Pick Our Brain - December 2011

How fat should a fish be?

How fat a fish should be depends on the species of fish. Since fish come in all shapes and sizes, the weight of a “healthy” fish is different for each species. A 2005 Interagency Ecological Program study of Delta fish condition found that a 54 millimeter (just over 2 inches) long delta smelt should weigh about 1.4 grams while a juvenile striped bass of the same length should weigh about 2.1 grams.

Like people, taller (longer) fish of the same species should weigh more so weight at a given length is a useful indication of fish condition and is a rough measure of how healthy a fish is. The photo below shows bay anchovies. The anchovy on the bottom was exposed to low dissolved oxygen conditions in the Alafia River estuary in Florida and is underweight for its length. This is an example where fish were clearly in poor condition due to adverse environmental conditions. Studies currently under way in the Delta are investigating the possible connection between fish condition (including weight) and environmental conditions such as water quality and food supply.

Bay Anchovies - Healthy (top) - Underweight (bottom)
Bay anchovies – Healthy (top) Underweight (bottom)
(Image courtesy of Ernst B. Peebles Ph.D.,
College of Marine Science, University of South Florida)

The weight of fish for its length is only one measure of the overall health of a fish. How fast a fish grows is another indicator of favorable environmental conditions and food supply. Scientists are continually improving and developing new tools to determine how fat (and fit) fish are. These tools include methods for assessing the condition of the liver and other organs, swimming performance, and genetic tests to determine how stressed a fish is as a result of exposure to toxic chemicals or starvation. All of these tools help scientists understand what causes fish populations to increase or decrease over time. Fish that are fat grow faster, have higher survival rates, and produce more offspring when the time comes to spawn.