging, suggesting differing group-foraging tactics for these marine predators. Group foraging could have consequences for models linking predator behaviour to prey field densities since this relationship may be affected by the presence of multiple predators at the same patch.Peer reviewe
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