Taking potluck: trophic guild structure and feeding strategy of an intertidal fish assemblage

Many estuarine fishes migrate with the tides onto the intertidal mudflats, which provide them with abundant food resources. The concentration of large numbers of juvenile fishes in these intertidal areas may give rise to competitive interactions. In this study, the seasonal change of the trophic guild on an estuarine mudflat in the Scheldt estuary was described and prey selection was compared with prey availability. Diet analysis showed that all fish species on the mudflat, without exception, fed to a more or lesser extent onCorophium volutator. The importance of prey species in the diet of fishes corresponded with the seasonal availability of prey in the field, confirming the generalist and opportunistic feeding nature of estuarine fishes. For all species, the niche width was larger when resources were less abundant or the population density was highest. A generalist and opportunistic feeding mode reduces the potential for competition resulting in an increasing population diet breadth with increasing population density. On the contrary, when fish abundance was lower, fish species were specializing onC. volutator, resulting in a narrowed niche. As most fishes were feeding on few prey species, niche overlap was generally high between species. A significant niche overlap was found between flounder and sole in summer. Direct competition for food was probably avoided by spatial niche separation, as flounder migrated relatively higher onto the mudflat when fish abundance increased. Compared to sole, flounder selected for smaller prey size classes, which suggests resource partitioning at the level of prey size. It is not clear whether this size selective strategy is adopted to reduce interspecific competition or is the result of foraging behaviour