Effect of Brief Navigation-Related Dewaterings on Fish Eggs and Larvae

Short-term dewatering of nearshore spawning areas often occurs during passage of commercial tows in the upper Mississippi River as well as in other navigated river systems. This phenomenon was examined experimentally to identify potential effects on survival of fish eggs and larvae. Early life stages of walleyes (Stizostedion vitreum vitreum) and northern pike (Esox lucius) were dewatered 2 min in laboratory studies at intervals of either 1, 3, 6, or 12 h from just after fertilization to 10-14 d posthatch. Dewatering did not cause mortality of eggs, but significant mortality of larvae of both species occurred at dewatering frequencies of 1 or 3 h. Therefore, significant mortality first was observed at a frequency equivalent to a mean passage of eight tows per day. This average passage rate is presently exceeded in about 50% of the upper Mississippi pools; it will be exceeded in all pools if proposed navigation modifications are implemented.