A new methodology for the analysis of fish breeding seasonality

The difficulty to dissect and weigh very tiny gonads and several fish specimens for the purpose of computing gonadosomatic indices (GSIs) due to non-availability of reliable electric power supply under field conditions, necessitated the study. This paper presents the performance of the gonad length indices vis-a-vis the traditional GSIs. The gonad elongation ratio (GER) (i.e. gonad length as a function of fish standard length expressed as a percentage) is here proposed for use in analyzing teleost fish breeding seasonality. The West African amphibious mudskipper, Periophthalmus barbarus (Linneaus, 1766) (Teleostei: Gobiidae) from the Imo River estuary, Nigeria, was used as the trial species. Monthly mean testicular and ovarian elongation ratios were examined vis–a–vis the classical testicular and ovarian gonadosomatic indices. The monthly mean gonad elongation ratios followed similar monthly mean sequences of the corresponding gonadosomatic indices. Mean GSI was 0.192 in males (range 0.097 – 0.347) and 1.197 in females (range 0.512 – 3.398); whereas mean GER was 16.5 for males (varied from 12.9 to 19.2) and 19.5 in females (ranged between 16.8 and 25.3). However, the gonad elongation ratios fluctuated more finely than the gonadosomatic indices. It is 2 - 3 times much easier and faster to determine gonad elongation ratio in the field than the gonadosomatic index. The determination of gonadosomatic indices require the use of sensitive electronic balance and reliable electric power supply while these are not necessities for the determination of gonad elongation ratios. Being a quantitative measure, the gonad elongation ratio can be subjected to statistical reliability estimates such as standard deviation, standard error and confidence intervals. The gonad elongation ratio which is quantitative and has biological interpretation that is analogous to the traditional gonadosomatic index, is recommended for use.