Estimates of fishing and natural mortality rates for four Lake Whitefish stocks in Northern Lakes Huron and Michigan

Abstract

We analyzed tag-recovery data to estimate instantaneous fishing ( F) and natural mortality ( M) rates of four lake whitefish stocks in lakes Michigan and Huron during 2004–2007. We tagged and released 22,452 adult lake whitefish of which 8.7% were subsequently recovered. Annual tag-reporting rates ranged from 17.8% to 56.2%. Tag retention was high for the first 5–6 months after tagging, but tag loss increased substantially thereafter. Nine tag-recovery models were evaluated with respect to whether F and/or M varied among stocks, lakes, or years. There was support for three models based on Akaike information criteria. The best model had yearly and stock-specific estimates of F of 0.03 to 0.79 and lake-specific estimates of M of 0.35 for Lake Michigan and 0.60 for Lake Huron. The second best model had yearly and stock-specific estimates of F of 0.04 to 0.71 and a constant estimate for M of 0.52. The third model had yearly and stock-specific estimates of F of 0.04 to 0.85 and stock-specific estimates of M of 0.32 to 0.67. Model-averaged estimates of F ranged from 0.04 to 0.78 and were substantially different than statistical catch-at-age estimates of F. Model-averaged estimates of M ranged from 0.40 to 0.59 and were greater than estimates obtained from prediction equations, possibly due to sea lamprey-induced mortality. We recommend that tag-recovery estimates of F and M be used as Bayesian priors in future lake whitefish stock assessments to help refine mortality estimates for the stocks.