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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    Fish abundance, fisheries, fish trade and consumption in sixteenth-century Netherlands as described by Adriaen Coenen
    Bennema, F.P. ; Rijnsdorp, A.D. - \ 2015
    Fisheries Research 161 (2015). - ISSN 0165-7836 - p. 384 - 399.
    pleuronectes-platessa l - atlantic bluefin tuna - southern north-sea - cod gadus-morhua - wadden sea - population-dynamics - medieval europe - resource use - lower rhine - history
    Concern about fisheries impact on marine ecosystems has raised the interest in the reconstruction of the state of marine ecosystems and the nature of the human activities in the past. We present late 16th century information on the occurrence and relative abundance of biota in Dutch coastal and inland waters (50 marine fish, 13 diadromous or freshwater and 4 marine mammal species), as well as a description of the sea fisheries (target species, fishing grounds, gear), fish trade, export, and fish consumption in Holland as documented in the handwritten Fish Book by Adriaen Coenen (1577–1581). The species composition and abundances are compared to published trawl survey data from around 1900 and in the 1990s. Fish species that have disappeared almost completely, were already rare around 1900 and are characterised by a large body size (rays and sharks, sturgeon, ling), whereas currently abundant species were already abundant in the 16th century. Intensive fisheries for herring occurred near Orkney, Fairhill and Shetland. Coastal and freshwater fisheries provided fresh fish for local as well as export markets, but also provided bait for the massive offshore hook and line fishery for the production of salted cod, which remained largely unnoticed. Dried flatfish were exported to Germany. Consumption of fish and marine invertebrates differed between social classes. Coenen distinguished eight consumer categories, a refinement of the categories ‘rich’ and ‘poor’ used in archaeological studies.
    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.
    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.
    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.
    Estimating spatial and temporal variability of juvenile North Sea plaice from opportunistic data
    Poos, J.J. ; Aarts, G.M. ; Vandemaele, S. ; Willems, W. ; Bolle, L.J. ; Helmond, A.T.M. van - \ 2013
    Journal of Sea Research 75 (2013). - ISSN 1385-1101 - p. 118 - 128.
    pleuronectes-platessa l - dutch wadden sea - marine ecosystems - climate-change - trawl fishery - flatfish - temperature - ecology - discard - growth
    Surveys are often insufficient to accurately capture the distribution of a species in both space and time. Complementary to the use of research vessel data, platforms of opportunity can be a powerful strategy to monitor species distributions at high temporal and spatial resolution. In this study we use data from commercial fishing vessels, collecting – under the European Union data collection framework – biological data on all species that are caught and subsequently discarded. Using such discard data in combination with a systematic trawl survey, we model the spatial and temporal distribution of juvenile plaice (Pleuronectes platessa) in the central North Sea. There is a clear age-dependent difference between the commercial fishing vessel data and the research vessel data, with age 1 being the dominating age in the survey catches, while age 2 is the dominating age in the discards. The results show how immature plaice, slowly migrate from the nursery areas, westwards into the deeper regions of the North Sea. Also, the results show that during the study period, juvenile plaice gradually moved to deeper waters at an earlier age. Finally we discuss how the framework can be applied to similar opportunistic data to monitor seasonal and inter-annual migration of marine organisms, and to quantify how they may be influenced by biotic and abiotic gradients, such as temperature.
    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.
    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.
    Implications of fisheries-induced changes in stock structure and reproductive potential for stock recovery of a sex-dimorphic species, North Sea plaice
    Rijnsdorp, A.D. ; Damme, C.J.G. van; Witthames, P.R. - \ 2010
    ICES Journal of Marine Science 67 (2010). - ISSN 1054-3139 - p. 1931 - 1938.
    cod gadus-morhua - pleuronectes-platessa l - maturation reaction norms - annual egg-production - atlantic cod - broadcast spawner - fecundity - growth - size - age
    A key assumption in stock assessment and stock forecasts often is that spawning-stock biomass (SSB) and egg production are proportional and that the reproductive potential is independent of stock structure (age composition and sex ratio). Based on a 60-year time-series of total egg production (TEP) of North Sea plaice, we demonstrate that this assumption could result in a biased perception of the temporal trend in reproductive potential. The time-series incorporates: (i) annual observations on maturity, growth, and condition, (ii) a predictive model for interannual variations in fecundity caused by variations in body condition and by the probability of being a recruit spawner, and (iii) a cohort analysis of sex-specific landings-at-age since 1948. Following an increase in fishing mortality rate, TEP declined by a factor of 7–8 from a peak in the 1970s to a minimum in 1999–2000. Concurrent with this decline, the contribution of recruit spawners and the size difference between spawning males and females decreased. The implications of phenotypic plasticity and fisheries-induced evolutionary changes in growth and maturation for the recovery potential of the plaice stock are discussed.
    Age-structure-dependent recruitment: a meta-analysis applied to Northeast Atlantic fish stocks
    Brunel, T.P.A. - \ 2010
    ICES Journal of Marine Science 67 (2010). - ISSN 1054-3139 - p. 1921 - 1930.
    cod gadus-morhua - pleuronectes-platessa l - spawning stock - reproductive success - spatial-distribution - maternal age - baltic cod - size - population - management
    Exploitation alters the age structure of fish stocks. Several stock-specific studies have suggested that changes in the age structure might have consequences for subsequent recruitment, but the evidence is not universal. To investigate how common such effects are among 39 Northeast Atlantic fish stocks, relationships were tested between age structure (spawner mean age, age diversity, and proportion of recruit spawners) and recruitment (number of recruits, sensitivity to environment, and recruitment variability). Significant correlations in the expected direction were observed for a few stocks, but not for the majority; significant correlations in the opposite direction were also found. Meta-analyses combining the stock-level tests revealed that none of the effects were significant overall. However, effects were significant for some species (cod, haddock, and plaice) and indices. The low variability in the age structure might explain the absence of significant effects for individual stocks. Other reasons could be the absence of a biological basis (reproductive characteristics not dependent on age) or the stronger influence of environmental variability than of age structure on recruitment.
    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
    A reanalysis of North Sea plaice spawning-stock biomass using the annual egg production method
    Damme, C.J.G. van; Bolle, L.J. ; Fossum, P. ; Kraus, G. ; Dickey-Collas, M. - \ 2009
    ICES Journal of Marine Science 66 (2009)9. - ISSN 1054-3139 - p. 1999 - 2011.
    pleuronectes-platessa l - herring clupea-harengus - gadus-morhua l. - solea-solea l. - irish sea - fecundity regulation - english-channel - fish community - oocyte growth - fisheries
    Uncertainty about the quality of current virtual population analysis-based stock assessment for North Sea plaice (Pleuronectes platessa) has led to various abundance indices. We compared biomass estimates from the annual egg production (AEP) method with current stock assessments based on catch-at-age to validate the current and historical perception of exploitation. The AEP method was also used to investigate the dynamics of the spatial components of plaice in the North Sea. We corrected for fecundity down-regulation and changes in sex ratio. Estimates from both methods were similar in trend and absolute biomass. On the Dogger Bank, there was a dramatic decline in biomass from 1948 and 1950 to 2004, and in the Southern Bight, the stock appeared to increase from 1987 and 1988 to 2004, although not reaching the historically high levels of 1948 or 1950. The timing of spawning of North Sea plaice does not appear to have changed throughout the period of high exploitation. We conclude that the AEP method is a useful way to hindcast the spatial dynamics of heavily exploited flatfish stocks
    Effects of climate change on growth of 0-group sole and plaice
    Teal, L.R. ; Leeuw, J.J. de; Veer, H.W. van der; Rijnsdorp, A.D. - \ 2008
    Marine Ecology Progress Series 358 (2008). - ISSN 0171-8630 - p. 219 - 230.
    vissen - klimaatverandering - gewichtstoename - mariene gebieden - noordzee - fishes - climatic change - weight gain - marine areas - north sea - north-sea plaice - pleuronectes-platessa l - western wadden sea - long-term changes - flounder platichthys-flesus - juvenile plaice - individual growth - regime shifts - irish sea - flatfish
    The effect of rising seawater temperature on growth of 0-group sole Solea solea and plaice Pleuronectes platessa in the southeastern North Sea was investigated for the period 1970 to 2004 using annual autumn pre-recruit survey data and frequent surveys on a nursery ground. Autumn length showed an increasing trend in sole but not in plaice. Increasing winter temperatures significantly increased the growing period of sole, a warm-water species that spawns in spring, but not of plaice, a temperate species that spawns in winter. Growth rate increased with higher summer temperatures in sole and to a lesser degree in plaice. Compared to experimental growth rates at ambient temperatures and unlimited food, observed growth rates were close to experimental values until mid-June but were much lower in July to September, suggesting food limitation in summer. The higher temperatures observed since 1989 positively affected the quality of the shallow coastal waters as a nursery area for sole but not for plaice. A further increase may negatively affect the nursery quality if the production rate of benthic food cannot meet the increase in energy requirements of 0-group flatfish.
    The effect of rising seawater temperature on growth of 0-group sole Solea solea and plaice Pleuronectes platessa in the southeastern North Sea was investigated for the period 1970 to 2004 using annual autumn pre-recruit survey data and frequent surveys on a nursery ground. Autumn length showed an increasing trend in sole but not in plaice. Increasing winter temperatures significantly increased the growing period of sole, a warm-water species that spawns in spring, but not of plaice, a temperate species that spawns in winter. Growth rate increased with higher summer temperatures in sole and to a lesser degree in plaice. Compared to experimental growth rates at ambient temperatures and unlimited food, observed growth rates were close to experimental values until mid-June but were much lower in July to September, suggesting food limitation in summer. The higher temperatures observed since 1989 positively affected the quality of the shallow coastal waters as a nursery area for sole but not for plaice. A further increase may negatively affect the nursery quality if the production rate of benthic food cannot meet the increase in energy requirements of 0-group flatfish.
    Fisheries-induced evolutionary changes in maturation reaction norms in North Sea sole Solea solea
    Mollet, F.M. ; Kraak, S.B.M. ; Rijnsdorp, A.D. - \ 2007
    Marine Ecology Progress Series 351 (2007). - ISSN 0171-8630 - p. 189 - 199.
    pleuronectes-platessa l - life-history evolution - long-term changes - cod gadus-morhua - coral-reef fish - phenotypic plasticity - natural-populations - social inhibition - sexual maturity - egg size
    Age and size at maturation decreased in several commercially exploited fish stocks, which, according to life history theory, may be due to fisheries-induced evolutionary change. However, the observed changes may also represent a plastic response to environmental variability. To disentangle phenotypic plasticity from evolutionary change, the probabilistic reaction norm approach was applied to 43 cohorts (1960 to 2002) of female sole Solea solea from market samples. The reaction norm for age and size at first maturation has significantly shifted towards younger age and smaller size. Size at 50% probability of maturation at Age 3 decreased from 28.6 cm (251 g) to 24.6 cm (128 g). This change was even stronger when condition was included as a third dimension in the reaction norm estimation. The influence of alternative factors was tested on the population level by regression of reaction norm midpoints on annual estimates of condition, temperature and competitive biomass. Although effects of temperature and competitive biomass were significant, the variation in the midpoints was best explained by the decreasing time trend. Therefore, the results provide strong evidence for a fisheries-induced evolutionary change in the onset of sexual maturity.
    The dynamics of small-scale patchiness of plaice and sole as reflected in the catch rates of the Dutch beam trawl fleet and its implications for the fleet dynamics
    Poos, J.J. ; Rijnsdorp, A.D. - \ 2007
    Journal of Sea Research 58 (2007)1. - ISSN 1385-1101 - p. 100 - 112.
    north-sea plaice - pleuronectes-platessa l - ideal free distribution - shelf nw spain - spatial-distribution - competitive interactions - geostatistical analysis - effort allocation - fishing vessels - diel variation
    Catch rates of sole Solea solea and plaice Pleuronectes platessa in the Dutch beam trawl fleet operating in the North Sea show spatio-temporal variation. The variation in catch rates reflects differences in abundance of the species. Up to 45% of the variation in catch rates can be explained by the time of day of the catch, the engine power of the vessel and the migration cycles of the species. Also, spatial covariance was found in the residual variation for both species using variograms and covariance functions. The ranges of the spatial structure were found to be between approximately 20 to 45 nautical miles, indicating patchy distribution of the species. No differences in the ranges of the spatial structures were found between different seasons. Cross-covariance analysis shows the patches lasted up to two weeks. The implication of the spatial pattern in flatfish for the dynamics of effort allocation is discussed.
    Three-dimensional maturation reaction norms for North Sea plaice
    Grift, R.E. ; Heino, M. ; Rijnsdorp, A.D. ; Kraak, S.B.M. ; Dieckmann, U. - \ 2007
    Marine Ecology Progress Series 334 (2007). - ISSN 0171-8630 - p. 213 - 224.
    pleuronectes-platessa l - life-history evolution - cod gadus-morhua - hippoglossoides-platessoides - sexual-maturation - back-calculation - growth - size - age - reproduction
    Probabilistic maturation reaction norms (PMRNs) with up to three explanatory dimensions were estimated for female North Sea plaice. The three-dimensional PMRNs reported here (1) are the first ones to be obtained for any organism, (2) reveal the differential capacity of alternative life-history state variables to predict the onset of reproduction, (3) document consistent temporal trends in maturation, and (4) help disentangle the contributions of genetic and plastic effects to these trends. We first show that PMRNs based on age and weight provide slightly more accurate approximations of maturation probabilities than PMRNs based on age and length. At the same time, weight-based PMRNs imply a much wider spread of maturation probabilities than length-based PMRNs. We then demonstrate that including condition as a third explanatory variable improves predictions of maturation probability. The resultant three-dimensional PMRNs for age-length-condition or age-weight-condition not only show how, at given size and age, maturation probability increases with condition, but also expose how this impact of condition decreases with age and has changed over time. Our analysis reveals several interesting temporal trends. First, it is demonstrated that, even after removing plastic effects on maturation captured by age, length, weight, and condition, residual trends towards maturation at younger ages and smaller lengths remain. Second, we find that the width of both length- and weight-based PMRNs decreased significantly over time. Third, age and condition are nowadays affecting maturation probabilities less than they did decades ago. We think that plaice is currently maturing at a very low age, size and body condition, and think that the narrow and steep reaction norms do not allow a strong continuation of the observed trends. All obtained findings are in good agreement with predictions from life-history theory based on the hypothesis of evolutionary change caused by heavy exploitation
    Seasonality only Works in Certain Parts of the Year: the Reconstruction of Fishing Seasons through Otolith Analysis
    Neer, W. van; Ervynck, A. ; Bolle, L.J. ; Millner, R.S. - \ 2004
    International Journal of Osteoarchaeology 14 (2004)6. - ISSN 1047-482X - p. 457 - 474.
    pleuronectes-platessa l - growth - cementum - death - age - increments - eruption - plaice - sites - teeth
    Seasonality estimations using incremental data usually suffer from small sample sizes and from the lack of comparison with sufficiently large modern samples. The present contribution reports on incremental studies carried out on large assemblages of plaice (Pleuronectes platessa) and haddock (Melanogrammus aeglefinus) from a late medieval fishing village (Raversijde, Belgium) on the North Sea coast. In an attempt to refine previous seasonality estimates made for this site, and to expand conclusions concerning general methodology, extensive monthly samples of modern otoliths of these species, caught within the North Sea, have also been investigated. The modern material shows that the timing of the seasonal changes in the edge type (hyaline or opaque) of the otoliths is extremely variable and that it is dependent on the fishing ground, the year considered, and the age of the fish. It also appears that the increase of the marginal increment thickness is highly variable, to such an extent that the thickness of the last increment of a single otolith is mostly useless for seasonality estimation. Where large archaeological otolith assemblages can be studied, preferably from single depositional events, seasonality determination becomes possible on the condition, however, that the archaeological assemblage corresponds to fish that were captured during their period Of fast growth. The growth ring study on the otoliths from Raversijde shows that plaice fishing took place in spring and that it was preceded by a haddock fishing season, probably in late winter/early spring. Copyright (C) 2004 John Wiley Sons, Ltd.
    Growth changes in plaice, cod, haddock and saithe in the North Sea: a comparison of (post-)medieval and present-day growth rates based on otolith measurements
    Bolle, L.J. ; Rijnsdorp, A.D. ; Neer, W. van; Millner, R.S. ; Leeuwen, P.I. van; Ervynck, A. - \ 2004
    Journal of Sea Research 51 (2004). - ISSN 1385-1101 - p. 313 - 328.
    pleuronectes-platessa l - size-selective mortality - length-at-age - dutch wadden sea - solea-solea l - juvenile growth - concentration hypothesis - back-calculation - reaction norms - density
    Fishing effort has strongly increased in the North Sea since the mid-19th century, causing a substantial reduction in the population size of exploited fish stocks. As fisheries research has developed simultaneously with the industrialisation of the fisheries, our knowledge of population dynamics at low levels of exploitations is limited. Otoliths retrieved from archaeological excavations offer a unique opportunity to study growth rates in the past. This study compares historical and present-day growth rates for four commercially important demersal fish species. A total of 2532 modern otoliths (AD 1984 1999) and 1286 historical otoliths (AD 1200 1925) obtained from archaeological excavations in Belgium and Scotland were analysed. Comparison of the growth patterns between eras revealed a major increase in growth rate of haddock, whereas growth changes were not observed in saithe and only in the smaller size classes of plaice and cod. Comparison of our results with literature data indicates that the observed growth rate changes in plaice and cod occurred within the 20th century. Apparently the onset of industrialised fisheries has not greatly affected the growth of plaice, cod and saithe populations in the North Sea. This result contradicts the expectation of density-dependent limitation of growth during the era of pre-industrialised fishing, but is in agreement with the concentration hypothesis of Beverton (Neth. J. Sea Res. 34 (1995) 1) stating that species which concentrate spatially into nursery grounds during their early life-history may saturate the carrying capacity of the juvenile habitat even though the adult part of the population is not limited by the adult habitat
    Fisheries-induced trends n reaction norms for maturation in North Sea plaice
    Grift, R.E. ; Rijnsdorp, A.D. ; Barot, S. ; Heino, M. ; Dieckmann, U. - \ 2003
    Marine Ecology Progress Series 257 (2003)(2003). - ISSN 0171-8630 - p. 247 - 257.
    pleuronectes-platessa l - life-history evolution - size-selective mortality - beam-trawl effort - back-calculation - growth - age - reproduction - population - maturity
    We analyse how intensive exploitation may have caused evolutionary changes in the age and length at maturation in North Sea plaice Pleuronectes platessa. Such evolutionary change in the onset of maturation is expected, given that fishing mortality is more than 4 times higher than natural mortality. In order to disentangle phenotypic plasticity from evolutionary change, we employ the probabilistic reaction-norm approach. This technique allows us to estimate the probabilities of maturing at each relevant age and size, and to disentangle the plasticity in age and size at maturation that results from changes in growth rates from evolutionary changes in maturation propensities themselves. This recently developed method is applied here to females of 41 cohorts (1955 to 1995) of North Sea plaice. We focus on trends in fishing mortality, in growth rates, and in the probabilities of maturing, and test the hypothesis that the decrease in age and length at maturation is partly caused by fisheries-induced adaptive change. We find that the reaction norm for age and length at maturation has indeed significantly shifted towards younger age and smaller length. The reaction-norm analysis suggests a picture in which short-term fluctuations originating from plastic responses are superimposed on a persistent long-term trend resulting from genetic responses and higher body growth.
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