2006. Recuperation and behaviour of Pacific cod after barotrauma. e ICES Journal of Marine Science, 63: 83e94.A 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 ÿ1 . 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.Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.

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, or approximately between 08:00 and 23:00 h Alaska Daylight Time (ADT; GMT -8) during August. Daytime vertical movements were correlated with light intensity, time of day and current velocity. The occurrence of vertical movements tended to increase with increasing light during the morning and early afternoon, but then decrease with increasing hour of the day after 13:00 h ADT. The magnitude of surface-directed vertical excursions was reduced during spring tide periods, when current velocities are highest. By comparison, the magnitude of slopedirected excursions was greater during spring tide periods and reduced during neap tide periods. Eight fish were at liberty for 42 to 44 d and 1 for 65 d. Two of the tagged males displayed nest guarding behavior for the majority of their time at liberty. Depths for these 2 males (115 to 117 m) were much deeper than previously observed for Atka mackerel spawning grounds. Given that Atka mackerel are more likely to be on the bottom during the night and less likely during the day, the variance of abundance estimates from bottom trawl surveys may be reduced by accounting for these diel differences.

Seasonal reproductive development, size and age at sexual maturity, and fecundity were described for darkblotched rockfish Sehasies crameri collected off the Oregon coast. Altogether, 1,060 fish captured by commercial groundfish and shrimp trawlers between July 1986 and July 1987 were examined. Reproductive events were protracted. Insemination of females occurred from August to December, and fertilization and parturition followed from December through March. Spermato/oa were observed within both vitcllogcnic and prcvitellogenic ovaries between July and November. Reproductive events in smaller males and females were delayed relative to those in larger individuals. Females attained 50% maturity at a greater size (36.5 cm total length) and age (8.4 years) than males (29.6 cm total length; 5.1 years). The unimodal development of eggs and larvae indicated one parturition per year. Most age-6, age-7, and agc-8 females possessed ovaries in an intermediate "maturing" condition. Histological analysis revealed that most of these females were immature; ovaries showed no evidence of previous spawning, oocytes never developed beyond an early vitellogenic stage, and during months of parturition, many of these females were resorbing their advancing oocytes. Total fecundities ranged from 19,815 to 489,064 oocytes per ovary pair and increased exponentially with increasing fish length, linearly with fish weight, and asymptotically with fish age (6 to 66 years).

The diel vertical migration (DVM) of Pacific cod Gadus macrocephalus was examined using depth and temperature data from 250 recaptured archival tags deployed on G. macrocephalus in the eastern Bering Sea and in the Gulf of Alaska near Kodiak Island. DVM of two types, deeper during daytime (type I) and deeper during night-time (type II), occurred frequently (15-40% of all days) in G. macrocephalus released at all sites. Most individuals displayed both diel types, with each type of behaviour lasting up to 58 contiguous days, and day and night depth differences averaging c. 8 m. Despite high among-individual variability, the occurrence of DVM varied significantly with the release site, season (i.e. day-of-year) and bottom depth, with the trend in seasonal occurrence nearly opposite for type I compared to type II DVM. No significance could be attributed to G. macrocephalus fork length, sex or ambient (tag) temperature. Trends in the magnitude of G. macrocephalus depth change were observed, with increased movement often occurring during night-time, dawn and dusk, and at release sites where the bathymetry was more complex. Both type I and type II DVMs were attributed to foraging on prey species that also undergo DVM, and increased vertical movements of G. macrocephalus during crepuscular and night-time periods were attributed to more active foraging during dim-light conditions when G. macrocephalus can potentially exploit a sensory advantage over some of their prey.

The Pacific Cod Gadus macrocephalus in the eastern Bering Sea is the target of one of the most lucrative fisheries in Alaska; however, relatively little is known about the movement of Pacific Cod and how this interacts with intense fishing on local spawning aggregations of cod every spring (January-April). This study aimed to draw inferences on Pacific Cod movement using a single tag release group of fish and the fishery as a representative for movement by qualitatively examining both temporal and spatial patterns of tag recoveries. Based on the tag recoveries in this study and past genetic studies, there is evidence that Pacific Cod show both homing tendencies and site fidelity during the spring when large aggregations of cod form to spawn. This study also supports results from an earlier study on Pacific Cod movement in this region and presents new insights into cod movement patterns. The cod in this tag release group were widely distributed across the Bering Sea during the summer and fall months and returned to the vicinity of the release site in the spring, presumably to spawn. Understanding the movement of cod and their interactions with the fishery is essential to the successful management of the Pacific Cod stock.

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