Didier Gascuel | Université Européenne de Bretagne (original) (raw)
Papers by Didier Gascuel
Marine Ecology Progress Series, 2014
Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic e... more Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic ecosystems and the induced biodiversity changes. However, much debate has ensued regarding discrepancies and challenges arising from the use of landings data from commercial fisheries to calculate TL indicators. Subsequent studies have started to examine survey-based and model-based indicators. In this paper, we undertake an extensive evaluation of a variety of TL indicators across 9 well-studied marine ecosystems by making use of model-as well as surveyand catch-based TL indicators. Using detailed regional information and data on fishing history, fishing intensity, and environmental conditions, we evaluate how well TL indicators are capturing fishing effects at the community level of marine ecosystems. Our results highlight that the differences observed between TL indicator values and trends is dependent on the data source and the TL cut-off point used in the calculations and is not attributable to an intrinsic problem with TLbased indicators. All 3 data sources provide useful information about the structural changes in the ecosystem as a result of fishing, but our results indicate that only model-based indicators represent fishing impacts at the whole ecosystem level.
ABSTRACT Ecopath et EcoTroph : des outils d'évaluation des effets des AMP à l'éch... more ABSTRACT Ecopath et EcoTroph : des outils d'évaluation des effets des AMP à l'échelle des réseaux trophiques
Marine protected areas (MPAs) are now viewed as an efficient tool to fight against the global det... more Marine protected areas (MPAs) are now viewed as an efficient tool to fight against the global deterioration of habitats and fish populations. However their efficiency and effects on the whole trophic network are little known. Based on the Bolong de Bamboung (Sénégal) case study, the objective of this study is to assess the impact of a MPA on the entire trophic network using trophodynamic models. This estuarine small MPA is scientifically surveyed since 2003, one year before the closure of the fishery. Using the resulting dataset, we calculated time series of abundance indices and constructed two Ecopath mass-balanced models for the year 2003 (fished) and the 2006–2008 period (unfished). Using EcoTroph, an ecosystem trophic-level based model, we compared the two periods and simulated a closure of the fishery starting from 2003 to assess the effect of the MPA. We observed a rather constant total biomass, but a biomass increase by a 2.5 factor in predators and a decrease by a 1.7 factor in their preys. Simulations showed that the increase in predators was too important to only being a direct consequence of the removal of the fishing mortality and a local production. This highlighted the role of the MPA as a refuge or a foraging arena for some predator species. What's more, the decrease of the preys, corresponding to pelagic species, was very important and couldn’t be explained only by an increase of the predation plus a release of the fishing pressure. This indicated other possible effects as environmental and behavioural ones, in addition of a direct MPA effect. Sensitivity testing and a comparison with another nearby similar but fished area, the Bolong de Sangako, validated the same global conclusion. We concluded that the fishing closure had direct effects but also indirect ones likely due to fish behaviour and environment, and that trophodynamic models are useful tools to analyse MPA effects on the whole trophic network.
Journal of Marine Systems, 2012
A C C E P T E D M A N U S C R I P T
Canadian Journal of Fisheries and Aquatic Sciences, 1994
EMSAWORSTOM/CWOBT, d6parternent HaBieutique, 65 route de 33542 Rennes CEQFX, France Gascarel, B. ... more EMSAWORSTOM/CWOBT, d6parternent HaBieutique, 65 route de 33542 Rennes CEQFX, France Gascarel, B. 1994, Une methde simple d'ajusternent des cies taille/Sge : application aux captures d'albacores (Thunnus aBbacares) de IfAtlantique Est. Can. ). Fish. Aquat. Sci. 51 : 723-733. Une methode d'ajustement des cles taiile/itge, qui tient compte de la variabilite interannuelle de I'abcsndance des cohortes, est presentee. Cette mkthode s'appuie sur une modelisation de la croissance et des variations des kcarts types de taille, en fonction de l'sge. Elle reprend un algorithme d'ajustement itkratif. L'algorithme converge rapidement vers une solution stable, qui est identique, sous certaines conditions, a celie obtenue par le maximum de vraisembiance. Il est lui-meme utilis6 pour estimer les pararn&tres du modele des &arts types de taille. La rnethode est appliquee aux captures d'aibacores (Thunnus albacares) de I'Atlantique Est de 1975 a 1989. Les cles mensuelles, ajustees aux captures de I'ensembie de la pet herie, presentent une variabi l it6 inter-annuel le importante. El les sont utiliskes pour une conversion tai!le/sge par metier et par carre! geographique de 5" de cote. Comparativement aux estimations anterieures, realiskes par la methode du (( slicing s, les captures par Age sont sensiblement corrigkes; elles presentent une plus grande variabilitk temporelle. Les resultats sont peu sensibles A I'estirnation des param5tres du modele des 6carts types de taille. La methode d'ajustement des cles est comparee aux methodes classiques de d6cornposition polymodale par le maximum de vraisemblance. Elle s'applique particulierement bien lorsque la croissance est connue; e l k presente l'avantage d'une grande simplicite et d'une grande souplesse. Elle permet notamment la prise en compte de modeles de croissance particuliers.
Using generalized additive models, we show evidence for nonlinear relationships between various h... more Using generalized additive models, we show evidence for nonlinear relationships between various hydrologi- cal factors and age-structured catch per unit effort of Atlantic yellowfin tuna ( Thunnus albacares) for two fishing fleets. Catchability effects are distinguished from tuna environmental preference effects in the catch per unit effort variability. With respect to catchability, an important nonlinear effect of local fishing effort
We used primary productivity data derived from remote sensing images and catch data for the perio... more We used primary productivity data derived from remote sensing images and catch data for the period 1998 to 2004 to characterize the productivity of eco-regions in the northeast Atlantic, Mediterranean, Black and Baltic Seas, and then analyzed the strength of the trophic linkage between primary productivity and marine fisheries production in European seas. Mean annual primary production (PP) derived from an ocean-color based model was highly contrasted among eco-regions, exceeding 500 g C m -2 yr -1 in the Marmara and North Seas but being less than 150 g C m -2 yr -1 in the Faroes, Adriatic-Ionian and Aegean-Levantine Seas. Fisheries production expressed in mean annual yield and primary production required to support catches (PPR) varied greatly among eco-regions, from 0.02 t km -2 and 0.7 g C m -2 yr -1 for the oceanic northeast Atlantic eco-region to 6 t km -2 and 130 g C m -2 yr -1 in the Marmara Sea respectively. Linear regression models showed significant positive relationships between PP and yield as well as between PP and PPR, suggesting a strong linkage between marine productivity and fisheries production in European seas. Moreover, this bottom-up control appeared consistent over time: recent mean annual yield for the period 1998 to 2004 reflected the long-term yield averaged over the last 3 decades. We argue that such large-scale coupling is due to energy transfer along the food web (from phytoplankton to predators) through predation processes, primary productivity driving the production of living marine resources and their exploitation at the scale of eco-regions. Our findings have an important bearing for ecosystem approaches to fisheries, particularly for the estimation of the capacity of eco-regions with regard to sustainable exploitation. This is further relevant in a context of climate change, because variations in PP linked to global warming could strongly modify fisheries production in the future.
![Research paper thumbnail of Fishing impact on the marine Guinean ecosystem: a 1985/2005 model using EwE [Impact de la pêche sur l'écosystème marin de Guinée : modélisation EwE 1985-2005]](https://attachments.academia-assets.com/45090323/thumbnails/1.jpg)
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The trophic level (T L ) mean and variance, and the degree of omnivory for five Celtic Sea fish p... more The trophic level (T L ) mean and variance, and the degree of omnivory for five Celtic Sea fish predators were estimated using a database of stomach content records characterized by a high level of taxonomic resolution. The predators occupied a high position in the food web, i.e. 4Á75 for Atlantic cod Gadus morhua, 4Á44 for haddock Melanogrammus aeglefinus, 4Á88 for European hake Merluccius merluccius, 5Á00 for megrim Lepidorhombus whiffiagonis and 5Á27 for whiting Merlangius merlangus. The level of taxonomic resolution of the prey did not greatly affect mean T L predator values; an effect on variance was evident, low resolution masking intra-population variability in T L . Generalized additive models (GAM) were used to explain the variability of predator T L caused by environmental variables (International Council for the Exploration of the Sea, ICES, division and season) and predator characteristics (total length, L T ). Significant year, location season and interaction effects were found for some species and with L T at the scale of ICES subdivision. The species-specific variability of T L could be due to spatio-temporal variations in prey availability and in predator selectivity following ontogenetic changes. Omnivorous fish T L was less affected by spatio-temporal variations. In addition, results showed that the omnivory index and T L variability provide dissimilar information on predator feeding strategy. Combining information on T L variability and omnivory allowed between withinindividual and between-individual components contributing to trophic niche width to be separated and the type of generalization of fish predators to be identified.
This paper presents a spatial multigear population dynamics model forced by the environment for A... more This paper presents a spatial multigear population dynamics model forced by the environment for Atlantic Ocean yellowfin tuna. The model simulates the population's distribution as a function of environmental variables and observed fishing effort. It is age structured to account for age-dependent population processes and catchability. It is based on an advection-diffusion-reaction equation in which the advective term is proportional to the gradient of a habitat suitability index derived from temperature, salinity and tuna forage data. Functional relationships between movement parameters, catchability and environmental variables are based on non linear relationships estimated with generalized additive models (GAM) to characterize, on the one hand, yellowfin environmental preferences and, on the other hand, their catchability to different gears. Analytically formalized, GAM's relationships characterizing environmental preferences enable the habitat index to be calculated at each point in time and space. Also formulated analytically, the relationships characterizing catchability to different gears enable the calculation of predicted catches, which are compared to observed catches to estimate the model parameters. In this paper, the problem of local overfishing of adult tuna in the Gulf of Guinea is addressed through different simulations and discussed.
Aquatic Living Resources
Intense local fishing pressure is likely to induce significant local decreases in both resource b... more Intense local fishing pressure is likely to induce significant local decreases in both resource biomass and fishing yields. This is what we term 'local overfishing'. Such a phenomenon and its consequences for stock assessment are investigated with the fisheries simulator SHADYS (Simulateur halieutique de dynamiques spatiales: spatialized simulator of fisheries dynamics). In SHADYS, resource dynamic is based on an advection-diffusion-reaction equation. After a brief presentation, the simulator is used to characterize the 'local overfishing' phenomenon as a 'wound' of the population. The latter is subsequently able to 'heal' and to recover its initial distribution. We show that, associated with cooperation tactics of the fishing fleets, local overfishing may induce non-linear relationships between catches per unit of effort (CPUE) and fishing effort calculated locally. In particular, it is shown that there is a strictly decreasing relationship between catchability and effort when fishing vessels apply intense local fishing pressure but do not cooperate; that the relationship may increase when vessels cooperate but do not apply intense local fishing pressures; and finally, that it may go through a maximum when, simultaneously, ships cooperate and have high local exploitation rates. These three main types of theoretical relationships correspond well with those observed in reality. The existence of such relationships could be an important source of biases in stock assessment. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Éditions scientifiques et médicales Elsevier SAS advection-diffusion / biomass nonlinearity / CPUE / fishermen cooperation / local fish depletion / spatial model / stock assessment Résumé − « Surexploitation locale » et tactiques de pêche : considérations théoriques et conséquences pratiques en évaluation des stocks étudiés avec un simulateur numérique de pêcheries. De fortes pressions de pêche locales sont susceptibles d'entraîner des diminutions locales significatives de la ressource et des rendements de pêche. C'est ce que nous appelons la « surexploitation locale ». Le phénomène et ses conséquences en évaluation des stocks sont étudiés à l'aide du simulateur de pêcheries « Shadys » (Simulateur halieutique de dynamiques spatiales) où la dynamique de la ressource est fondée sur une équation d'advection-diffusion-réaction. Après avoir été rapidement présenté, le simulateur est utilisé pour caractériser le phénomène de « surexploitation locale » qu'on décrit ici comme une « blessure » de la population. Celle-ci peut, ensuite, « cicatriser » et retrouver sa répartition initiale. Associé à des stratégies de coopération des flottilles de pêche, le phénomène de surexploitation locale est susceptible d'entraîner l'existence d'une relation non linéaire entre les captures par unité d'effort (CPUE) et l'effort de pêche, calculés localement. En particulier, nous montrons qu'il existe une relation entre la capturabilité et l'effort de pêche, et que cette fonction est strictement décroissante quand les navires développent de fortes pressions de pêche locales mais ne coopèrent pas entre eux, qu'elle peut être croissante si les navires coopèrent mais ne développent pas de fortes pressions de pêche locales, et qu'elle peut passer par un maximum si les navires coopèrent et développent de fortes pressions de pêche locales. Ces trois grands types de relations obtenues théoriquement correspondent aux relations observées dans la réalité. L'existence de telles relations est une source de biais potentiellement importants en évaluation des stocks. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Éditions scientifiques et médicales Elsevier SAS advection-diffusion / non linéarité de la relation CPUE-biomasse / coopération des pêcheurs / surexploitation locale / modèle spatial / évaluation des stocks
Marine Ecology Progress Series, 2014
Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic e... more Trophic level (TL)-based indicators have been widely used to examine fishing impacts in aquatic ecosystems and the induced biodiversity changes. However, much debate has ensued regarding discrepancies and challenges arising from the use of landings data from commercial fisheries to calculate TL indicators. Subsequent studies have started to examine survey-based and model-based indicators. In this paper, we undertake an extensive evaluation of a variety of TL indicators across 9 well-studied marine ecosystems by making use of model-as well as surveyand catch-based TL indicators. Using detailed regional information and data on fishing history, fishing intensity, and environmental conditions, we evaluate how well TL indicators are capturing fishing effects at the community level of marine ecosystems. Our results highlight that the differences observed between TL indicator values and trends is dependent on the data source and the TL cut-off point used in the calculations and is not attributable to an intrinsic problem with TLbased indicators. All 3 data sources provide useful information about the structural changes in the ecosystem as a result of fishing, but our results indicate that only model-based indicators represent fishing impacts at the whole ecosystem level.
ABSTRACT Ecopath et EcoTroph : des outils d'évaluation des effets des AMP à l'éch... more ABSTRACT Ecopath et EcoTroph : des outils d'évaluation des effets des AMP à l'échelle des réseaux trophiques
Marine protected areas (MPAs) are now viewed as an efficient tool to fight against the global det... more Marine protected areas (MPAs) are now viewed as an efficient tool to fight against the global deterioration of habitats and fish populations. However their efficiency and effects on the whole trophic network are little known. Based on the Bolong de Bamboung (Sénégal) case study, the objective of this study is to assess the impact of a MPA on the entire trophic network using trophodynamic models. This estuarine small MPA is scientifically surveyed since 2003, one year before the closure of the fishery. Using the resulting dataset, we calculated time series of abundance indices and constructed two Ecopath mass-balanced models for the year 2003 (fished) and the 2006–2008 period (unfished). Using EcoTroph, an ecosystem trophic-level based model, we compared the two periods and simulated a closure of the fishery starting from 2003 to assess the effect of the MPA. We observed a rather constant total biomass, but a biomass increase by a 2.5 factor in predators and a decrease by a 1.7 factor in their preys. Simulations showed that the increase in predators was too important to only being a direct consequence of the removal of the fishing mortality and a local production. This highlighted the role of the MPA as a refuge or a foraging arena for some predator species. What's more, the decrease of the preys, corresponding to pelagic species, was very important and couldn’t be explained only by an increase of the predation plus a release of the fishing pressure. This indicated other possible effects as environmental and behavioural ones, in addition of a direct MPA effect. Sensitivity testing and a comparison with another nearby similar but fished area, the Bolong de Sangako, validated the same global conclusion. We concluded that the fishing closure had direct effects but also indirect ones likely due to fish behaviour and environment, and that trophodynamic models are useful tools to analyse MPA effects on the whole trophic network.
Journal of Marine Systems, 2012
A C C E P T E D M A N U S C R I P T
Canadian Journal of Fisheries and Aquatic Sciences, 1994
EMSAWORSTOM/CWOBT, d6parternent HaBieutique, 65 route de 33542 Rennes CEQFX, France Gascarel, B. ... more EMSAWORSTOM/CWOBT, d6parternent HaBieutique, 65 route de 33542 Rennes CEQFX, France Gascarel, B. 1994, Une methde simple d'ajusternent des cies taille/Sge : application aux captures d'albacores (Thunnus aBbacares) de IfAtlantique Est. Can. ). Fish. Aquat. Sci. 51 : 723-733. Une methode d'ajustement des cles taiile/itge, qui tient compte de la variabilite interannuelle de I'abcsndance des cohortes, est presentee. Cette mkthode s'appuie sur une modelisation de la croissance et des variations des kcarts types de taille, en fonction de l'sge. Elle reprend un algorithme d'ajustement itkratif. L'algorithme converge rapidement vers une solution stable, qui est identique, sous certaines conditions, a celie obtenue par le maximum de vraisembiance. Il est lui-meme utilis6 pour estimer les pararn&tres du modele des &arts types de taille. La rnethode est appliquee aux captures d'aibacores (Thunnus albacares) de I'Atlantique Est de 1975 a 1989. Les cles mensuelles, ajustees aux captures de I'ensembie de la pet herie, presentent une variabi l it6 inter-annuel le importante. El les sont utiliskes pour une conversion tai!le/sge par metier et par carre! geographique de 5" de cote. Comparativement aux estimations anterieures, realiskes par la methode du (( slicing s, les captures par Age sont sensiblement corrigkes; elles presentent une plus grande variabilitk temporelle. Les resultats sont peu sensibles A I'estirnation des param5tres du modele des 6carts types de taille. La methode d'ajustement des cles est comparee aux methodes classiques de d6cornposition polymodale par le maximum de vraisemblance. Elle s'applique particulierement bien lorsque la croissance est connue; e l k presente l'avantage d'une grande simplicite et d'une grande souplesse. Elle permet notamment la prise en compte de modeles de croissance particuliers.
Using generalized additive models, we show evidence for nonlinear relationships between various h... more Using generalized additive models, we show evidence for nonlinear relationships between various hydrologi- cal factors and age-structured catch per unit effort of Atlantic yellowfin tuna ( Thunnus albacares) for two fishing fleets. Catchability effects are distinguished from tuna environmental preference effects in the catch per unit effort variability. With respect to catchability, an important nonlinear effect of local fishing effort
We used primary productivity data derived from remote sensing images and catch data for the perio... more We used primary productivity data derived from remote sensing images and catch data for the period 1998 to 2004 to characterize the productivity of eco-regions in the northeast Atlantic, Mediterranean, Black and Baltic Seas, and then analyzed the strength of the trophic linkage between primary productivity and marine fisheries production in European seas. Mean annual primary production (PP) derived from an ocean-color based model was highly contrasted among eco-regions, exceeding 500 g C m -2 yr -1 in the Marmara and North Seas but being less than 150 g C m -2 yr -1 in the Faroes, Adriatic-Ionian and Aegean-Levantine Seas. Fisheries production expressed in mean annual yield and primary production required to support catches (PPR) varied greatly among eco-regions, from 0.02 t km -2 and 0.7 g C m -2 yr -1 for the oceanic northeast Atlantic eco-region to 6 t km -2 and 130 g C m -2 yr -1 in the Marmara Sea respectively. Linear regression models showed significant positive relationships between PP and yield as well as between PP and PPR, suggesting a strong linkage between marine productivity and fisheries production in European seas. Moreover, this bottom-up control appeared consistent over time: recent mean annual yield for the period 1998 to 2004 reflected the long-term yield averaged over the last 3 decades. We argue that such large-scale coupling is due to energy transfer along the food web (from phytoplankton to predators) through predation processes, primary productivity driving the production of living marine resources and their exploitation at the scale of eco-regions. Our findings have an important bearing for ecosystem approaches to fisheries, particularly for the estimation of the capacity of eco-regions with regard to sustainable exploitation. This is further relevant in a context of climate change, because variations in PP linked to global warming could strongly modify fisheries production in the future.
The trophic level (T L ) mean and variance, and the degree of omnivory for five Celtic Sea fish p... more The trophic level (T L ) mean and variance, and the degree of omnivory for five Celtic Sea fish predators were estimated using a database of stomach content records characterized by a high level of taxonomic resolution. The predators occupied a high position in the food web, i.e. 4Á75 for Atlantic cod Gadus morhua, 4Á44 for haddock Melanogrammus aeglefinus, 4Á88 for European hake Merluccius merluccius, 5Á00 for megrim Lepidorhombus whiffiagonis and 5Á27 for whiting Merlangius merlangus. The level of taxonomic resolution of the prey did not greatly affect mean T L predator values; an effect on variance was evident, low resolution masking intra-population variability in T L . Generalized additive models (GAM) were used to explain the variability of predator T L caused by environmental variables (International Council for the Exploration of the Sea, ICES, division and season) and predator characteristics (total length, L T ). Significant year, location season and interaction effects were found for some species and with L T at the scale of ICES subdivision. The species-specific variability of T L could be due to spatio-temporal variations in prey availability and in predator selectivity following ontogenetic changes. Omnivorous fish T L was less affected by spatio-temporal variations. In addition, results showed that the omnivory index and T L variability provide dissimilar information on predator feeding strategy. Combining information on T L variability and omnivory allowed between withinindividual and between-individual components contributing to trophic niche width to be separated and the type of generalization of fish predators to be identified.
This paper presents a spatial multigear population dynamics model forced by the environment for A... more This paper presents a spatial multigear population dynamics model forced by the environment for Atlantic Ocean yellowfin tuna. The model simulates the population's distribution as a function of environmental variables and observed fishing effort. It is age structured to account for age-dependent population processes and catchability. It is based on an advection-diffusion-reaction equation in which the advective term is proportional to the gradient of a habitat suitability index derived from temperature, salinity and tuna forage data. Functional relationships between movement parameters, catchability and environmental variables are based on non linear relationships estimated with generalized additive models (GAM) to characterize, on the one hand, yellowfin environmental preferences and, on the other hand, their catchability to different gears. Analytically formalized, GAM's relationships characterizing environmental preferences enable the habitat index to be calculated at each point in time and space. Also formulated analytically, the relationships characterizing catchability to different gears enable the calculation of predicted catches, which are compared to observed catches to estimate the model parameters. In this paper, the problem of local overfishing of adult tuna in the Gulf of Guinea is addressed through different simulations and discussed.
Aquatic Living Resources
Intense local fishing pressure is likely to induce significant local decreases in both resource b... more Intense local fishing pressure is likely to induce significant local decreases in both resource biomass and fishing yields. This is what we term 'local overfishing'. Such a phenomenon and its consequences for stock assessment are investigated with the fisheries simulator SHADYS (Simulateur halieutique de dynamiques spatiales: spatialized simulator of fisheries dynamics). In SHADYS, resource dynamic is based on an advection-diffusion-reaction equation. After a brief presentation, the simulator is used to characterize the 'local overfishing' phenomenon as a 'wound' of the population. The latter is subsequently able to 'heal' and to recover its initial distribution. We show that, associated with cooperation tactics of the fishing fleets, local overfishing may induce non-linear relationships between catches per unit of effort (CPUE) and fishing effort calculated locally. In particular, it is shown that there is a strictly decreasing relationship between catchability and effort when fishing vessels apply intense local fishing pressure but do not cooperate; that the relationship may increase when vessels cooperate but do not apply intense local fishing pressures; and finally, that it may go through a maximum when, simultaneously, ships cooperate and have high local exploitation rates. These three main types of theoretical relationships correspond well with those observed in reality. The existence of such relationships could be an important source of biases in stock assessment. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Éditions scientifiques et médicales Elsevier SAS advection-diffusion / biomass nonlinearity / CPUE / fishermen cooperation / local fish depletion / spatial model / stock assessment Résumé − « Surexploitation locale » et tactiques de pêche : considérations théoriques et conséquences pratiques en évaluation des stocks étudiés avec un simulateur numérique de pêcheries. De fortes pressions de pêche locales sont susceptibles d'entraîner des diminutions locales significatives de la ressource et des rendements de pêche. C'est ce que nous appelons la « surexploitation locale ». Le phénomène et ses conséquences en évaluation des stocks sont étudiés à l'aide du simulateur de pêcheries « Shadys » (Simulateur halieutique de dynamiques spatiales) où la dynamique de la ressource est fondée sur une équation d'advection-diffusion-réaction. Après avoir été rapidement présenté, le simulateur est utilisé pour caractériser le phénomène de « surexploitation locale » qu'on décrit ici comme une « blessure » de la population. Celle-ci peut, ensuite, « cicatriser » et retrouver sa répartition initiale. Associé à des stratégies de coopération des flottilles de pêche, le phénomène de surexploitation locale est susceptible d'entraîner l'existence d'une relation non linéaire entre les captures par unité d'effort (CPUE) et l'effort de pêche, calculés localement. En particulier, nous montrons qu'il existe une relation entre la capturabilité et l'effort de pêche, et que cette fonction est strictement décroissante quand les navires développent de fortes pressions de pêche locales mais ne coopèrent pas entre eux, qu'elle peut être croissante si les navires coopèrent mais ne développent pas de fortes pressions de pêche locales, et qu'elle peut passer par un maximum si les navires coopèrent et développent de fortes pressions de pêche locales. Ces trois grands types de relations obtenues théoriquement correspondent aux relations observées dans la réalité. L'existence de telles relations est une source de biais potentiellement importants en évaluation des stocks. © 2001 Ifremer/CNRS/Inra/IRD/Cemagref/Éditions scientifiques et médicales Elsevier SAS advection-diffusion / non linéarité de la relation CPUE-biomasse / coopération des pêcheurs / surexploitation locale / modèle spatial / évaluation des stocks