Changes in the abundance of small littoral-zone fishes in Lake Mendota, Wisconsin

Since 1900, major changes have occurred in the assemblage of small littoral-zone fishes (maximum total length < 150 mm; usually encountered in the nearshore area) that inhabits Lake Mendota, Wisconsin. A diverse assemblage that included several environmentally sensitive species has been replaced by an assemblage dominated by a single species, the brook silverside (Labidesthes sicculus), whose abundance fluctuates dramatically from year to year. Between 1900 and 1981, eight species, the pugnose shiner (Notropis anogenus), common shiner (Notropis cornutus), blackchin shiner (Notropis heterodon), blacknose shiner (Notropis heterolepis), tadpole madtom (Noturus gyrinus), banded killifish (Fundulus diaphanus), blackstripe topminnow (Fundulus notatus), and fantail darter (Etheostoma flabellare), disappeared from the lake. The blackchin shiner and banded killifish were the most abundant small littoral-zone species in 1914–1916, and remained common until the late 1960's. Their decline was associated with the invasion and explosive increase in abundance of an exotic macrophyte, the Eurasian water milfoil (Myriophyllum spicatum), in the mid-1960's. The decline and disappearance of the blackchin shiner and banded killifish in three other Wisconsin lakes was also associated with the invasion of Eurasian water milfoil. Changes in the assemblage of small littoral-zone fishes in Lake Mendota indicate environmental degradation in the nearshore area, and may have important implications for the entire fish community of the lake.