The Role of Lake Trout as a Keystone Species in the Recovery of Native Fish Community of Lake Superior

The Lake Superior fish community underwent massive changes in the second half of the 20^th century as exhibited by large cycles in abundance of principal prey species, the ciscoes (Coregonus spp.) and the invasive rainbow smelt Osmerus mordax, and the principal predator, lake trout Salvelinus namaycush.  In the late 1950s – early 1960s, smelt abundance was at its maximum, wild lake trout was at its minimum, and abundance of hatchery lake trout was increasing rapidly.  The bloater Coregonus hoyi was the prevalent cisco in the lake; abundance was more than three-times greater than the next most abundant cisco, shortjaw cisco C. zenithicus, followed by kiyi C. kiyi, and cisco C. artedi.  By the mid-1960s, abundance of hatchery lake trout was nearing maximum, smelt abundance was just beginning to decline, and abundances of all ciscoes declined but especially that of shortjaw cisco and kiyi.  Even though levels of the principal predator (lake trout) had been low since the late 1950s, all cisco species appeared to be in a state of chronic recruitment failure.  By the late 1970s, recovery of wild lake trout stocks was well underway and abundances of hatchery lake trout and smelt were declining and the ciscoes were reaching their nadir.  During 1980-1990, the fish community underwent a dramatic shift in organization and structure.  The rapid increase in abundance of wild lake trout concurrent with a rapid decline in hatchery lake trout signaled an impending recovery.  Abundance of rainbow smelt dropped precipitously and within four years the ciscoes rebounded on the heels of a series of strong recruitment events.  High abundance of adult smelt prior to 1980 appears to be the likely factor linked to recruitment failure in the ciscoes.  The recovery of lake trout in Lake Superior reestablished this species as a keystone species exerting a strong top-down influence on the entire fish community.  This influence was instrumental in the reestablishment of the native fish community and a return of the lake ecosystem to a relatively natural state.