PATTERN OF SHORELINE SPAWNING BY SOCKEYE-SALMON IN A GLACIALLY TURBIDLAKE - EVIDENCE FOR SUBPOPULATION DIFFERENTIATION

Alaskan sockeye salmon Oncorhynchus nerka typically spawn in lake trib utaries during summer (early run) and along clear-water lake shoreline s and outlet rivers during fall (late run). Production at the glaciall y turbid Tustumena Lake and its outlet, the Kasilof River (south centr al Alaska), was thought to be limited to a single run of sockeye salmo n that spawned in the lake's clear-water tributaries. However, up to 4 0% of the returning sockeye salmon enumerated by sonar as they entered the lake could not be accounted for during lake tributary surveys, wh ich suggested either substantial counting errors or that a large numbe r of fish spawned in the lake itself. Lake shoreline spawning had not been documented in a glacially turbid system. We determined the distri bution and pattern of sockeye salmon spawning in the Tustumena Lake sy stem from 1989 to 1991 based on fish collected and radiotagged in the Kasilof River. Spawning areas and times were determined for 324 of 413 sockeye salmon tracked upstream into the lake after release. Of these , 224 fish spawned in tributaries by mid-August and 100 spawned along shoreline areas of the lake during late August. In an additional effor t, a distinct late run was discovered that spawned in the Kasilof Rive r at the end of September. Between tributary and shoreline spawners, r un and spawning time distributions were significantly different. The n umber of shoreline spawners was relatively stable and independent of a nnual escapement levels during the study, which suggests that the shor eline spawning component is distinct and not surplus production from a n undifferentiated run. Since Tustumena Lake has been fully deglaciate d for only about 2,000 years and is still significantly influenced by glacier meltwater, this diversification of spawning populations is pro bably a relatively recent and ongoing event.