Vulnerability, Stability, and Coherence of the Fish Community in Lake Superior

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Abstract
Vulnerability analysis and spectrum analysis were found useful in examining potential structural changes in fisheries systems influenced by large perturbations. In the 1950's the Ontario fisheries of Lake Superior experienced a major perturbation due to invasion by sea lamprey (Petromyzon marinus). Vulnerability analysis indicated that invasion by sea lamprey and the consequent shifting of the fisheries to more intensive fishing on lower trophic level species resulted in higher vulnerability of the predatory fish species; i.e. likelihood of extinction increased. Spectrum analysis was then applied to the yield series of five fish species from Lake Superior. Analysis of the data before and after invasion by sea lamprey indicated major structural changes in the fishery: (1) except for lake trout (Salvelinus namaycush), either the dominant amplitude, the dominant frequency, or both decreased; (2) partial coherencies between pairs of yield series changed after the invasion; (3) lake whitefish (Coregonus clupeaformis) and lake trout replaced lake herring (C. artedii) as the species whose fluctuations in commercial yield were most highly synchronized with those of the other species of commercial importance.