Abstract
I constructed an improved index for measuring the frequency of wounds in prey fish stocks caused by sea lampreys Petromyzon marinus in the Great Lakes. Previous indices did not make use of all of the available data and did not correct for sampling and observational errors. I developed two statistical models and fitted them to sea lamprey wounding data for lake trout Salvelinus namaycush in Lake Superior and lake whitefish Coregonus clupeaformis in Lake Huron. The models were useful for tracking seasonal variation in wounding frequencies and provided estimates of the sampling variance for one index (proportion of wounded fish) used to measure wounding frequencies on Lake Superior lake trout. Seasonal patterns in wounding differed between species and between locations for the same species. The estimated sampling variance for the proportion of wounded lake trout was about three times higher than could be expected on theoretical grounds. The excess variance suggests that sources of “extra” observational error may contribute substantial components to the variability of wounding data or that the data were obtained from a heterogeneous mixture of stocks. It may be necessary to either increase the size of samples or to collect and tabulate data on the basis of smaller statistical zones that coincide with the geographic distribution of individual stocks. The precision of measurement achieved with an approach like the one used for this analysis could be increased by distributing sampling effort so that samples were collected near the middle as well as at the beginning and end of each annual wounding cycle.
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