An evaluation of restoration efforts in fishless lakes stocked with exotic trout, Mt. Rainier National Park, Washington, USA
Only about 5% of the approximately 16,000 montane lakes in the western USA naturally contained fish, but now most are stocked with sport fishes. Several studies have shown detrimental effects of stocking, especially on native amphibian populations, which prompted removal of introduced cutthroat Oncorhynchus clarki, rainbow Oncorhynchus mykiss and brook trout Salvelinus fontinalis from naturally fishless lakes at Mt. Rainier National Park, northwest USA. In this study, using paleolimnological indicators, the effects of stocking eight naturally fishless lakes and whether they had returned to prestocking conditions after fish removal (restoration) was assessed.
Study site: The study was undertaken in Mt. Rainier National Park Washington, USA. Eight high-elevation lakes were selected: three contained trout; three were previously stocked but had had fish removed; and two were unstocked. Measurements of transparency, dissolved oxygen, conductivity, pH and nutrient concentrations, made between 1989 and 1998, were similar among the lakes.
Field and laboratory procedures: Sediment cores were collected in the summers of 1996 and 1997; rates of sediment accumulation were determined. Evidence from diatoms, invertebrates and loss-on-ignition were used to reconstruct the ecological histories of the lakes.
Diatoms and invertebrate remains were prepared in the laboratory for taxonomic analysis and examined under a microscope to determine identification.
Diatom floras: Analysis of the core samples revealed that the diatom floras were relatively stable between 315 and 90 years before present in all lakes. A dramatic change in diatom flora occurred approximately 80 years ago coinciding with fish introduction in four of five stocked lakes, whereas the floras in two unstocked lakes had not changed significantly over the last 315 years. Diatoms were not preserved in an eighth lake, thus precluding analysis.
Invertebrates: Preserved invertebrate densities fluctuated dramatically over time in all cores, and were a poor reference for assessing the effects of fish stocking
Conclusions: Diatom communities have not returned to assemblages present prior to fish stocking in restored lakes, even 20–30 years after fish removal. The authors conclude that Mt. Rainier lakes have not been successfully restored by fish removal.
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