Vulnerability to Harvest By Anglers Differs Across Climate, Productivity and Diversity Clines: Or Why Are Northern Fisheries More Vulnerable to Overharvest?

We contrast catchability of walleye and northern pike populations to angling fisheries across regions that differ two-fold in growing-degree-days and productivity and 6-fold in fish diversity. Populations of both species in Alberta, Wisconsin, Minnesota and Oneida Lake, NY had density-dependent catchability with approximately ten-fold higher catchability in Alberta than in the other regions. There is no evidence that the higher catchability estimates for Alberta walleye and northern pike are due to differential spatial distributions, enhanced hook avoidance due to catch and release or to differential size structure of the populations or differences in harvest regulations. We argue that the most likely explanation for the 10-fold higher catchability is increased hunger resulting in enhanced foraging activity in the region with a substantially shorter growing season, lower prey productivity and lower prey community diversity. Regardless of the proximate causes, higher catchability of fish harvested in recreational fisheries in Alberta substantially increases their vulnerability to over-harvest and collapse if angling effort is unabated.