Water quality monitoring and fertilizer application during the restoration of sockeye salmon at Coghill Lake, Alaska 1986-1996

Study History: Following the 1989 Exxon Valdez oil spill off Bligh Reef in Prince William Sound, Coghill Lake was selected as a system for sockeye salmon restoration to replace fishery stocks damaged by the oil spill. Edmundson et al. (1992) suggested that a decrease in macrozooplankton from excessive foraging by high densities of rearing fry produced from large escapements caused the decline in returns of Coghill Lake sockeye salmon. In 1993, a planned 5-year nutrient enrichment and evaluation continued in 1994 (Restoration Project 94259), 1995 (Restoration Project 95259) and 1996 (Restoration Project 96259). The fifth year of nutrient enrichment (1997) was not funded and this report summarizes results from the 4 years of treatment.

Abstract: Prior to the recent run decline, Coghill Lake consistently produced the highest runs of sockeye salmon Oncorhynchus nerka in Prince William Sound. Although limnological and juvenile fisheries data are not available before the run decline, it is believed that sockeye salmon fry produced from consecutive years of high escapements overgrazed the forage base (zooplankton) and reduced the lake's rearing capacity. In 1993, the Exxon Valdez Oil Spill Trustee Council approved a stock restoration project (nutrient enrichment) for Coghill Lake as mitigation for damaged fishery resources. During 4 years of nutrient enrichment (1993 to 1996), the seasonal mean phosphorous concentration increased 220 percent, and the standing stock (density) of the prevalent zooplankton (Cyclops) increased 117 percent compared to pre-enrichment years. The sockeye salmon smolt population abundance averaged 263,604 before enrichment compared to 940,411 during treatment. This increased smolt production is attributed to increases in primary and secondary production because the number of smolts produced per spawner also increased during enrichment. The trophic level responses to nutrient enrichment in this meromictic, glacial were similar to those observed for other holomictic and meromictic (clear-water) lakes treated in Alaska.

Publications:
Edmundson, J.A., G.B. Kyle, S.R. Carlson, and P.A. Shields. 1997. Trophic-level responses to nutrient treatment of meromictic and glacially influenced Coghill Lake, Exxon Vuldez Oil Spill Restoration Project Final Report (Restoration Project 96259), Alaska Department of Fish and Game, Division of Commercial Fisheries, Soldotna, Alaska.