Research Document - 1999/191
A Habitat Based Evaluation of Okanagan Sockeye Salmon Escapement Objectives.
By K.D. Hyatt and D.P. Rankin
Abstract
Information from both historic and recent assessment activities has been assembled in this report to determine the current status of Okanagan sockeye. Subsets of the information are then used to: (i) evaluate the utility of historic escapement data, (ii) identify factors that may limit stock production within the Okanagan Basin and (iii) identify escapement objectives that are consistent with what is known about spawning and rearing habitat limitations in the Okanagan River and Osoyoos Lake.
Okanagan sockeye stock production is not large (average return of 56,147 sockeye per annum; range 1,666-199,832) compared to that found in other areas on the B.C. coast, but is very important locally to meet ceremonial and subsistence needs of First Nations people of the U.S. and Canadian portions of the Columbia and Okanagan River basins. Returns, averaging 18,148 adult sockeye per year in the current decade, are the lowest for any decade within the 45 year period of record for the stock. In addition, spawner abundance has declined to match record lows in 3 of the past 5 years and has stimulated expressions of concern by fisheries personnel and resource stakeholders about future prospects for long term persistence of Okanagan sockeye.
Analysis of spawning habitat capacity in terms of quantity and quality of gravel available for redds and egg incubation indicated that the Upper Okanagan River could accommodate several times the number of spawners that currently return annually. Similarly, analysis of rearing habitat in Osoyoos Lake based on both its nutrient status (total phosphorus load) and limits placed on sockeye use by changes in seasonal water quality parameters (temperature and oxygen levels) suggests that Osoyoos Lake has ample capacity to support good growth and survival of fry from no fewer than 58,730 spawners (as enumerated at Wells Dam). Analysis for possible associations between annual variations in spawner abundance and subsequent production variations of both smolt numbers and biomass also supports conclusions that neither spawning habitat in the Okanagan River or rearing habitat in Osoyoos Lake currently limit Okanagan sockeye population levels.
Recommendations supported by the paper are that: (1) minimum escapement objectives for Okanagan sockeye be set at 58,730 adults (in Wells Dam count units) or 29,365 adults as peak visual counts on the spawning grounds, (2) biological consequences of smolt size management of Osoyoos Lake sockeye be explored as a requisite to refining escapement objectives that might be adopted if Okanagan sockeye stock rebuilding is successful in future years, (3) annual sampling for size and age composition of Okanagan sockeye smolts be completed whenever feasible to facilitate future analysis of smolt-to-adult survival trends and smolt size management options in Osoyoos Lake, (4) annual sampling for age, size and sex composition of sockeye in catch and escapement be completed whenever feasible to facilitate future analysis of stock and recruit relations and to follow smolt-to-adult survival trends, (5) retrieval and review of source documentation pertaining to SEDS estimates of Okanagan sockeye escapement plus entry of supplemental comments describing methods used to generate escapement estimates be completed, (6) annual summary estimates of Okanagan sockeye escapement from Wells Dam counts be incorporated into SEDS as a formal alternative to relatively imprecise spawning ground counts and finally (7) given that neither spawning gravel capacity or lake rearing capacity appear to set the principal limits on Okanagan sockeye population size at current stock levels, additional effort should be expended by the Okanagan Basin Fisheries Working Group to determine the reasons for recent stock declines in the absence of any targeted exploitation of Okanagan sockeye.
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