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Shellfish Assessment Program - Kodiak Laboratory

Gulf of Alaska Small-Mesh Trawl Surveys 1953-2007

Figure 6 graphs, see caption
Figure 6.  Trends in CPUE values observed in the Gulf of Alaska small-mesh trawl surveys for Pacific cod, northern shrimp, and jellyfish of the class Scyphozoa.

Scientists at the RACE Division’s Kodiak Laboratory and Alaska Department of Fish and Game (ADF&G) have been conducting small-mesh bottom trawl surveys in the Gulf of Alaska (GOA) since 1953. Surveys locations have ranged from Yakutat Bay southwest to Unalaska Island. During this period 9,973 hauls have been completed with catches with contents representing 483 different taxonomic groups. Twenty-six different vessels have participated in the survey and included in that list is the venerable NOAA ship John N. Cobb. The small-mesh trawl survey, originally designed to track the trends in abundance and assess the status of pandalid shrimp populations in the GOA, has become an important source of additional information on groundfish and invertebrates.

In the earliest decades of the survey, the ADF&G conducted the surveys around Kodiak Island and the north GOA coast, while the RACE Division surveyed the central shrimp grounds around the lower Alaska Peninsula, consistently in Pavlof Bay. With some variations, this was the case until 2005 when the ADF&G began conducting the entire survey, with financial and personnel assistance from the RACE Division. In the early decades of the survey, the survey gear, although designed as shrimp trawls, was inconsistent, but in 1972 the RACE Division developed a high opening, three bridle net with a 3.1-cm (stretch) mesh liner which was adopted as the standard by both ADF&G and RACE.

The small-mesh shrimp surveys in the GOA are widely recognized as the most comprehensive and longest-standing continuous time series of its type in the North Pacific. As such, it has proved a treasure trove of information of marine living resources for researchers at the Kodiak Laboratory and elsewhere. Research has focused not only on basic shrimp biology but also on the effects of climate change on the marine ecosystem.

The most complete portion of the time series comes from the work in Pavlof Bay, and it was this time series that Paul Anderson (retired) and John Piatt (U.S. Geological Service) used as the basis of their seminal paper demonstrating the dramatic transition from a community dominated by northern shrimp, Pandalus borealis, and forage fishes to one with a preponderance of groundfish. Their paper was one of the first to demonstrate that marine community reorganization was closely associated with climate change – in this case a change seen in 1976 and 1977 that followed an interdecadal climate shift known as the Pacific Decadal Oscillation (PDO). The importance of the paper is reflected in the fact that since its publication, it has been cited in the literature at least 128 times. Abundance data from the small-mesh survey database has been further analyzed recently, further detailing the mechanisms of how climate change affects the North Pacific marine ecosystem.

At the heart of the analysis of community reorganization is the examination of trends in relative abundance of selected taxa as reflected in catch-per-unit effort (CPUE) statistics in the survey. To illustrate some key trends in the marine ecosystem from 1972 to 2007, CPUE values were calculated and are presented here for three ecologically important taxa: Pacific cod, Gadus macrocephalus, northern shrimp, and jellyfish of the class Scyphozoa (Fig. 6). We considered seven areas which were the most consistently sampled over the time period: Marmot Bay, Two-Headed Gully, Pavlof Bay, Kuiukta Bay, Kiliuda Bay, Alitak Bay, and Chignik-Castle Bays (Fig. 7 below). The relative abundance (log-transformed) for each species, expressed as kilograms of catch by the distance fished in kilometers (kg km-1), is shown along with the running average.

Northern shrimp declined following the 1976 and 1977 climate regime shift, and the average CPUE remains an order of magnitude lower than 1970s’ levels. At the same time Pacific cod CPUE rose sharply post-shift but has declined from highs of the 1980s and 1990s. Jellyfish remain at the high levels observed since the 1980s. There is evidence from the North Atlantic that cod predation on shrimp may act as a top-down control on shrimp populations, although the large commercial catches of shrimp during the 1970s and early 1980s also may have played a role in their decline (e.g., over 13,000 metric tons from Pavlof Bay in 1977). Jellyfish in the North Pacific are known to have a positive response to warmer water temperatures such as those seen post-PDO.

Figure 7 map, see caption
Figure 7.  Map showing sites sampled in Gulf of Alaska small-mesh trawl surveys, 1953-2007.

We anticipate the small-mesh survey time series will continue to play an important role in efforts to understand the combined effects of climate change and fishery removals on marine communities. In recognition of that importance, we are currently working in collaboration with the ADF&G on a project to update, audit, and conduct a series of quality assurance measures to ensure that the database is as accurate and complete as possible. Part of that process will include moving the complete data set to an Oracle database schema. To obtain more information about the database or for access to the database contact the Shellfish Assessment Program at the Kodiak Laboratory at (907) 481-1711.

By Brian O’Gorman and Dan Urban

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