Michigan Department of Natural Resources Fisheries Technical Report No. 2000-2, 2000

Evaluation of the Effects of the Herbicide Sonar on Sport Fish Populations in Michigan Lakes

James C. Schneider Michigan Department of Natural Resources Institute for Fisheries Research 212 Museums Annex Building Ann Arbor, Michigan 48109-1084

Abstract.–The effects of applications of Sonar A.S.® on populations of bluegill Lepomis macrochirus and other species of fish were monitored at nine Michigan lakes for up to six years. Monitored population attributes included trap net catch per effort of larger size fish, average length, size composition, growth, and year class strength. Two additional lakes were surveyed less intensively, and four unaltered lakes served as reference sites. Most Sonar treatments eliminated nearly all Eurasian milfoil and other macrophytes for approximately a year, but often the loss of fish cover was partially ameliorated by increases in Chara or Vallisneria. Modest responses occurred in most fish populations, many of which were statistically meaningful. From a fisheries perspective, all lake responses except one were improvements because all treatment lakes except one had a history of small-size, slow-growing, over-abundant bluegills. Bluegill size and growth characteristics significantly (P<0.25) improved for 29 comparisons, remained the same for 3 comparisons, and declined for 3 comparisons. The composite bluegill size score improved from poor to average at five lakes but remained poor at three severely stunted lakes. Bluegills as large as 7 inches total length increased in abundance, but bluegills as large as 8 inches remained sparse. Improvements in size of pumpkinseed Lepomis gibbosus and black crappie Pomoxis nigromaculatus were also noted for most lakes. Year class strength in the treatment year(s) may have been slightly reduced for a few lakes. Results demonstrated the resiliency of sport fish population dynamics over a broad range of habitat perturbations. Fish populations with normal or good growth and lower density and recruitment may not respond favorably to macrophyte alteration.

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