Surface Trawling for Young-of-the-Year Rockfish, Young-of-the-Year Sablefish, and Juvenile Salmon in Offshore Waters of the Eastern Gulf of Alaska
Young-of-the year rockfish caught by the Medeia in August 2006.
During the period 11-21 August 2006, ABL used the Alaska Department of Fish and Game (ADF&G) research vessel Medeia to conduct research directed at diel sampling of epipelagic ichthyofauna in offshore waters of the eastern Gulf of Alaska. The general objective of this cruise was to evaluate procedures for establishing a trawl indexing survey for young-of-the-year (YOY) rockfish and sablefish and for juvenile salmon in offshore waters of southeastern Alaska.
The project was a continuation of similar research conducted in mid-August 2005. Specific objectives included: 1) conducting an experiment to determine if surface trawling at night is more effective than surface trawling in daytime for capturing YOY rockfish, YOY sablefish, and juvenile salmon; 2) determining the spatial distribution of these three taxa at selected stations along transects off the northern region of southeastern Alaska; and 3) collecting physical oceanographic data coincident with the trawl hauls and vessel track to investigate the relationship between oceanographic conditions and the distribution and abundance of fish captured in the trawl.
Three ABL programs collaborated on this study: Marine Ecology and Stock Assessment (formerly ABL’s Groundfish Program), Marine Salmon Interactions, and Ocean Carrying Capacity.
Day and night surface trawl sampling with a Nordic 264 rope trawl was successfully completed by the Medeia at each of the six planned stations. These stations were located up to 75 km offshore in southeastern Alaska along two transects: Icy Point and Cape Edward. Catches totaled nearly 10,000 fish representing 25 species in 35 hauls. Catch in numbers for the target species of the study were YOY rockfish, 6,283; YOY sablefish, 276; and juvenile salmon, 206. Young-of-the-year rockfish species identification will be made in the laboratory at a later date based on retained samples.
Juvenile salmon catch numbers were pink salmon, 154; chum salmon, 44; coho salmon, 5; and sockeye salmon, 5. Relatively high catches of YOY rockfish were found in several hauls at stations along both transects. The largest catch of YOY rockfish in a single haul was about 3,700 fish, which appears to be the most YOY rockfish ever taken in a pelagic research haul in Alaska. The large numbers of YOY rockfish encountered in this cruise may be an indication of an abundant year class in this region.
There was a marked difference in the catches between day and night. Day tows yielded much higher catches of the target species. In particular, 6,283 YOY rockfish were caught in the day, versus only 21 at night. For salmon, 184 were caught in the day, as compared to 22 at night, and the predominance of day catches held true for each salmon species. These results suggest that for the Nordic 264 rope trawl in 2006, daytime surface tows may have been more effective at sampling these species than were surface tows at night.
The 2006 results were very different than results of ABL’s 2005 Medeia surface trawling cruise. This was true despite the fact that both cruises were conducted during almost the same dates in August, and they each sampled the same stations along the Icy Point and Cape Edward transects. In contrast to the large catches of YOY rockfish in 2006, a total of only 11 YOY rockfish were caught in 2005. The results of the day-night comparison for salmon were opposite between cruises; in 2006, salmon catches were predominantly in the day, whereas in 2005 they were mostly at night. Also, unusual species not commonly found as far north as Alaska, such as Humboldt squid and Pacific sardines, were caught in 2005 but not in 2006.
These differences may be explained by the variable oceanographic conditions encountered in each year’s cruise, especially water temperatures. Summer 2005 was unusually warm in the eastern Gulf of Alaska, and average water temperatures near the surface at stations along our transects were 2.4°C higher than in 2006.
Two piggyback projects also occurred on the cruise: 1) one day was spent doing exploratory surface trawls in nearshore waters near Icy Point in an unsuccessful attempt to locate spiny dogfish for food habits and tagging studies, and 2) two days were spent in inside waters of northern Southeast Alaska (Icy and Chatham Straits) sampling standard stations for ABL’s Southeast Alaska Coastal Monitoring project. This sampling was scheduled at the last minute because the NOAA ship John N. Cobb, which normally does this work, was unavailable due to mechanical problems.
By Dave Clausen and Alex Wertheimer
Juvenile Sablefish Sampling in Saint John Baptist Bay
The juvenile sablefish tagging cruise took place on the John N. Cobb in Saint John Baptist Bay, from 14 to 20 May 2006. The goal of the cruise was to tag and release 1,000 juvenile sablefish with numerical spaghetti tags and 80 sabelfish with surgically implanted electronic archival tags. Juvenile sablefish catches were low in 2006, and only 88 juvenile sablefish were captured with 174 rod hours. Of these fish, 20 received spaghetti tags, and 66 were implanted with electronic archival tags. Juvenile sablefish ranged in size from approximately 270 to 360 cm FL. Archival tags were placed in individuals that were longer than 295 cm FL.
Electronic archival tags were programmed to continuously record temperature and depth and will be recovered as sablefish recruit to the commercial fishery. Temperature and depth profiles of prerecruit juvenile sablefish will provide a valuable tool for gathering information on the geographic distribution and vertical habitat of juvenile sablefish in Alaska waters. To date, a total of 388 electronic archival tags have been released in age 0+ and age 1+ juvenile sablefish from the 2003 (235 tagged), 2004 (87 tagged), and 2005 (66 tagged) year
classes in Saint John Baptist Bay. These tagged sablefish should begin to recruit in the commercial fishery at age 4 and 5 in 2007 and 2008,
respectively.
