Survey design considerations for estimating the length composition of the commercial catch of some deep-water species in the northeast Atlantic

For many deep-water species it is impractical to conduct scientific Surveys to assess the state of the stocks. Therefore, the only means available to monitor these species is through assessing changes in the commercial catch composition over time. A pilot study was conducted to determine a suitable sampling strategy for estimating the age and length distribution of the commercial catch of ling, tusk and blue ling in the northeast Atlantic. Based on this study, it is shown that the precision of estimates of the length distribution is a function of not just the number of fish sampled but more importantly, how the fish were sampled. It appears that the most efficient survey design is to collect relatively small samples from as many vessels as possible. For example, the standard error for estimating the mean length Of tusk if based on approximately 20,000 fish sampled from fishing trips made by 3 vessels Would be 1.61, while if 5000 fish were collected from trips made by 5 vessels, the standard error Would be equal to 1.26. The gain in precision by spreading out the sampling is due to the fact that fish caught in a particular small area tend to be more similar than those in the entire Population. (C) 2004 Elsevier B.V. All rights reserved.