62 research outputs found

    The consequences of balanced harvesting of fish communities

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    Balanced harvesting, where species or individuals are exploited in accordance with their productivity, has been proposed as a way to minimize the effects of fishing on marine fish communities and ecosystems. This calls for a thorough examination of the consequences balanced harvesting has on fish community structure and yield. We use a size- and trait-based model that resolves individual interactions through competition and predation to compare balanced harvesting with traditional selective harvesting, which protects juvenile fish from fishing. Four different exploitation patterns, generated by combining selective or unselective harvesting with balanced or unbalanced fishing, are compared. We find that unselective balanced fishing, where individuals are exploited in proportion to their productivity, produces a slightly larger total maximum sustainable yield than the other exploitation patterns and, for a given yield, the least change in the relative biomass composition of the fish community. Because fishing reduces competition, predation and cannibalism within the community, the total maximum sustainable yield is achieved at high exploitation rates. The yield from unselective balanced fishing is dominated by small individuals, whereas selective fishing produces a much higher proportion of large individuals in the yield. Although unselective balanced fishing is predicted to produce the highest total maximum sustainable yield and the lowest impact on trophic structure, it is effectively a fishery predominantly targeting small forage fish

    最近の經濟學界

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    Size spectrum models have emerged from 40 years of basic research on how body size determines individual physiology and structures marine communities. They are based on commonly accepted assumptions and have a low parameter set, which make them easy to deploy for strategic ecosystem oriented impact assessment of fisheries. We describe the fundamental concepts in size-based models about food encounter and the bioenergetics budget of individuals. Within the general framework three model types have emerged that differs in their degree of complexity: the food-web, the trait-based and the community model. We demonstrate the differences between the models through examples of their response to fishing and their dynamic behavior. We review implementations of size spectrum models and describe important variations concerning the functional response, whether growth is food-dependent or fixed, and the density-dependence imposed on the system. Finally we discuss challenges and promising directions.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

    Limited impact of big fish mothers for population replenishment

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    A recent meta-analysis by Barneche et al. (Science 360(6389): 642) show that fish reproductive output scales hypergeometrically with female weight. This result challenges the common assumption that reproductive output is proportional to weight. The implication made is that current theory and practice severely underestimates the importance of larger females for population replenishment. Their example for cod shows that current practice makes an error of 149%. By properly accounting for fish demography we show that the error is maximally on the order of 10%, and in most other fish stocks likely much less.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

    Workshop to compare the indicators for CFP and MSFD D3 management objectives through simulations (WKSIMULD3)

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    Rapports Scientifiques du CIEM. Volume 6, nº 4The WKSIMULD3 meeting provided a platform for experts from the EU member states to meet and progress the assessment methodology on Criteria 3 of Descriptor 3 upon request by EC (DGENV). WKSIMULD3 is the third of a series of three workshops (WKD3C3SCOPE and WKD3C3THRESHOLDS being the first two) to identify operational indicators for MSFD D3C3. The workshop was organised as a series of presentations with intermittent discussions. The group agreed on a number of stocks to run simulations to explore the relationships between indicators of population traits/dynamics and healthy population structure. The participants found that for demersal stocks in the North Sea, MSE simulations showed decreasing values of D3C3 indicators with increasing fishing mortality. However, the use of age-structure indicators for these stocks is likely to lead to frequent false positives due to the high interannual variability. Age-based simulations were implemented also for pelagic and short-lived stocks, such as mackerel, sprat, and sandeel. The distribution of the threshold values implemented for the stocks did not show a clear response to the fishing regime. As a result, the use of age-structure indicators for these stocks is likely to lead to frequent false positives/negatives. The simulations aimed to validate the population size structure and calculate indicators under different fishing scenarios. In the Mediterranean Sea, length-based simulations were conducted for two hake stocks and one sole stock. The length indicators obtained were compared to historical data to assess the responsiveness of the indicators to exploitation. These results show the behaviour of the length based and of the age-based indicators calculated on observed data. Overall, the indicator status can be well below the average value at Fmsy. Additionally, an exploratory run tested a harvest control rule driven by age-based indicator targets. The outputs can provide directions on how to choose a threshold that can ensure the stock to be in GES. Overall, the workshop findings highlighted the complexity of evaluating indicators for CFP and MSFD D3 management objectives, particularly in relation to the responsiveness of the indicators to fishing pressure and environmental variation. The group emphasized the need for robust and peer-reviewed models to ensure reliable results when evaluating indicators for fish stocks. Additionally, it underscores the challenges associated with using age- and length-based indicators for different species and the importance of considering environmental and recruitment variability in simulations. In conclusion, the series of the 3 workshops contributed valuable insights into the complexities of assessing and comparing indicators for MSFD D3 management objectives, providing important considerations for future assessments and management strategies.info:eu-repo/semantics/publishedVersio

    The second workshop on lists of commercial fish and shellfish species for reporting of MSFD D3 (WKD3Lists2)

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    WKD3Lists2 created lists of regionally relevant commercial fish and shellfish species (and higher order taxa) for the use of Article 8 reporting by EU member states under Descriptor 3 of the Marine Strategy Framework Directive (MSFD). The regional taxa lists were based on landings data from the Fisheries Dependent Information data base (FDI) provided by EU member states and compiled by the Joint Research Centre (JRC). The taxonomy of landings data was consolidated by regional experts and the consolidated data were combined to obtain absolute and proportional landing weights and values for each (sub)re-gion, which were used to apply dual (weight and vale) selection thresholds to compile (sub)re-gional D3-taxa-lists. Regional D3-taxa-lists were produced for two MSFD regions (Baltic Sea & Black Sea) and eight MSFD subregions: The Greater North Sea, Celtic Seas, the Bay of Biscay and Iberian Coast, Mac-aronesia, Western Mediterranean, the Ionian Sea & Central Mediterranean, the Adriatic Sea and the Aegean-Levantine Sea. To exclude taxa with very low landing weights or value from the final lists, two types of thresh-olds (cumulative and minimum) with differing cut-off values were evaluated (90%, 95%, 98% and 99% for cumulative and 0.1% and 1% for minimum thresholds). Depending on the cut-off value, the number of taxa included varied substantially and in most (sub)regions the application of thresholds reduced the initial number of taxa by more than 50%. WKD3Lists2 did not recommend any threshold type or cut-off value to be applied generically in all (sub)regions, but identified trade-offs between inclusiveness and parsimony of relevant con-tent i.e. higher cut-off values will lead to longer lists including many taxa with relatively low landings weights/values. In some (sub)regions, thresholds with lower cut-off values (90% to 95%) were considered feasible by regional experts (Mediterranean subregions, Bay of Biscay and Iberian Coast, Macaronesia), whereas in other MSFD (sub)regions cut-off values in the range of 98-99% were considered as appropriate (Baltic Sea, Greater North Sea, Celtic Seas). The regional D3-taxa lists by WKD3Lists2 were created without considering the availability of data or assessments i.e. many species are included, for which no assessment information is avail-able. WKD3Lists2 decided on this approach because a representative selection of commercially targeted taxa was considered to indicate knowledge and data gaps in current data collection and assessment schemes. Regional species lists shall be used by EU member states for the national reporting of D3. Stocks and species from the regional lists shall be considered by member states, and additional stocks/species can be added where appropriate (e.g. those stocks/species of national or local of importance that do not appear on the regional lists). x WKD3Lists2 discussed and compiled recommendations on how Member States can complement the regional lists of D3-taxa. A key recommendation is to maintain taxa reported in 2018 under D3, even if they are not part of the regional D3-taxa list for 2024. Wherever possible, Member States should report on stock level. WKD3Lists2 also discussed linkages between D1 and D3-reporting of commercial taxa.info:eu-repo/semantics/publishedVersio

    Workshop to scope and preselect indicators for criterion D3C3 under MSFD decision (EU) 2017/848 (WKD3C3SCOPE)

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    The workshop to scope and preselect indicators for Descriptor 3 criterion 3 under MSFD Commission Decision (EU) 2017/848 (WKD3C3SCOPE) provided a platform for experts from the EU member states and relevant regional bodies to meet and support development and progress the assessment methodology, based on a request by the EC (DGENV). WKD3C3SCOPE is the first of a series of three workshops (WKD3C3THRESHOLDS and WKSIMULD3) to provide guidance in relation to operational indicators for MSFD D3C3. The workshop was organized as a series of presentations with intermittent group discussions. On the first day of the workshop the participants discussed what defines a ‘healthy population structure’ for species with different life history traits (ToR a). During the following days, the group discussed and identified relevant D3C3 indicators (ToR b) and developed criteria to select among the identified D3C3 indicators to allow further testing and setting of thresholds at WKD3C3THRESHOLDS (ToR c). The participants found that overall, healthy fish stocks are characterized by high productivity, wide age and size structuring in the population, and the ability to quickly recover from disturbances. The groups noted that environmental factors, along with stock biomass and fishing pressure, influence the productivity and health of a stock, with environment playing a particularly large role in the recruitment of short-lived stocks. It was suggested that the age structure of a stock might be more relevant for evaluating the health of long-lived stocks. However, it was acknowledged that not all stocks have sufficient data to evaluate all proposed indicators, and a single indicator is unlikely to suffice for all stocks. Data availability, species- specific factors and regional or sub-regional variation are thus also important considerations. In relation to ToR b, the participants presented their work on potential indicators including: recruitment time-series, proportion of fish larger than the mean size of first sexual maturation, F rec/Fbar, length distribution L 90, relative proportion of old fish above A 90, indicators of spawner quality, and SSB/R. A discussion on pros/cons, benefits to the population of high or low indicator values, benefits supported by empirical evidence, applicability to data-poor stocks and benefits supported by simulation/theoretical considerations followed the presentations. Finally, in relation to ToR c, the difficulty emerged in ranking the indicators alone without considering the data used to estimate them and a new set of evaluation criteria for use in WKD3C3THRESHOLDS were defined. Based on the outputs of the meeting a list of indicators to be further evaluated has been drafted, which also emphasizes the stocks for which studies have empirically demonstrated effects on productivity. In addition to the listed indicators, indicators of genetic diversity and proportion of fish with parasite infestation were mentioned but to the knowledge of the participants, widespread data for these are currently not publicly available.info:eu-repo/semantics/publishedVersio

    Joint ICES-SEAwise workshop to quality assure methods to incorporate environmental factors and quantifying ecological considerations in management strategy evaluation tools (WKEcoMSE)

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    The EU project SEAwise (https://seawiseproject.org/) endeavours to enhance existing multi-stock multi-species Management Strategy Evaluation (MSE) models so that they can be used to define and evaluate fisheries management strategies that address broad Ecosystem-Based Fisheries Management (EBFM) objectives, including in particular identifying Harvest Control Rules (HCRs) that are robust to changes in productivity. The WKEcoMSE workshop was held to: (1) benchmark the approaches used or developed in the project to develop robust and consistent environment-productivity relationships for commercial stocks across selected case studies and integrate them in MSE models used by the SEAwise project and by ICES; (2) to provide context for those approaches within the general field of “environment-enriched” MSEs; and (3) to draw from the participants collective experience some general guidelines about the integration of environmental impacts on stock productivity in MSE tools. 23 presentations were given, both about the work carried out within SEAwise but also by international colleagues working toward similar objectives, and various topics were discussed over eight sessions designed to accommodate participants spread across Europe and Northern America. “Good practices” to incorporate environmental considerations in MSE modelling were then drafted collectively and have been summarized in the panels below. These rely on the experiences of the WKEcoMSE participants and are not exhaustive.info:eu-repo/semantics/publishedVersio
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