Research Document - 2013/121
Pre-COSEWIC Assessment of Interior Fraser Coho Salmon (Oncorhynchus kisutch)
By A.S. Decker and J.R. Irvine
Abstract
Genetically distinct Interior Fraser Coho Salmon (Oncorhynchus kisutch) were recognized as a Designatable Unit (DU) when their status was assessed by COSEWIC in 2002. Genetic and ecological studies revealed geographically based population structure within the Interior Fraser Coho aggregate, resulting in five Conservation Units (CUs) under Canada’s Wild Salmon Policy (North, South, and Lower Thompson, Middle/Upper Fraser, and Fraser Canyon). We provide updated time series of data for escapement, total return, exploitation, productivity, size and fecundity, and distribution and summarize this information to facilitate a reassessment of the status of Interior Fraser Coho using primarily COSEWIC-based quantitative criteria. When possible, we present information for individual CUs as well. We examine major threats to Interior Fraser Coho, including threats not identified in the 2002 COSEWIC assessment. We also review previous status assessments and recovery plans.
Escapement of wild Coho Salmon to the interior Fraser River watershed declined from an average of 60,000 spawners during 1975-1988, to a low of 9,000 spawners in 1996. The most recent generational average (2009-2011, geometric mean) was 27,000. Recent generational average escapements for CUs ranged from 2,200 (Fraser Canyon) to 8,800 (North Thompson). The estimated decline in escapement for the Interior Fraser Coho aggregate for the entire time series (1975-2011) was 72%. Annual escapements to the interior Fraser River watershed have varied 8‑fold during the most recent 10 years, but without any clear trend (estimated 8% decline). Rates of change in escapement for individual CUs were similar to those for the aggregate, with the exception of the Lower Thompson CU that increased 72% in escapement during the last 10 years, and the Fraser Canyon CU that decreased 58%. Limited data for several enhanced streams suggest size and fecundity of Interior Fraser Coho decreased in the 1990s, concurrent with decreasing escapement, and then increased during the 2000s. Estimated extent of occurrence (EO) was 110,000 km² for the Interior Fraser Coho aggregate, and ranged from 110 km² (Fraser Canyon) to 76,100 km² (Middle/Upper Fraser) for individual CUs. Area of occupancy (AO) for the Fraser Canyon CU was 32 km² and 14 km², based on 2x2 km and a 1x1 km grids, respectively. During the most recent three years, Coho Salmon were detected at 75 locations (streams) in the interior Fraser River watershed, and at one location to 30 locations within individual CUs. During 1998-2011, there was either no trend (North Thompson and Lower Thompson CUs), or a weak positive trend (South Thompson and Middle/Upper Fraser CUs and Interior Fraser Coho aggregate) in the number of locations where Coho Salmon were detected annually. The proportion of surveyed streams with Coho Salmon detected was positively correlated with escapement at both the aggregate and CU levels, with the exception of the Lower Thompson CU.
Declines in escapements and total returns of Interior Fraser Coho during the 1990s were primarily the result of declining smolt-adult survivals exacerbated by overfishing. An abrupt decrease in productivity (recruits per spawner) coincided approximately with the 1989-1990 shift in marine conditions in the North Pacific; average recruits/spawner decreased from 3.1 during 1975-1990 (return years) to 1.3 during 1991-2011. Fishing restrictions introduced in 1998 reduced average exploitation from 66% to 10%. During the most recent 10 years, productivity of the Interior Fraser Coho aggregate has been highly variable (0.4‑3.2 recruits per spawner), but has remained low relative to the pre-1991 period as a result of continuing poor smolt-adult survival; the aggregate has been below replacement (<1 recruit/spawner, or negative population growth) four of the last 10 years. Since the 2002 COSEWIC assessment, fishing, habitat perturbations, and climate change remain the most important threats to the long-term viability of the Interior Fraser Coho aggregate. Alien invasive species may be an emerging threat. The single watershed Fraser Canyon CU is particularly vulnerable due to its highly restricted EO and AO, small population size, and 58% decline in escapement during the last 10 years.
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