Research Document - 2000/127
Forecast for southern British Columbia coho salmon in 2000
By L.B. Holtby, K. Simpson, R.W. Tanasichuk, and J.R. Irvine
Abstract
This research paper documents forecasts of marine survival, abundance and distribution for the coho salmon of southern British Columbia (interior Fraser including the Thompson River, lower Fraser, Strait of Georgia, and West Vancouver Island) for return year 2000.
Marine survival:
Recommendations for the marine survival forecast for the five hatchery indicators and one wild coho indicator are given in the following Table. For populations around the Strait of Georgia and in the Fraser River, survivals are forecast to be either unchanged or higher in 2000 compared to those observed in 1999. Survival will remain poor throughout southern BC and survival is forecast to improve at Black Creek only because survivals were particularly poor there in 1999. In 1999, the sibling-regression models generally performed better than the statistical models so both models have been tabulated in the following Table. The two forecasts are similar only for the Big Qualicum hatchery population and there is no apparent geographic pattern to the forecast survivals. Two survival forecasts are presented for Robertson Creek coho on the west coast of Vancouver Island. The sibling regression forecast is similar to forecasts made over the past two years, which have tended to be too optimistic. The euphausiid model outperformed the sibling model in 1999 and provides a forecast for 2000 that is considerably lower than the forecast from the sibling regression. Survival of West Vancouver Island (wVI) coho might have been well below average for fish entering the ocean in 1999.
| Indicator | Best model | Ŝ2000 | (50% CI) | change relative to observed survival for 1999 return |
Ŝ2000 (sibling) | (50% CI) |
|---|---|---|---|---|---|---|
| Big Qualicum | LLY 1 | 0.015 | (0.006-0.04) | same | 0.012 | (0.007-0.021) |
| Quinsam | LLY | 0.01 | (0.006-0.016) | same | 0.012 | (0.007-0.021) |
| Chilliwack | RAT32 | 0.014 | (0.008-0.025) | same | 0.008 | (0.024-0.066) |
| Itch Creek | LLY | 0.019 | (0.009-0.040) | same | 0.040 | (0.024-0.066) |
| Black Creek | 3YRA3 | 0.033§ | (0.024-0.046) | higher | - | - |
| Robertson Creek | sibling regression | 0.033§ | (0.023-0.046) | higher | - | - |
| Robertson Creek | euphausiid | 0.019 | (0.015-0.023) | lower | - | - |
1 LLY - Like Last Year
2 RAT3 - Average 3-year trend
3 YRA - 3-year average
§ The similarity of the forecasts for Black Creek and Robertson Creek is coincidental and is not a typographic error.
Abundance forecast:
Without fisheries information, forecasting abundance is highly problematic, and because we are using time-series models, the forecast is dependent on the highly uncertain estimates of abundance in 1998 and 1999. Although the observed abundance of the Strait of Georgia-Fraser River (StG-Fr) aggregate in 1999 (3.3 105) was well above the forecast (2.0´ 105; 50% CI: 1.5´ 105-2.8´ 105), the RAT3 model continues to be the best performing model. The RAT3 model forecast of the abundance of the StG-Fr aggregate in 2000 is 2.5´ 105 (50% CI: 1.8´ 105-3.4´ 105) or 15% of the long term average abundance of 1.6´ 106.
The estimated abundance of the wVI aggregate in 1999 (2.6´105) was considerably less than the forecast abundance (4.5´ 105; 50% CI: 3.1´ 105-6.5´ 105). The estimate of abundance is consistent with preliminary escapement records, which indicate that there were declines in abundance in 1999 compared to 1998 (-29% for swVI streams and -49% for nwVI streams relative to 1998 (KS, unpublished and preliminary data). The 3YRA forecast for wVI abundance in 2000 is 2.7´ 105 (50% CI: 2.0´ 105-3.7´ 105) or 48% of the overall average abundance of 5.7´ 105.
The estimated total abundance of interior Fraser coho in 1999 was 2.1´ 104 or 62% of the forecast abundance. The abundance forecast for interior Fraser coho for 2000 is 2.2´104, or 20% of the long term mean abundance. Thus, the forecast is for continued depression with little change from the last three years. Brood year escapements in the Lower and South Thompson were respectively the lowest and second lowest on record since records began. Since there is no indication of improved marine survival, continuation of poor escapement is likely in those areas and it is unlikely that total stock size will increase in 2000.
Distribution forecast:
In the hypothetical circumstance of historical patterns of fishing, the predicted proportion of catch inside the Strait of Georgia (pinside) would be 0.31 (50%CI 0.21-0.44), which can be characterized as a moderately strong outside distribution. The confidence interval suggests that an extreme outside year (pinside < 0.2) is less likely than a return to a "normal" distribution (pinside>0.4). This forecast of distribution is based on incomplete salinity data.
Accessibility Notice
This document is available in PDF format. If the document is not accessible to you, please contact the Secretariat to obtain another appropriate format, such as regular print, large print, Braille or audio version.
- Date modified: