Recuperation and behaviour of Pacific cod after barotrauma

Abstract

AbstractA total of 624 Pacific cod was captured, tagged with data-storage tags, and released in the Gulf of Alaska and eastern Bering Sea from 2001 to 2003. Cod were captured with pot or jig gear at depths ranging from 32 to 127 m. As of January 2004, 272 tags (44%) were recovered, with fish at liberty from 2 days to 1.5 years. The tags, which collected time, depth, and temperature information, revealed behaviour patterns common to nearly all recaptured fish. Analysis of swimbladder function suggests that these patterns resulted from swimbladder ruptures and deflation. In most cases, fish immediately dived to the bottom and then, within hours, returned to shallower depths. Fish that subsequently descended back to the depth at which they were captured, did so at rates ranging from 4.9 to 23.2 m day−1. Observations of bubbles being released from cod as they neared the surface during capture, indicated that cod swimbladders can rupture. A series of X-rays taken of live cod immediately after capture and subsequently at 24 h, revealed that ruptured swimbladders were sealed within 24 h. The loss of gas from the swimbladder, and the subsequent loss in buoyancy, inhibited most cod from remaining near the bottom. Their quick return to shallow water after an initial escape response indicates either a need or preference to reside at a depth at which they are more neutrally buoyant. Although rates of descent were highly variable among individuals, smaller individuals tended to descend faster than larger ones. Rates of descent were most likely limited by the secretion rate of gas into the swimbladder. Future tagging work for species such as Pacific cod need to recognize the recuperation period that is necessary before natural vertical or horizontal migrations can be evaluated.