Research Document - 1999/140
Biological Assessment of Skeena River coho salmon.
By B. Holtby, B. Finnegan, D. Chen and D. Peacock
Abstract
Marine survivals were higher in the 1997-sea entry than they had been in the 1996-entry year. The increase relative to the previous year was largest for Toboggan wild and hatchery smolts but was slight for Fort Babine smolts. Survival was average for Lachmach coho but was below average for both Toboggan hatchery and wild smolts. Survivals remain below levels required for sustaining populations at the two Skeena hatcheries but there is growing evidence that hatchery survivals are less than 30% of wild survivals. Exploitation rates ranged between 28% for Toboggan coho to 60% for Babine coho. Exploitation in Canadian fisheries was due entirely to small incidental catches in some FW fisheries and to release mortality in all marine fisheries and the remaining FW fisheries. The total exploitation rate on upper Skeena coho in all Canadian fisheries was reduced to less than 2%. Exploitation rates in Alaskan fisheries remained largely unchanged from recent years.
Juvenile densities in 1998 provided a complex picture of 1997 escapement. Juvenile densities were lower in 1998 than in 1997 in five of eight summary areas. The largest decrease was seen in the upper Skeena (0.12´ ). No juvenile coho were detected in the Sustut River sites despite an expanded search very few juveniles and no young of the year were detected in the upper Bulkley. Decreases in the middle Skeena areas and the Bulkley/Morice ranged from 0.59´ to 0.68´ . Large increases were seen in the Lachmach (1.7´ ) and the coastal streams (1.8´ ). Juvenile densities also increased in the Babine by a factor of 1.3´ . This increase was general throughout the Babine. Despite the increase juvenile densities remained well below levels we would interpret as indicating an adequately seeded system.
Escapement was much improved in 1998 compared to 1997 throughout the Skeena Basin. The test-fishery index to August 25th was about the 29th percentile in a 43-year time series. The index value was similar to values seen in the 1980's. However, the value is consistent with a simple transfer of catch to escapement. Escapement to the Babine was 4,291 or over 9-times the escapement in 1997. Compared to historic escapement the value in 1998 was at the 34th percentile, which is significantly less than the median and is again comparable to escapement in the 1980's and early 1990's. However, total stock size was lower than the brood years and did not represent a departure from the downward trend in stock size that began sometime in the 1970's. Visual escapement indices increased relative to 1997 in all Statistical Areas except Area 5. The largest proportional increase was in the upper Skeena (13.9´ ) but only six streams were included in the index. More escapement work in the upper Skeena would be required to have increased confidence in the visual escapement index. Escapement to the Bulkley/Morice above Moricetown falls was 2.3´ 104 or 3.5-times the 1997 escapement. With only four observations but covering a very wide range of escapement, the Moricetown estimate is significantly correlated with the Skeena test-fishery index (unadjusted). Tagging at this site could potentially yield escapement estimates for Morice pink, Nanika/Morice sockeye and Bulkley/Morice chinook. The coho-tagging program should become a core assessment program. Escapement to the upper Bulkley increased from 88 in 1997 to 317, an increase of 3.6´ . However, escapement to the upper Bulkley remains less than 10% of historic averages. A new index site on the Sustut River was introduced in this report. Coho escapement there increased to 64 from 5 (all males) in 1997 but was only 46% of the escapement in the dominant brood year (1994). Historic data from this area is very unreliable but habitat measures suggest that current escapements are less than 10% of carrying capacity. Overall the escapement measures present a consistent picture of the status of Skeena Basin coho. Although escapement improved throughout the Basin, status remains very poor in the high interior and the upper Bulkley and well below carrying capacity throughout the interior. Coastal and middle Skeena areas appear to have recovered to average levels. The reappearance of fish in all areas is an encouraging sign that recovery is possible.
Very simple characterizations of average productivity for the Statistical Area aggregates and for the indicator streams confirm large productivity differences between interior streams (and Area 6) and streams in the lower and middle Skeena, Area 3 and SE Alaska. It is apparent that relative productivity is strongly related to population and aggregate status, as measured by two measures of status. This is compelling evidence that the root cause of declines in coho abundance in the Skeena interior is a chronic mismatch of exploitation rate and productivity.
A simple simulation of future population size for Babine coho indicated that recovery is contingent on both future survival and exploitation rates. With fishing levels similar to those in 1998 and a continuation of present survivals, slow recovery to escapement near carrying capacity is expected. With average fishing rates recovery is uncertain unless survivals improve substantially.
Finally a provisional escapement target to the Babine of 1.15´ 103 is suggested. At average survival the corresponding exploitation rate would be approximately 46%. A Limit Reference Escapement of 1.2´ 103 is also provided.
1Escapement should not be allowed to fall below this level.
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