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Atka Mackerel Tag Recovery Cruise in the Aleutian Islands and Examination of Tagged Steller Sea Lion Prey Field In Situ

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Apr-May-June 2012
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Figure 6(a,b,c).  Click to enlarge


The objective of the Fisheries Interaction Team's (FIT) tag release-recovery studies is to determine the efficacy of trawl exclusion zones as a management tool to maintain prey abundance/availability for Steller sea lions at local scales. Trawl exclusion zones were established around sea lion rookeries as a precautionary measure to protect critical sea lion habitat, including local populations of prey such as Atka mackerel. Localized fishing may affect Atka mackerel abundance and distribution near sea lion rookeries. Tagging experiments are being used to estimate abundance and movement of Atka mackerel between areas open and closed to the Atka mackerel fishery.

This study is an ongoing research effort. From 1999 to 2006, approximately 80,000 tagged fish were released during AFSC chartered tag release cruises near Seguam Pass, Tanaga Pass, Amchitka Island, and Kiska Island. In May to June of 2011, a cooperative venture between the North Pacific Fisheries Foundation and the AFSC released approximately 8,500 fish near the Seguam Pass area, 9,000 fish at Tanaga Pass, and 10,000 at Petrel Bank. In August 2011, we conducted a summer recovery cruise in the same area (Fig.6). The recent winter recovery cruise was conducted from 27 March to 17 April.

The cruise had three objectives. The first objective was to recover previously tagged fish in the open areas outside the trawl exclusion zones during the winter months. Even though tags were released inside the closed areas, during the recent 2011 recovery cruise, recoveries were not conducted inside the trawl exclusion zones to minimize potential negative impacts of Atka mackerel removal to the Steller sea lion prey fields inside the closed areas. The second objective of this study was to use catch composition data from the tows to estimate relative abundance indexes (CPUEs) for all major fish and invertebrate species present in the study areas. The third objective of this study was to characterize Atka mackerel habitat and develop methods for estimating indices of abundance of sea lion prey species with non-extractive methods such as camera tows.

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Figure 7.  Length Frequency distribution in the three study areas. Solid line: Petrel Bank, Large stippled line: Tanaga Island, small stippled line: Seguam Pass.

During the 2012 cruise we conducted 54 hauls and examined 1,529 metric tons (t) of Atka mackerel for tags, equivalent to approximately 2.6 million individual fish. We recovered 49 tags: 13 at Seguam pass, 25 at Tanaga pass, and 11 at Petrel Bank, all of which were released during the 2011 tag release charter. All hauls were sampled for species composition and sexed length frequencies. In addition, we collected 420 biological samples such as stomachs, gonads, and age structures and obtained sexed length frequencies from 4,697 individual fish. Length distribution of Atka mackerel differed by area with the smallest fish at Petrel Bank, medium sized fish at Tanaga Pass, and the largest fish at Seguam Pass (Fig.7).

In order to examine the habitat and develop indices of abundance, we conducted 12 underwater tows with a portable underwater camera. We conducted the camera tows at the same locations as the tag recovery hauls. We were able to conduct five camera tows at Seguam Pass, four camera tows at Tanaga Island, and three camera tows at Petrel Bank.

Further analysis will be conducted to estimate population sizes of Atka mackerel in these study areas and understand relative abundance of other SSL prey species, invertebrates, and habitat types associated with those populations.

Satellite tagged Steller sea lion prey study

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Figure 8.  Click image to enlarge

During November 2011, the AFSC's National Marine Mammal Laboratory tagged a female adult Steller sea lion with a satellite tag. The female was also branded with the brand '=24', hence for the purposes of this report we will refer to her under this name.

The female =24 had been transmitting location data since November 2011. She was located at Semisnopochnoi Island and travelled to the southern part of Petrel Bank at regular intervals, presumably to feed. FIT staff took the opportunity during this recovery cruise to run a hydroacoustic transect at the southern end of Petrel Bank. We conducted five tows in areas where the sea lion was frequently observed and where we found fish signal during the transects. We also conducted two camera tows in the vicinity of one of the feeding 'hot spots' of =24. The location of the transect, bottom, and camera tows are presented in Figure 8.

It appeared that the sea lion was diving consistently in two locations— one close to the canyon edge (haul 21) and one in the flat area to the south of the edge (haul 16). Future analysis of the hydroacoustic data will give further insight of the size of the fish aggregations in this area.

We conducted two tows in the vicinity of the canyon; at the canyon edge we found mostly Pacific ocean perch (78%) and sponges (15%) with a trace of Atka mackerel (2%). In the canyon itself we found a mix of adult walleye pollock (59%) and Pacific ocean perch (31%). We conducted two tows in the flat area to the south of the canyon. In haul 16 we found mostly northern rockfish (50%) and Pacific cod (30%) and in haul 20 we found mostly northern rockfish (65%) and Atka mackerel (18%). We also conducted a tow along the eastern edge of the shelf (haul 22) where we found mostly Atka mackerel (75%) and Pacific cod (12%).

By Susanne McDermott

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