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Shellfish Assessment Program - Kodiak Laboratory

Gulf of Alaska Small-mesh Trawl Survey, 2008

sorting shrimp
Figure 1.  Kurt Peterson and Aaren Ellsworth sort the shrimp catch during the 2008 small-mesh survey.  Photo by Dave Jackson.
 

The annual small-mesh trawl survey for shrimp and forage fish again was conducted jointly by scientists from the AFSC's RACE Division and the Alaska Department of Fish and Game (ADF&G) Division of Commercial Fisheries (Fig. 1).

The ADF&G research vessel Resolution was deployed for 129 tows in embayments around Kodiak Island and south of the Alaska Peninsula as far west as Pavlof Bay (Fig. 2 below) from 29 September to 24 October 2008. The 2008 effort marks the latest in a series that started in 1953.

As background, the Resolution has been used to conduct the survey since it was acquired by the ADF&G in 1971. Prior to 1971, only the AFSC was conducting the small-mesh trawl surveys and the original goal of this survey was to determine the potential for a commercial shrimp fishery in the Gulf of Alaska and Bering Sea.

A major shrimp fishery did develop in the Gulf of Alaska during the 1970s, and at that point the ADF&G also began regular trawl surveys. With the subsequent decline of this fishery in the 1980s, survey efforts also declined; the RACE Division surveys were limited mainly to Pavlof Bay while ADF&G surveys covered the waters around Kodiak Island and the Alaska Peninsula triennially.

figure 2 map, see caption
Figure 2.  Map of the central and western Gulf of Alaska showing haul locations (solid circles) from the 2008 small mesh survey.
 

Since 2001, ADF&G surveys have again been conducted annually and though they no longer conduct their own independent survey, since 2005 the RACE Division has provided support and has participated with the ADF&G in this survey. The ADF&G is responsible for survey design, providing the vessel platform, most personnel, data editing, and survey reports. It is now widely acknowledged that the span of the time series makes it one of the premier ecological data sets in the North Pacific.

Both RACE Division and ADF&G staff are actively working to edit, update, and combine their time series databases. For example, the ADF&G is editing haul location information and also station areas to allow for comparable biomass calculations for the period of their time series.

Scientists with the RACE Division are examining small-mesh survey data archives, including both the original deck logs and associated computer files that once resided on the now long retired Burroughs computer system but were subsequently "lost." Retrieval of these files has allowed us to compile a far more complete version of the time series than was previously available.

The continual editing and updating of the databases present a challenge, however, to respond to requests for these comprehensive small-mesh data in a timely and accurate manner. Methodology and protocols must be developed across the two agencies (RACE SAP Oracle software-based and ADF&G Java software-based) which are needed to combine the databases and incorporate the latest corrections and updates, provide metadata, and also address the technology security concerns of each agency.

Since the survey was originally conducted primarily for shrimp stock assessment, minimal information was initially collected about other organisms in the remainder of the catch. Gradually more comprehensive information was recorded and has evolved to where the catch is currently identified to the species level for nearly all fish and invertebrates with catch weights and counts recorded for each taxa.

see caption
legend
Figure 3.  The catch per unit effort (CPUE as ln (x + 1) kg km-1 ) of selected species from Marmot Bay (left plot) on northeast Kodiak Island and Pavlof Bay (right plot) on the western end of the Alaska Peninsula, 1972 to 2008.
 

Lengths are recorded from a representative sample of forage fish and commercially important groundfish and shrimp. Samples of shrimp are also preserved for sex determination and length measurement in the lab after the survey. Bottom temperatures are recorded with a sensor on the trawl headrope, and bathymetry and bottom hardness are recorded using both the Quester Tangent QTC4 and Simrad Olex systems. A photograph is taken of the catch from each haul as a visual record of the general catch composition.

The ancillary information collected on groundfish and other invertebrates, in combination with the shrimp data, is now viewed as being extremely valuable for the analysis of long-term changes in the marine community in the Gulf of Alaska.

Community changes are commonly examined by tracking trends in relative abundance of selected taxa, as exemplified by the trends observed in Marmot and Pavlof Bays (Fig. 3 ). These two bays have been some of the most consistently surveyed since the small-mesh trawl net was standardized by NMFS in 1971; both bays also formerly supported major shrimp fisheries.

Both Marmot and Pavlof bays experienced a decline in shrimp and forage fish densities and a concurrent rise in Pacific cod following the well-documented climate regime shift in the late 1970s. Northern pink shrimp (Pandalus borealis) have continued to decline in Pavlof Bay throughout the last three decades, while in Marmot Bay the decline has not been as dramatic or long lasting.

Shrimp populations in Marmot Bay have remained an important component in the catch and have actually shown some increases in density since 1990. In addition, eulachon (Thaleichthys pacificus) populations have rebounded in Marmot Bay, a trend not seen in Pavlof Bay. The reason for the differences in community structure between these bays in not clear but certainly warrants further investigation.

Also of interest in the 2008 survey results was the widespread capture of ocean shrimp (Pandalus jordani) around Kodiak and also along the Alaska Peninsula as far west as the Shumagin Islands. P. jordani is a lower-latitude pandalid shrimp species that is commercially fished off British Columbia and the west coast of the United States. Although it has been sporadically caught in the small-mesh survey since 1974, more consistent catches starting with the 2004 survey may signal a northward distribution shift in response to recent warming in the Gulf of Alaska.

by Dan Urban and Brian O'Gorman
 

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