Larval Fish Transport Dynamics in an Arid Estuary Do Not Follow Selective Tidal Stream Transport Mechanisms
2025
Sloterdijk, Hans | Brehmer, Patrice | Ndiaye, Ibrahima | Capet, Xavier | Ekau, Werner
The mechanisms governing larval fish transport between low-flow estuaries and coastal waters in seasonally arid climates remain poorly understood. To address this, we conducted a field study in the Sine Saloum Estuary, a tropical inverse estuary in West Africa. We used simultaneous measurements of current profiles and larval fish densities during semi-diurnal tidal cycles at three strategic locations. We focused on economically and ecologically important fish families (Clupeidae, Cynoglossidae, Gerreidae, and Mugilidae) known for their contrasting life histories and reliance on estuarine environments. The estuarine system was characterized by inverse salinity gradients, partial stratification, and gravitational circulation, offering natural pathways for larval retention. Unlike traditional estuarine systems, no evidence of selective tidal stream transport (STST) was found; instead, fish larvae exhibited stable depth preferences independent of diel and tidal cycles. This simpler mechanism, requiring minimal energy expenditure, appears to rely on gravitational circulation for effective larval transport. The prevalence of the gravitational circulation and the low energy cost for larvae to maintain themselves over a defined depth range vs the desired direction of mean displacement make the proposed larval transport mechanism potentially important most of the year. These findings provide critical insights into the biological and physical processes shaping larval fish recruitment in arid estuarine systems and emphasize the need for adaptive management strategies to support fisheries under changing environmental conditions.
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