Ocean model-based covariates improve a marine fish stock assessment when observations are limited

The productivity of many fish populations is influenced by the environment, but developing environment-linked stock assessments remain challenging and current management of most commercial species assumes that stock productivity is time-invariant. In the Northeast United States, previous studies suggest that the recruitment of Southern New England-Mid Atlantic yellowtail flounder is closely related to the strength of the Cold Pool, a seasonally formed cold water mass on the continental shelf. Here, we developed three new indices that enhance the characterization of Cold Pool interannual variations using bottom temperature from a regional hindcast ocean model and a global ocean data assimilated hindcast. We associated these new indices to yellowtail flounder recruitment in a state–space, age-structured stock assessment framework using the Woods Hole Assessment Model. We demonstrate that incorporating Cold Pool effects on yellowtail flounder recruitment reduces the retrospective patterns and may improve the predictive skill of recruitment and, to a lesser extent, spawning stock biomass. We also show that the performance of the assessment models that incorporated ocean model-based indices is improved compared to the model using only the observation-based index. Instead of relying on limited subsurface observations, using validated ocean model products as environmental covariates in stock assessments may both improve predictions and facilitate operationalization.publishedVersio

Global change in the trophic functioning of marine food webs

The development of fisheries in the oceans, and other human drivers such as climate warming, have led to changes in species abundance, assemblages, trophic interactions, and ultimately in the functioning of marine food webs. Here, using a trophodynamic approach and global databases of catches and life history traits of marine species, we tested the hypothesis that anthropogenic ecological impacts may have led to changes in the global parameters defining the transfers of biomass within the food web. First, we developed two indicators to assess such changes: the Time Cumulated Indicator (TCI) measuring the residence time of biomass within the food web, and the Efficiency Cumulated Indicator (ECI) quantifying the fraction of secondary production reaching the top of the trophic chain. Then, we assessed, at the large marine ecosystem scale, the worldwide change of these two indicators over the 1950-2010 time-periods. Global trends were identified and cluster analyses were used to characterize the variability of trends between ecosystems. Results showed that the most common pattern over the study period is a global decrease in TCI, while the ECI indicator tends to increase. Thus, changes in species assemblages would induce faster and apparently more efficient biomass transfers in marine food webs. Results also suggested that the main driver of change over that period had been the large increase in fishing pressure. The largest changes occurred in ecosystems where 'fishing down the marine food web' are most intensive

An iron cycle cascade governs the response of equatorial Pacific ecosystems to climate change

Earth System Models project that global climate change will reduce ocean net primary production (NPP), upper trophic level biota biomass and potential fisheries catches in the future, especially in the eastern equatorial Pacific. However, projections from Earth System Models are undermined by poorly constrained assumptions regarding the biological cycling of iron, which is the main limiting resource for NPP over large parts of the ocean. In this study, we show that the climate change trends in NPP and the biomass of upper trophic levels are strongly affected by modifying assumptions associated with phytoplankton iron uptake. Using a suite of model experiments, we find 21st century climate change impacts on regional NPP range from −12.3% to +2.4% under a high emissions climate change scenario. This wide range arises from variations in the efficiency of iron retention in the upper ocean in the eastern equatorial Pacific across different scenarios of biological iron uptake, which affect the strength of regional iron limitation. Those scenarios where nitrogen limitation replaced iron limitation showed the largest projected NPP declines, while those where iron limitation was more resilient displayed little future change. All model scenarios have similar skill in reproducing past inter‐annual variations in regional ocean NPP, largely due to limited change in the historical period. Ultimately, projections of end of century upper trophic level biomass change are altered by 50%–80% across all plausible scenarios. Overall, we find that uncertainties in the biological iron cycle cascade through open ocean pelagic ecosystems, from plankton to fish, affecting their evolution under climate change. This highlights additional challenges to developing effective conservation and fisheries management policies under climate change

Spatial synchrony in the response of a long range migratory species ( Salmo salar ) to climate change in the North Atlantic Ocean

International audienceA major challenge in understanding the response of populations to climate change is to separate the effects of local drivers acting independently on specific populations, from the effects of global drivers that impact multiple populations simultaneously and thereby synchronize their dynamics. We investigated the environmental drivers and the demographic mechanisms of the widespread decline in marine survival rates of Atlantic salmon (Salmo salar) over the last four decades. We developed a hierarchical Bayesian life cycle model to quantify the spatial synchrony in the marine survival of 13 large groups of populations (called stock units, SU) from two continental stock-groupings (CSG) in North America (NA) and Southern Europe (SE) over the period 1971-2014. We found strong coherence in the temporal variation in post-smolt marine survival among the 13 SU of NA and SE. A common North Atlantic trend explains 37% of the temporal variability of the survivals for the 13 SU and declines by a factor 1.8 over the 1971-2014 time series. Synchrony in survival trends is stronger between SU within each CSG. The common trends at the scale of NA and SE capture 60% and 42% of the total variance of temporal variations, respectively. Temporal variations of the post-smolt survival are best explained by the temporal variations of sea surface temperature (SST, negative correlation) and net primary production indices (PP, positive correlation) encountered by salmon in common domains during their marine migration. Specifically, in the Labrador Sea/Grand Banks for NA populations 26% and 24% of variance is captured by SST and PP, respectively and in the Norwegian Sea for SE populations 21% and 12% of variance is captured by SST and PP, respectively. The findings support the hypothesis of a response of salmon populations to large climate induced changes in the North Atlantic simultaneously impacting populations from distant continental habitats

Dynamique spatio-temporelle de l’exploitation de la Sole commune, Solea solea, en Manche Est

The Eastern English Channel (EEC) common sole (Solea solea) stock is one of the most important stocks for the French fisheries in the EEC. Low recruitments in 2012 and 2013, coupled with a fishing mortality well above Fmsy led to repeated cuts in TAC over the last years. This master thesis aimed to study the spatiotemporal dynamics of the French fisheries exploiting sole in the EEC and their impact on this stock; it also allowed drawing assumptions on populations’ dynamics. We first showed a spatial structuration of the French fisheries (mainly trammel net and bottom otter trawl). A focus on trammel netters which represent the main share of the landings and which are highly dependent on common sole showed spatial differences on the length structures of the catches. Regression trees on mean lengths of catches and a multinomial logistic regression on the length structures of the catches revealed that these differences can be explained by the mesh size used, fishing period and the fishing area (including nursery area). The impact of potential subpopulations with different biological parameters in the EEC was tested focusing on growth parameters in these subareas, using a non-linear mixed effect model. It showed differences in growth rates and asymptotic lengths between the Northeast and the Southwest of the EEC. Finally, differences in the landings were checked through exploitation patterns to describe links and differences between catches and landings in the different regions and areas.Le stock de sole commune (Solea solea) de Manche Est (ME) est d’une grande importance économique. La conjonction de faibles recrutements en 2012 et 2013 et d’une mortalité par pêche au-dessus du Frmd a conduit à des diminutions répétées du TAC. Ce mémoire a pour objectif d’étudier la dynamique spatio-temporelle de l'exploitation française de la sole commune en ME et d'en tirer des hypothèses sur la dynamique de la population. En premier lieu, nous avons mis en évidence la structuration spatiale de la pêcherie française de sole commune dominée par les filets type trémail et les chaluts de fond à panneaux. Dans un second temps, en se concentrant sur les fileyeurs qui représentent la majeure partie des débarquements (65%) et qui sont fortement tributaires de la sole commune, nous avons montré l’existence d’une variabilité spatiale des structures en taille des captures. L’analyse, basée sur deux arbres de régression (longueurs moyennes) et sur une régression logistique multinomiale (distributions en tailles), a révélé que ces différences peuvent être expliquées par le maillage utilisé, la période de pêche et la zone de pêche. L’impact de potentielles sous-populations présentant des différences de paramètres biologiques a, ensuite, été testé à partir des paramètres de croissance en utilisant un modèle non linéaire à effets mixtes. Nous avons ainsi montré des différences de taux de croissance et de longueur asymptotique entre le Nord-est et le Sud-Ouest de la ME. Dans une dernière partie, les liens entre les captures et les débarquements ont été analysés dans les différentes régions et zones

Impacts of climate change on the trophic functioning of the world ocean

Climate change impacts on marine life in the world ocean are expected to increase over the 21st century. In this thesis, I investigated the effects of climate change on biomass flows in marine food webs and their consequences on ecosystem structure and functioning. First, the transfer efficiency and biomass residence time are estimated in the world’ shelf seas from 1950 to 2010. Based on the projected ocean warming under two climate scenarios, I highlighted that biomass transfers may be faster and less efficient by 2100 without mitigation of greenhouse gases emissions. Then, using a modelling framework called EcoTroph that is based on a representation of biomass flow, I projected the future of consumer biomass in marine food webs. From the projected changes in temperature and primary production, marine animal biomass is estimated at each trophic level on a 1° x 1° grid of the global ocean from 1950 to 2100. The projections showed that the projected alteration of biomass flows may lead to a global decline in consumer biomass by 2100 under the “no mitigation policy” climate scenario, with more pronounced impacts at higher trophic levels. In the European waters, the EcoTroph model forced by a coupled hydrodynamic-ecosystem model is used to investigate the potential climate change effects on the ecosystem structure and functioning. The results revealed that biomass and catch may decrease by 2100 under the “no mitigation policy” scenario and if fishing mortality remains constant at its current value. Overall, this thesis showed that climate change would alter biomass flows in marine ecosystems, causing a decrease in the future ocean animal biomass and direct repercussions on fisheries.Science, Faculty ofZoology, Department ofGraduat

Investigating spatial heterogeneity of von Bertalanffy growth parameters to inform the stock structuration of common sole, Solea solea, in the Eastern English Channel

In fisheries science, a mismatch between the delineation of a fish stock and the underlying biological population can lead to inaccurate assessment and management. Previous results suggested a potential spatial structuration of the Eastern English Channel (EEC) stock of common sole, Solea solea, in three sub-populations. In this article, we propose to investigate the spatial population structure of common sole in the EEC using the von Bertalanffy Growth Function parameters as indicators of population segregation. In order to test the sub-population hypothesis and evaluate its robustness to data sources, we developed three models, all including an area effect on growth parameters. The first model was aimed at testing a potential data source effect (in addition to the area effect) using commercial and scientific survey data jointly. The two other models used either scientific survey or commercial fishery data and focused on spatial differences in growth parameters. Our results showed that the growth parameter estimates indeed differed depending on the type of data used, with higher estimated asymptotic length and length at age two (L2) using commercial data. They also highlighted spatial differences in asymptotic length, consistent between models, which tend to confirm a spatial structuration of sole in the EEC. While these results need to be strengthened by marking and genetic studies, they constitute a first step towards a better understanding of the population spatial structuration of common sole in the EEC

Climate change undermines the global functioning of marine food webs

International audienceSea water temperature affects all biological and ecological processes that ultimately impact ecosystem functioning. In this study, we examine the influence of temperature on global biomass transfers from marine secondary production to fish stocks. By combining fisheries catches in all coastal ocean areas and life history traits of exploited marine species, we provide global estimates of two trophic transfer parameters which determine biomass flows in coastal marine food web: the trophic transfer efficiency and the biomass residence time in the food web. We find that biomass transfers in tropical ecosystems are less efficient and faster than in areas with cooler waters. In contrast, biomass transfers through the food web became faster and more efficient between 1950 and 2010. Using simulated changes in sea water temperature from three Earth system models, we project that the mean trophic transfer efficiency in coastal waters would decrease from 7.7% to 7.2% between 2010 and 2,100 under the ‘no effective mitigation’ Representative Concentration Pathway (RCP 8.5), while biomass residence time between trophic level 2 and 4 is projected to decrease from 2.7 to 2.3 year on average. Beyond the global trends, we show that the trophic transfer efficiencies and biomass residence times may vary substantially among ecosystem types and that the polar ecosystems may be the most impacted ecosystems. The detected and projected changes in mean trophic transfer efficiency and biomass residence time will undermine food web functioning. Our study provides quantitative understanding of temperature effects on trophodynamic of marine ecosystems under climate change

Ocean model-based covariates improve a marine fish stock assessment when observations are limited

The productivity of many fish populations is influenced by the environment, but developing environment-linked stock assessments remain challenging and current management of most commercial species assumes that stock productivity is time-invariant. In the Northeast United States, previous studies suggest that the recruitment of Southern New England-Mid Atlantic yellowtail flounder is closely related to the strength of the Cold Pool, a seasonally formed cold water mass on the continental shelf. Here, we developed three new indices that enhance the characterization of Cold Pool interannual variations using bottom temperature from a regional hindcast ocean model and a global ocean data assimilated hindcast. We associated these new indices to yellowtail flounder recruitment in a state–space, age-structured stock assessment framework using the Woods Hole Assessment Model. We demonstrate that incorporating Cold Pool effects on yellowtail flounder recruitment reduces the retrospective patterns and may improve the predictive skill of recruitment and, to a lesser extent, spawning stock biomass. We also show that the performance of the assessment models that incorporated ocean model-based indices is improved compared to the model using only the observation-based index. Instead of relying on limited subsurface observations, using validated ocean model products as environmental covariates in stock assessments may both improve predictions and facilitate operationalization

Spatial synchrony in the response of a long distant migratory specie (Salmo salar) to climate change in the North Atlantic Ocean

International audienc