Biology of Finfish and Shellfish

4.4.1.3. Automatic egg counter

Total count of eggs in an ovary can also be made using automatic egg counters. The advantage of using this machine is that sampling error in any sub-sampling technique is avoided but the only disadvantage is the slowness of this machine.

Fecundity can be estimated by removing the ovaries from females in stages III to V in the case of total spawners. Fecundity analysis is largely confined to total spawners, because it is difficult to estimate the fecundity of partial spawners. All the oocytes to be spawned in one spawning cycle might not have been differentiated in the early stages in the case of partial spawners. In later stages, some developing eggs might be the leftover of already occurred spawning. Adequate estimate of annual fecundity in such species, depends upon the data on the number of spawning per year, number of eggs shed at each spawning and the relation between these factors and size & age of the fish.

Sex Ratio

The knowledge of sex composition of catches is of help in understanding whether any differential fishery exists, its possible bearing of fish stocks and whether sexual congregation takes place during spawning. It indicates the proportion of males and females in the population. The expected sex ratio is 1:1 in the nature. Variations from this are often observed in fish because of differential behaviour of sexes, environmental conditions, fishing, etc. The number of male to female ratio is observed for a minimum period of one year. This data is pooled in two ways – month wise and lengthwise to study the distribution of sexes according to seasons and size of fish. It also indicates the segregation or aggregation of sexes according to feeding, breeding or migratory behaviours. The sex ratio is calculated by using the chi-square formula,

x² = ∑ (O – E)² / E to test the homogeneity in distribution of males and females.

Where, O – Observed value; E – Expected value