A conceptual framework for the management of steelhead, Oncorhynchus mykiss

Abstract

This paper outlines a possible framework for the management of steelhead in British Columbia and provides a rationale for the elements of the framework. The intent of the management framework is to conserve the productive capacity of steelhead stocks (i.e., populations or aggregates of populations with similar dynamics) by maintaining spawner abundance at levels that potentially provide sustainable benefits to society. The framework consists of: (1) abundance-based biological reference points that define management zones, and (2) associated sets of management actions (decision rules) that adjust either mortality rates or stock productivity to move population abundance towards a desired endpoint within a given time. The framework provides an explicit link between habitat management and harvest management for a stock by defining the reference points in terms of a habitat-based maximum smolt production. The key reference point is the “conservation concern threshold” (CCT) below which the stock is regarded as overfished. For a stock whose recruitment dynamics can be described by a deterministic Beverton-Holt type spawner-recruit relationship, this threshold is at 0.25·B, where B is the asymptotic maximum adult recruitment. The CCT has the useful property of being largely independent of stock productivity. We further define a limit reference point (LRP) as the spawner abundance from which a stock can recover to the CCT within a defined time (e.g., one generation) in the absence of harvest. Although the LRP varies with stock productivity, simulations show that it can be approximated by a fixed value near 0.15·B over a wide range of stock productivity if management actions progressively reduce mortality below the CCT. At abundance levels below the LRP, the stock is considered to be an “extreme conservation concern” and extraordinary management actions may be required to eliminate controllable mortality and to increase productivity. Because the LRP and CCT jointly determine the rate at which anthropogenic mortality changes with abundance, it is possible to alter the pair of deterministic values slightly with little impact on the performance of the management system, e.g., for steelhead sport fisheries we could set the CCT at 0.3·B to 0.35·B to accommodate parameter uncertainty and set the LRP at 0.1·B to 0.15·B. In conjunction with appropriate management regulations, the system of management zones established by the CCT and LRP will generally maintain stocks at levels well above those at which population viability is a concern. The social cost may be foregone harvest opportunities, particularly for other fisheries that take steelhead as a bycatch.