Pattern of shoreline spawning by sockeye salmon in a glacially turbid lake: evidence for subpopulation differentiation

Alaskan sockeye salmon typically spawn in lake tributaries during
summer (early run) and along clear-water lake shorelines and outlet
rivers during fall (late run). Production at the glacially turbid
Tustumena Lake and its outlet, the Kasilof River (south-central
Alaska), was thought to be limited to a single run of sockeye salmon
that spawned in the lake's clear-water tributaries. However, up to
40% of the returning sockeye salmon enumerated by sonar as they
entered the lake could not be accounted for during lake tributary
surveys, which suggested either substantial counting errors or that a
large number of fish spawned in the lake itself. Lake shoreline
spawning had not been documented in a glacially turbid system. We
determined the distribution and pattern of sockeye salmon spawning in
the Tustumena Lake system from 1989 to 1991 based on fish collected
and radiotagged in the Kasilof River. Spawning areas and time 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 effort, a distinct late run was
discovered that spawned in the Kasilof River at the end of September.
Between tributary and shoreline spawners, run and spawning time
distributions were significantly different. The number of shoreline
spawners was relatively stable and independent of annual escapement
levels during the study, which suggests that the shoreline spawning
component is distinct and not surplus production from an
undifferentiated run. Since Tustumena Lake has been fully deglaciated
for only about 2,000 years and is still significantly influenced by
glacier meltwater, this diversification of spawning populations is
probably a relatively recent and ongoing event.