Size structure and feeding ecology of fish communities in the surf zone of the Eastern Baltic

Food and feeding of juvenile turbot Scophthalmus maximus and flounder Pleuronectes flesus were studied in five nursery areas at Gotland, Central Baltic Sea, ICES SD 27 and SD 28. Ontogeny involved partitioning of available food resources. The food choice of turbot < 30 mm standard length (L-S) included both planktonic-hyperbenthic prey (calanoid copepods and mysids) and epibenthic-endobenthic prey (chironomids and amphipods), whereas turbot >= 30 mm L-S fed mainly on hyperbenthic species (mysids and fishes). Conversely, for flounder, epibenthic-endobenthic prey were the most abundant prey items throughout development (harpactocoid copepods, oligochaetes and chironomids for fish < 40 mm L-S and oligochaetes, chironomids and amphipods for flounder >= 40 mm L-S). Thus, the highest degree of dietary overlap occurred between turbot < 30 mm and flounder >= 40 mm. Food composition for both turbot and flounder varied, however, according to exposure and predominant wind direction in the nursery area. For example, expressed as the ratio between the biomass of mysids and fishes consumed, the relative importance of mysids v. fishes as food source for turbot, varied from < 1 in the most sheltered area to 16 and 27 in the more open areas. Considerable differences in feeding incidence were recorded; mean +/- S.D. 58 +/- 20% for turbot < 30 mm L-S and 83 +/- 8% for turbot >= 30 mm L-S, as opposed to >= 85-90% for flounder irrespective of size. The lower feeding success of turbot < 30 mm L-S was related to mysid abundance, shown to vary spatially and temporally, and to density of flounder, indicating that food availability, and potentially interspecific competition, influence feeding of early juvenile turbot with implications for survival following settlement. Regarding variability in abundance, hyperbenthic prey, as mysids, are considered more variable than epi- and endobenthic organisms. Hence, in addition to the 'nursery size hypothesis', i.e. the positive relationship between abundance of recruits and extension of nursery areas, variability in food availability may explain the average lower recruitment of turbot as compared to other flatfishes, e.g. flounder. (C) 2007 The Authors.