Mechanisms for longitudinal transport on early life stages in benthic-pelagic fishes within a tide-dominated estuary

Abstract

Mechanisms that control the longitudinal transport of larvae and juveniles in nursery grounds such as estuaries are reported for some species. However, the behaviour and population consequences of these mechanisms are still uncertain. In this study, we tested selective tidal-stream transport from the along-channel (up- and downstream) and cross-channel (from one margin to the other) perspectives for two kinds of fish: estuarine-resident gobies (Pomatoschistus spp.) and marine estuarine-opportunistic anchovies (Engraulis encrasicolus). Three cruises were conducted in the lower Guadalquivir estuary, on the ebb and on the flood of spring tides in summer. Plankton samples were collected across a channel section, at three stations (one in the middle of the channel and two in adjacent shallower areas), near the surface and near the bottom simultaneously. In addition, multiple physico-chemical variables (temperature, turbidity, dissolved oxygen, pH, salinity, chlorophyll-a, along with wind and current direction and velocity) were measured to examine the different correlations used by the studied fish in their strategies. The benthic distribution of gobies indicated that they used flood currents near the bottom of lateral (shallow) areas to ingress into and remain in the estuary, temperature and/or dissolved oxygen being their main possible cues. On the contrary, the anchovies were more abundant near the surface, especially on the ebb tide, showing downstream advection, which was mainly influenced by salinity. However, the largest individual anchovies in the lateral/shallow zones suggested a behavioural ontogeny, which, together with wind induced transport, could contribute to their retention. This comparison also enhanced the knowledge of the habitat distribution of two species common and abundant in estuaries, anchovies and gobies.