Tagging Mortality of Greenland Halibut Reinhardtius hippoglossoides (Walbaum)

Tagging mortality for Greenland halibut (Reinhardtius hippoglossoides) was studied under summer and winter conditions. The fish were caught using longlines and tagged with a T-bar tag. The winter experiment was conducted in Cumberland Sound, Canada in May 1997. Air temperatures were below 0°C and cold water-masses were present at 0-300 m. Fish were immediately placed in a tub of water after capture and transported by snowmobile to a heated tent for tagging and then placed in cages that were submerged to 300 m depth. The summer experiment was conducted in Upernavik, Greenland in August 1998. Air temperatures were above 0°C but intermediate cold water-masses were present at 60-200 m. In the summer experiment, fish were tagged and released in a observation tank to assess immediate tagging mortality (1 to 18 hr). They were then placed in specially designed cages and submerged to 300-500 m to assess short-term tagging mortality (up to 117 hr). A total of 155 Greenland halibut were included in the study. Overall tagging mortality was estimated to be 7%. Immediate handling and tagging mortality in both winter and summer experiments was low (< 5%). Several factors were shown to have significant effects on the outcome (level of condition). Fish held in the tanks for longer time periods were in better condition. Females had a tendency to be in poorer condition than males immediately following tagging. Overall short-term mortality was 4%. There was no significant difference in mortality rates between seasons. There was no effect on mortality of the covariates size, time held in the cage and several other factors examined, but overall mortality was so low that differences would be difficult to detect.ÿ However, there was a significant correlation between the fish condition and mortality. The study showed that tagging under harsh winter conditions is just as possible as under summer conditions as long as exposure to sub-zero air/water temperatures are minimized. Our study further suggests that holding the tagged fish in an observation tank for a period of 5 hr or more could reduce the tagging mortality on released fish.