Date of Award

August 2021

Degree Type

Thesis

Degree Name

Master of Science

Department

Freshwater Sciences and Technology

First Advisor

Harvey Bootsma

Committee Members

Dave Clapp, Brenda Moraska Lafrancois, Dong-Feng Deng, John Janssen

Abstract

Few organisms are well adapted to efficiently feed on invasive dreissenid mussels, a dominant primary consumer in Lake Michigan and other lower Great Lakes. As a result, these mussels represent a potential trophic dead-end. However, round gobies (Neogobius melanostomus), an invasive species introduced to the Great Lakes region at the end of the 20th century, possess several adaptive advantages that allow them to make dreissenid mussels a significant portion of their diet. Since their invasion, round gobies have become the predominant shallow nearshore fish in Lake Michigan and their success, along with the success of dreissenid mussels, has caused major shifts in regional productivity, trophic structure, and energy flow pathways in the lake.

In the Great lakes, round gobies have been incorporated into the diets of numerous piscivorous species, and therefore may serve as a conduit of energy, nutrients, and contaminants to higher trophic levels. This potential has made round gobies a critical species to consider in management plans, especially in regions important for Great Lakes fisheries. For management to be successful, a deeper understanding of round gobies' effect on food web structure and energy flow is needed. This research aimed to quantify round goby abundance and productivity in the rocky nearshore zone of Lake Michigan, focusing on a rocky reef (10-11 m depth) in Good Harbor Bay near Sleeping Bear Dunes National Lakeshore (SLBE) that has historically been used for spawning and feeding by native fish species such as lake trout and lake whitefish.

Productivity was estimated by quantifying several population and bioenergetic parameters in June-October 2020. Benthic sampling provided biomass estimates of nearshore primary producers and consumers as well as a stable isotope trophic baseline. Round goby population density and size-frequency were determined using visual and video transects. Age structure was estimated from sagittal otoliths and combined with length data to model growth. Round goby diet composition was determined based on gut content and stable isotope analysis and used to estimate the population's reliance on benthic algae production vs. dreissenid grazing of phytoplankton. The combination of these methods allowed for an estimate of total round goby productivity on the rocky reef. A comparison of round goby productivity with energy inputs in the rocky nearshore zone allowed for trophic transfer efficiency to be estimated.

Mean round goby density was 2.6 individuals ? m-2. The population was found to have a right-skewed unimodal size distribution with a mean size of 7.3 ± 2.4 cm (n = 1304) and a maximum size of 15.9 cm. Males from Good Harbor Reef have a faster growth rate and obtain a greater maximum size and age than females. Diet analysis indicated an ontogenetic diet shift, with larger gobies being more reliant on invasive mussels than smaller gobies. However, at the population level, non-mussel benthic invertebrates accounted for over half of round goby prey. Round goby productivity was estimated to be 0.009 g wet weight ? day-1 ? 0.041 kJ ? day-1. This resulted in an estimated reef transfer efficiency of 1.3 - 1.8% when accounting for both dreissenid and non-dreissenid benthic invertebrates. This low efficiency is due to only a small fraction of dreissenid production (3%) being consumed by round gobies. By contrast, round gobies appear to be consuming virtually all (81 - 122%) non-dreissenid benthic invertebrate productivity. On a lake-wide scale, annual round goby productivity was estimated to be four times that of recent estimates of alewife production. These results suggest that round gobies represent a substantial portion of Lake Michigan prey fish biomass and have the potential to serve as an important energetic conduit from the benthic region and invasive mussels to upper trophic levels.

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