The Food and Feeding Relationships of Fish Species from the Campbell Plateau, New Zealand.

Abstract

Detailed studies are made on the food and feeding habits of seven fish species from the Campbell Plateau: hoki (Macruronus novaezelandiae), southern blue whiting (Micromesistius australis), javelin fish (Lepidorhynchus denticulatus), ling (Genypterus blacodes), rattail (Coelorinchus fasciatus and C. aspercephalus), silverside (Argentina elongata) and small-scaled notothenid (Notothenia microlepidota). Brief notes are also presented on the feeding of 15 other fish and squid species. The importance of prey items in the diet of a predator has been assessed by an 'index of relative importance' (IRI) which combines measurements of frequency of occurrence, number and weight of prey. Significant changes in prey importance in the diet of the seven major fish species occurred with season (autumn and spring), geographical region and length of predator. Hoki are pelagic feeders, which probably feed at night. Fishes examined preyed largely on natant decapod crustaceans, amphipods, and myctophid and photichthyid fishes. Natant decapods became more important in the diet of larger hoki (up to 100 cm) whereas the importance of amphipods decreased. Southern blue whiting, like hoki, feed pelagically at night. Major prey were amphipods, natant decapods and euphausiids. With increasing length of fish, prey dominance changed from euphausiids to amphipods, to natant decapod crustaceans. Javelin fish are also pelagic feeders. Natant decapod crustaceans, amphipods and small squids were the main prey. Ling are benthic feeders. Prey consisted largely of natant decapod crustaceans, macrourid fishes and hoki. The diet of rattail examined comprised small benthic organisms, including natant decapod crustaceans, isopods, opal fishes and Polychaetes. Silverside feed near the bottom. Salps were the most important prey, irrespective of season, region and length of fish. Small-scaled notothenid are also predominantly benthic feeders. Salps, amphipods, brachyuran crustaceans and opal fishs were the main prey. Feeding relationships between these seven fish species, including aspects of dietary overlap, predator-prey size selection, feeding diversity, niche overlap and similarity of prey composition of fishes caught in the same trawl have been examined. Dietary overlap has been assessed by 'percentage similarity' (PS). PS values between the diet of hoki, southern blue whiting and javelin fish were often high, but the diets of the other fish species were generally dissimilar. The size relationship between predator and prey was examined for hoki, southern blue whiting, javelin fish, ling, rattail and silverside. Ling and silverside preyed on organisms approximately 0.2% of their body weight; rattail, javelin fish and hoki on organisms 0.1% of their weight; and southern blue whiting on prey 0.02% of their weight. Feeding diversity has been assessed by H'. Prey diversity values for hoki, southern blue whiting, javelin fish and rattail were similar. The diet of silverside was the least diverse. A niche overlap analysis was conducted to examine the possibility that competition for food between hoki, southern blue whiting and javelin fish affects their geographical distributions. Results suggest that feeding competition is not an important factor determining the distribution of the fishes. Hoki, southern blue whiting and javelin fish caught in the same trawl were found generally to have consumed different prey. 'Pelagic' and 'demersal' feeding guilds are described. The former includes hoki, southern blue whiting and javelin fish; the latter, ling, silverside, rattail and small-scaled notothenid. The extent of food resource partitioning among the fishes is discussed. Spatial and temporal feeding differences, and variations in prey type and prey size were evident for most of the predators. Possible consequences of an increased trawl fishery for southern blue whiting, hoki and ling on the Campbell Plateau are considered. Comparisons are made with other ecosystems, in particular the North Sea, for which changes in the structure of the fish community associated with exploitation have been studied. A simple energy flow model of the Campbell Plateau fish community has been constructed, from which predictions of changes in relative species abundances have been made. It is suggested that reduced stocks of southern blue whiting and hoki could 'release' food energy which may be utilised by other pelagically feeding animals, in particular javelin fish, which could become more abundant. Smaller stocks of ling may reduce predation on rattail macrourids, which could consequently increase in abundance. Larger stocks of rattail and javelin fish could, under certain circumstances, be exploited by an 'industrial fishery', reducing whole fish to fish meal and oil.