Low activity and seasonal change in population size structure of grenadiers in the oligotrophic abyssal central North Pacific Ocean

Abstract

Analysis of video recordings of swimming in abyssal grenadiersCoryphaenoidesspp. revealed site differences in tail‐beat frequencies. At the highly oligotrophic deep central North Pacific (CNP; 5800 m depth) station fishes had significantly lower tail‐beat frequencies (0·73 ± 0·02 Hz, mean ± s.e.) than fishes of similar size at the shallower ‘Station F’ (Sta. F; 4400 m depth) beneath the more productive waters of the California Current Upwelling (1·06 ± 0·04 Hz). These behavioural differences may be evidence for the proposed physiological adaptations ofCoryphaenoides armatusandCoryphaenoides yaquinae, to different depths and food supply levels. At CNP, smaller fishes (38·9 cm meanLT) were present in autumn than in summer (59·4 cmLT) suggesting large‐scale migrations across the abyssal ocean floor despite the observed slow swimming speeds.