Research Document - 2000/158
tatus in 1999 of Coho Stocks Adjacent to the Strait of Georgia.
By K. Simpson, R. Semple, D. Dobson, J. Irvine, S. Lehmann, and S. Baillie
Abstract
Escapements of 1996 brood coho were poor relative to 1998 and 10-year averages in areas of the Georgia Basin other than the lower Fraser. Compared to ten year averages, one lower Fraser wild indicator, Salmon River, was very poor and another, Upper Pitt, quite good. 1999 escapements were better than 1998 escapements in this area with the notable exception of the Salmon River indicator stock. In terms of the provisional limit reference point of 3 females/km, virtually all enumerated stocks in the Basin were above the limit. Escapements were the result of poor escapements in 1996 and poor marine survival. Exploitation due to release mortalities and catch in Alaska was 13.1% for ECVI and lower Fraser coho. Most of the exploitation was from release mortalities in BC, as estimated using DNA estimates of stock composition and estimates of regional escapements. If accurate, and our confidence in the escapement estimates is low, these values approximate the exploitation of wild coho. Extremely low marine survival is the driving short-term cause of poor abundances. A slight increase in 1999 everywhere except in the northern Strait provides some hope that the decline has stopped.
Based on smolt estimates at Black Creek and Salmon River and using fry densities and sizes, the 1997 brood smolt runs were probably below average in 1999 and possibly well below average on the Sunshine Coast. With marine survivals forecast to remain poor, we expect escapements in 2000 to be well below 1990's averages, similar to 1999 except in the Fraser Valley where escapements were not as depressed in 1999. Nevertheless, assuming continued near-abatement of exploitation, most monitored stocks will probably exceed the provisional limit reference point of three females per kilometre of stream as they did in 1999.
Considering the current low productivity of Georgia Basin coho, we recommend that fishing mortality remain similar to existing minimal levels in order to ensure that there is a sufficient proportion of escapements that exceed the provisional limit reference point.
The abundance of 1998 brood smolts this spring will probably be better than the 10-year average everywhere except possibly on the Sunshine Coast. Excluding this part of the Basin, fry densities were above average in 1999 in response to average to better than average escapements. Their sizes were probably sufficient to provide average winter survival with some regional variation. With inadequate data coverage, we think fry abundances were probably poor on the Sunshine Coast and winter survival, as inferred from fry size, was likely average despite lower densities. Smolt runs may be poor in this area but sample sizes were too small to conclude this with any confidence.
Fry densities at both the individual stream level and summarised over the Basin are correlated with parental escapements throughout the 1990's, which is the period of the fry survey. Fry surveys are an economical and effective way to determine trends (at least) in escapements when escapements are low to moderate. Continuing in a tactical vein, a 'full' indicator facility is needed on the Sunshine Coast where juveniles are enumerated and tagged and adults are accurately counted and sampled. Another is required in the Fraser Valley. The existing indicator of Salmon River and the escapement indicator of Upper Pitt have different escapement trends and the area requires another full indicator facility.
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