1,721,152 research outputs found
Temporal and spatial dynamics of hake Merluccius merluccius recruitment in the Tyrrhenian and Ligurian Sea (Mediterranean)
No full textThe present doctoral thesis investigated fundamental aspects of temporal and spatial dynamics of hake (Merluccius merluccius) recruitment in the Tyrrhenian and Ligurian Sea (Mediterranean).
We identified recruit and post-recruit as two well distinct phases of hake juvenile life stage according to a marked bathymetric segregation. Hake juveniles enter the post-recruit stage at a length range between 13.2 cm and 15.8 cm through a sharp migration from the margin of the shelf-break and upper slope to the shallower continental shelf waters. Spatial models provided accurate identification of the main areas of aggregation of hake recruits before their migration and spreading on the shelf. We found that nurseries in the Tyrrhenian and Ligurian Sea are characterised by some of the highest densities of recruits observed in the Mediterranean, suggesting these areas could play an important role for the dynamics of hake population at a larger scale far beyond the study area. The analysis of time series data for two different periods of the year demonstrated an inter-annual and seasonal stability in the spatial location of the main nursery areas and a strong association for specific habitats of the shelf-break. Together with these main nurseries, a certain number of secondary nurseries were detected only in certain years, but their variability and importance for the population remain to be addressed.
Wide fluctuations in the abundance of hake recruits were found to be strongly influenced by both oceanographic and biological processes. Thermal anomalies in summer, characterised by high peaks in water temperature, revealed a negative effect on the abundance of recruits in autumn, possibly due to a reduction in hake egg and larval survival rates. Recruitment was also reduced when elevated sea-surface temperatures in summer were coupled with low levels of water circulation. Enhanced spring primary production related to late winter low temperatures are known to affect water mass productivity in the following months, and according to our results hake spring recruitment. We found a dome-shaped relationship between wind mixing in early spring and recruitment that was interpreted as an “optimal environmental window” in which intermediate water mixing level can play a positive role in phytoplankton displacement, larval feeding rate and appropriate larval drift.
The temporal and spatial persistence of hake nurseries represented fundamental features for testing the effects of a spatial management of the population particularly during the highly vulnerable recruit phase. The effect of different levels of protection on recruits were tested through model simulations and compared with a more general reduction of fishing effort, providing unexpected results on the mature portion of the population.Partially funded by BECAUSE project (Critical Interactions BEtween Species and their Implications for a PreCAUtionary FiSheries Management in a variable Environment — a Modelling Approach), EU contract no. 502482
Bathymetric preferences of juvenile European hake (Merluccius merluccius)
No full textThe concept of a recruit is a basic notion in fisheries science, but it is still far from being an unequivocal term, and many diverse, even ambiguous, definitions can be found in the literature. We propose a more objective and biologically meaningful way to de. ne the length range of recruits for species that have clear bathymetric segregation during the early stages of their life cycle. The bathymetric distribution of juvenile European hake was studied by fitting a thin plate spline to data from the national autumn trawl survey. Hake showed a stable pattern of depth preference in the 6-year dataset examined. Small hake had the greatest preference for depths of 170 - 220 m and appeared to move slightly deeper when they reached 10-cm total length. Larger hake persisted on the continental shelf with a preference for water 70 - 100 m deep, especially when they reached 18 - 20 cm long. The length at migration was defined as the length at which the minimum depth preference was shown, and it ranged between 13.2 and 15.8 cm depending on the year. There was a relationship between length at and depth of migration, and we provide a full description of the depth preference of juvenile hake, and test the effectiveness of the analytical approach used
Modelling recruitment dynamics of hake, Merluccius merluccius, in the central Mediterranean in relation to key environmental variables
No full textHake recruitment has been examined in relation to environmental variables in two of the main reproductive areas of the central Mediterranean, the northern and central Tyrrhenian Sea. Seventeen years time series data from trawl surveys revealed high fluctuations in recruit abundance that could not be just explained by spawning biomass estimations. Generalized additive models were developed to investigate hake recruitment dynamics in the Tyrrhenian Sea in relation to spawner abundance and selected key oceanographic variables. Environmental data were explored in attempt to explain survival processes that could affect early life history stages of hake and that accounted for high fluctuations in its recruitment. Thermal anomalies in summer, characterised by high peaks in water temperature, revealed a negative effect on the abundance of recruits in autumn, probably due to a reduction in hake egg and larval survival rates. In the northern Tyrrhenian, recruitment was reduced when elevated sea-surface temperatures were coupled with lower levels of water circulation. Enhanced spring primary production, related to late winter low temperatures could affect water mass productivity in the following months, thus influencing spring recruitment. In the central Tyrrhenian a dome-shaped relationship between wind mixing in early spring and recruitment could be interpreted as an "optimal environmental window" in which intermediate water mixing level played a positive role in phytoplankton displacement, larval feeding rate and appropriate larval drift. Results are discussed in relation to the decline in hake stock biomass and within the present climate change and global warming context
Frequency distribution curves and the identification of hotspots: response to comments
No full textThis is a response to the criticism by Cayuela et al. (2011) of our proposal for a frequency distribution approach (Bartolino et al. 2011) to the identification of hotspots in conservation and ecology
A frequency distribution approach to hotspot identification
No full textWe present a new global method for the identification of hotspots in conservation and ecology. The method is based on the identification of spatial structure properties through cumulative relative frequency distributions curves, and is tested with two case studies, the identification of fish density hotspots and terrestrial vertebrate species diversity hotspots. Results from the frequency distribution method are compared with those from standard techniques among local, partially local and global methods. Our approach offers the main advantage to be independent from the selection of any threshold, neighborhood, or other parameter that affect most of the currently available methods for hotspot analysis. The two case studies show how such elements of arbitrariness of the traditional methods influence both size and location of the identified hotspots, and how this new global method can be used for a more objective selection of hotspots
Conservation and management of Soqotra Archipelago’s marine resources.
No full textThe paper descrive the marine environment of Socotra island and the activities carried out in the framework of the italian project. The sectors of interest were "monitoring of fishery activities and landing", "monitoring of coral reef and coral fish", "mark and recapture of nesting loggerhead turtles", Marine Protected Areas"
Conservation value of historical data: reconstructing stock dynamic of turbot (Psetta maxima) during the last century in the Eastern North Sea.
No full textApprocci per l'individuazione di aree marine protette per la pesca basati su dati di trawl survey
No full textIdentifying fish nurseries using density and persistence measures
No full textWe propose a 3-step methodological approach to identify and classify fish nurseries for fisheries management purposes. We applied our approach to juvenile European hake Merluccius merluccius in the central Mediterranean Sea. Time series of trawl-survey fish-density data were used to map juvenile hake distribution with Bayesian kriging, while geostatistical aggregation curves were used to find density hot-spots. Persistence measures were adopted to identify nurseries on the basis of their spatio-temporal persistence. We found that areas with a high density of juvenile hake showed a high temporal persistence on both a seasonal and annual basis, with the most persistent nursery areas covering about 5% of the study areas while including about 39% of hake recruitment (averaged over 10 yr). We believe the persistence of these areas is indirect evidence of their importance to the productivity of the population, with many potentially important implications for fisheries management. The approach that we developed to identify hake nurseries can be applied to different species and life stages to improve knowledge of the role of habitat for populations and communities
Incidence of lessepsian migrants on landings of the artisanal fishery of south Lebanon
No full textIn the last decade, a growing amount of evidence incorporated by several authors as signals of global changes, defined a trend of expansion of thermophilic species in the Mediterranean. This phenomenon is markedly shown by the spread of some non-indigenous fish beyond their natural limits and by their success in the new colonized areas. The incidence of those non-indigenous fish in the catch composition of the artisanal fishery of Tyre (south Lebanon) was investigated for the first time using both official data and daily landing site surveys. The investigatory fleet consisted of 250 small vessels (4–10 m length) with old and not very powerful engines, and about 400–550 fishermen at the end of 2005. Most of the fleet used different types of bottom standing gear, such as trammel nets, set gill-nets and bottom longlines, whereas purse seines and other fishing gear (floating longlines and traps) were used less frequently. Landings comprised a great number of species, many of which were lessepsian migrants. We recorded a total of 25 lessepsian species, representing 17 families and comprising 37% of the total landing by weight. Some of these non-indigenous species have become important components of local fisheries in the area
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