Hydrobiologia 386: 167–182, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 167 Abundance and distribution of Neomysis mercedis and a major predator, longfin smelt (Spirinchus thaleichthys) in Lake Washington Paulinus Chigbu 1,∗ , Thomas H. Sibley & David A. Beauchamp 2 School of Fisheries WH-10, University of Washington, Seattle, WA 98195, U.S.A. 1 Present address: Department of Biology, P.O. Box 18540, Jackson State University, Jackson, MS 39217, U.S.A. E-mail: pchigbu@stallion.jsums.edu ( ∗ author for correspondence) 2 Present address: Department of Fish and Wildlife, Utah State University, Logan, UT 84322-5255, U.S.A. E-mail: fadave@cc.usu.edu Received 17 October 1997; in revised form 13 October 1998; accepted 23 October 1998 Abstract Seasonal variations in the horizontal and depth distributions of Neomysis mercedis and longfin smelt (Spirinchus thaleichthys) were examined using night-time mid-water trawl and Bongo net samples collected in Lake Washing- ton from July 1989 to February 1992. Mysid density varied spatially, seasonally and yearly. For example, during summer, and fall (odd years), mysid abundance was highest in the northern, and lowest in the southern sections of the lake, except in December 1991 when they were uniformly distributed. In fall (November 1990), mysid density was highest in the central basin of the lake. Furthermore, in winter of even years, highest mysid density occurred in the southern region of the lake, but in the central region in winter (February) of odd year. Longfin smelt horizontal distribution also varied seasonally. For example, density of the 1988 YC smelt (1+) was highest in the northern area of Lake Washington in summer but highest in the southern area in fall. During winter, distribution seemed random. The abundance of the 1990 YC smelt (YoY) was also highest in the northern section of the lake in summer, but highest in the southern section in fall; density remained high in the southern section in winter. But, by late spring when they were more than one year old, the distribution had changed such that highest abundance occurred in the northern and mid-section of the lake. By winter when they were about two years old and about to begin spawning, density had become highest again in the southern section. These suggest extensive movement of mysids and smelt from one area to another, perhaps driven by wind-induced water currents in the lake. Depth distribution patterns of mysids and smelt are discussed. Smelt were captured mainly in the shallow strata (8 m) of the lake during all seasons except during winter when they predominated at 50 m. Mysids were also mainly caught in the shallow strata of the lake during all seasons, although a significant proportion occurred at greater depths (> 30 m). The abundance of both species was positively correlated in spring and summer but negatively correlated in fall. A poor correlation was observed in winter. Negative correlation in fall was primarily due to the occurrence of mysids and smelt in different areas of the lake whereas poor correlation in winter was particularly due to their occurrence at different depths. Because of considerable overlap in the distribution of both species in the lake, mysids face a high risk of predation by smelt. This piece of information is consistent with the hypothesis that smelt control mysid abundance in Lake Washington. Introduction Fish predation is a major factor affecting the abund- ance of cladoceran zooplankton (Brooks & Dodson, 1965; Post & Mcqueen, 1987) and mysids (Siegfried, 1987; McDonald et al., 1990; Kjellberg et al., 1991; Chigbu & Sibley, 1998) in lentic freshwater eco- systems. Heavy predation on prey populations may depress prey biomass, reduce the size of individual organisms and alter prey distributions. For predators to control their prey, there must be a considerable de- gree of spatial overlap between them (Williamson et al., 1989; Roche, 1990) as well as high prey vulnerab- ility. To assess the impact of a predator on their prey