Results from the 2001 Transboundary Trawl Survey of the Southern Strait of Georgia, San Juan Archipelago and Adjacent Waters

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Published: May 2003

Pages: 117

Author(s): Wayne A. Palsson, Suzanne Hoffmann, Paul Clarke, and James Beam

Abstract

During May and June 2001, a synoptic survey of the southern Strait of Georgia, San Juan Archipelago and adjacent waters was conducted in the transboundary waters of Washington and British Columbia. The survey was designed to estimate the numerical and biomass abundances of key benthic species, identify population trends, and quantify the impact of fisheries. The 2001 survey was also designed to describe the distribution of key commercial fishes that inhabit the Strait of Georgia, San Juan Archipelago and adjacent waters. The survey was also designed to determine which species are likely move between Washington and British Columbian waters and which species are vulnerable to fisheries on either side of the border.

Standard trawl survey methodology was used to design the stratified random survey. A 400 mesh Eastern Trawl was towed by a chartered fishing vessel. The bottom trawl was fitted with a codend net liner with a 3 cm mesh opening, and the trawl was towed at predetermined stations for approximately10 minutes. The survey was stratified by country and by five depth strata: 5- 20 fathoms, 21-40 fm, 41-60 fm, 60-120 fm, and >120 fm. A total of 115 trawl samples were collected in the 2,313 km2 survey area. Fifty stations were sampled the in the 876 km2 Washington Strait of Georgia, 19 samples were collected in the 361 km2 of the B.C. Strait of Georgia, 40 samples in the 864 km2 San Juan Archipelago, and 6 samples in the 274 km2Canadian Haro Strait. The two shallowest strata were not sampled in BC waters due to extensive rocky habitats or a lack of time.

Ninety identifiable species of fish were collected during the trawl survey of the four regions. Sixty-seven species of fish were collected in the Washington Strait of Georgia, and 43 fishes were collected in the BC Strait of Georgia, 65 species were collected in the San Juan Archipelago, and 30 species were collected in the B.C. Haro Strait and Boundary Pass. An estimated 112,108 individual fish were caught during the trawl survey, and they weighed 18 mt.

There was an estimated population of 251.3 million fish weighing 39,535 mt living in the southern Strait of Georgia and Archipelago. Washington contained 220 million bottomfish while B.C. had 31 million. The B.C. bottomfish resource constituted an estimated 8,811 mt while the Washington resource weighed an estimated 30,723 mt.

Spotted ratfish was the most abundant taxon of any region surveyed. They comprised almost 40% of the fish in the WA Strait of Georgia, and almost 60% of the deep waters surveyed in the BC Strait of Georgia. Ratfish made up almost half of the San Juan Archipelago fish populations and almost 70% of the BC Haro Strait and Boundary Pass. Flatfish as a group was the second most dominant species group in WA and BC Strait of Georgia while other species contributed together to form the third most common group in these waters. Dogfish was the third most abundant species in the San Juan Archipelago and the second most abundant fish in the BC Haro Strait and Boundary Pass.

Biomass and numerical abundance estimates and occurrence patterns were presented for key species including spiny dogfish, spotted ratfish, Pacific cod, walleye pollock, Pacific whiting (hake), lingcod, English sole, rock sole, starry flounder, Pacific sanddab, sand sole, Dover sole,

Dungeness crab, and spotted prawn. This survey was the sixth bottom trawl survey conducted in the Washington Strait of Georgia since 1987. Overall, total fish biomass has not changed since the surveys were initiated, and the 2001 survey had the greatest point estimate of any survey. Most flatfish species have apparently increased in abundance in recent years, however, other key fishery species are in decline. In 2001, Pacific cod biomass was at an all-time low, and spiny dogfish biomass while higher in 2001 than the 1997 survey, is also at very low levels compared to the late 1980s and early 1990s.

The limited survey of the southern Strait of Georgia in 2001 confirmed the general observations made during the more extensive bottom trawl survey conducted in 1997. The earlier survey in the Strait of Georgia found that only deep water species such as Pacific cod, Pacific hake (whiting), walleye pollock, English sole, and Dover sole were candidates for transboundary management because the deep Malaspina Trough aggregated these species in the area around the international border. Shallow-water species such as the rock and sand soles and starry flounder in the Strait of Georgia were more restricted to the perimeter of the basin and were less likely candidates in their adult stages for two independent fisheries targeting a common stock..

The biomass estimates resulting from the 2001 bottom trawl were compared to the commercial catches from the subsequent 12-month period providing a robust measure of the annual exploitation rate. The exploitation rates for the Washington survey areas were greater for the British Columbian survey areas. The latter were far less than 1% per year but the Washington exploitation rates were as high as 15% for halibut, 11% for English sole. Most of the other Washington exploitation rates 3% to 6% for key species including skates, Pacific cod, starry flounder and sand sole, and the exploitation rate for dogfish was 1.1%, a rate that is likely commensurate for this late-maturing and long-lived species.

Transboundary bottom trawl surveys are needed on a periodic basis in the future and more resources are needed to conduct more effective surveys in British Columbian waters than were available for the 2001 survey. Clearly, many key fish and invertebrate resources intermingle between the common border and water bodies shared between the United States and Canada. Fish populations need to be assessed on a broader, synoptic scale so that fisheries and populations in common water bodies can be effectively evaluated for harvest rates, movement patterns, habitat preference, and productivity. Greater cooperation and shared resources between Washington and Canadian fishery scientists can be a key factor in achieving these understandings of shared fish populations.