Research Document - 2001/088
Forecast for northern British Columbia coho salmon in 2001
By L. Blair Holtby and Barry Finnegan
Abstract
This Research Document documents forecasts of marine survival and abundance for the coho of selected areas of northern British Columbia including the upper Skeena conservation area.
Marine survival
In 2001, marine survival at the three northern indicators is expected to be well above the mean for their respective periods of observation.
| Indicator | Model | (50% CI) | Observed meanand period of observation (year of sea-entry) |
|---|---|---|---|
| Lachmach | Sibling regression | 0.29 (0.23-0.35) | 0.098 1987-1999) |
| Toboggan Creek hatchery | From Lachmach | 0.09 (0.05-0.14) | 0.036 (1987-1999) |
| Fort Babine hatchery | From Lachmach | 0.08 (0.04-0.15) | 0.023 (1993-1999) |
The period of observation is short for all three indicators. The survival rate of wild Toboggan Creek coho should be comparable to Lachmach but cannot be reliably forecast.
Abundance forecast:
The forecast total return of Lachmach coho is 3.5x103 (50%CI: 2.8x103 - 4.2x103) which is above the mean of 2.7x103 observed over the period 1988 to 2000. Smolt production at Lachmach has remained modest. The estimated smolt production in 2000 was 1.4x104, which was well below the observed mean of 3.2x104 (1987 - 1999). Therefore, the above average forecast for Lachmach is due to the very high forecast survival rather than to large smolt production. In contrast, wild smolt production from Toboggan Creek in 2000 was estimated to have been 89x103, indicating high freshwater survival. Assuming that wild survival will be 1.7-times that of the forecast survival for hatchery coho (the same expansion observed for the 2000 return) the total return of wild Toboggan coho could be 13x103, which is well above the mean of 4.7x103 (1988 - 2000). Assuming that the exploitation rate in 2001 is unchanged from 2000 the forecast wild escapement to Toboggan is 8.1x103, which is well above the mean of 2.0x103 (1988 - 2000).
After the application of stock-recruitment and time-series models to reconstructions of abundance in the Babine Lake aggregate and the Area 6 average-stream, we conclude the following about abundance in 2001:
| Aggregate | Abundance | Escapement | Eharacterization of forecast abundance |
|---|---|---|---|
| Area 6 | 0.08 | 0.25 | Well below average |
| Babine | 0.33 | 0.49 | Average |
++Assuming a log-normal cumulative probability distribution with mean and standard deviation calculated over the observation period 1950 (1946 for Babine) to 2000 (return years).
Caution remains warranted in the conduct of northern BC fisheries that could impact coho because Area 6 coho continues to be depressed. There are also some indications that high-interior Skeena coho, specifically the Sustut, have not recovered, as has the Babine aggregate. However, with expectations of well above average survivals at the Area 3 indicator, similarly high survivals at the two upper Skeena hatchery indicators and strong forecast returns in all areas but Area 6, we conclude that the conservation concerns of the past decade are considerably reduced.
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