Larval fish catches in the lower Milk River, Montana in relation to timing and magnitude of spring discharge

Abstract

AbstractLarval fishes were sampled in the Milk River, Missouri River drainage, Montana from May to August 2002, 2003 and 2004 to describe temporal spawning distribution in relation to spring discharge. Total larval catch‐per‐unit‐effort (CPUE) in 2002 (28.9 fish/100 m3) was an estimated 29 times greater than in 2003 (0.99 fish/100 m3) and 16 times greater than in 2004 (1.78 fish/100 m3). In 2003 and 2004 more than one third of the total catch occurred before 12 June whereas in 2002, only 5% of the total catch occurred before 12 June. Marked differences in larval species composition were also observed between years, suggesting that a later peak in discharge may benefit some species and an earlier peak others. In 2002, when flows peaked later (at 77 m3 s−1), common carp Cyprinus carpio represented 37% of the total larval catch. Common carp were proportionally less abundant in 2003 (7.2%) and 2004 (1.4%) than in 2002. In 2004, when flows peaked (at 163 m3 s−1) 32 days earlier than in 2002 but only 15 days earlier than in 2003 (at 73 m3 s−1), shorthead redhorse Moxostoma macrolepidotum and suckers Catostomus sp. were the numerically dominant taxa. These results indicate that the timing, not necessarily the magnitude, of peak spring discharge may influence spawning success in the lower Milk River, as indicated by larval fish catches. Copyright © 2008 John Wiley & Sons, Ltd.