Utility of the steelhead harvest analysis in determining population trends and estimating escapement

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Executive Summary

Previous work has shown the Steelhead Harvest Analysis (SHA) to be biased because it is derived from mail out surveys, thus actual total catch and effort are difficult to determine. However, often it is assumed that catch-per-angler day (CpAD) derived form the survey can be used as abundance trend indicators under the assumption that CpAD is proportional to abundance (ß=1 equation 3.0). Examination of bias in trend information derived from the SHA indicates that CpAD is hyper-stable, not declining as quickly as stock size, when compared to available run size estimates. Given currently available run size information, for a few steelhead rivers within British Columbia, ß was found to be on average 0.37 with variation amongst rivers. The Keogh and Vedder rivers CpAD were found to be the most hyper-stable (ß=0.287, ß=0.104) and the Skeena River system, Chilcotin River and Little Campbell River having moderate degrees of hyper-stability (ß=0.44, ß=0.46, ß=0.62). These result are not unexpected as in most instances catch-per-effort data is found to be hyper-stable however, failure to correct for hyper-stability in steelhead CpAD when making inferences about stock performance will result in a more optimistic assessment of stock status. The analysis also indicates a substantial increase in angler efficiency (~75%) over time within the Skeena, Chilcotin and Keogh rivers likely the result of changes in angler efficiency and noticeably correlated with a regulatory switch to catch and release.

A simple regression model for determining stock size from SHA derived catch and angler density (angler per kilometre of anandromous length) was found to predict stock size with reasonable accuracy. Current escapement estimates were calculated for all rivers in the SHA having non-zero catch. 87% of assessed rivers were found to have less than 20 steelhead per kilometre and 61% were found to have less than 5. Based of previously assessed smolt capacity for 86 rivers within the province a habitat based regression model was developed to determine smolt capacity for all steelhead stocks within the province. When adult capacity estimates (assuming a 15% smolt to adult survival) were compared to estimates of current stock size 68% of rivers were determined to have less than 30% of capacity. If a more recent estimate of smolt survival (3%) is used, 35% of rivers assessed are below 30% of current capacity.