The diets of two co-occurring marine teleosts, Parequula melbournensis and Pseudocaranx wrighti, and their relationships to body size and mouth morphology, and the season and location of capture

Abstract

The dietary compositions of the co-occurring gerreid Parequula melbournensis and the carangid Pseudocaranx wrighti have been determined, using samples collected seasonally from a 200 km stretch of coastal water in temperate Australia, in which these species are very abundant. Although the small representatives of P. melbournensis and P. wrighti both fed to a large extent on copepods, the latter ingested a wider variety of prey and thus had a greater dietary breadth. The diets of both species changed markedly as body size increased. The larger representatives of P. melbournensis fed mainly on onuphid and other polychaetes, whereas those of P. wrighti consumed a considerable volume of crustaceans, molluscs, polychaetes other than onuphids, and echinoderms. The above differences account for the dietary breadth being far greater in large P. wrighti than large P. melbournensis. Schoener's index and classification showed that dietary overlap between P. melbournensis and P. wrighti was low, suggesting that food resources were partitioned between these two demersal species. Intraspecific overlap was less amongst the length classes of small fish than among those of larger fish, indicating that any competition for food would be less among small fish, when growth would have been most rapid. The larger P. melbournensis fed mainly on prey types which were relatively sessile and can protrude from the substrate, such as tube-dwelling onuphid polychaetes, whereas the larger P. wrighti fed on a variety of epibenthic and/or more mobile species, such as mysids, amphipods, bivalves, gastropods, nereid polychaetes and echinoderms. Although P. melbournensis foraged to a far greater extent on a sessile fauna that occurs on or in the substrate, it ingested a far smaller amount of sand than P. wrighti, even though this latter species fed on a more epibenthic fauna. The presence of smaller amounts of sand in the stomach contents of P. melbournensis than of P. wrighti, presumably reflects the possession of a smaller and far more protrusible mouth, which enabled its sessile prey to be targetted more precisely. The dietary composition of P. melbournensis and P. wrighti underwent some seasonal changes, presumably reflecting seasonal fluctuations in the densities of prey species, and that of P. melbournensis differed slightly between some sites.