Fish Population Dynamic and Stock Assessment

1.8.3. Types of sampling

1) Sampling onboard the fishing vessels

Sampling for length composition of commercial catch should best be done on board of the fishing vessel during fishing operation. Because all fish caught are not always landed, smaller specimes are discarded at sea; it is of no market value. There are several practical difficulties with sampling programmes for collecting length data on onboard fishing vessels. It is difficult and costly to send scientists onboard in all selected vessels, even if the number of selected vessels is small.

2) Sampling at landing sites

Most convenient place for sampling fish for most fisheries is the site where fish are landed after capture, since a number of boats land at a landing site within a short period of time. It is possible for a single scientist to examine and measure sample from selected vessels.

The compositions of catch may sometimes vary between centres if vessels from these centres fish in different areas containing possibly of different stocks. The most practicable solution in this case would be to continue sampling at the convenient centres, but sampling may be done separately or combined after giving due weightage, according to landings from each area of fishing. When the fish are lying in a large heap on the beach, a small number has to be selected as sample. The usual tendency in the absence of a proper sampling system is to take the desired number of fish from the top of a heap and since it is a larger fish.

Most important raw materials required for the stock assessment studies are catch data, effort data and length frequency distribution of the catch. All these data should be properly collected and tabulated for analysis.