Geology topics

Brandon S. Gerig

Publications and source records attributed to Brandon S. Gerig.

5 recordsLinked to original sources

USGS2025

Larval fish export in response to altered discharge in the St. Marys river rapids

The St. Marys River (SMR) forms the border between Michigan, USA, and Ontario, Canada, connecting lakes Superior and Huron. Discharge is controlled by a 16-gate water control structure upstream of the SMR rapids. The SMR rapids are a critical spawning habitat in the Great Lakes, yet the influence of regulated discharge on larval fish export remains poorly understood. From 2018 to 2021, larval fish were sampled across four locations to assess community composition and export dynamics under varying discharge releases. Rainbow smelt ( Osmerus mordax ) dominated catches (87.9 %), followed by species in the family Leuciscidae (i.e., native minnows) and burbot ( Lota lota ). Larval densities were strongly influenced by water temperature and, to a lesser extent, discharge. Bayesian state-space models estimated a total export of ∼ 41.3 million larvae from the rapids across 3 years, with the highest export in 2019. Rainbow smelt larvae exhibited peak densities at intermediate flows (300–600 m 3 s −1 ) and temperatures (∼18–19 °C), while non-rainbow smelt taxa showed species-specific, less consistent responses. Nighttime sampling consistently yielded higher larval densities. Export estimates varied by analysis but both manual and Bayesian approaches captured similar interannual and taxonomic trends. Despite past expectations, the larval community was largely composed of prey species, with limited representation of lake sturgeon ( Acipenser fulvescens ), lake whitefish ( Coregonus clupeaformis ), or walleye ( Sander vitreus ). These results suggest the SMR rapids currently support significant forage fish production and highlight the importance of discharge and temperature in structuring larval export.

St Mary's River rapids↗

USGS2025

Food web structure of the Lake Superior fish community in 2021–2022

The trophic linkages and ecological requirements of the Lake Superior fish community have not been assessed on a whole lake scale in over a decade. Here, we investigated the trophic dynamics across multiple species and habitat zones of Lake Superior. From April to October of 2021 and 2022, a total of five piscivore, four planktivore, and six benthivore species were collected by region and length class during bottom-trawl, standard gillnet, commercial cisco gillnet, and recreational angler surveys. To assess trophic linkages, stomach contents were measured to estimate biomass consumed and a multivariate analysis was used to assess diet composition by species, length, and region. We found a high degree of interconnectedness in the Lake Superior food web, with Mysis as a critical diet item for most fishes. Native piscivore diets varied by region and with ontogeny. Lake charr were important habitat couplers in the lake, exhibiting a diverse diet and opportunistic foraging strategy. Conversely, Pacific salmon were more restricted in their foraging. Planktivores and benthivores primarily consumed Mysis , with less reliance on Diporeia compared to previous studies. Lake whitefish and cisco were the exception to this pattern, with broader bathymetric depth distributions represented in their diets. We found the food web to be supported by a predominantly native species assemblage, with redundancies at all trophic levels.

Lake Superior↗

USGS2024

Trophic ecology of juvenile lean and siscowet lake charr (Salvelinus namaycush) in Lake Superior: Assessing for potential competition

We investigated the spatial overlap, diet, isotopic niche, and growth of juvenile lean and siscowet lake charr ( Salvelinus namaycush ) in Lake Superior to address concerns of potential competition with implications to the study of resource polymorphism. Catch data revealed the greatest levels of sympatry in waters from 40 to 60 m. Juvenile lean and siscowet diet changed ontogenetically with Mysis being the dominant prey item for the smallest lake charr but differentiating with onset of piscivory. As ecotypes increased in size, lean diets became dominated by pelagic prey, whereas siscowets had equal proportions of benthic and pelagic prey. Isotopic niche overlap declined between ecotypes coincident with siscowet lake charr shifting to deeper habitats around 400 mm. Lean and siscowet exhibited different growth trajectories. However, length at age-4 declined in parallel for both ecotypes with no trend in condition suggesting that lake charr growth is sensitive to prey biomass and unlikely related to competition. Our findings indicate minimal evidence of competition and support the concept that multiple sympatric ecotypes of lake charr in Lake Superior are maintained by resource polymorphism.

Lake Superior↗

USGS2021

Metal accumulation in Lake Michigan prey fish: Influence of ontogeny, trophic position, and habitat

Developing an understanding of factors that influence the accumulation and magnification of heavy metals in fish of the Laurentian Great Lakes is central to managing ecosystem and human health. We measured muscle tissue concentrations of heavy metals in Lake Michigan prey fish that vary in habitat use, diet, and trophic position, including alewife, bloater, deepwater sculpin, round goby, rainbow smelt, and slimy sculpin. For each individual, we measured tissue concentrations of four metals (chromium [Cr], copper [Cu], manganese [Mn], and total mercury [THg]), stable isotope ratios for trophic position (δ 15 N and δ 13 C), and individual fish attributes (length, mass). Total mercury concentration was positively related to total length and δ 15 N. Of all species, round goby displayed one of the greatest increases in mercury per unit growth and was most isotopically distinct from other species. Profundal species (bloater, deepwater sculpin, slimy sculpin) had similar high THg tissue concentrations, possibly due to slower growth due to cold temperatures, whereas other species (alewife, round goby, rainbow smelt) showed more variation in THg. In contrast, other metals (Cr, Cu, Mn) had either a negative or no relationship to total length and δ 15 N, suggesting no bioaccumulation or biomagnification. Potential incorporation of mercury by sportfish may thus be related to species, age, diet, trophic position, and habitat of prey fish. Our findings serve as a foundation for understanding how heavy metals accumulate in Lake Michigan food webs and highlight the continued need for management of metal input and cycling in Lake Michigan.

Illinois, Indiana, Michigan, Wisconsin↗

USGS2015

Assessing juvenile native fish demographic responses to a steady flow experiment in a large regulated river

The Colorado River below Glen Canyon Dam, Arizona, is part of an adaptive management programme which optimizes dam operations to improve various resources in the downstream ecosystem within Grand Canyon. Understanding how populations of federally endangered humpback chub Gila cypha respond to these dam operations is a high priority. Here, we test hypotheses concerning temporal variation in juvenile humpback chub apparent survival rates and abundance by comparing estimates between hydropeaking and steady discharge regimes over a 3-year period (July 2009–July 2012). The most supported model ignored flow type (steady vs hydropeaking) and estimated a declining trend in daily apparent survival rate across years (99.90%, 99.79% and 99.67% for 2009, 2010 and 2011, respectively). Corresponding abundance of juvenile humpback chub increased temporally; open population model estimates ranged from 615 to 2802 individuals/km, and closed model estimates ranged from 94 to 1515 individuals/km. These changes in apparent survival and abundance may reflect broader trends, or simply represent inter-annual variation. Important findings include (i) juvenile humpback chub are currently surviving and recruiting in the mainstem Colorado River with increasing abundance; (ii) apparent survival does not benefit from steady fall discharges from Glen Canyon Dam; and (iii) direct assessment of demographic parameters for juvenile endangered fish are possible and can rapidly inform management actions in regulated rivers.

River Research and Applications↗