Nutrient Treatment of 3 Coastal Alaskan Lakes: Trophic Level Responses and Sockeye Salmon Production Trends

Abstract

Three coastal oligotrophic lakes with variable interannual production of sockeye salmon Oncorhynchus nerka were treated with additions of fertilizer to increase zooplankton biomass for the purpose of producing more numerous and larger smolts. Nutrient additions increased primary production (chlorophyll-a) by as much as 500% and resulted in a sustained higher level of zooplankton biomass than before treatment. Zooplankton biomass increased from 40% to 700% after nutrient treatment in the 3 lakes. For the 2 lakes in which data were available, sockeye smolt biomass increased from 50% to 250%, and in the third lake the mean weight of age-1 smolts increased by 35%. For all 3 lakes subsequent adult sockeye production increased. The phosphorus loading rates for these lakes exceeded that of treated coastal lakes in Canada by 2–5 times; however, the oligotrophic status of these lakes was not altered. The sustained high production of zooplankton and the consistent production of larger or more abundant smolts indicated that the higher phosphorus loading rates for these Alaskan lakes were advantageous to rearing sockeye salmon fry. Alaska Fishery Research Bulletin 1(2):153–167. 1994. Copyright © 1994 by the Alaska Department of Fish and Game 153 Author: GARY B. KYLE is a regional limnologist with the Alaska Department of Fish and Game, Commercial Fisheries Management and Development Division, 34828 Kalifornsky Beach Road, Suite B, Soldotna, Alaska 99669. Acknowledgments: Many fishery technicians and biologists — assisted with nutrient application and data collection. The Limnology Laboratory in Soldotna — analysis of water and zooplankton samples. Jeffery Koenings — reviewed an earlier version of this manuscript. Project Sponsorship: This investigation was partially funded by the Kodiak and Cook Inlet Regional Aquaculture Associations and the Lower Cook Inlet Seiners Association. juvenile sockeye production is not an exclusive function of spawner density, and that a high-quality rearing environment improves juvenile sockeye growth and survival. The goal of Alaska’s lake enrichment program is to increase zooplankton biomass without negatively altering the zooplankton species composition or changing the lake’s oligotrophic state (Kyle et al. in press). This goal is based on the assumption that lake fertilization will lead to a higher-quality rearing environment for planktivorous juvenile sockeye salmon, and ultimately increase smolt production. In this paper I present the results from 3 coastal Alaskan lakes treated with nutrient additions and examine whether (1) nutrient treatment simultaneously increased zooplankton biomass and sockeye smolt production (size/number), and (2) adding nutrients at phosphorus loading rates 2–5 times greater than those used in Canadian coastal lakes increased sockeye smolt production without adversely altering the lake’s trophic state. Of the 10 lakes fertilized in southcentral Alaska (Kyle et al. in press), Frazer, Leisure, and Packers Lakes were selected for this paper because they were the only lakes that provided concurrent data for all trophic levels, as needed to examine the above-stated objectives. These 3 lakes also experienced interannual INTRODUCTION Addition of nutrients to oligotrophic lakes in Canada stimulated productivity of lower trophic levels, which in turn, improved the growth and survival of juvenile sockeye salmon Oncorhynchus nerka and produced larger adult returns (LeBrasseur et al. 1978; Hyatt and Stockner 1985; Stockner and Shortreed 1985). The treated Pacific-coast lakes of Canada experienced smaller increases in zooplankton biomass than Canada’s interior lakes and lower-than-expected production of adult sockeye salmon (Stockner 1987). The Canadian lakes were fertilized at phosphorus (P) loading rates of 3–5 mg P·m·week for 18–20 weeks (Stockner 1987). The Alaska Department of Fish and Game has investigated trophic responses to nutrient additions, salmon fry stocking, adult salmon escapements, and environmental factors in a variety of lakes in Alaska (Koenings and Burkett 1987; Kyle et al. 1988; Koenings et al. 1990; Koenings and Kyle in press; Koenings and Edmundson 1991; Edmundson et al. 1993; Kyle et al. in press; Kyle 1994). These investigations revealed a basic tenet of rearing efficiency of sockeye salmon in nursery lakes: that