Shifts in environmental conditions can impact fish distributions, phenology, community composition, and population abundance. Here, we use 18 years of larval fish data to investigate bluegill (Lepomis macrochirus) hatch seasonality, abundance, and survival rate in a hypereutrophic reservoir. Along with gizzard shad (Dorosoma cepedianum), bluegill is one of the most abundant and ecologically important fish species in midwestern reservoirs. Larval bluegill were collected weekly via tows at two offshore sites in Acton Lake (USA) (April - August, 2007 - 2024) to quantify larval abundance and size. For determining the age of collected fish, estimated bluegill growth rates were used, allowing us to predict hatch dates and assess long-term trends. We found that larval bluegill density in the reservoir is increasing over time (18 years), and a preliminary analysis on a subset of years suggests that both the yearly date of peak abundance and peak hatch may be occurring earlier in the summer compared to the beginning of our study. We also observed that the density of bluegill at hatch decreases over the course of a summer, and hatch dynamics vary substantially year-to-year (i.e., one major hatch vs up to 8 hatch cycles per year). Additionally, our study shows that several abiotic and biotic drivers mediate long-term larval bluegill trends and seasonal hatch dynamics. Our results demonstrate that while larval bluegill hatch dynamics are highly variable, we can improve our understanding by incorporating key physical and biological conditions of the lake. 

Larval fish were sampled weekly using a 200 µm neuston (May-Aug, 2007-2025) in Acton Lake at two sites. Upstream and Downstream sites are averaged. 

Larval fish are counted and identified as gizzard shad, bluegill, and “other.”

Adult and YOY fish were electroshocked and measured on a monthly basis to track populations. 

Turbidity (NVSS) and temperature data are part of Acton’s routine monitoring. Zooplankton are sampled weekly with a 63 µm net and identified as rotifers, cladocerans, and copepods. 

A sub-sample of 25 fish is measured (mm) from each site. 

Length data is used to split fish into “age classes”

Key Takeaways

We observed larger larval bluegill hatches over time. 

Future work will incorporate more parameters and analyses to explore the mechanisms driving increases of larval bluegill in Acton Lake. 

Hatch dynamics are highly variable; while standardizing our data by month/year enabled initial analyses, we may be missing how physical and biological conditions of the lake impact hatches on a short-term basis.

See Preliminary Linear Regression Results Here 

1. Vanni, MJ, Arend, KK, Bremigan, MT, Bunnell, DB, Garvey, JE, González, MJ, Stein, RA. (2005). Linking landscapes and food webs: Effects of omnivorous fish and watersheds on reservoir ecosystems. BioScience, 55: 155-167.

2. Breck, J. E. (1993). Hurry up and wait: Growth of young bluegills in ponds and in simulations with an individual‐based model. Transactions of the American Fisheries Society, 122(3), 467-480.

3. Showalter, A, Vanni, MJ, González, MJ. (2016). Ontogenetic diet shifts produce trade-offs in elemental imbalance in bluegill sunfish. Freshwater Biology, 61: 800–813.

This work was funded by the Miami University Undergraduate Summer Scholars Program, the Honors College, as well as the National Science Foundation: #2427185, #1930655, #1255159, #0743192, #0235755O