Recently, there have been large discrepancies between the estimates of abundance derived from the Mission hydroacoustic surveys, and those made on the spawning grounds for some Fraser River sockeye stocks. Also recently, large proportions of Late-run Fraser River sockeye salmon have died in fresh water before reaching their spawning grounds. Early river entry has been associated with increased levels of pre-spawn mortality for Late-run sockeye stocks. A large scale radio-telemetry study was conducted to provide estimates of river entry timing, in-river survival, migration rates and the impact of fisheries on the survival of all run-timing groups of Fraser River sockeye salmon. Fishwheels were deployed in the lower Fraser River to provided Early Stuart sockeye for tagging and assess the near-shore species composition needed to convert hydroacoustic counts into species-specific estimates. In 2010, 728 sockeye were radio-tagged, including 67 at the Crescent Island fishwheels, 303 at the Juan de Fuca reef-nets, and 358 on the Johnstone Strait troll boat near Crescent Island. Each radio-tagged fish was also measured and spaghetti-tagged, and a small adipose tissue sample was taken for microsatellite stock identification. Radio-tagged fish were tracked using 28 fixed-station receivers in 27 locations along the Fraser River and within major tributaries. Fifty-two percent of the radio-tagged sockeye were detected at least once after release, and 46% were known to pass Mission. In all, 9% of tags were returned from marine and in-river fisheries, and 24% were tracked to the vicinity of stock-specific spawning areas. The majority (43%) of radio-tagged sockeye were identified to Late-run stocks.
‘After-harvest’ survival to spawning areas was significantly higher for Late-run sockeye than for all other run timing groups. Survival estimates derived from radio-telemetry data were compared with the Difference Between Estimates (DBEs) derived from the Mission and spawning ground estimates. The previous observed pattern, of telemetry based survival estimates being lower than those derived from the DBEs, continued in 2010. The telemetry data indicated that the survival rates were lowest for Early Stuart (53% ±14%) and Early Summer (46% ±12%) while DBEs suggested survival rates of 61% for both of these timing groups. The DBE for Summer-run stocks suggests a rather unbelievable survival rate of 108% compared to the radio-telemetry estimate of 71% ±12%. The DBE for Late-run Shuswap stocks (89%) was reasonable close to the comparable and most precise of the radio-telemetry estimates (84% ±6%). The distribution of the tags relative to the abundance measured at Mission was substantially better for the Early Stuart and Late-run components than for the other two run-timing groups. The poor representation of the latter half of the Early Summer and Summer-run migrations could have contributed to underestimating survival rates for these run-timing groups where in-river survival tends to increase as water temperature decline.
The proportion of Late-run sockeye that delayed river entry increased over time. No Thompson-bound Late-run sockeye that passed Mission before 30 August survived to a spawning location. The highest rate of en-route loss for sockeye was observed in the reach between Hell’s Gate and Kelly Creek, and for Thompson-bound stocks between Hell’s Gate and Ashcroft.
Two fishwheels were operated from late June to early October in a relatively fast-flowing section of the Fraser River near Crescent Island. All captured fish were identified to species, and species composition was calculated daily. The fishwheels caught 7,346 sockeye, 1,094 steelhead, 1,079 coho, 616 Chinook, 81 chum, and 5 pink salmon along with 14 other species. The 2010 sockeye return to the Fraser River was the largest since the 1913 Hells Gate slide and the total sockeye abundance estimated at Mission was the largest recorded at this site. This large abundance of sockeye dwarfed the numbers of other co-migrating species and thus sockeye usually represented over 85% of the salmon captured by the fishwheels and Whonnock gillnet test fishery. Consequently, these two test fisheries produced very similar stock composition estimates for most of the sockeye migration period.