Factors influencing the partitioning of food resources among six fish species in a large embayment with juxtaposing bare sand and seagrass habitats

Six abundant fish species were collected from a large embayment during both day and night and at bimonthly intervals for a year. Gerres subfasciatus a nd Upeneus tragula occurred mainly over bare sand, while Psammoperca waigie nsis, Centrogenys vaigiensis and Apogon victoriae lived mainly in seagrass (Amphibolis antarctica) and A. rueppellii often migrated from seagrass to o ver bare sand at night. All species except U. tragula fed at night, as well as during the day. The overall dietary compositions of the six species wer e significantly different from each other, even when the species occurred i n the same habitat. G. subfasciatus and U. tragula consumed greater volumes of errant polychaetes, which could be readily targeted in a substrate that does not contain dense rhizome mats. In contrast, P. waigiensis, C vaigien sis and A. victoriae ingested greater volumes of carid decapods, which are particularly abundant in seagrass and, thus, within the water column. Furth ermore, when A. rueppellii moved at night from seagrass to over sand, the c onsumption of carid decapods declined, whereas that of polychaetes and part icularly mysids, which are very abundant over bare sand, increased. However , the composition of the prey consumed by different species within the same habitat also often varied markedly. For example, unlike U. tragula, G. sub fasciatus ingested not only sedentary polychaetes, but also considerable vo lumes of errant polychaetes, reflecting its ability to use a combination of vision and its highly protrusible mouth to target prey both on and just be low the substrate surface. Differences in the types and range of prey inges ted by the six species could often be related to differences in the overall size, width and/or protrusibility of the mouth. Furthermore, dietary bread th was greatest in species with the largest mouth dimensions. The diets of three species underwent diel changes that could be related to differences i n foraging mode and/or prey availability. Thus, a use of vision to detect p rey would account for the greater consumption during the day of copepods by G. subfasciatus and of small teleosts by A. rueppellii, while the nocturna l emergence of amphipods and/or tanaids from the substrate explains their g reater ingestion by G. subfasciatus, A. victoriae and A. rueppellii at nigh t. Although the smaller individuals of each species consumed larger volumes of prey, such as copepods and mysids, and the larger fish ingested greater volumes of prey, such as decapods and teleosts, the extent of the size-rel ated changes in diet varied markedly amongst species. (C) 2001 Elsevier Sci ence B.V. All rights reserved.