In situ comparison of activity in two deep-sea scavenging fishes occupyingdifferent depth zones

The activity of two scavenging deep-sea fishes occupying the same niche in overlapping depth zones were compared by in situ measurements of swimming s peeds, tail-beat frequencies and by arrival time at baits. At 4800 m on the Porcupine Abyssal Plain, the grenadier Coryphaenoides (Nematonurus) armatu s was the dominant scavenger, arriving at baits after 30 min, and swimming at relatively slow speeds of 0.17 body lengths (BL) sec(-1). At 2500 m in t he relatively food rich Porcupine Seabight both C. (N.) armatus and the blu e-hake, Antimora rostrata, were attracted to bait, but A. rostrata was alwa ys the first to arrive and most of the bait was consumed before the C. (N.) armatus arrived. A. rostrata swam at mean speeds of 0.39 BL sec(-1), simil ar to related shallow water species at equivalent temperatures. Observation s on tail-beat frequency from video sequences confirmed the greater activit y of A. rostrata. The data indicate that, given sufficient food supply high pressure and low temperature do not limit activity levels of demersal deep -sea fishes. Low activity of C. (N.) armatus is an adaptation to poor food supply in the abyss, where these fishes dominate, but prevents it competing with the more active A. rostrata in shallower depths.