Body condition and consequences on life-history trait plasticity in small pelagic fish of the Mediterranean Sea

Multiple changes have been described since 2008 in the Gulf of Lions ecosystem and particularly in small pelagic fish dynamic. In particular, the two main exploited species, i.e. anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) are now smaller and in poor condition while a third species, the sprat (Sprattus sprattus) strongly increased in terms of biomass and abundance. This PhD thesis investigated these changes through the analysis of small pelagic fish body condition (i.e individual energy stores) variations, its causes and consequences. Anchovy and sardine body condition was optimal in 2005 and 2006 and decreased after 2008 to remain steady at low level since 2010. Moreover, older sardine have particularly displayed poor body condition since 2008. A part of these changes is associated with changes in zooplankton concentration for both species, but also with both sea surface temperature and diatoms for sardine and Rhône outflow for anchovy. Thus, as bottom-up control was highlighted, the following chapter investigated potential changes in diet through isotope and stomach content analyses. Isotopic niche varied temporally and have been overlapping since 2010 for both anchovy and sardine with the sprat, resulting in a potential new trophic competition. Further, preys are also smaller and probably less energetic. Both studies may explain the poorer body condition by a reduced food supply. Although smaller energy stores are available, both species still allocate a large part of their energy to reproduction, as observed through e.g. gonado-somatic indices. This bias in energy allocation towards reproduction might impair survival and explain the disappearance of oldest mature sardine in the Gulf of Lions. Maternal effects were also underlined. Indeed, large individuals spawn more eggs while fatter individual spawn higher quality eggs. The decrease in size and condition has thus led to a lower production of eggs since 2010 for sardines while anchovy managed to maintain if not increase its egg production thanks to the expanding number of individuals and the decline of anchovy size and age at maturity. At a broader scale, we pointed out that fish body condition decreased in other Mediterranean areas such as the Adriatic Sea, the Catalan Sea or the Strait of Sicily. Nonetheless, body condition variations were not synchronous in the Mediterranean Sea, pointing out the importance of local factors in this quasi-enclosed basin. Furthermore, anchovy displays a better body condition in high energetic areas (fronts areas, local upwellings) when sardine prefers areas with high primary productivity. Those differences could be linked to the different reproductive cycles and trophic behaviours. This PhD thesis complements the previous work made on top-down processes and strengthens the bottom-up importance to determine forage fish body condition both in the Gulf of Lions and the Mediterranean Sea. Studying body condition allowed understanding the reasons of the sardine demographic truncation. Indeed, the older the individual the poorer the body condition, preventing them to achieve both reproduction and survival. This PhD thesis reaches new conclusions and improves knowledge on small pelagic fish dynamic in the Gulf of Lions. Although, further work is still necessary to estimate experimentally if a body condition threshold exists and to improve zooplanktonic monitoring to better understand the link between planktonic productivity and small pelagic fish dynamic. This will help to investigate deeply the ecological and economical consequences of small pelagic fish decreasing body condition in the Mediterranean.L’écosystème pélagique du Golfe du Lion a subi un changement très marqué de la dynamique de population des poissons petits pélagiques depuis 2008. L'anchois (Engraulis encrasicolus) et la sardine (Sardina pilchardus), exploités économiquement, sont devenus plus petits et plus maigres tandis que le sprat (Sprattus sprattus), non exploité car de faible valeur commerciale a fortement augmenté en abondance et en biomasse. Cette thèse analyse les changements observés en se basant sur la condition corporelle, i.e. les réserves énergétiques des individus, et les causes et conséquences de ses variations. La condition corporelle était optimale, en 2005 et 2006 pour l’anchois et la sardine, puis s’est dégradée depuis 2008 pour rester à de faibles niveaux depuis 2010. Les principaux paramètres expliquant les changements de condition corporelle sont la concentration zooplanctonique pour les deux espèces ainsi que la température de surface pour la sardine et le débit du Rhône pour l’anchois. Depuis 2008, les sardines les plus âgées semblent être les plus touchées par cette chute de condition. Le chapitre suivant a permis d'identifier des changements temporels des niches isotopiques qui sont aujourd’hui partagées par l’anchois et la sardine avec le sprat alors qu’elles ne l’étaient pas en 2004-2005. En plus de cette compétition potentielle, les proies aujourd’hui consommées sont de plus petite taille et probablement moins énergétiques. Ces deux résultats peuvent expliquer la plus faible condition actuellement observée par un apport nutritif actuellement réduit. Malgré les plus faibles réserves disponibles, les deux espèces investissent toujours autant dans la reproduction, voire même plus pour la sardine, comme suggéré par plusieurs proxys que sont l’indice gonado-somatique ou la durée de la période de reproduction. L’investissement énergétique semble donc se faire vers la reproduction au détriment de la survie, ce qui explique la disparition des sardines les plus âgées (celles en âge de se reproduire). Des effets maternels ont aussi été montrés, les plus grands individus pondant plus d’œufs et ceux en meilleure condition produisant des œufs de meilleure qualité. Le stock de sardine produit ainsi moins d’œufs depuis 2010 alors que l’anchois en produit plus, ce qui pourrait venir de l’augmentation du nombre d’individus et de l’abaissement de la taille et de l’âge à maturité. Dans une étude à large échelle, nous avons mis en évidence que la baisse de la condition est également perceptible dans d’autres zones de Méditerranée. Néanmoins, les variations de condition ne sont pas régies de façon synchrone en Méditerranée, pointant l’importance des facteurs locaux dans cette mer quasi-fermée. De plus, les anchois présentent une meilleure condition corporelle dans les zones de convergence des masses d’eaux (e.g. fronts, upwellings locaux) alors que les sardines préfèrent les zones de forte production primaire. Ces différences pourraient être liées aux différences dans le cycle de reproduction et dans le comportement alimentaire. Ces travaux de thèse complètent les précédents travaux faits sur les mécanismes top-down et soulignent ici le rôle que joue la ressource alimentaire (contrôle 'bottom-up') dans les variations de condition corporelle dans le Golfe du Lion et en Méditerranée. L’étude de la condition corporelle a aussi permis de comprendre les raisons de la troncation démographique observée chez la sardine. Malgré cela, il est toujours nécessaire d’estimer si un seuil létal de la condition corporelle existe chez nos espèces et d’améliorer le suivi zooplanctonique pour mieux comprendre le lien entre la productivité planctonique et la dynamique de population des petits pélagiques. Cette thèse permet une avancée notable dans la compréhension de la dynamique de population des petits pélagiques et permettra d’appréhender au mieux les répercussions écologiques et économiques de la baisse générale de leur condition dans toute la Méditerranée

Condition corporelle et conséquences sur la plasticité des traits d’histoire de vie chez les petits pélagiques de Méditerranée

Multiple changes have been described since 2008 in the Gulf of Lions ecosystem and particularly in small pelagic fish dynamic. In particular, the two main exploited species, i.e. anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) are now smaller and in poor condition while a third species, the sprat (Sprattus sprattus) strongly increased in terms of biomass and abundance. This PhD thesis investigated these changes through the analysis of small pelagic fish body condition (i.e individual energy stores) variations, its causes and consequences. Anchovy and sardine body condition was optimal in 2005 and 2006 and decreased after 2008 to remain steady at low level since 2010. Moreover, older sardine have particularly displayed poor body condition since 2008. A part of these changes is associated with changes in zooplankton concentration for both species, but also with both sea surface temperature and diatoms for sardine and Rhône outflow for anchovy. Thus, as bottom-up control was highlighted, the following chapter investigated potential changes in diet through isotope and stomach content analyses. Isotopic niche varied temporally and have been overlapping since 2010 for both anchovy and sardine with the sprat, resulting in a potential new trophic competition. Further, preys are also smaller and probably less energetic. Both studies may explain the poorer body condition by a reduced food supply. Although smaller energy stores are available, both species still allocate a large part of their energy to reproduction, as observed through e.g. gonado-somatic indices. This bias in energy allocation towards reproduction might impair survival and explain the disappearance of oldest mature sardine in the Gulf of Lions. Maternal effects were also underlined. Indeed, large individuals spawn more eggs while fatter individual spawn higher quality eggs. The decrease in size and condition has thus led to a lower production of eggs since 2010 for sardines while anchovy managed to maintain if not increase its egg production thanks to the expanding number of individuals and the decline of anchovy size and age at maturity. At a broader scale, we pointed out that fish body condition decreased in other Mediterranean areas such as the Adriatic Sea, the Catalan Sea or the Strait of Sicily. Nonetheless, body condition variations were not synchronous in the Mediterranean Sea, pointing out the importance of local factors in this quasi-enclosed basin. Furthermore, anchovy displays a better body condition in high energetic areas (fronts areas, local upwellings) when sardine prefers areas with high primary productivity. Those differences could be linked to the different reproductive cycles and trophic behaviours. This PhD thesis complements the previous work made on top-down processes and strengthens the bottom-up importance to determine forage fish body condition both in the Gulf of Lions and the Mediterranean Sea. Studying body condition allowed understanding the reasons of the sardine demographic truncation. Indeed, the older the individual the poorer the body condition, preventing them to achieve both reproduction and survival. This PhD thesis reaches new conclusions and improves knowledge on small pelagic fish dynamic in the Gulf of Lions. Although, further work is still necessary to estimate experimentally if a body condition threshold exists and to improve zooplanktonic monitoring to better understand the link between planktonic productivity and small pelagic fish dynamic. This will help to investigate deeply the ecological and economical consequences of small pelagic fish decreasing body condition in the Mediterranean.L’écosystème pélagique du Golfe du Lion a subi un changement très marqué de la dynamique de population des poissons petits pélagiques depuis 2008. L'anchois (Engraulis encrasicolus) et la sardine (Sardina pilchardus), exploités économiquement, sont devenus plus petits et plus maigres tandis que le sprat (Sprattus sprattus), non exploité car de faible valeur commerciale a fortement augmenté en abondance et en biomasse. Cette thèse analyse les changements observés en se basant sur la condition corporelle, i.e. les réserves énergétiques des individus, et les causes et conséquences de ses variations. La condition corporelle était optimale, en 2005 et 2006 pour l’anchois et la sardine, puis s’est dégradée depuis 2008 pour rester à de faibles niveaux depuis 2010. Les principaux paramètres expliquant les changements de condition corporelle sont la concentration zooplanctonique pour les deux espèces ainsi que la température de surface pour la sardine et le débit du Rhône pour l’anchois. Depuis 2008, les sardines les plus âgées semblent être les plus touchées par cette chute de condition. Le chapitre suivant a permis d'identifier des changements temporels des niches isotopiques qui sont aujourd’hui partagées par l’anchois et la sardine avec le sprat alors qu’elles ne l’étaient pas en 2004-2005. En plus de cette compétition potentielle, les proies aujourd’hui consommées sont de plus petite taille et probablement moins énergétiques. Ces deux résultats peuvent expliquer la plus faible condition actuellement observée par un apport nutritif actuellement réduit. Malgré les plus faibles réserves disponibles, les deux espèces investissent toujours autant dans la reproduction, voire même plus pour la sardine, comme suggéré par plusieurs proxys que sont l’indice gonado-somatique ou la durée de la période de reproduction. L’investissement énergétique semble donc se faire vers la reproduction au détriment de la survie, ce qui explique la disparition des sardines les plus âgées (celles en âge de se reproduire). Des effets maternels ont aussi été montrés, les plus grands individus pondant plus d’œufs et ceux en meilleure condition produisant des œufs de meilleure qualité. Le stock de sardine produit ainsi moins d’œufs depuis 2010 alors que l’anchois en produit plus, ce qui pourrait venir de l’augmentation du nombre d’individus et de l’abaissement de la taille et de l’âge à maturité. Dans une étude à large échelle, nous avons mis en évidence que la baisse de la condition est également perceptible dans d’autres zones de Méditerranée. Néanmoins, les variations de condition ne sont pas régies de façon synchrone en Méditerranée, pointant l’importance des facteurs locaux dans cette mer quasi-fermée. De plus, les anchois présentent une meilleure condition corporelle dans les zones de convergence des masses d’eaux (e.g. fronts, upwellings locaux) alors que les sardines préfèrent les zones de forte production primaire. Ces différences pourraient être liées aux différences dans le cycle de reproduction et dans le comportement alimentaire. Ces travaux de thèse complètent les précédents travaux faits sur les mécanismes top-down et soulignent ici le rôle que joue la ressource alimentaire (contrôle 'bottom-up') dans les variations de condition corporelle dans le Golfe du Lion et en Méditerranée. L’étude de la condition corporelle a aussi permis de comprendre les raisons de la troncation démographique observée chez la sardine. Malgré cela, il est toujours nécessaire d’estimer si un seuil létal de la condition corporelle existe chez nos espèces et d’améliorer le suivi zooplanctonique pour mieux comprendre le lien entre la productivité planctonique et la dynamique de population des petits pélagiques. Cette thèse permet une avancée notable dans la compréhension de la dynamique de population des petits pélagiques et permettra d’appréhender au mieux les répercussions écologiques et économiques de la baisse générale de leur condition dans toute la Méditerranée

Body condition and consequences on life-history trait plasticity in small pelagic fish of the Mediterranean Sea

L’écosystème pélagique du Golfe du Lion a subi un changement très marqué de la dynamique de population des poissons petits pélagiques depuis 2008. L'anchois (Engraulis encrasicolus) et la sardine (Sardina pilchardus), exploités économiquement, sont devenus plus petits et plus maigres tandis que le sprat (Sprattus sprattus), non exploité car de faible valeur commerciale a fortement augmenté en abondance et en biomasse. Cette thèse analyse les changements observés en se basant sur la condition corporelle, i.e. les réserves énergétiques des individus, et les causes et conséquences de ses variations. La condition corporelle était optimale, en 2005 et 2006 pour l’anchois et la sardine, puis s’est dégradée depuis 2008 pour rester à de faibles niveaux depuis 2010. Les principaux paramètres expliquant les changements de condition corporelle sont la concentration zooplanctonique pour les deux espèces ainsi que la température de surface pour la sardine et le débit du Rhône pour l’anchois. Depuis 2008, les sardines les plus âgées semblent être les plus touchées par cette chute de condition. Le chapitre suivant a permis d'identifier des changements temporels des niches isotopiques qui sont aujourd’hui partagées par l’anchois et la sardine avec le sprat alors qu’elles ne l’étaient pas en 2004-2005. En plus de cette compétition potentielle, les proies aujourd’hui consommées sont de plus petite taille et probablement moins énergétiques. Ces deux résultats peuvent expliquer la plus faible condition actuellement observée par un apport nutritif actuellement réduit. Malgré les plus faibles réserves disponibles, les deux espèces investissent toujours autant dans la reproduction, voire même plus pour la sardine, comme suggéré par plusieurs proxys que sont l’indice gonado-somatique ou la durée de la période de reproduction. L’investissement énergétique semble donc se faire vers la reproduction au détriment de la survie, ce qui explique la disparition des sardines les plus âgées (celles en âge de se reproduire). Des effets maternels ont aussi été montrés, les plus grands individus pondant plus d’œufs et ceux en meilleure condition produisant des œufs de meilleure qualité. Le stock de sardine produit ainsi moins d’œufs depuis 2010 alors que l’anchois en produit plus, ce qui pourrait venir de l’augmentation du nombre d’individus et de l’abaissement de la taille et de l’âge à maturité. Dans une étude à large échelle, nous avons mis en évidence que la baisse de la condition est également perceptible dans d’autres zones de Méditerranée. Néanmoins, les variations de condition ne sont pas régies de façon synchrone en Méditerranée, pointant l’importance des facteurs locaux dans cette mer quasi-fermée. De plus, les anchois présentent une meilleure condition corporelle dans les zones de convergence des masses d’eaux (e.g. fronts, upwellings locaux) alors que les sardines préfèrent les zones de forte production primaire. Ces différences pourraient être liées aux différences dans le cycle de reproduction et dans le comportement alimentaire. Ces travaux de thèse complètent les précédents travaux faits sur les mécanismes top-down et soulignent ici le rôle que joue la ressource alimentaire (contrôle 'bottom-up') dans les variations de condition corporelle dans le Golfe du Lion et en Méditerranée. L’étude de la condition corporelle a aussi permis de comprendre les raisons de la troncation démographique observée chez la sardine. Malgré cela, il est toujours nécessaire d’estimer si un seuil létal de la condition corporelle existe chez nos espèces et d’améliorer le suivi zooplanctonique pour mieux comprendre le lien entre la productivité planctonique et la dynamique de population des petits pélagiques. Cette thèse permet une avancée notable dans la compréhension de la dynamique de population des petits pélagiques et permettra d’appréhender au mieux les répercussions écologiques et économiques de la baisse générale de leur condition dans toute la Méditerranée.Multiple changes have been described since 2008 in the Gulf of Lions ecosystem and particularly in small pelagic fish dynamic. In particular, the two main exploited species, i.e. anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) are now smaller and in poor condition while a third species, the sprat (Sprattus sprattus) strongly increased in terms of biomass and abundance. This PhD thesis investigated these changes through the analysis of small pelagic fish body condition (i.e individual energy stores) variations, its causes and consequences. Anchovy and sardine body condition was optimal in 2005 and 2006 and decreased after 2008 to remain steady at low level since 2010. Moreover, older sardine have particularly displayed poor body condition since 2008. A part of these changes is associated with changes in zooplankton concentration for both species, but also with both sea surface temperature and diatoms for sardine and Rhône outflow for anchovy. Thus, as bottom-up control was highlighted, the following chapter investigated potential changes in diet through isotope and stomach content analyses. Isotopic niche varied temporally and have been overlapping since 2010 for both anchovy and sardine with the sprat, resulting in a potential new trophic competition. Further, preys are also smaller and probably less energetic. Both studies may explain the poorer body condition by a reduced food supply. Although smaller energy stores are available, both species still allocate a large part of their energy to reproduction, as observed through e.g. gonado-somatic indices. This bias in energy allocation towards reproduction might impair survival and explain the disappearance of oldest mature sardine in the Gulf of Lions. Maternal effects were also underlined. Indeed, large individuals spawn more eggs while fatter individual spawn higher quality eggs. The decrease in size and condition has thus led to a lower production of eggs since 2010 for sardines while anchovy managed to maintain if not increase its egg production thanks to the expanding number of individuals and the decline of anchovy size and age at maturity. At a broader scale, we pointed out that fish body condition decreased in other Mediterranean areas such as the Adriatic Sea, the Catalan Sea or the Strait of Sicily. Nonetheless, body condition variations were not synchronous in the Mediterranean Sea, pointing out the importance of local factors in this quasi-enclosed basin. Furthermore, anchovy displays a better body condition in high energetic areas (fronts areas, local upwellings) when sardine prefers areas with high primary productivity. Those differences could be linked to the different reproductive cycles and trophic behaviours. This PhD thesis complements the previous work made on top-down processes and strengthens the bottom-up importance to determine forage fish body condition both in the Gulf of Lions and the Mediterranean Sea. Studying body condition allowed understanding the reasons of the sardine demographic truncation. Indeed, the older the individual the poorer the body condition, preventing them to achieve both reproduction and survival. This PhD thesis reaches new conclusions and improves knowledge on small pelagic fish dynamic in the Gulf of Lions. Although, further work is still necessary to estimate experimentally if a body condition threshold exists and to improve zooplanktonic monitoring to better understand the link between planktonic productivity and small pelagic fish dynamic. This will help to investigate deeply the ecological and economical consequences of small pelagic fish decreasing body condition in the Mediterranean

Spatial and ontogenetic variations in sardine feeding conditions in the Bay of Biscay through fatty acid composition

International audienceFood characteristics are amongst the most influential factors determining the fish life history traits as quantitative and qualitative changes in individuals' diet can lead to a decline in the energy allocated to their growth, and hence influence natural populations' characteristics. The size-at-age and weight of European sardines (Sardina pilchardus) in the Bay of Biscay (BoB) have decreased substantially over the last decade, especially for the youngest age classes, and the factors underlying such changes have not yet been identified. We therefore analysed the fatty acid (FA) composition in the neutral (NL) and polar (PL) lipids in samples collected across the BoB to determine whether the diet of sardines changes with their ages. We found that the total FA contents in both lipid fractions varied mainly with the sampling locations and age. Indeed, sardines aged 1 and 2 years living in South BoB had particularly high contents in FA specific to non-diatom phytoplankton, while older sardines living in the Northern part had higher total FA content and more FA specific to copepods. These differences probably resulted from differences in prey availability and to a lesser extend a change in feeding behaviour with age. The strong dependence of younger sardines’ diet to phytoplankton in spring suggests that changes in primary production may explain their decline in size-at-age. Finally, NL clearly reflect finest feeding variations in comparison to PL imprinted by diet variations at longer time scale. Future studies should consider separately NL and PL fractions

Fish population growth in the Gulf of St Lawrence: effects of climate, fishing and predator abundance

Climate variability, fishing and predation are the main factors affecting fish population dynamics. In this study, the drivers of population growth variability were investigated for 7 fish stocks (2 cod, 4 herring and 1 mackerel stock) in the Gulf of St. Lawrence (NW Atlantic). The annual population growth sensitivity to both recruitment and survival (i.e. how much population growth depends on recruitment and survival) was estimated through elasticity analyses before being linked to harvesting history as well as environmental conditions (climate and predation). Stock-specific generalized additive models showed that population growth was most sensitive to recruitment variability with decreasing fish generation time, increasing water temperature and, in some cases, with predation. The dependence of population growth on recruitment, however, was generally close to 0, albeit higher for pelagic stocks than for demersal ones. This indicates that adult survival was more important than recruitment in shaping population growth and thus population size. As climate mainly acts on recruitment while fishing affects adult survival, management efforts seem more important than climate variability in regulating population growth, especially for long-lived cod. Nevertheless, population growth may become increasingly dependent on recruitment variability with warming waters; therefore, more flexible management strategies should be developed to cope with these oscillations

Assessment modelling approaches for stocks with spawning components, seasonal and spatial dynamics, and limited resources for data collection.

The true spatiotemporal structure of a fish population is often more complex than represented in assessments because movement between spawning components is disregarded and data at the necessary scale are unavailable. This can generate poor advice. We explore the impacts of modelling choices and their associated risks given limited data and lack of biological knowledge on spawning component structure and connectivity. Pseudo-data for an age structured fish population were simulated with two spawning components that experience various levels of connectivity and that might overlap during a certain period but segregate during reproduction. A variety of implicit spatiotemporal and simpler models were fitted to the pseudo-datasets, mimicking different situations of data availability. To reproduce the true stock characteristics, the spatiotemporal models required total catch data disaggregated by spawning component; however, catch-at-age was not as important nor were disaggregated biomass indices to reproduce true dynamics. Even with just 5% connectivity between spawning components, both the spatiotemporal models and simpler alternatives generally overestimated stock biomass. Although bias was smallest when considering one unit population, spawning components might still need to be considered for management and conservation. In such case, the spatiotemporal model was less influenced by ignored connectivity patterns compared to a model focussing on one spawning component only

Physiological biomarkers and fisheries management

The benefits of physiological biomarkers, knowledge and concepts are well-established in fish and wildlife management as they confer the ability to understand mechanistic processes, identify cause-and-effect relationships, and develop predictive models. Although this approach is gaining momentum in the context of species conservation, the use of physiological biomarkers in exploited marine fish stock management and recovery plans remains relatively rare. Here, we present five essential issues to consider to implement physiological biomarkers in fisheries management: (i) choice of relevant biomarkers that have a well-known mechanistic basis, (ii) identification of species-specific biomarkers reflecting a meaningful timespan for management, (iii) selection of biomarkers compatible with data collection during routine scientific fisheries surveys, (iv) use of biomarkers as early-warning signals and complementary indicators of population-level changes in life history traits and (v) how physiological biomarkers may help to refine long-term population dynamic projections under climate change and management scenarios. Overall, if based on well-established mechanisms linked to individuals’ fitness, a focus on physiological biomarkers should help to better understand the mechanisms behind stock declines, changes in stock characteristics, and thus more efficiently manage marine fisheries and conserve populations. As this approach is transferable among species, locations, and times, the integration of physiological biomarkers in fisheries science has the potential to more broadly enhance assessments and management of fish stocks

Fish morphometric body condition indices reflect energy reserves but other physiological processes matter

Morphometric indices of body condition are assumed to reflect an animal’s health and ultimately its fitness, but their physiological significance remains a matter of debate. These indices are indeed usually considered as proxies of energy reserves, ignoring other physiological processes involved in animal health such as nutritional, immune and hormonal states. Given the wide variety of ecological processes investigated through morphometric body condition indices in marine sciences, there is a clear need to determine whether morphometric indices reflect primarily individuals’ energy reserves or their broader integrated physiological status. To address this issue, we used morphometric and physiological data (cortisol level, oxidative stress, digestive enzymes activity, and both fatty acids percentage and total amount) collected in three stocks of the European sardine (Sardina pilchardus) presenting contrasted patterns of growth and body condition. We found that morphometric body condition indices are indeed mainly and consistently linked to a proxy of the amount of lipid reserves (i.e., fatty acids amount), but also significantly to quality of lipid reserves (i.e., fatty acids percentage) and fish chronic stress (scale cortisol levels). We found no significant relationship between morphometric indices of body condition and both oxidative stress proxies and variables measuring digestive enzymes activity. Our study confirmed that morphometric body condition indices primarily reflect the variance in individuals’ lipid reserves and to a lesser extent the actual composition of these reserves (linked to differences in fish diet) and scale cortisol levels (indicating fish metabolism and/or their chronic stress levels). Therefore, some non-energetic aspects should be considered when studying individuals’ responses to environmental changes and other key physiological processes (oxidative stress proxies, activity of digestive enzymes) should be investigated directly to support scientific-based decision-making in the context of climate change

The fisheries history of small pelagics in the Northern Mediterranean

Since 2007, the biomass of sardine and anchovy in the NW Mediterranean has remained persistently low, whereas the biomass of the commercially low-valued sprat has exploded. Also, simultaneous decreases in condition, size, and/or age of these populations were observed. Altogether, this resulted in a drop in landings of small pelagics. To understand the amplitude of these events and to provide a baseline scenario against which current changes can be compared, we compiled exceptionally long landing series (1865–2013) of sardine, anchovy, and mackerel for different subregions of the southern French coast. We characterized the fluctuations of these landings and compared these with environmental drivers (sea surface temperature, Rhône river discharge, North Atlantic Oscillation, Western Mediterranean Oscillation—WeMO, and Atlantic Multidecadal Oscillation—AMO), using different time-series analyses. We also collated historical data to infer qualitative changes in fishing effort over time. A fishing effort related increase in landings was observed around 1962 for all three species, although current sardine landings have dropped below levels observed before this period. Sardine and anchovy landings were, respectively, positively and negatively related to the AMO index and anchovy landings were also positively related to the WeMO. We finished by discussing the potential role of the environmental variables and fishing on long-term fishery landings trends

How membrane fatty acids influence sardine size across diverse marine environments

International audienceDifferences in diet quality and quantity may influence trophodynamic processes in small pelagic fish. Yet, we currently lack direct and comprehensive information on how large-scale areas differ in dietary resources and the degree to which these differences influence fish physiological performances (i.e., growth), ultimately influencing entire fish stocks. Fatty acid composition is one of the bioindicator that can provide insights on how dietary provisions of essential lipids influence the structure of the membrane fatty acids and subsequently fish growth among contrasted habitats. To address this issue, we conducted a large-scale sampling of European sardine (Sardina pilchardus) a species with major socioeconomic importance that plays a key role as an energy pathway linking lower and upper trophic levels in pelagic ecosystems. We sampled individuals from the Gulf of Lions (Mediterranean Sea), the Bay of Biscay, and the English Channel (Atlantic Ocean) of age-0 to-3 + and found clear spatial differences in the quantity and quality of dietary lipids. Sardines from the Gulf of Lions fed on trophic food web based on dinoflagellates, with greater proportions of DHA (22:6n-3; docosahexaenoic acid) in reserve lipids. Sardines' reserve lipids had important proportions of zooplankton biomarkers in the English Channel (e.g., 20:1n-9 and 20:1n-11), and diatoms biomarkers such as 16C fatty acids and EPA (20:5n-3; eicosapentaenoic acid) in the Bay of Biscay. The relationship between sardines' membrane fatty acid composition and individuals' length changed progressively with individuals' age, a result consistent across areas, indicating ontogenetic abilities between largest and smallest individuals. Before maturity, largest sardines had higher DHA proportions, followed after maturity by higher proportions of ARA (20:4n-6; arachidonic acid), EPA and DPA (22:5n-3; docosapentaenoic acid). Finally, the study highlights the importance of considering the quality and diversity of dietary resources to better understand how individuals cope with their physiological needs. It is thus important to consider combined aspects (e.g., diet quality and diversity, influence of particular nutrients on length) to better understand the underlying mechanistic processes influencing fish physiology, likely cascading to different expression of their life history traits and affecting fisheries stocks