DIET AND FOOD-CONSUMPTION OF A DEEP-SEA FISH, ORANGE ROUGHY HOPLOSTETHUS-ATLANTICUS (PISCES, TRACHICHTHYIDAE), OFF SOUTHEASTERN AUSTRALIA

We examined the diet of nearly 7500 orange roughy Hoplostethus atlanticus Collett, 1889 from southeastern Australian waters caught during trawl surveys in 1988 and 1989. Juveniles f ed mainly on bentho- and meso-pelagic Crustaceans, while mature fish consumed predominantly fish and squid. The composition of the diet changed significantly with depth, geographical area, and year. Evidence of diel feeding periodicity found in adults at a station sampled over 42 h was supported by patterns of stomach fullness and digestion stage in the full data set. Based upon observed rates of stomach evacuation, daily rates of food consumption were estimated as 1.15 % body weight for adults and 0.91 % for juveniles. The metabolism of orange roughy, estimated from a mass balance model, is similar to that of active, migratory mesopelagic fishes and substantially higher than that of non-migratory bathypelagic fishes. The bodily composition of orange roughy, which is high in lipid and protein and low in water, is also similar to that of active mesopelagic fishes. Non-migratory bathypelagic fishes typically have exceptionally low metabolic rates, which, despite low food consumption, lead to high growth rates and high growth efficiency. Orange roughy, however, appears to have very high metabolic costs, which lead to an exceptionally low growth rate and growth efficiency, despite relatively high rates of food consumption.