Population biology of longfin smelt and aspects of the ecology of other major planktivorous fishes in Lake Washington

I assessed the relationship between Cedar River flow (a major input to Lake Washington) during egg incubation and fly emergence period and longfin sme lt (Spirinchus thaleichthys) recruitment to Lake Washington. There was a si gnificant inverse relationship between an index of abundance of odd-year cl ass smelt and Cedar River mean flow during peak spawning period (February). There was no significant relationship between smelt abundance and peak flo w during spawning period (March) of the even-year class smelt. Cedar River discharge greater than 28.33 m(3)/s occurred more frequently during odd yea rs than even years. This, in addition to other factors, may be responsible for the relatively low recruitment of the odd-year spawning smelt and thus the population cycle observed in the Lake Washington smelt. The positive re lationship between smelt recruitment and flow during spring, when smelt fly are migrating into the lake, supports other studies that suggest that an i ncrease in turbidity at high flows reduces juvenile fish susceptibility to predation. There have been significant changes in the planktivorous fish as semblage in Lake Washington since the 1960s. Notably, longfin smelt, which was < 12% of the planktivorous fish abundance, now accounts for 58 to 84 % of the planktivore abundance. Recent increases in abundance of smelt may be related to reduced Cedar River mean flow that has enabled a larger proport ion of spawned eggs to hatch and survive.