Intra-specific Variation in Resource Use by Red Mullet, Mullus barbatus

The distribution patterns and feeding habits of red mullet were studied for fish collected during experimental seasonal trawl surveys along the Cretan continental shelf (1988–1991) and monthly sampling of Iraklion Gulf (1991–1993). Depth, temperature and salinity data were combined with biological data on abundance, biomass, and fish size, to establish the ranges of bottom depth, temperature and salinity over which red mullet are distributed. Despite their distribution overlap, depth and temperature selection differed considerably between the size classes of the species. In general, fish size increased with depth, and smaller individuals tended to occur in shallower and warmer water. Abundance increased in mid-shelf waters during spring, indicating a movement across the shelf towards deeper water, associated with the spawning behaviour of the species. The factor controlling the timing of that movement to deep water seems to be the maturity of individual fish. Stomach content analysis revealed that red mullet fed mainly on benthic invertebrates, and that each size class consumed a narrow range of prey species. However, the composition of the prey consumed varied considerably with fish size and there were differences in mean prey size for each size class. Trophic diversity was low and decreased significantly with fish size, but in general was not affected by season. Intra-specific differences in preference and utilization of prey are related to the distribution patterns of the size classes. In particular, younger individuals exhibited the most restricted distribution and appeared to have a broader trophic niche than the larger size classes that occurred over a wider depth range.