Foraging behavior of abyssal grenadier fish: inferences from acoustic tagging and tracking in the North Pacific Ocean

Abstract

Abyssal grenadier fish Coryphaenoides yaquinae were Coryphaenoides armatus and observed arriving at baits deployed within view of a free-fall video vehicle (FVV) camera on the sea floor at two stations in the North Pacific, Sta. F 32°50′N, 124°W, 4400 m deep in the vicinity of the California current and Sta. CNP 31°N, 159°W, a 5900 m deep oligotrophic station. Included within each bait deployment were one or two ingestible acoustic transmitters. A total of 23 fish at Sta. F and 13 fish at Sta. CNP ingested transmitters and were tracked using an acoustic tracking system (ATEX). The number of fish within view of the camera increased to a mean maximum of 4.7 at 60 min at Sta. F and 11.8 by 400 min at Sta. CNP, a paradox in view of presumed lower fish population density at Sta. CNP. Fish that ingested transmitters moved away at radial velocities between 1 and 15 cm s −1, reaching a mean radius of 233 m by 370 min at Sta. F and 622 min at Sta. CNP. Fish appear to be active foragers with no evidence for a “sit and wait” foraging strategy. Grenadiers generally remained near the sea floor as they dispersed. Only one vertical movement to an altitude of ca 25 m was recorded and this comprised less than 0.2% of tracking time. The number of fish present at the bait was found to correspond to the following relationship: N t = α 0 x (1 − c −xf) t ⩽ β α 0 x c −xt(c βx − 1) t > β where N t is number of fish present at time t min after bait reaches the sea floor, α 0 is initial arrival rate of fish, β is mean fish staying time and x is the bait decay constant. In accordance with optimal foraging theory staying time (β) is longer at Sta. CNP.