Research Document - 2002/062
Forecast for northern and central coastal British Columbia coho salmon in 2002
By L.B. Holtby, B. Finnegan, J. Gordon
Abstract
This Research paper documents forecasts of marine survival and abundance for the coho of northern and central coastal British Columbia (Statistical Areas 1 to 13), including the upper Skeena conservation area.
Marine survival:
In 2002, marine survival at the three northern indicators is expected to be below the means of their respective periods of observation.
| Indicator | Model | Ŝ2002 | (50% CI) | Observed mean | Observation (year of sea-entry) |
|---|---|---|---|---|---|
| Lachmach | Sibling regression | 0.075 | (0.06-0.09) | 0.10 | (1987 - 2000) |
| Toboggan Creek hatchery | From Lachmach | 0.025 | (0.016-0.04) | 0.039 | (1987 - 2000) |
| Fort Babine hatchery | From Lachmach | 0.011 | (0.007-0.02) | 0.025 | (1991 - 2000) |
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
Estimated smolt production from Lachmach in 2001 was 3.6x104, which is slightly above the observed mean of 3.1x104 (1987 - 2000). That combined with below-mean marine survival produce a forecast return of 2.7x103 (50%CI: 2.2x103 - 3.3x103) which is the mean return observed over the period 1988 to 2001 (return years). The forecast of abundance for wild Toboggan coho is 1.4x103, which is considerably less than the mean total return of 4.7x103 (return years 1988 - 2001). Assuming an exploitation rate of 36% (i.e., same as 2001), the wild escapement to Toboggan would be 8.7x102, including terminal sport fisheries. That escapement is considerably below the mean of the available observations (2.1x103; 1988 -2001). Abundance of Babine Lake coho is forecast to be 2.2x104 (50%CI: 1.7x104 - 3.0x104) using the preferred S-R model. This return is above the mean of the time series (1.2x104; 1946 to 2001). Assuming an exploitation rate of 0.55, escapement would be 9.4x103, which is approximately 78% of the provisional escapement target for the aggregate (1.2x104; Holtby et al. 1999b).
The stock-recruit and time series forecasts of abundance for Babine coho and the average-stream indices of the 12 north and central coastal aggregates show some indication of geographic patterning but do not indicate any conservation concerns in the area, with the possible exceptions of Areas 4C/5 and 13. Escapement data are very poor in Area 4C/5 so it is difficult to determine the extent to which the poor escapements are due simply to limited data. Escapement data are better in Area 13 and there are other indications, including fresh water juvenile surveys, that are consistent with the poor status indicated by the escapement index. The total abundance and the escapement of coho in the northern part of the forecast region (Areas 1, 3, 4L and 4U) will average to above-average in 2002. In the areas around Hecate Strait (Areas 2E, 4C/5, 6 to 12), total abundance will be well below average to average but provided fisheries do not expand much over levels in 2001, escapements will be average in most of those areas. Forecast abundance in Johnston Strait streams (Area 13) can be characterized as well below the mean. Without further investigation of this situation and a demonstration that status is actually better than indicated by the index used here, expansion of fisheries in the part of the coast should be discouraged.
Forecast characterizations for the aggregates considered. Probability values between 35% and 65% were characterized as average; probabilities less than 15% or greater than 85% were characterized as either well below or well above average respectively. We have arranged the aggregates into six geographical groups based on geography, distributions of CWT's in fisheries and on productivity (Holtby et al. 1999b). This is a convenient way to summarize the forecasts because forecasts of abundance and escapement for average stream indices are useful only in the context of how far they deviate from the long-term means of their respective time series.
| Aggregate | Group | Model | Forecast P† | Charecterization |
|---|---|---|---|---|
| Area 2W | 1 | 3YRA | 35% | average |
| Area 1 | 2 | 3YRA | 38% | average |
| Area 3 | 2 | S-R | 71% | above average |
| Area 4L | 3 | S-R | 54% | average |
| Area 4U | 3 | S-R | 73% | above average |
| Babine Lake aggregate |
3 | S-R | 63% | above average |
| Area 2E | 4 | 3YRA | 11% | well below average |
| Area 4C/Area 5 | 4 | LLY | 5% | well below average |
| Area 6 | 4 | 3YRA | 23% | below average |
| Area 7 | 4 | 3YRA | 29% | below average |
| Area 8 | 5 | 3YRA | 11% | well below average |
| Area 9/12 | 5 | 3YRA | 38% | average |
| Area 13 | 6 | 3YRA | 4% | well below average |
| Aggregate | Group | Model | Forecast P | Charecterization | % of Smax |
|---|---|---|---|---|---|
| Area 2W | 1 | 3YRA | 56% | average | 43% |
| Area 1 | 2 | 3YRA | 70% | above average | 97% |
| Area 3 | 2 | S-R | 90% | well above average | 96% |
| Area 4L | 3 | S-R | 90% | well above average | 111% |
| Area 4U | 3 | S-R | 85% | well above average | 125% |
| Babine Lake aggregate |
3 | S-R | 75% | above average | 85% |
| Area 2E | 4 | 3YRA | 46% | average | 31% |
| Area 4C/Area 5 | 4 | LLY | 25% | below average | 40% |
| Area 6 | 4 | 3YRA | 55% | average | 36% |
| Area 7 | 4 | 3YRA | 62% | average | 65% |
| Area 8 | 5 | 3YRA | 43% | average | 43% |
| Area 9/12 | 5 | 3YRA | 78% | above average | 55% |
| Area 13 | 6 | 3YRA | 28% | below average | 13% |
† Proportions of observed abundance or escapement less than the forecast value. These calculations assume a log-normal cumulative probability distribution with mean and standard deviation calculated over the observation period 1950 (1946 for Babine) to 2001 (return years).
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