Inter-annual fluctuation in the population structure of an estuarine fish: Influence of environmental drivers

Abstract

The environmental influence on the long-term variations of fish population structure, mainly that which drives the movements between segregated life stages, is not yet fully understood in estuarine systems. As a case study, we used seasonal (winter, spring) fishery-independent data of whitemouth croaker (Micropogonias furnieri) to determine the influence of environmental drivers (salinity, temperature, depth, freshwater outflow, and wind) on the long-term inter-annual scale (1993–2006) variation in the population structure. We used an approach based on size assemblages to represent and model the length distribution of the population structure. The different size assemblages (four in winter, five in spring) identified and verified by ordination analyses (cluster and nMDS) indicated a demographic structure that resembled juveniles, adolescent, and adult stages. The size assemblage composition showed significant inter-annual differences in both seasons. Additionally, there was significant decrease in the abundance of most size assemblages, except for juveniles which increased in winter. A multivariate regression tree analysis indicated that salinity expressed a greater influence on the size assemblage composition than the temperature. Moreover, the river outflow showed greater localized effects at the upper estuary across the variation in the spatial cover of salinity, which favored juveniles. On short time scales (between consecutive years), demographic variations responded to changes in the availability of environmental conditions preferred by each size assemblage, whereas long-term variations could be attributed to other effects (e.g. fishing mortality, habitat degradation).