Gonad weight

“If I’d caught her before she spawned, she would’ve weighed over 9 pounds,” proclaimed the enthused angler as he showed off his trophy 7-pound bass.

The angler might have been right, but many anglers have a somewhat exaggerated idea of how much weight fish lose when they spawn.

For most male fish, the testes weight right before spawning is, at the most, about 3 percent of the fish’s weight. For females, the weight of the egg mass is considerably more but still less than 14 percent of the fish’s weight.

Fisheries biologists sometimes use gonad weight of fish as an indicator of health. Gonad weight generally increases with fish weight.

To standardize gonad weight, biologists calculate the gonadosomatic index, which is simply the gonad weight divided by the fish weight. The gonadosomatic index, or GSI for short, allows fish of various sizes to be used to assess health and spawning potential.

Published GSI values can also be used to estimate how much weight a fish gains prior to spawning, and how much weight the fish loses when it does spawn. The following table reports estimates for maximum percentages of weight change associated with the spawn:

Male                    Female

Largemouth bass               1 percent              8 percent

Bluegill                              2 percent            12 percent

Crappies                           2 percent             11 percent

Channel catfish                  3 percent             14 percent

Applying this biologist-generated data, the enthused angler’s 7-pound bass would have weighed, at the most, approximately 7.56 pounds (7+ [7 X 0.08]).