ICHTHYOPLANKTON OF A SHALLOW COASTAL INLET IN SOUTH-WEST SPAIN - FACTORS CONTRIBUTING TO COLONIZATION AND RETENTION

The ichthyoplankton of a shallow inlet was sampled for 13 months using conical tide-strained nets. The utilization of this habitat by the young stages of different fish species is analysed. Temporal and spatial distributions are discussed in relation to environmental cycles and gradients, and to feeding rhythms. A total of 110 971 individuals, belonging to 39 species and 19 families of teleosts, was collected. The postlarval stage was the most represented in the samples. Ichthyoplankton density peaked during late winter and early spring. Density was higher during flood tide, resulting in a net input of postlarvae from the bay to the inner inlet. This ecosystem functions primarily as a nursery ground for coastal pelagic spawners and, secondarily, as a spawning area for some benthic egg spawners and pouch-brooder species. Most postlarvae migrate from their spawning area (open sea) to the inlet probably cued by a decreasing gradient of water temperature and salinity, and an increasing gradient of water turbidity and suitable food concentration. The attraction of postlarvae towards the inlet mouth probably results from a combination of biotic (availability of suitable food) and abiotic (temperature, salinity and turbidity) factors. Passive transport contributes to the subsequent penetration and retention of the postlarvae within the shallow, flood-dominated inlet. Vertical migrations observed among some planktonic fish stages were related to light periods and feeding activity rather than tidal phases, and cannot act as a retention mechanism. © 1991.