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Research Document - 2004/116

Critical Habitat Case Study – Sakinaw Lake Sockeye Salmon

By Godbout, L., J.R. Irvine, C.C. Wood, C. Fu, G. Jamieson

Abstract

Sockeye salmon (Oncorhynchus nerka) from Sakinaw Lake, southern British Columbia were considered for legal listing under the Species at Risk Act (SARA), which would have required that critical habitat be identified, to the extent feasible. Sakinaw sockeye were abundant 15 years ago, but have declined to such low levels that they have a high probability of extirpation.

We describe a methodology we developed to identify potential critical habitat for Sakinaw sockeye. Critical habitat is defined in SARA as the habitat necessary for the survival or recovery of the population. Our methodology consists of 1) describing the present status and life history of Sakinaw sockeye, 2) operationally defining critical habitat, and 3) modellling critical habitat requirements. A habitat was considered critical if the habitat (or improved habitat) was part of the minimal configuration leading to population viability, defined as a population with <10% probability of quasi-extinction and ≥ 95% probability of achieving recovery. To illustrate our approach we used a threshold of 250 female spawners by 2017 as a recovery goal. We recognize that this underestimates the habitat needed to meet longer term (~100 years) more ambitious recovery goals (e.g. 5000 spawners or 2500 female spawners).

We developed a stochastic age structured model using vital rate data chiefly from other (non-endangered) sockeye populations. We evaluated linkages between possible critical habitat combinations and management actions (e.g. levels of enhancement and fishery exploitation) using Monte Carlo simulation. Our habitat scenarios focused on the lake outlet and spawning beaches, both habitats likely to be limiting and where changes might result in improved survival. Changes at the outlet that might alter survival (e.g. predator control and flow adjustments) were modeled by varying pre-spawner survival rates while improvements to spawning beaches were modeled by increasing egg-to-fry survival rates and the carrying capacity for female spawners. Results from our preliminary model suggest that spawning habitat for 280 to 360 female spawners is required (depending on scenario) for Sakinaw sockeye to have <10% probability of quasi-extinction and ≥ 95% probability of meeting the recovery goal assuming no fishing exploitation. To offset 15% exploitation, spawning habitat for an additional 7-110 female spawners (depending on scenario) would be needed. All scenarios include artificial propagation. Potential critical habitat also includes habitat at the lake outlet sufficient to ensure that 90%-95% of the pre-spawners can reach the lake.

These results should be considered preliminary until more comprehensive sensibility analysis, evaluation of the effects of alternative density dependent responses, and criteria for quasi-extinction are completed. Retrospective simulations using relatively high initial population sizes (as occurred in the 1970’s and 1980’s) to investigate the degree to which vital rates must be reduced to mimic the rate of decline in abundance observed since 1987 should also be done. This will allow re-evaluation of the potential overall role of critical habitat on the ability of the population to survive and recover.

We suggest that this overall approach is applicable for other sockeye populations and, with some limitations, other species of salmon. However, success requires a good understanding of the life cycle of the species under investigation, and under what conditions, if any, that habitat may be limiting.

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