Research Document - 2000/128
Forecast for northern British Columbia coho salmon in 2000
By B. Holtby, B. Finnegan, and B. Spilsted
Abstract
This Research Paper documents forecasts of marine survival and abundance for the coho of northern British Columbia including the upper Skeena conservation area.
Marine survival:
In 2000, marine survival at the three northern indicators is expected to be above the mean of their respective periods of observation.
| Indicator | Model | Ŝ2000 | (50% CI) | Observed mean and period of observation (year of sea-entry) |
|---|---|---|---|---|
| Lachmach | sibling regression | 0.14 | (0.11-0.17) | 0.091 (1987-1998) |
| Toboggan Creek hatchery | from Lachmach | 0.05 | (0.03-0.08) | 0.029 (1987-1998) |
| Fort Babine hatchery | from Lachmach | 0.03 | (0.02-0.06) | 0.024 (1993-1998) |
The period of observation is short for all three indicators. The forecast for Fort Babine is poorly defined. 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 2.3x103, which is below the mean of 2.8x103 observed over the period 1988 to 1999. Smolt production from Toboggan Creek in 1999 was estimated to have been 44x103, indicating high fresh water (FW) survival. Assuming that wild survival will be 1.2-times that of the forecast survival for hatchery coho (the same expansion observed for the 1999 return) the total return of wild Toboggan coho could be 2.7x103, or 56% of the mean total return of 4.8x103 observed between 1988 and 1999.
After the application of stock-recruitment and time-series models to reconstructions of abundance in 11 aggregate stocks in north coastal British Columbia, the following forecasts of abundance in 2000 were made.
| Aggregate | Proportions of observed abundance and escapement less than forecasts‡ Abundance |
Proportions of observed abundance and escapement less than forecasts‡ Escapement |
Characterization of Forecast Abundance |
|---|---|---|---|
| Area 6 | 0.07 | 0.16 | well below average |
| Babine | 0.09 | 0.22 | well below average |
| Area 4-U | 0.10 | 0.23 | well below average |
| Area 2E | 0.16 | 0.31 | below average |
| Area 7 | 0.22 | 0.43 | below average |
| Area 2W | 0.23 | 0.39 | below average |
| Area 4-L | 0.23 | 0.63 | below average |
| Area 5 | 0.31 | 0.64 | below average |
| Area 3 | 0.33 | 0.77 | below average |
| Area 8 | 0.33 | 0.76 | below average |
| Area 1 | 0.51 | 0.81 | average |
‡ Assuming a log-normal cumulative probability distribution with mean and standard deviation calculated over the observation period 1950 (1946 for Babine) to 1999 (return years).
Impacts of incremental Canadian fisheries
In the context of a simple approach to determining the status of coho within large geographic aggregates relative to two potential Reference Points (RP), the aggregates of Area 2W and Area 6 appear unable to support incremental Canadian fisheries. There are potentially some limited incidental fishing opportunities available on coho from the upper Skeena that would have limited risk of irreversible damage. However, forecasts for this area have not proven sufficiently reliable to proceed with incremental fishing, even if modest, without an early in-season indicator that would warn of unforeseen survival disasters such as the one that occurred in 1996 sea-entry. Such a system has been developed for the 2000 fishing season (Holtby 2000). We also note that the properties of the approach presented here for the estimation of RPs have not been explored in the context of aggregates represented by the average-stream index.
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