Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Effects of fishing during the spawning period: implications for sustainable management
    Overzee, H.M.J. van; Rijnsdorp, A.D. - \ 2015
    Reviews in Fish Biology and Fisheries 25 (2015)1. - ISSN 0960-3166 - p. 65 - 83.
    cod gadus-morhua - loligo-vulgaris-reynaudii - north-sea plaice - halibut hippoglossus-stenolepis - palinurus-elephas fabricius - pleuronectes-platessa l - life-history evolution - coral-reef fish - atlantic cod - closed areas
    While fishery closures during the spawning season are commonplace, direct evidence for their benefit is mainly restricted to species forming large spawning aggregations. This paper analyses the conditions under which spawning closures could contribute to sustainable fisheries management by reviewing how fishing during spawning may affect the physiology, behaviour and ecology of individuals and how this may influence the dynamics and the genetics of the population. We distinguish between the effects of fishing activities in relation to mortality, disturbance of spawning activity, and impact on spawning habitat. Spawning closures may be of benefit it they: (1) reduce the fishing mortality of the large and older spawners; (2) avoid negative effects on spawning habitats; (3) reduce the risk of over-exploitation in species which form large spawning aggregations; (4) reduce the evolutionary effects on maturation and reproductive investment; and (5) reduce the risk of over-exploitation of specific spawning components. The contribution of spawning closures to sustainable fisheries will differ among species and depends on the complexity of the spawning system, the level of aggregation during spawning and the vulnerability of the spawning habitat. The importance of these closures depends on the degree of population depletion but does not cease when populations are ‘healthy’ (i.e. no sign that recruitment is impaired).
    Fecundity regulation in horse mackerel
    Damme, C.J.G. van; Thorsen, A. ; Fonn, M. ; Alvarez, P. ; Garabana, D. ; O'Hea, B. ; Perez, J.R. ; Dickey-Collas, M. - \ 2014
    ICES Journal of Marine Science 71 (2014)3. - ISSN 1054-3139 - p. 546 - 558.
    daily egg-production - herring clupea-harengus - trachurus-trachurus l. - pleuronectes-platessa l - saronikos gulf greece - scomber-scombrus l. - north-sea plaice - cod gadus-morhua - batch fecundity - spawning frequency
    Egg production methods have been used successfully in the provision of advice for fisheries management. These methods need accurate and unbiased estimates of fecundity. We explore the reproductive strategy of horse mackerel and estimation of fecundity. Fecundity and fecundity regulation in relation to condition was investigated over a number of years. Fulton's K, lipid content, and hepatosomatic index increased after the start of spawning, though decreased again at the end of spawning. The increase in the gonadosomatic index, fecundity, and body condition after the onset of spawning suggests that horse mackerel utilizes food resources during the spawning season and might be an income breeder. However, the decline in K and lipid before the spawning season suggests that the first batch of oocytes is developed on stored energy. Fecundity varied between years and within a spawning season. Over latitude, variations in fecundity were small. K and lipid content are not reliable indices as proxy for fecundity. Batch fecundity appears to be heterogeneous across the spawning season but homogeneous across latitude. The homogeneity of batch fecundity over latitude could indicate that the daily egg production method is an appropriate approach for estimating the abundance of a wide ranging species, as horse mackerel.
    Evolutionary impact assessment: accounting for evolutionary consequences of fishing in an ecosystem approach to fisheries management
    Laugen, A.T. ; Engelhard, G.H. ; Whitlock, R. ; Mollet, F.M. ; Rijnsdorp, A.D. - \ 2014
    Fish and Fisheries 15 (2014)1. - ISSN 1467-2960 - p. 65 - 96.
    cod gadus-morhua - maturation reaction norms - effective population-size - life-history evolution - north-sea plaice - pike esox-lucius - herring clupea-harengus - eco-genetic model - atlantic cod - marine fish
    Managing fisheries resources to maintain healthy ecosystems is one of the main goals of the ecosystem approach to fisheries (EAF). While a number of international treaties call for the implementation of EAF, there are still gaps in the underlying methodology. One aspect that has received substantial scientific attention recently is fisheries-induced evolution (FIE). Increasing evidence indicates that intensive fishing has the potential to exert strong directional selection on life-history traits, behaviour, physiology, and morphology of exploited fish. Of particular concern is that reversing evolutionary responses to fishing can be much more difficult than reversing demographic or phenotypically plastic responses. Furthermore, like climate change, multiple agents cause FIE, with effects accumulating over time. Consequently, FIE may alter the utility derived from fish stocks, which in turn can modify the monetary value living aquatic resources provide to society. Quantifying and predicting the evolutionary effects of fishing is therefore important for both ecological and economic reasons. An important reason this is not happening is the lack of an appropriate assessment framework. We therefore describe the evolutionary impact assessment (EvoIA) as a structured approach for assessing the evolutionary consequences of fishing and evaluating the predicted evolutionary outcomes of alternative management options. EvoIA can contribute to EAF by clarifying how evolution may alter stock properties and ecological relations, support the precautionary approach to fisheries management by addressing a previously overlooked source of uncertainty and risk, and thus contribute to sustainable fisheries.
    Fluctuating quota and management costs under multiannual adjustment of fish quota
    Dijk, D. van; Haijema, R. ; Hendrix, E.M.T. ; Groeneveld, R.A. ; Ierland, E.C. van - \ 2013
    Ecological Modelling 265 (2013). - ISSN 0304-3800 - p. 230 - 238.
    north-sea plaice - pleuronectes-platessa l. - fisheries management - stock - uncertainty - sustainability - maturation - failure - trends - growth
    North Sea fisheries are managed by the European Union (EU) through a system of annual quota. Due to uncertainty about future fish stocks, yearly revisions of these policies lead to fluctuation in quota, which in turn affects harvest and investment decisions of fishermen. Determination of quota requires high management costs in terms of obtaining information and negotiations between experts and policy makers. To reduce both quota fluctuation and management costs, the EU has proposed a system of multiannual quota. In this paper we study the effect of multiannual quota on quota volatility and resource rents, while accounting for management costs. We develop a bi-level stochastic dynamic programming model, where at level one, the EU determines the quota that maximizes resource rents. At level two, fishermen decide myopically on their harvest and investment levels, subject to the quota. Results show that policy makers can reduce quota volatility and improve resource rents from the fishery with multiannual quota. Important trade-offs are involved in the accomplishment of these objectives: fish stock and investments become more volatile, which leads to more overcapacity.
    Can fisheries-induced evolution shift reference points for fisheries management?
    Heino, M. ; Baulier, L. ; Boukal, D.S. ; Mollet, F.M. ; Rijnsdorp, A.D. - \ 2013
    ICES Journal of Marine Science 70 (2013)4. - ISSN 1054-3139 - p. 707 - 721.
    cod gadus-morhua - north-sea plaice - life-history evolution - exploited fish stocks - pleuronectes-platessa l - eco-genetic model - atlantic cod - population-dynamics - reproductive investment - natural mortality
    Biological reference points are important tools for fisheries management. Reference points are not static, but may change when a population's environment or the population itself changes. Fisheries-induced evolution is one mechanism that can alter population characteristics, leading to “shifting” reference points by modifying the underlying biological processes or by changing the perception of a fishery system. The former causes changes in “true” reference points, whereas the latter is caused by changes in the yardsticks used to quantify a system's status. Unaccounted shifts of either kind imply that reference points gradually lose their intended meaning. This can lead to increased precaution, which is safe, but potentially costly. Shifts can also occur in more perilous directions, such that actual risks are greater than anticipated. Our qualitative analysis suggests that all commonly used reference points are susceptible to shifting through fisheries-induced evolution, including the limit and “precautionary” reference points for spawning-stock biomass, Blim and Bpa, and the target reference point for fishing mortality, F0.1. Our findings call for increased awareness of fisheries-induced changes and highlight the value of always basing reference points on adequately updated information, to capture all changes in the biological processes that drive fish population dynamics.
    Evaluating the effect of fishery closures: Lessons learnt from the Plaice Box
    Beare, D.J. ; Rijnsdorp, A.D. ; Rockmann, C. ; Tulp, I.Y.M. ; Hal, R. van; Kooten, T. van - \ 2013
    Journal of Sea Research 84 (2013). - ISSN 1385-1101 - p. 49 - 60.
    marine protected areas - north-sea plaice - pleuronectes-platessa l. - beam-trawl effort - solea-solea l. - closed areas - georges bank - management - disturbance - ecosystems
    To reduce discarding of plaice Pleuronectes platessa in the North Sea flatfish fisheries, the major nursery areas were closed to large trawlers in 1995. The area closed was named the ‘Plaice Box’ (PB) and beam trawl effort fell by over 90%,while the exemption fleets of small flatfish beam trawlers, gill netters targeting sole (Solea solea) and shrimp (Crangon crangon) trawlers increased their effort. Contrary to the expectation, plaice landings and biomass declined. The initial support for the PB from the fisheries was lost, whereas other stakeholder groups claimed that any failure was due to the fact that fishing had never been completely prohibited in the area. To evaluate whether the PB has been an effective management measure, the changes in the ecosystem (plaice, demersal fish, benthos) and fisheries are analyzed to test whether the observed changes are due to the PB or to changes in the environment unrelated to the PB. Juvenile growth rate of plaice decreased and juveniles moved to deeper waters outside the PB. Demersal fish biomass decreased, whereas the abundance of epibenthic predators (Asterias rubens and Cancer pagurus) increased in the PB. Endobenthos, in particular the main food items of plaice (polychaetes and small bivalves) remained stable or decreased both inside and outside the PB. Currently catches of both plaice and sole from within the PB are lower than in the late 1980s and the exemption fleet often prefers to fish outside the Plaice Box alongside much larger competitors. It is concluded that the observed changes are most likely related to changes in the North Sea ecosystem, which may be related to changes in eutrophication and temperature. It is less likely that they are related to the change in fishing. This case study highlights the importance of setting testable objectives and an appropriate evaluation framework including both ecological and socio-economic indicators when implementing closed areas
    Impacts of climate change on the complex life cycles of fish
    Petitgas, P. ; Rijnsdorp, A.D. ; Dickey-Collas, M. ; Engelhard, G.H. ; Peck, M.A. ; Pinnegar, J.K. ; Drinkwater, K. ; Huret, M. ; Nash, R.D.M. - \ 2013
    Fisheries Oceanography 22 (2013)2. - ISSN 1054-6006 - p. 121 - 139.
    cod gadus-morhua - plaice pleuronectes-platessa - anchovy engraulis-encrasicolus - herring clupea-harengus - north-sea plaice - juvenile atlantic cod - arcto-norwegian cod - flounder platichthys-flesus - dependent development rates - western wadden sea
    To anticipate the response of fish populations to climate change, we developed a framework that integrates requirements in all life stages to assess impacts across the entire life cycle. The framework was applied on plaice (Pleuronectes platessa) and Atlantic herring (Clupea harengus) in the North Sea, Atlantic cod (Gadus morhua) in the Norwegian/Barents Seas and European anchovy (Engraulis encrasicolus) in the Bay of Biscay. In each case study, we reviewed habitats required by each life stage, habitat availability, and connectivity between habitats. We then explored how these could be altered by climate change. We documented environmental processes impacting habitat availability and connectivity, providing an integrated view at the population level and in a spatial context of potential climate impacts. A key result was that climate-driven changes in larval dispersion seem to be the major unknown. Our summary suggested that species with specific habitat requirements for spawning (herring) or nursery grounds (plaice) display bottlenecks in their life cycle. Among the species examined, anchovy could cope best with environmental variability. Plaice was considered to be least resilient to climate-driven changes due to its strict connectivity between spawning and nursery grounds. For plaice in the North Sea, habitat availability was expected to reduce with climate change. For North Sea herring, Norwegian cod and Biscay anchovy, climate-driven changes were expected to have contrasting impacts depending on the life stage. Our review highlights the need to integrate physiological and behavioural processes across the life cycle to project the response of specific populations to climate change.
    Estimating age at maturation and energy-based life-history traits from individual growth trajectories with nonlinear mixed-effects models
    Brunel, T.P.A. ; Ernande, B. ; Mollet, F.M. ; Rijnsdorp, A.D. - \ 2013
    Oecologia 172 (2013)3. - ISSN 0029-8549 - p. 631 - 643.
    herring clupea-harengus - pleuronectes-platessa l - north-sea plaice - ontogenic growth - back-calculation - somatic growth - reaction norms - fish age - size - maturity
    A new method is presented to estimate individuals’ (1) age at maturation, (2) energy acquisition rate, (3) energy expenditure for body maintenance, and (4) reproductive investment, and the multivariate distribution of these traits in a population. The method relies on adjusting a conceptual energy allocation model to individual growth curves using nonlinear mixed-effects modelling. The method’s performance was tested using simulated growth curves for a range of life-history types. Individual age at maturation, energy acquisition rate and the sum of maintenance and reproductive investment rates, and their multivariate distribution, were accurately estimated. For the estimation of maintenance and reproductive investment rates separately, biases were observed for life-histories with a large imbalance between these traits. For low reproductive investment rates and high maintenance rates, reproductive investment rate estimates were strongly biased whereas maintenance rate estimates were not, the reverse holding in the opposite situation. The method was applied to individual growth curves back-calculated from otoliths of North Sea plaice (Pleuronectes platessa) and from scales of Norwegian spring spawning herring (Clupea harengus). For plaice, maturity ogives derived from our individual estimates of age at maturation were almost identical to the maturity ogives based on gonad observation in catch samples. For herring, we observed 51.5 % of agreement between our individual estimates and those directly obtained from scale reading, with a difference lower than 1 year in 97 % of cases. We conclude that the method is a powerful tool to estimate the distribution of correlated life-history traits for any species for which individual growth curves are available.
    Beam trawlermen take feet off gas in response to oil price hikes
    Beare, D.J. ; Machiels, M.A.M. - \ 2012
    ICES Journal of Marine Science 69 (2012)6. - ISSN 1054-3139 - p. 1064 - 1068.
    north-sea plaice - fishing effort - fisheries - diets
    Average towing speed by Dutch beam trawlermen has fallen substantially between 2002 and 2009. Changes in towing speed are related to changes in oil price. The price of their valuable main target species (sole, Solea vulgaris) did not influence towing speed.
    Ecological and economic trade-offs in the management of mixed fisheries: a case study of spawning closures in flatfish fisheries
    Rijnsdorp, A.D. ; Overzee, H.M.J. van; Poos, J.J. - \ 2012
    Marine Ecology Progress Series 447 (2012). - ISSN 0171-8630 - p. 179 - 194.
    north-sea plaice - pleuronectes-platessa l - life-history evolution - evolving fish stocks - demersal fisheries - ecosystem approach - reaction norms - adult plaice - impact - communities
    As a contribution to the ecosystem approach to fisheries management, we estimated the effects of spawning closures on stock status, ecosystem impacts and economic performance. We focused on the flatfish fishery in the North Sea and explored how spawning closures for plaice and sole contribute to sustainable management of 4 target species (sole, plaice, turbot and brill). Seasonal patterns in fishing effort and catchability by age group and area were estimated to quantify the effect of different spawning closure scenarios on the selection pattern. The scenario performance was evaluated using indicators of stock status (spawning stock biomass), economic performance of the fishery (yield, revenue) and ecosystem impact (discards, bycatch of cod and rays, seabed integrity, fisheries-induced evolution). In a single-species context, spawning closures may be beneficial for the target species, while in a mixed fisheries and ecosystem context, negative effects may occur. A spawning closure for plaice combines positive effects on the plaice stock and the revenue with reductions of the negative impact for several ecosystem indicators and only a small negative effect on sea bed integrity. The effects did not differ when evaluated at current levels of effort or at maximum sustainable yield (MSY) effort. Tailor-made solutions are required that need to be developed in stakeholder consultation to trade-off the ecological and economic objectives. Mixed-species MSY was lower than the sum of the single-species MSYs.
    Where do egg production methods for estimating fish biomass go from here?
    Dickey-Collas, M. ; Somarakis, S. ; Witthames, P.R. ; Damme, C.J.G. van; Uriarte, A.R. ; Lo, N.C.H. ; Bernal, M. - \ 2012
    Fisheries Research 117-118 (2012). - ISSN 0165-7836 - p. 6 - 11.
    anchovy engraulis-encrasicolus - sardine sardinops-sagax - gadus-morhua-l. - pleuronectes-platessa l. - north-sea plaice - solea-solea l. - postovulatory follicles - population-dynamics - trachurus-trachurus - multinomial models
    The special theme volume of Fisheries Research is intended to synthesise the current understanding of the methods and applicability of egg production methods (EPM). It originates from a workshop in Athens which also focused on the future challenges to both the science and logistics of carrying out and using egg production methods. This synthesis addresses three interlinked challenges for those using EPM; how methods have, and need to be, improved, what added value can EPM provide directly to aid advice for management of the marine environment and lastly what extra understanding can EPM bring to marine science? EPM surveys offer some of the most intensive sampling of plankton and adult fish populations in fisheries science. They provide, and will probably provide further insights into fish reproductive processes, embryonic development and spatial and temporal variability in fish populations. Researchers should be encouraged to examine new methods for representative real-time sampling, swift processing of samples and integration of sampling of adults and plankton. EPM provides managers with many “added value” products on habitats and spawning and already provides platforms for monitoring hydrography, zooplankton distributions and acoustic back scatter. Some EPM surveys also incorporate monitoring of birds and sea mammals. EPM, together with aquaculture, has progressed understanding of fish reproductive biology and embryo development. EPM provides long time series of both the ichthyoplankton and fish reproductive traits thus enabling informed study of regime change, variability and ecosystem status. As the EPM become more developed, we expect that these contributions to marine science will increase
    Harvest-induced maturation evolution under different life-history trade-offs and harvesting regimes
    Poos, J.J. ; Brannstrom, A. ; Dieckman, U. - \ 2011
    Journal of Theoretical Biology 279 (2011)1. - ISSN 0022-5193 - p. 102 - 112.
    north-sea plaice - fisheries-induced evolution - herring clupea-harengus - evolving fish stocks - cod gadus-morhua - reaction norms - population-dynamics - marine reserves - brook charr - arctic cod
    The potential of harvesting to induce adaptive changes in exploited populations is now increasingly recognized. While early studies predicted that elevated mortalities among larger individuals select for reduced maturation size, recent theoretical studies have shown conditions under which other, more complex evolutionary responses to size-selective mortality are expected. These new predictions are based on the assumption that, owing to the trade-off between growth and reproduction, early maturation implies reduced growth. Here we extend these findings by analyzing a model of a harvested size-structured population in continuous time, and by systematically exploring maturation evolution under all three traditionally acknowledged costs of early maturation: reduced fecundity, reduced growth, and/or increased natural mortality. We further extend this analysis to the two main types of harvest selectivity, with an individual's chance of getting harvested depending on its size and/or maturity stage. Surprisingly, we find that harvesting mature individuals not only favors late maturation when the costs of early maturation are low, but promotes early maturation when the costs of early maturation are high. To our knowledge, this study therefore is the first to show that harvesting mature individuals can induce early maturation.
    Individual quotas, fishing effort allocation, and over-quota discarding in mixed fisheries
    Poos, J.J. ; Bogaards, J.A. ; Quirijns, F.J. ; Gillis, D.M. ; Rijnsdorp, A.D. - \ 2010
    ICES Journal of Marine Science 67 (2010)2. - ISSN 1054-3139 - p. 323 - 333.
    north-sea plaice - multispecies trawl fishery - pleuronectes-platessa l - transferable quotas - fleet dynamics - beam trawlers - tagging data - management - flatfish - movement
    Many fisheries are managed by total allowable catches (TACs) and a substantial part by individual quotas. Such output management has not been successful in mixed fisheries when fishers continue to fish while discarding marketable fish. We analyse the effects of individual quotas on spatial and temporal effort allocation and over-quota discarding in a multispecies fishery. Using a spatially explicit dynamic-state variable model, the optimal fishing strategy of fishers constrained by annual individual quotas, facing uncertainty in catch rates, is studied. Individual fishers will move away from areas with high catches of the restricted quota species and, depending on the cost of fishing, will stop fishing in certain periods of the year. Individual vessels will discard marketable fish, but only after their individual quota for the species under consideration has been reached. These results are in line with observations on effort allocation and discarding of marketable fish, both over-quota discarding and highgrading, by the Dutch beam-trawl fleet. The models we present can be used to predict the outcomes of management and are therefore a useful tool for fisheries scientists and managers.
    Multiple growth-correlated life history traits estimated simultaneously in indivuals
    Mollet, F.M. ; Ernande, B. ; Brunel, T.P.A. ; Rijnsdorp, A.D. - \ 2010
    Oikos 119 (2010)1. - ISSN 0030-1299 - p. 10 - 26.
    north-sea plaice - pleuronectes-platessa l - herring clupea-harengus - evolving fish stocks - indeterminate growth - back-calculation - somatic growth - reaction norms - resource-allocation - arctica-islandica
    We present a new methodology to estimate rates of energy acquisition, maintenance, reproductive investment and the onset of maturation (four-trait estimation) by fitting an energy allocation model to individual growth trajectories. The accuracy and precision of the method is evaluated on simulated growth trajectories. In the deterministic case, all life history parameters are well estimated with negligible bias over realistic parameter ranges. Adding environmental variability reduces precision, causes the maintenance and reproductive investment to be confounded with a negative error correlation, and tends, if strong, to result in an underestimation of the energy acquisition and maintenance and an overestimation of the age and size at the onset of maturation. Assuming a priori incorrect allometric scaling exponents also leads to a general but fairly predictable bias. To avoid confounding in applications we propose to assume a constant maintenance (three-trait estimation), which can be obtained by fitting reproductive investment simultaneously to size at age on population data. The results become qualitatively more robust but the improvement of the estimate of the onset of maturation is not significant. When applied to growth curves back-calculated from otoliths of female North Sea plaice Pleuronectes platessa, the four-trait and three-trait estimation produced estimates for the onset of maturation very similar to those obtained by direct observation. The correlations between life-history traits match expectations. We discuss the potential of the methodology in studies of the ecology and evolution of life history parameters in wild populations
    Genetic population structure of marine fish: mismatch between biological and fisheries management units
    Reiss, H. ; Hoarau, G. ; Dickey-Collas, M. ; Wolff, W.J. - \ 2009
    Fish and Fisheries 10 (2009)4. - ISSN 1467-2960 - p. 361 - 395.
    cod gadus-morhua - herring clupea-harengus - plaice pleuronectes-platessa - capelin mallotus-villosus - hake merluccius-merluccius - dna-sequence variation - mackerel trachurus-trachurus - north-sea plaice - pan-i locus - halibut reinhardtius-hippoglossoides
    An essential prerequisite of a sustainable fisheries management is the matching of biologically relevant processes and management action. In fisheries management and assessment, fish stocks are the fundamental biological unit, but the reasoning for the operational management unit is often indistinct and mismatches between the biology and the management action frequently occur. Despite the plethora of population genetic data on marine fishes, to date little or no use is made of the information, despite the fact that the detection of genetic differentiation may indicate reproductively distinct populations. Here, we discuss key aspects of genetic population differentiation in the context of their importance for fisheries management. Furthermore, we evaluate the population structure of all 32 managed marine fish species in the north-east Atlantic and relate this structure to current management units and practice. Although a large number of studies on genetic population structure have been published in the last decades, data are still rare for most exploited species. The mismatch between genetic population structure and the current management units found for six species (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Micromesistius poutassou, Merluccius merluccius and Clupea harengus), emphasizes the need for a revision of these units and questions the appropriateness of current management measures. The implementation of complex and dynamic population structures into novel and less static management procedures should be a primary task for future fisheries management approaches
    Biased stock assessment when using multiple, hardly overlapping, tuning series if fishing trends vary spatially
    Kraak, S.B.M. ; Daan, N. ; Pastoors, M.A. - \ 2009
    ICES Journal of Marine Science 66 (2009)10. - ISSN 1054-3139 - p. 2272 - 2277.
    north-sea plaice - age data - catch
    Fishing-effort distributions are subject to change, for autonomous reasons and in response to management regulations. Ignoring such changes in a stock-assessment procedure may lead to a biased perception. We simulated a stock distributed over two regions with inter-regional migration and different trends in exploitation and tested the performance of extended survivors analysis (XSA) and a statistical catch-at-age model in terms of bias, when spatially restricted tuning series were applied. If we used a single tuning index that covered only the more heavily fished region, estimates of fishing mortality and spawning-stock biomass were seriously biased. If two tuning series each exclusively covering one region were used (without overlap but together covering the whole area), estimates were also biased. Surprisingly, a moderate degree of overlap of spatial coverage of the two tuning indices was sufficient to reduce bias of the XSA assessment substantially. However, performance was best when one tuning series covered the entire stock area
    Resolving the effect of climate change on fish populations
    Rijnsdorp, A.D. ; Peck, M.A. ; Engelhard, G.H. ; Moellmann, C. ; Pinnegar, J.K. - \ 2009
    ICES Journal of Marine Science 66 (2009)7. - ISSN 1054-3139 - p. 1570 - 1583.
    vissen - migratie - mariene gebieden - atlantische oceaan - lichaamsgewicht - fishes - migration - marine areas - atlantic ocean - body weight - cod gadus-morhua - plaice pleuronectes-platessa - haddock melanogrammus-aeglefinus - individual-based models - north-sea plaice - sole solea-solea - marine ecosystems - trophic cascades - regime shifts - baltic sea
    This paper develops a framework for the study of climate on fish populations based on first principles of physiology, ecology, and available observations. Environmental variables and oceanographic features that are relevant to fish and that are likely to be affected by climate change are reviewed. Working hypotheses are derived from the differences in the expected response of different species groups. A review of published data on Northeast Atlantic fish species representing different biogeographic affinities, habitats, and body size lends support to the hypothesis that global warming results in a shift in abundance and distribution (in patterns of occurrence with latitude and depth) of fish species. Pelagic species exhibit clear changes in seasonal migration patterns related to climate-induced changes in zooplankton productivity. Lusitanian species have increased in recent decades (sprat, anchovy, and horse mackerel), especially at the northern limit of their distribution areas, while Boreal species decreased at the southern limit of their distribution range (cod and plaice), but increased at the northern limit (cod). Although the underlying mechanisms remain uncertain, available evidence suggests climate-related changes in recruitment success to be the key process, stemming from either higher production or survival in the pelagic egg or larval stage, or owing to changes in the quality/quantity of nursery habitats
    Comprehensive discard reconstruction and abundance estimation using flexible selectivity functions
    Aarts, G.M. ; Poos, J.J. - \ 2009
    ICES Journal of Marine Science 66 (2009)4. - ISSN 1054-3139 - p. 763 - 771.
    north-sea plaice - pleuronectes-platessa - fisheries management - stock assessment - mixed fisheries - beam-trawl - age data - catch - survival - flatfish
    The additional mortality caused by discarding may hamper the sustainable use of marine resources, especially if it is not accounted for in stock assessment and fisheries management. Generally, long and precise time-series on age-structured landings exist, but historical discard estimates are often lacking or imprecise. The flatfish fishery in the North Sea is a mixed fishery targeting mainly sole and plaice. Owing to the gear characteristics and a minimum landing size for these species, considerable discarding occurs, especially for juvenile plaice. Discard samples collected by on-board observers are available since 1999 from a limited number of commercial fishing trips. Here, we develop a statistical catch-at-age model with flexible selectivity functions to reconstruct historical discards and estimate stock abundance. We do not rely on simple predefined selectivity ogives, but use spline smoothers to capture the unknown non-linear selectivity and discard patterns, and allow these to vary in time. The model is fitted to the age-structured landings, discards, and survey data, the most appropriate model is selected, and estimates of uncertainty are obtained
    The discard problem - A comparative analysis of two fisheries: The English Nephrops fishery and the Dutch beam trawl fishery
    Catchpole, T. ; Keeken, O.A. van; Gray, T. ; Piet, G.J. - \ 2008
    Ocean & Coastal Management 51 (2008)11. - ISSN 0964-5691 - p. 772 - 778.
    north-sea plaice - catch - sole
    Discarding is the throwing overboard of unwanted fish or benthic animals [In this study, we define discards in terms of discarded edible fish, crustaceans and benthic organisms. We exclude, therefore, discards of plant material, offal, sea mammals and sea birds.] that have been caught by sea fishing vessels. Some estimates suggest that up to 25% of the global amount of fish caught is discarded annually. In this comparative analysis of discarding in two contrasting North Sea fisheries ¿ the English Nephrops fishery and the Dutch beam trawl fishery ¿ we discuss the effectiveness of measures that might reduce their rates of discarding, including management measures, market forces and fisher's behaviour. Our findings are that despite the many differences between the two fisheries and their respective discard problems, one common thread ¿ the importance of appropriate incentives ¿ runs through both of them.
    The arms race between fishers
    Rijnsdorp, A.D. ; Poos, J.J. ; Quirijns, F.J. ; Hille Ris Lambers, R. ; Wilde, J.W. de; Heijer, W.M. den - \ 2008
    Journal of Sea Research 60 (2008)1-2. - ISSN 1385-1101 - p. 126 - 138.
    north-sea plaice - hake merluccius-bilinearis - maturation reaction norms - fishing vessels - mixed fisheries - fleet dynamics - behavioral inferences - population-dynamics - beam trawlers - catch
    An analysis of the changes in the Dutch demersal fishing fleet since the 1950s revealed that competitive interactions among vessels and gear types within the constraints imposed by biological, economic and fisheries management factors are the dominant processes governing the dynamics of fishing fleets. Double beam trawling, introduced in the early 1960s, proved a successful fishing method to catch deep burying flatfish, in particular sole. In less than 10 years, the otter trawl fleet was replaced by a highly specialised beam trawling fleet, despite an initial doubling of the loss rate of vessels due to stability problems. Engine power, size of the beam trawl, number of tickler chains and fishing speed rapidly increased and fishing activities expanded into previously lightly fished grounds and seasons. Following the ban on flatfish trawling within the 12 nautical mile zone for vessels of more than 300 hp in 1975 and with the restriction of engine power to 2000 hp in 1987, the beam trawl fleet bifurcated. Changes in the fleet capacity were related to the economic results and showed a cyclic pattern with a period of 6¿7 years. The arms race between fishers was fuelled by competitive interactions among fishers: while the catchability of the fleet more than doubled in the ten years following the introduction of the beam trawl, a decline in catchability was observed in reference beam trawlers that remained the same. Vessel performance was not only affected by the technological characteristics but also by the number and characteristics of competing vessels.
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