Characteristics of a Naturalized Kokanee Salmon Oncorhynchus nerka Population in Atypical Habitat

Abstract

Kokanee salmon are a dwarf freshwater form of the anadromous sockeye salmon (Oncorhynchus nerka) native to the Pacific coast of North America and Asia and widely introduced in western North America as a sport and prey fish. Few kokanee salmon introductions have succeeded in eastern North America. A naturalized population of kokanee salmon has persisted for over 50 yr in Florence Lake, Wisconsin, an unlikely location given its relatively small size (21.4 ha), shallow maximum depth (7.6 m), low elevation (381 m), and southerly latitude (45.12°N). Kokanee salmon completed their entire life cycle within the lake and rarely entered an accessible coldwater inlet stream. Spawning occurred during late October and early November along the lake shoreline. Spawning redds with defending males were observed in 1–2 m of water on sand/gravel/ cobble substrate. Kokanee salmon growth in the lake was relatively rapid, and fish had a mean total length of 117 mm after their first growing season (age 0+), 212 mm after their second (age 1+), begin to mature and spawn after their third (age 2+) at 317 mm for females and 324 mm for males, and were at 344 mm for females and 354 mm for males after their fourth (age 3+). Few fish reproduced at age 4+. Despite limited oxy-thermal habitat during summer, kokanee salmon appear to thrive in Florence Lake, perhaps because of cold inflows from the inlet and springs within the lake.