Relative demand by double-crested cormorants and anglers for fish production from lakes on Manitoulin Island, Lake Huron

Abstract

The magnitude of angler harvest (kg·ha−1·yr−1) and cormorant consumption (kg·ha−1·yr−1) were compared for a set of lakes (N=11) on Manitoulin Island. Empirical models relating total phosphorus to total fish production as well as production to body mass were used to scope the possible range of fish production and to partition production among small, medium and large size segments of fish populations, respectively. Medium (66–112g) and large (>200g) size segments were defined as size categories targeted by cormorants (stomach diet analysis) and anglers (creel interviews), respectively. Angling effort and cormorant density were estimated from aerial surveys of the lake set during the open water season and for anglers during the winter ice-fishing season. Results showed that anglers harvested almost all large fish production, assuming the mean total fish production model, and 43% of large fish production under the more optimistic upper 80% prediction limit of total fish production. Cormorant consumption of medium fish production was less (39% using mean regression model; 15% using upper 80% prediction model) than angler consumption of large fish production. Anglers therefore imposed more population stress on their preferred sizes of fish than cormorants imposed on their preferred sizes of fish. Population stress was increased when cormorant consumption of medium size fish was discounted from contributing to large fish production. Angler harvest near (or above) sustainable yield levels will be exacerbated and appear as a fish collapse when cormorants consume fish production destined for fish size segments preferred by anglers.