Impacts that cause the highest direct mortality of individuals do not necessarily have the greatest influence on temperate eel escapement

Abstract

Temperate eels are three panmictic catadromous species with a long period of oceanic passive larval drift and large distribution areas in contrasting environments. Spatial patterns of life history traits have been observed, and are correlated with environmental gradients, and may arise from both adaptive phenotypic plasticity and genetic polymorphism. This raises the question of the effect of spatially heterogeneous anthropogenic pressures on these populations. In this context, we used Geneveel, an individual-based optimization model that includes both phenotypic plasticity and genetic polymorphism, to explore the effects of different kinds of anthropogenic pressures: glass eel and silver eel fisheries, obstacles to upstream migration, and turbine mortality. More specifically, we analyzed the effects of these pressures on five output variables: the number of escapees, the proportion of females, the proportion of slow growers, the mean length-at-silvering and the resulting egg production. Our results suggest that phenotypic plasticity could act as a compensatory mechanism that mitigates the effects of some pressures (glass eel fishery and obstacles to upstream migration) and could be a source of resilience for the population, while other pressures did not show any compensatory effect (silver eel fishery and turbine mortality). Therefore, global impacts are very hard to assess, and the pressure that kills the most individuals does not necessarily have the biggest impact on the spawning biomass.

Freshwater eels: A symbol of the effects of global change

“…At last, obstacles to upstream migration can act as a permanent selection pressure (Mateo, Lambert, Tétard, & Drouineau, ; Podgorniak, Angelini, et al., ; Podgorniak, Milan, et al., ). Côté et al.…”

“…() demonstrated the existence of genetically distinct ecotypes, with different growth rates and different sex ratios. By impairing migration within catchments, obstacles can decrease the fitness of some types of individuals; those individuals who genetically belong to the “freshwater habitat” may not be able to reach suitable habitats or will suffer damage during their downstream migration (Mateo, Lambert, Tétard, & Drouineau, ).…”

“…At last, similarly to obstacles to upstream migration, obstacles to downstream migration affect specific types of individual: individuals that settle upstream of the obstacle (i.e. individuals that settle in upstream habitats and individuals that were able to pass the obstacle), as such, obstacles may have the potential to exert a selection pressure on the population (Mateo, Lambert, Tétard, & Drouineau, ).…”

“…More generally, the large diversity of tactics during the continental phase and presumably during the spawning migration may correspond to remarkable bet‐hedging well suited to address environmental variability (Daverat et al., ; Righton et al., ). The environmental sex determination may also be a compensatory mechanism: The higher production of females in a context of depleted population may mitigate the reduction in eggs production that would result from the decline in silver eel abundance (Geffroy & Bardonnet, ; Mateo, Lambert, Tétard, & Drouineau, ), especially since eels have a high fecundity.…”

“…On the other hand, all the other anthropogenic pressures tend to affect ecotypes corresponding to more upstream habitats. As such, these anthropogenic pressures reduce the fitness of those individuals and can become an important selective pressure (Mateo, Lambert, Tétard, Castonguay, et al., ; Mateo, Lambert, Tétard, & Drouineau, ). For example, half the American silver eels migrating down the St. Lawrence River, one of the most productive areas for American eel (Casselman, ), have been killed by hydropower dams and fisheries (Verreault & Dumont, ).…”