Food and feeding habits of fish associated with skipjack tuna in New Zealand

Abstract

The food and feeding habits of 29 fish species taken in the New Zealand purse-seine fishery for skipjack tuna Katsuwonus pelamis are described. Ten species are considered in detail, of which the flyingfish Cypselurus pinnatibarbatus melanocercus, saury Scomberesox saurus scombroides, Ray's bream Brama brama, blue mackerel Scomber australasicus, starry toado Arothron firmamentum and porcupinefish Allomycterus jaculiferus are residents of northern North Island waters, and the pilotfish Naucrates ductor, frigate tuna Auxis thazard, albacore Thunnus alalunga and sunfish Mola mola are summer time visitors from the tropics. Feeding was assessed by the points and numerical methods and investigated with respect to fish sex and length, geographical area and the physical environment. All species are pelagic, day time feeders. Flyingfish and porcupinefish preyed largely on hyperiid amphipods. Saury, pilotfish and Ray's bream fed on hyperiids and euphausiids, with the latter group usually predominant. The diets of blue mackerel and frigate tuna consisted almost entirely of euphausiids. Albacore exhibited a pronounced change in diet with increasing length. Prey dominance passed from euphausiids, hyperiids and cephalopods in small and medium sized fish to teleosts such as blue mackerel in large specimens. The diet of starry toado examined consisted of diphyid siphonophores and their hyperiid parasitoids, and euphausiids. Sunfish fed on salps, hyperiids, scyllarid phyllosomae and pteropod molluscs. Of the remaining 19 fish species, the following are of particular interest: dasyatid rays Dasyatis brevicaudatus and D. thetidis preyed on hyperiids, pteropods and small fish, echeneids Remora remora and R. brachyptera consumed hyperiids and euphausiids, and marlins Makaira indica M. nigricans and Tetrapturus audax fed on skipjack tuna, arrow squid Nototodarus sloani sloani and other nekton. Feeding data, including that of the skipjack tuna, are summarized in a diagrammatic representation of the upper levels of the epipelagic food web. Euphausiid Nyctiphanes australis is probably the single most important food item, supporting a considerable number of valuable fish species. Hyperiids often complemented the role of N. australis but are ranked second in their overall contribution to the web. Minor crustacean groups are of localized importance while pteropods, salps and siphonophores occur as major prey of a few species. Apex predators are represented by the marlins, sharks and large tunas. There are between four and five trophic levels in the food web and two or three steps to commercial quantities of fish. For the tropical species, in particular the juvenile albacore, frigate and skipjack tunas, this means an opportunity to feed at lower trophic levels than they are usually accustomed to. Thus the summer migration to subtropical coastal areas may be of considerable survival value. The food webs present on the north-west and north-east coasts of North Island are compared but little major difference is apparent.