Science: Seasonal Weight Gain in Spawners

Did you see Shaw’s massive 30 pound sack from Day 2 down at the Harris Chain? The man’s still got eyes like a hawk for sight fishing those big girls. How much extra weight do you think those females were packing thanks to being near their prime for the spawn? Probably not as much as you might imagine.

How many times have you heard it said, when a guy catches a nice 6 or 7 pound bass, that if only that fish had been caught during pre-spawn, that fish would probably go closer to 9 or 10 pounds? Sounds good – problem is, that’s rarely the case.

Biologists have looked at this extra weight deal at great lengths. You probably already guessed, but there’s a fancy term for this calculation: gonadosomatic index, or you can simply refer to it as the GSI. Bass carry their eggs with them most all year. They just don’t start enlarging until a couple months before they’re ready to spawn – then they swell quick. Think sowbelly.

Still, a fish taken at it’s peak is only going to have, on average, about a 3%-8% gain in weight due to the eggs it’s carrying, with a figure around 10% being toward the high end of the curve. This obviously depends on timing, the individual health of the fish, and the water body the fish is taken from, plus it varies from species to species. You can check out the chart from a USGS study* below for a general guideline of this yearly fluctuation in Florida bass, both males and females.

* Seasonal Reproductive Cycles In Florida Largemouth Bass. Timothy S. Gross, W.E. Johnson, T. Schoeb, N.D. Denslow, C.M.Wieser, J.J.Wiebe, and D.A. Gross.USGS – Florida Integrated Science Center, the University of Florida, and Florida Fish and Wildlife Conservation Commission.