Temporal Trends in Nutritional State and Reproduction of Quagga Mussels (Dreissena rostriformis bugensis) in Southern Lake Michigan

Abstract

Currently little is known about the nutritional state and spawning patterns of quagga mussels (Dreissena rostriformis bugensis) in deep regions of the Great Lakes. This lack of information severely limits our ability to predict the future ecological impacts of D. r. bugensis throughout the Great Lakes and other large, deep lakes. To address these issues indices of nutritional state and spawning patterns were determined at monthly intervals (April-September) at three established sites along a depth transect (25, 45, and 93 m) in Southern Lake Michigan in 2013. These indices were compared to values at the 25-m and 45-m sites in 2004 and 2008. In addition, density and AFDW biomass were measured seasonally in March, July, and August at the same three sites. Condition index (CI), a ratio of dry soft tissue weight to internal shell capacity, was used to assess nutritional state and a gametogenic index was used to monitor reproductive activity. In 2013 annual mean CI was 56.6, 32.4, and 37.9 at 25, 45, and 93 m, respectively. Densities at 25 m (8811/m2) and 45 m (8796/m2) were similar however biomass was higher at 45 m (53.65 g/m2) than at 25 m (35.98 g/m2). Density was highest at 93 m (12152/m2) but biomass was lowest at this depth (8.22 g/m2). In terms of spawning patterns, mussels at 25 m had not yet spawned by September. Reproductive patterns were more variable at 45 m than at 25 m; however, spawning began earlier at 45 m with half or more mussels being spent beginning in July; at 93 m spawning began in August. Results indicate that competition for food was high, particularly at intermediate depths where biomass was greatest, and that this competition along with decreasing chlorophyll a concentrations in the southern basin of Lake Michigan likely caused decreases in nutritional state. At 45 m and 93 m CI decreased with increasing shell length; this also suggested that food availability may be limiting for larger individuals. A comparison of this study to earlier studies indicated that the population of D. rostriformis

bugensis continues to grow and expand into deeper offshore regions of Lake Michigan as based upon an increase in biomass and mean size, along with evidence of reproduction at 93 m. Although expansion continues into deeper regions, results indicate that growth is stabilizing at intermediate depths as densities and biomass appear to be stabilizing and CI is in decline. Densities and CI continue to decline at 25 m. If populations continue to decline in intermediate and shallow regions the negative effects that D. r. bugensis has had on the Great Lakes food web may be lessened, however the possibilities for ecosystem recovery remain unknown.