A couple of weeks ago, I was driving around with my dog and decided to stop at the Hoover Reservoir to take a quick walk. As my dog and I approached the edge of the water, I quickly noticed dozens of little silver fish floating on the surface (pictured below). I initially assumed all of the fish were dead, but after some observation I noticed that some were still trying to swim (rather unsuccessfully). I was taken aback by the number of fish that were either already dead or well on their way there, and I began to wonder what kind of fish they were and why this was happening.
After trying to remember all of the fish I learned in my taxonomy class last semester, I guessed that the fish in question were Gizzard Shad (Dorosoma cepedianum), which I later confirmed with a google search. I then made a few searches to see if something like this has happened before, and quickly found that Gizzard Shad are very sensitive to cold weather and die-offs like these happen relatively often with this species. In fact, it has been noted to be a normal annual occurrence in spring for some areas (Muzyk, 2022).
Studies have shown that Gizzard Shad are often susceptible to starvation over the winter, as they cannot take in nutrients as efficiently in cold water. For smaller fish, these energy reserves are even smaller and result in them being more sensitive to and dying much quicker in colder waters (Fetzer et al., 2011). This explains why all of the fish I saw were relatively the same size (less than 3 inches). This spring die-off, while it may be s(h)ad to see, is a part of the life history of this species and is a normal occurrence that plays into their population dynamics.
Gackenbach 2022
References
Fetzer, W. W., Brooking, T. E., Jackson, J. R., & Rudstam, L. G. (2011). Overwinter mortality of gizzard shad: Evaluation of starvation and cold temperature stress. Transactions of the American Fisheries Society, 140(6), 1460–1471. https://doi.org/10.1080/00028487.2011.630281
Muzyk, C. (2022, February 15). Nothing’s fishy: Gizzard shad die-off at Lake Brittle is a common, natural occurrence. Prince William Times. Retrieved March 7, 2022, from https://www.princewilliamtimes.com/news/nothing-s-fishy-gizzard-shad-die-off-at-lake-brittle-is-a-common-natural-occurrence/article_c47023de-8e60-11ec-bed3-5fc279928cd3.html
Photographs by Madelene Gackenbach (2022)