Research Document - 2014/086
Assessment of the Interior Fraser Coho Salmon (Oncorhynchus kisutch) Management Unit Relative to the 2006 Conservation Strategy Recovery Objectives
By A. Scott Decker, Michael A. Hawkshaw, Bruce A. Patten, Joel Sawada and A. Lester Jantz
Abstract
Interior Fraser Coho is the name for the management unit (MU) which refers to the Coho Salmon (Oncorhynchus kisutch) that return to the Fraser River and tributaries upstream of Hells Gate in the Fraser Canyon. In response to low abundance of southern BC Coho Salmon stocks, fisheries that intercept Interior Fraser Coho have been restricted since 1998. This document describes the fishery management actions that have been taken since 2006 to meet the 2006 Conservation Strategy recovery objectives for Interior Fraser Coho, provides an assessment of current status of the MU against these objectives, quantifies annual exploitation rates and the level of uncertainty in these estimates, and provides estimates of the probability of achieving the 2006 Conservation Strategy Recovery Objectives at a range of potential exploitation rates.
Given that Wild Salmon Policy (WSP) benchmarks are not currently available for Interior Fraser Coho, the status of Interior Fraser Coho was assessed against two ‘short term’ and one ‘long term’ recovery objective originally proposed by the Interior Fraser Coho Recovery Team (IFCRT 2006). Short-term objective #1 consists of maintaining a minimum of 1,000 naturally spawning wild Coho Salmon (3-year geometric mean) in at least half of the 11 subpopulations that have been proposed within the five conservation units (CUs) (IFCRT 2006), while the long-term objective consists of maintaining 1,000 or more wild Coho Salmon in all 11 subpopulations. Our analyses showed that escapement levels of 20,000 and 40,000 spawners, respectively, would result in near 100% probability that these two objectives would be met. Generational average escapements for the Interior Fraser Coho aggregate exceeded 20,000 wild adults in every year from 2008 onward, but exceeded 40,000 wild adults in only the two most recent return years (2012 and 2013).
Short-term objective #2 focuses on maintaining the productivity of Interior Fraser Coho. After controlling for brood escapement, we found strong evidence for two distinct periods of productivity: a relatively high productivity period during 1978-1993 return years, and a low productivity period during 1994-2012. There is no evidence that Interior Fraser Coho have moved above the ‘low’ productivity’ regime that has persisted since 1994 (return year).
Modelled estimates of exploitation rates averaged 10% during 1998-2012. From 1986-1997, the average exploitation rate was nearly seven fold higher. The reliability of exploitation rate estimates for Interior Fraser Coho from 1998 onward is uncertain, owing primarily to uncertainty around the assumption that base period effort and exploitation rate (16-26 years ago) are representative of current effort and exploitation rate.
A model selection analysis indicated that the Ricker model best explained the stock-recruitment relationship for Interior Fraser Coho during the period of low productivity (1994-2012), and was used to form the basis of the harvest impact projections. Assuming low productivity, there is a strong trade-off between the probability of achieving the short-term objective of 20,000 spawners (3-year geometric mean) and exploitation rate. The probability of meeting the long-term objective of 40,000 spawners at low productivity is low regardless of exploitation rate. A declining population trend was predicted at exploitation rates exceeding 30%. Assuming a continuation of the 1994-2012 low productivity regime, there appears to be limited potential for recovering the MU to abundance levels higher than 20,000-40,000 spawners. A relatively low escapement target within this range (20,000 spawners) could maximize harvest opportunities for other stocks while still maintaining Interior Fraser Coho at a relatively productive point along the stock-recruitment curve. However, this represents a riskier management strategy compared to a strategy of maximizing escapement given current productivity.
This assessment must be considered as preliminary and not equivalent to a Wild Salmon Policy (WSP) status assessment since it relies primarily on the IFCRT recovery objectives and not formal WSP benchmarks.
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