Fish Spawning Aggregations: A Reef Resilience Toolkit Module

The Gonadosomatic Index

The gonadosomatic index (GSI) is the ratio of fish gonad weight to body weight.

The GSI is particularly helpful in identifying days and seasons of spawning, as the ovaries of gravid females swiftly increase in size just prior to spawning. If it is possible to collect reasonable numbers of gonads each month from adults (e.g., 30-50 individuals) over a year, one can usually determine the spawning season for the fish.

Seasons are determined by plotting monthly mean GSI values along the X (Month)-Y (GSI value) axis. Spawning times are shown when GSI values increase and peak. The GSI is determined by the following formula:

Gonad weight/body weight X 100 = GSI

Gonadosomatic index for adult male (open circle) and female (closed circle) squaretail coralgrouper (Plectropomus areolatus) taken from Pohnpei January 2006 – January 2007 during a combined market and life history survey. A total of 330 of samples across all available size ranges were used to calculate the GSI shown here, with ~ 30-40 samples taken monthly throughout the year. The GSI shows that in 2006, the reproductive season was February – May. Elevated GSI values in January 2007 point to the initiation of reproductive activity in 2007. From Rhodes and Sadovy 2002b.

Gonadosomatic index (mean ± standard deviation) for camouflage grouper (Epinephelus polyphekadion). From Rhodes and Sadovy 2002b.


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