Research Document 1997/138
A biological assessment of the coho salmon of the Skeena river, British Columbia, and recommendations for fisheries in 1998
By B. Holtby and B. Finnegan
Abstract
This assessment covers coho salmon of the Skeena River. We rely on two indices of aggregate abundance, two hatchery indicator populations, two wild indicators, one associated with a hatchery and the other not actually in the Skeena but close by, and surveys of juvenile densities that were made in 45 to 52 streams throughout the Skeena watershed.
Conservation concerns for the early run-timing component of Skeena coho were first raised by the Pacific Stock Assessment Review Committee (PSARC) in 1986 (Stocker 1987). Coho populations throughout most of the upper Skeena (Bear-Sustut, Babine and Bulkley-Morice subareas) continue to be severely depressed and heavily exploited, a combination which makes them extremely vulnerable to episodes of poor marine survival. The near-zero ocean survival that appears to have occurred in the 1996 smolt year (1997 returns) has produced a situation in the upper Skeena, particularly the upper Bulkley, the Babine and the high interior, that can only be described as perilous. Coastal populations appear to be more productive and have been stable despite high exploitation rates, probably because of a prolonged period of good marine survival. The status of Skeena coho epitomizes the problems faced in attempting to manage the coho salmon of a large and varied geographic region when nearly all of the exploitation is exerted in mixed-stock ocean fisheries. Although coastal populations may be able to withstand the prevalent exploitation rates even during episodes of relatively poor marine survival, the interior populations clearly cannot.
Marine survival rates for the smolts of 1997 are forecast to be 9.1%, or near average, for coastal populations and 8.2%, or below average, for upper Skeena populations. For a variety of reasons, the most important of which being that similar forecasts failed in 1996, we recommend that these forecasts be treated with extreme caution. Four approaches are outlined to provide recommended exploitation rate targets or floors for 1998 fisheries. The approaches yield target or maximum exploitation rates that range from 44% to 67% for coastal populations and 9% to 71% for interior populations. The wide range in recommended harvest rates is the consequence of different objectives and levels of assumed risk, and is not solely a reflection of uncertainty. The indicator populations used to generate these recommended exploitation rates appear to be among the most productive of some 45 streams where fry densities have been routinely surveyed, and the status of the most upper Skeena systems was judged to be considerably worse than the interior indicator (Toboggan Creek).
Given the continuing conservation concerns for upper Skeena area coho, the alarming further decline in abundance in 1997, and uncertainty in survival rates for coho returning in 1998, we caution that any exploitation of upper Skeena area coho poses a high risk to the viability of coho populations in this area.
Although conservation problems for lower and middle area Skeena coho were not indicated to 1996, because of the precipitous decline in abundance in 1997 and uncertainty in survival rates for coho returning in 1998, we recommend a more conservative approach to the harvest of these coho stocks.
Assessment of Skeena coho continues to be affected by limited information, and in particular the lack of effective forecasting tools for the Skeena coho and the lack of wild indictor sites where wild smolt production can be measured and their survival determined. Consequently we recommend the development of additional wild indicator sites in the Skeena, particularly in the interior, as well of the development of more effective forecasting tools for Skeena coho.
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