Assessing trawl-survey estimates of frequency distributions
Original version
This report is not to be cited without prior reference to the authorsAbstract
Marine trawl surveys catch a cluster of fish at each station and fish caught
together tend to have more similar characteristics, such as length, age, stomach contents
etc., than those in the entire population. When this is the case, the effective sample size of
estimates of the frequency distribution of a population characteristic can be much smaller
than the number of fish sampled during a survey. As examples, it is shown that the
effective sample size for estimates of length-frequency distributions generated by trawl
surveys conducted in the Barents Sea, off Namibia and off South Africa is on average
approximately one fish per tow. It is concluded that many more fish than necessary are
measured at each station and that one way to increase the effective sample size for these
surveys and, hence, increase the precision of the length-frequency estimates, is to reduce
tow duration and use the time saved to collect samples at more stations.