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Electronic Tagging of Juvenile Sablefish in Southeast Alaska

From 29 September to 4 October 2003, ABL scientists on the NOAA ship John N. Cobb captured juvenile sablefish in St. John Baptist Bay, a rearing location in coastal Southeast Alaska near Sitka. Thirteen of the fish were surgically implanted with electronic, acoustically transmitting tags that were programmed to transmit a record of temperature and depth experienced by the fish. Mobile acoustic receivers located onboard the Cobb were used to monitor juvenile sablefish behavior and habitat utilization in this rearing location during the cruise. Fixed acoustic receivers located at the head of the bay leading from the rearing location to the open ocean are also being used to monitor the timing of juvenile sablefish emigration from their nearshore rearing habitat to the more open waters of the Gulf of Alaska. Short-term objectives are to use the electronic acoustic tags to provide information on juvenile sablefish behavior and habitat use in nearshore rearing areas and on the timing and duration of the emigration from nearshore rearing habitat.

During the same cruise, 736 juvenile, age 0+ sablefish were tagged with spaghetti tags, and 76 additional juvenile sablefish were also tagged with electronic archival tags that were surgically implanted into the fish. Long-term objectives are to utilize the electronic archival tags in addition to the electronic acoustically transmitting tags to provide information on juvenile sablefish behavior and habitat during their transition from nearshore rearing areas to the age at which they are intercepted by the fishery. The electronic archival tags monitor the temperature and depth experienced by juvenile sablefish from the time they leave nearshore rearing areas at age 1+ or greater until the time they recruit to the fishery at age 2+ or greater. The electronic archival tags record temperature and depth and are designed for recovery in the commercial fishery at age 2+ or greater.

By Tom Rutecki.

Analysis of Pacific Sleeper Shark Relative Abundance Trends in the Gulf of Alaska and Bering Sea

Pacific sleeper sharks (Somniosus pacificus) are deepwater sharks of the North Pacific Ocean that have been suggested as a source of Steller sea lion mortality. Some information also suggests that abundance of the sharks is increasing. For this study, Pacific sleeper shark data from fishery-independent longline surveys in the Gulf of Alaska and the Bering Sea was analyzed to determine abundance trends and whether any changes were statistically significant. Relative population numbers (RPNs) of Pacific sleeper sharks captured in the NMFS domestic sablefish longline surveys ranged from a low of 79 in 1988 to a high of 2,980 in 2001. The most substantial increase in RPNs occurred between 1992 and 1993, and RPNs remained high from 1994 to 2003. Not all the 95% bootstrap confidence intervals calculated for annual Pacific sleeper shark RPNs in the longline surveys from 1989 to 2003 overlapped, indicating a significant increase (at the 95% confidence level) in relative abundance occurred during this period. The increase in RPNs was driven largely by Pacific sleeper shark catches in Shelikof Trough, a deepwater Gulf of Alaska gully on the continental shelf between Kodiak Island and the Alaska Peninsula. The sharks appeared to be relatively abundant in this area. Pacific sleeper sharks may be relatively abundant in other areas, such as on the continental shelf of the Gulf of Alaska and Bering Sea at depths less than 200 m, but these areas are not routinely sampled by the sablefish longline survey.

By Dean Courtney.

2003 Pacific Sleeper Shark Electronic Archival Tagging in Upper Chatham Strait, Southeast Alaska

For this study, 49 Pacific sleeper sharks were tagged and released in upper Chatham Strait in Southeast Alaska for recovery as bycatch from commercial longline fisheries. Little is known about the life history or ecological role of Pacific sleeper sharks in the North Pacific Ocean, where the sharks could potentially interact with marine mammals such as Steller sea lions (Eumetopias jubatus) or harbor seals (Phoca vitulina). The recovery of electronic archival tags from the sharks will aid in identifying Pacific sleeper shark habitat utilization and distribution in Southeast Alaska and the potential for interactions between Pacific sleeper sharks and marine mammals.

The study area was adjacent to Point Howard in upper Chatham Strait/lower Lynn Canal at approximately lat. 58.34N, long. 135.04W. ABL scientists aboard the chartered fishing vessel Williwaw conducted 5 days of longline operations on 3-5 June and 4-5 September 2003. Sleeper sharks were caught using standard halibut longline gear provided by the vessel. The sharks were tagged with electronic archival temperature and depth recording tags mounted externally on the first dorsal fin. The archival tags record temperature and depth for 2-5 years and store the recorded data for 5-10 years. The ABL is offering a $200 reward for the return of the electronic archival temperature and depth recording tags and has initiated communications with ADF&G and with Southeast Alaska commercial longline fishing organizations regarding the reward program.

By Dean Courtney.

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