Life history strategies of fishes across environmental gradients in estuaries | Histoire de vie des poissons au sein des gradients estuariens

The demographic strategies of fishes can be defined by specific combinations of traits expressed by organisms to cope with environmental constrains. Species traits thus influence the distribution and abundance of organisms by excluding species with traits poorly adapted to local conditions and enhancing performances of organisms possessing advantageous attributes. Fishes in estuaries belong to several ecological guilds, i.e. marine, freshwater, diadromous, and resident species, which perceive and respond to environmental components at different scales. In the present study, we hypothesized that the distributions of fish species across to gradient of estuarine habitats are related to their life history strategies. Eight life history traits were considered to reflect the demographic strategies of 168 taxa caught in 47 estuaries of the North East Atlantic coast. Our findings were replaced within a conceptual triangular model of life history, composed on three endpoint strategies: periodic (large, long generation time, high fecundity), opportunistic (small, short generation time, high reproductive effort) and equilibrium (low fecundity, large egg size, parental care). Various life history strategies co-occur in estuarine systems depending on fish ecological guilds, but our results demonstrated that species were mainly distributed along to an opportunistic-periodic gradient. The opportunistic attributes of estuarine resident species provided them high degree of demographic resilience, so that they can deal with a large range of estuarine environmental settings. However, the increasing of stressful conditions over the salinity gradient was appeared as crucial factor structuring the assemblages of resident fishes based on their demographic strategies. Our results highlighted a filtering effect associated with the system size. In small estuaries, fish assemblages are mainly composed on resident species with opportunistic traits, whereas the proportion of individuals with periodic attributes increased with system size. Contrary to our expectations, the decline of both species richness and abundance in estuaries subjected to human disturbances did not appeared significantly related to the life history attributes of fish assemblages. Our findings were discussed in relation the degree of environmental predictability at various spatio-temporal scales to underline general patterns in assembly processes of estuarine fauna.