Migration speed, run timing, and migration route for interior Fraser Steelhead Trout

Abstract

In this paper, we assess and validate the information available to inform key input parameters required for the development of future fishery planning models for Interior Fraser River Steelhead Trout, including migration speed, migration route, and run timing. This assessment includes the development and evaluation of models to estimate run timing of Interior Fraser River Steelhead Trout in the lower Fraser River. Our literature review revealed that relatively little is known about migration speed and migration route of Steelhead Trout in British Columbia, and even less is known specifically about Interior Fraser River Steelhead Trout. Migration speed is highly variable both within and among populations, but generally Steelhead Trout appear to migrate faster in marine waters than in rivers. The proportion of fish migrating along either side of Vancouver Island is not well-studied for most salmonids, but there is evidence that adult returning Steelhead Trout (and in particular, Interior Fraser River Steelhead Trout) are present on both sides. Despite knowing this, we have limited information to determine the relative proportions that take each route. Run timing of Interior Fraser Steelhead through the lower Fraser River is much better informed due to their incidental catch in Chinook and Chum directed test fisheries at the Albion test fishery, which coincides with their fall migration. While the standard practice in estimating run timing of salmonids has been to use a normal distribution, here we present evidence that an alternative (asymmetric normal distribution) better captures the observed patterns in catch. This approach results in estimating that the majority (95%) of Interior Fraser River Steelhead Trout migrate through the lower Fraser River between September 8 and November 23, on average. The information presented in this paper is intended to support future work on a fishery planning model that estimates exposure of Interior Fraser River Steelhead Trout to fisheries throughout southern British Columbia. Given the uncertainty in modelled estimates of run-timing parameters, migration route around Vancouver Island, and migration speed throughout the migration corridor, it is strongly recommended that the future exposure model not use one value for each of these parameters; rather, a better understanding of the risk of fishery plans to Interior Fraser River Steelhead Trout would be obtained by drawing input parameters from a distribution and/or ensuring a sensitivity analysis is completed to understand how variation in these biological parameters may affect exposure estimates and subsequent management decisions.