Fish Population Dynamic and Stock Assessment

1.8.2. Sampling fish

Length distributions of landed fish can be obtained from samples taken in the auction halls, ports, quays side sampling or at sea on commercial vessels. Generally, the length sampling is carried out by area, gear and time to ensure that the estimated size distribution is accurate. Age and weight-at-length can be obtained from these samples. Sampling throughout the year allows scientists to monitor differences in growth and age over the year. An unbiased sample should have fishes of a wide range of lengths and weights.

The sampled length frequency information for either landings or catch can be converted to ages using age-length keys derived from the relationship between age and length. Ages are estimated using  otoliths. Counting the rings on these otoliths gives an estimate of the age of the fish. The numbers of fish at different ages for various gears, areas, quarters and nations are combined to produce annual catch numbers-at-age for each assessed stock.