Sylvain Lenoir

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Age-class structure truncation in fish population

Proportion of spawner in the stock

The patterns of variations in fisheries time series are known to result from complex combinations of species and fisheries dynamics all coupled with environmental forcing.

Accumulating evidence shows that the oldest and largest individuals are disappearing from fish stocks, triggered by overexploitation. Such age- or size-truncation for fish stocks may impair subsequent recruitment, through:

- 1) decreasing the reproductive capacity of the spawners,
- 2) decreasing the number and the size of eggs and
- 3) reducing spatial and temporal dispersion of eggs.

A truncated age-class structure may decrease population growth rates as well as increase the risk of collapse due to poor environmental conditions events or long-term environmental changes.

In this project we examine the hypothesis that truncation of the age structures increases population sensitivity to environmental variability using data on the American plaice (Hippoglossoides platessoides) stocks in the North Atlantic Ocean.
The history of American plaice fisheries dated back to 1960, and age structures of several stocks are observed to decline. We investigate the Gulf of Maine and Georges Bank regions. Specifically, for each stock we examine the relationships between stock dynamics (indexed by annual abundance, spawning stock biomass) and both the North Atlantic temperature variability and fishery pressure (annual catch per unit effort, fishing mortality). Then, we relate these effects to changes in age structure, taking into account changes in growth rates, elasticity of vital rates to the recruitment, and mean age of spawners.

Keywords: Age-truncation - Life history traits - Age/Size Composition - Fish Population Dynamic - American Plaice - Fish Population sensitivity to Climate Changes - Sustainable Exploitation

Seabird, Fish Preys and Climate warming

Junk-Food hypothesis and Dead chick

A combination of overfishing and the effects of hydroclimatic changes have been a source of major alteration of the trophodynamics of the North Sea ecosystem since the 1960s.

One manifestation of change has been a decline in seabirds breeding success due changes in their preferred fish prey of lesser sandeels (Ammodytes marinus) and sprat (Sprattus sprattus), together with a recent increase in the abundance of snake pipefish (Entelurus aequoreus), which is a poor food for seabirds.

In this study, we studied the former, current and future spatial probability of occurrence of three fish species, caught by seabirds in the North Sea: the lesser sandeel, the European sprat and the snake pipefish that are known to influence seabird breeding success. We showed that climate warming is likely to be the cause of already observed changes in the abundance and community composition of small pelagic fish in the North Sea. These changes are likely to have significant implications for diet and breeding of seabirds around the North Sea coasts.

Keywords: Climate Warming - North Sea trophodynamic - Seabirds Breeding succes - Ecological-Niche Model - Seabirds Diet - Prey Abundance - Trophic Amplification of Climate Warming

Marine fish Distribution and Climate Warming

Change in Sprat distribution

In the marine biosphere, current climate change is affecting the abundance, spatial distribution and the phenology of species and altering prey-predator interactions. Pronounced climate change may become a confounding factor of fishing and both driving forces may act in synergy to precipitate the collapse of fish stocks around the world. To better evaluate the effect of climate on a species, it is essential to know its spatial distribution; this information is often lacking in the marine realm.

We studied the impact of climate warming on the spatial distribution of fish in the North Atlantic , using the new habitat model called the Non-Parametric Probabilistic Ecological Niche Model (NPPEN) . The model NPPEN is nonparametric and requires only presence data. It is based on concept of the ecological niche sensu Hutchinson. The model NPPEN tests the Mahalanobis generalised distance by permutations to produce and map the probability of species occurrence. The model is therefore well suited to study expected changes in the biogeography of marine species at macro-scale .

Applying this new model on more than fifty marine species in the North Atlantic, has highlighted the impact of global warming on the biogeography of species , structure and trophodynamic of the marine ecosystem. Disruption, already observed in spatial distribution and abundance (probability of occurrence) of fish species such as Atlantic cod were found again. The majority of species will move northward to stay in an environment consistent with their ecological niche.

Change in Thinlip mullet distribution

The intensity and rapidity of the biogeographic movements expected, as the balance of gains or losses in the spatial range differ among fish; governed by the ability of species movement, their range of environmental tolerance (niche breadth) and the intensity of global warming . In the North Sea, species such as pollack, with tight and strict requirements ecological niche, may disappear following the contraction of their niche. In the English Channel the abundance of adapted species, as the Thinlip mullet, has increased and their upper limit of distribution extended northward. These changes might alter the functioning of the food web by changing the availability and quality of fish resources to upper level consumers as seabirds, reducing their reproductive success. These trophic and biogeographic changes are more pronounced when the species is at the limit of its ecological niche . The new model NPPEN habitat provides essential information to consider in order anticipating changes in marine resources, particularly in the context of management plans for exploited fish stocks .

Keywords: Biogeography - Climate Change, Ecological Niche - Ecological Niche Modelling - Marine Fish Distribution - Projections - Marine Resources Management - Sustainable Exploitation