Research Document - 2003/064

Forecast for Northern British Columbia coho salmon in 2003

By Sawada, J. , Holtby L. B. and Finnegan B.

Abstract

This research documents abundance trends and forecasts of marine survival for the coho of northern coastal British Columbia (Statistical Areas 1 to 6), including the upper Skeena conservation area.

Observed Marine Survival in 2002:

Due to problems with the MRP database, marine survival for 2002 could not be determined at time of publication.

Forecasted Marine survival for 2003:

In 2003, marine survival at the three northern indicators is expected to be above or equal to the means of their respective periods of observation.

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.

2003 Abundance forecast

Estimated smolt production from Lachmach in 2002 was 2.7x10⁴, which is below the observed mean of 3.1x10⁴ (1987 - 2000). That combined with above-mean marine survival produce a forecast return of 3.5x10³ (50%CI: 2.9x10³ - 4.2x10³) which is above mean (2.8x10³) return observed over the period 1988 to 2002 (return years). The forecast of abundance for wild Toboggan coho is 1.8x10³, which is considerably less than the mean total return of 4.7x10³ (return years 1988 - 2002). Assuming an exploitation rate of 16% (i.e., same as 2002), the wild escapement to Toboggan would be 1.5x10³, including terminal sport fisheries. That escapement is considerably below the mean of the available observations (2.1x10³; 1988 - 2002). Abundance of Ft. Babine hatchery coho is forecast to be 6.8x10² (50%CI: 4.3x10² - 1.1x10³). This return is below the mean of the time series (1.0x10³; 1994 to 2001). Assuming an exploitation rate of 0.21, the mean exploitation rate of Lachmach, Toboggan and Zolzap in 2002, escapement of Ft. Babine hatchery coho would be 5.3x10² .

The time series of abundance and the average-stream indices of the 6 north coastal aggregates show some indication of geographic patterning but do not indicate any conservation concerns in the area, with the possible exceptions of Area 4 upstream of the Babine confluence and Area 5. Escapement data are very poor in these Areas so it is difficult to determine the extent to which the poor escapements are due simply to limited data. The total abundance and the escapement of coho to the Lachmach River, Toboggan Creek and Ft. Babine hatchery will average to above-average in 2003. Without further investigation and a demonstration that status is actually better than indicated by the index used here, expansion of fisheries in the northern part of the coast should be discouraged.

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