Research Document - 2000/113
Review of 1999 Terminal Run of Somass (Stamp) River Chinook Salmon and 1999 Escapement to WCVI Extensively Surveyed Indicators, and Forecast for 2000.
By B. Riddell, W. Luedke, J. Till, and R. Ferguson
Abstract
The detailed assessments and abundance forecasts of the Robertson Creek Hatchery (RCH) and Stamp River chinook are undertaken annually for management of ocean and terminal fisheries, and as an indicator of the expected returns to the naturally spawning chinook populations along the west coast of Vancouver Island (WCVI). Forecasts presented in this research paperdocument indicate a conservation concern developing for naturally spawning chinook populations along the WCVI during the next few years. The minimum escapement goal established for the Stamp/RCH chinook stock will not be met in 2000 and total terminal return for this stock is projected to decline by 66% relative to 1999. If this decline is assumed for the naturally spawning stocks along the WCVI, the numbers of females expected to spawn naturally in 2000 will vary from as low as 30 females to over a couple of hundred females in each river system.
1999 Terminal return of the WCVI chinook:
The 1999 terminal return of chinook to the Stamp River/RCH indicator stock was estimated to be 30,500 (a 57% decline from the 1998 return) (Table 4). Age-5 chinook comprised the majority (69%) of the spawning stock and returns of age-3 and age-4 chinook were much lower than average. The age 3 return is production from the 1996 brood, which was the lowest escapement on record.
Returns to another 22 WCVI streams that were monitored for chinook spawning escapements did not indicate a proportional reduction as large as the indicator stock but the age compositions were generally very similar. In all extensively surveyed systems along the WCVI (excluding the two rivers, Nitinat and Conuma, with major hatcheries), the 1999 total escapement declined 28% from 1998 levels. Escapements to the Nitinat and Conuma rivers each declined by about 40% from 1998. However, there was significant variation between systems with two rivers (San Juan River and Sarita River in southwest Vancouver Island) indicating greater declines than for the Stamp River/RCH indicator stock. On average, the age-5 chinook comprised 64% of the spawning populations along the WCVI, and age-3 and age-4 components were weak. The overall age composition was very similar to that in the Stamp River/RCH terminal run.
Forecast for the 2000 terminal return of the WCVI chinook:
The forecasting methods applied have been reviewed and accepted previously by PSARC. However, for the first time, the method could not be applied to one age-class since no coded-wire tags were recovered from age-2 chinook. Consequently, the forecast of age-3 chinook in 2000 is based on the lowest cohort size for age-3 chinook observed from past brood years (cohort sizes for 1983 and 1996 broods were about 4000 chinook).
For 2000, the forecasted total return of Stamp River/RCH chinook to the terminal area of Barkley Sound and Alberni Inlet is estimated to be 10,000 based on averaging the Prod2 forecast of 12,000 and the Prod3 forecast of 7,800. The mean absolute percent error in the average forecast (1985-1999 returns) is 10%. The age structure of the 2000 return is projected to be: 13% Age 3, 52% Age 4, and 35% Age 5; with an expected sex ratio of 50% females (note that the forecast of Age 3 is very uncertain). At this time, the forecast only assumes fishing mortality in South East Alaska (SEAK). Harvest rate factors in SEAK were based on the Pacific Salmon Treaty agreements and we initially used a harvest rate scalar of 0.5 in SEAK troll fishery. The remaining cohort is identified as the expected terminal run assuming no fishing mortality on this stock in Canada.
At this level of terminal run to the Stamp River/RCH, the indicator stock will not achieve the minimum target escapement goal established by PSARC in 1995. The forecast represents a further two-third (66%) reduction in terminal abundance relative to 1999 returns and would be the smallest return since 1985 (Fig. 7), when the indicator stock program began. However, given the expected sex ratio, the expected number of eggs available will be the fourth worst since 1985 (Table 5).
A slightly more conservative terminal run is predicted if the forecast is expressed as a cumulative probability distribution as previously requested by PSARC. Based on the annual deviations from forecasts observed between 1988 and 1999, the 50% value of the cumulative distribution is 9000 chinook in the terminal run and the 50% confidence interval is 8200 to 9800 chinook (Fig. 8). However, given that this distribution is based on only 12 years of observations, the authors recommend continuing with past methods and applying the average forecast model that predicts 10036 chinook returning to the terminal area of Barkley Sound.
The more serious concern for conservation is the expected run size to the naturally spawning chinook populations along the WCVI. While the relative decline from 1998 to 1999 was not as large in many of these populations, relative to the Stamp River/RCH stock, we are unable to make specific forecasts for these natural populations. Applying the expected decline for the Stamp River/RCH indicator stock to these naturally spawning populations provides a conservative expectation of their returns in 2000 (Table 6).
Returns to most of these streams in 2000 are not likely to constitute a serious conservation concern with the possible exception of returns to the Area 24 streams. However, these are returns projected for 2000 only and do not indicate the declines expected in future years. Marine survival rates for the 1995 through 1997 brood years from RCH indicate that production from WCVI populations is expected to be poor for a few years (likely through 2002).
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