Diet and prey selection of six species of tuna baitfish in three coral reef lagoons in the Solomon Islands

Abstract

The diets and prey selection of six species of tuna baitfish (the engraulids Stolephorus devisi and S. heterolobus, three dussumierids of genus Spratelloides, and the apogonid Archamiazosterophora. were examined at two heavily fished and one unfished site in the Solomon Islands. All species ate almost exclusively nektonic zooplankton, especially crustaceans. Calanoid copepods were the most importantpreyofallspeciesexcept A. zosterophora, whichateagreater biomassoflarvalfish. Baitfishcould be divided into two groups according to prey taxa: the apogonid A. zosterophora and the engraulids S. devisi and S. heterolobus ate similar prey in similar proportions; the three dussumierid species (Spratelloides) ate smaller zooplankton and a greater proportion of calanoids than the other species.The size of prey eaten differed greatly between baitfish species. A. zosterophora ate larger prey than other species. The two Stolephorus species ate similar‐size prey, which were larger than the prey of the three species of Spratelloides. Among the Spratelloides, S. delicatulus ate smaller prey than the other two species. All species differed in the size of calanoids, carids and brachyurans they ate. In all cases, prey differed more by the minimum dimension (width) than by volume. There were significant relationships between fish length and prey width and volume, except for Archamia. However, in terms ofvolume, the prey of Stolephorus increased with fish size at a much greater rate than in Spratelloides species.Baitfish diets were compared to the zooplankton available. Prey selection by the three Spratelloides species and two Sfolephorus species was density‐dependent for all common prey types, although Stolephorus positively selected hyperiid amphipods and brachyurans. Archamia showed strong negative selection for calanoids and selected larger zooplankters, including fish larvae and carids, avoiding smaller, more common, prey.Overall baitfish density and zooplankton density did not correlate at either of the heavily fished sites. Nor were there consistent differences for any species in diet or changes in prey selection between the heavily fished and unfished sites. This suggests that, in these lagoons, commercial baitfish catches are not directly influenced by the composition or density of prey.