Maxime Olmos

Populations change as a result of many interacting processes, each of which may be moderated by density dependence and/or climate. Models and projections of population dynamics support conservation and economic planning, but the extent to which the influence of climate or density is incorporated varies. We develop a population dynamics model for eastern Bering Sea snow crab ( Chionoecetes opilio ) that fits to data collected from 1982 to 2023 to estimate variability in recruitment and mortality. We then use generalized additive models (GAMs) to explain the variability in recruitment, mortality and maturity using environmental covariates and density. Finally, we project the population forward under various assumptions about climate and density dependence on recruitment, mortality and maturity. Feedbacks among population processes result in different short‐ and long‐term population trajectories. For example, including density dependence in mortality projections allows for a stronger rebound in the population in the short term, but the harmful effects of warming outweigh the protective effect of lower density in the long term. Projected long‐term crab abundances were much lower than historically observed under receding ice cover in the Bering Sea regardless of the combination of processes driven by density and climate. Synthesis and applications . Our results underscore the need for management capable of addressing the impacts of both density and climate effects on multiple processes in harvested populations. Non‐stationarity in drivers of population dynamics (e.g. climate change) will further affect the ability of management to achieve sustainable yields. Current frameworks built around equilibrium‐based maximum sustainable yield are not well‐suited to address either of these issues.

Improving our understanding of the ecological and demographic mechanisms that underlie changes in wild population productivity is critical to support ecosystem‐based management. Yet, population productivity results from a combination of demographic rates (such as mortality, growth and recruitment) driven by intrinsic and extrinsic factors which interact through density‐, size‐ or environment‐dependent mechanisms. The interdependence between demographic rates, and intrinsic and extrinsic factors, has rarely been studied throughout the life cycle of a wild population. This is particularly needed for short life species, such as small pelagic fish, which are subject to inter‐annual abrupt changes in abundances. Here, we developed an age‐based life‐cycle model to investigate the relative influence of intrinsic and extrinsic factors on successive transitions rates along the life cycle of a system composed of two small pelagic species of the Bay of Biscay, the European anchovy ( Engraulis encrasicolus ) and European sardine ( Sardina pilchardus ). Our model allows to disentangle the influence of density dependence, size dependence and environmental factors throughout the life cycle of short life species. Our results highlight that sardine demography is density‐dependent, with significant effects of density on natural mortality at age 1 and age 3. In contrast, anchovy demography is both size‐ and density‐dependent, with density‐dependent effects on natural mortality at age 1, age 2 and on recruitment, and a size‐dependent effect on age‐1 mortality. Moreover, we found that natural mortalities of both anchovy and sardine were related to large‐scale environmental indicators, such as the Atlantic Multidecadal Oscillation and the North Atlantic Oscillation. The originality of this approach lies in its ability to integrate the effects of size, density and environment on a multitude of demographic processes throughout the life cycle for wild species, and can be seen as a step towards supporting ecosystem‐based management.

The productivity of Atlantic salmon (Salmo salar) has declined markedly since the 1980s, in part because of changing ocean conditions, but mechanisms driving this decline remain unclear. Previous research has suggested differential recruitment dynamics between the continental stock groups, with post-smolt growth influencing the survival of populations in Europe, but not North America. We used a large, representative archive of North American, multi sea-winter salmon scales to reconstruct long-term changes in growth between 1968 and 2018. We then modeled relationships between annual growth indices, estimates of maturation rates, and post-smolt survival, while allowing for the possibility of non-stationary dynamics. We found that marine growth of MSW salmon has changed over the past 50 years, generally increasing despite declining survival. However, we found strong evidence of a non-stationary influence of post-smolt growth on survival. Prior to a period of rapid change in the ocean environment during the late 1980s, post-smolt growth was positively related with survival, similar to the pattern observed in European populations. These findings suggest that the mechanisms determining marine survival of North American and European salmon populations may have diverged around 1990. More generally, our results highlight the importance of considering non-stationary dynamics when evaluating linkages between the environment, growth, and survival of Atlantic salmon.

A 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 groups (CSG) in North America (NA) and Southern Europe (SE) over the period 1971–2014. We found strong coherence in the temporal variation in postsmolt 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 of 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 postsmolt 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 populations from NA, 26% and 24% of variance is captured by SST and PP, respectively and in the Norwegian Sea for populations from SE, 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.

A hierarchical Bayesian life cycle model is presented that considers spatial covariation of marine life history traits of Atlantic salmon (Salmo salar) populations in the North Atlantic. The model is based on a collective analysis of the dynamics of 13 stock units (SUs) from two continental stock groups (CSGs) in North America and Southern Europe in a single hierarchical model over the period 1971–2014. The model sets up a new assessment framework for Atlantic salmon stocks. It also provides a framework to investigate the drivers of changes in Atlantic salmon population dynamics including disentangling the effects of fisheries from those of environmental factors in a hierarchy of spatial scales. It is used to test the hypothesis of a strong spatial synchrony in marine life history dynamics of Atlantic salmon populations. The trends in two key parameters associated with the early marine phase of the life cycle are estimated: (i) the marine survival during the first summer–autumn spent at sea and (ii) the proportion of fish maturing after the first winter at sea. The results provide evidence of a decline in the marine survival together with an increase in the proportion of fish that mature after the first winter at sea, common to all SUs. Our results show an increased coherence in the covariations of trends in these two marine life history traits related to geographic proximity of SUs which support the hypothesis of a coherent response of geographically proximate Atlantic salmon populations that likely share similar migration routes.

Alterations in predation pressure can have large effects on trophically-structured systems. Modification of predator behaviour via ocean warming has been assessed by laboratory experimentation and metabolic theory. However, the influence of ocean acidification with ocean warming remains largely unexplored for mesopredators, including experimental assessments that incorporate key components of the assemblages in which animals naturally live. We employ a combination of long-term laboratory and mesocosm experiments containing natural prey and habitat to assess how warming and acidification affect the development, growth and hunting behaviour in sharks. Although embryonic development was faster due to temperature, elevated temperature and CO₂ had detrimental effects on sharks by not only increasing energetic demands, but also by decreasing metabolic efficiency and reducing their ability to locate food through olfaction. The combination of these effects led to considerable reductions in growth rates of sharks held in natural mesocosms with elevated CO₂, either alone or in combination with higher temperature. Our results suggest a more complex reality for predators, where ocean acidification reduces their ability to effectively hunt and exert strong top-down control over food webs.