Comparison of horse mackerel length frequency from research vessel and commercial vessel in February 2002

Abstract

Abstract by author:

The study compares the length frequency of cape horse mackerel Trachurus capensis caught by the research vessel Welwitschia and commercial vessel Eysk during February 2002. The data were obtained from the area of high concentration of horse mackerel for length frequency comparison. The t-test on the data indicated that there was a significant difference between length frequencies from these two vessels, with the commercial vessel catching larger fish than the research vessel. This difference was due to differences in trawl size, trawl speed and trawl duration. The result of the study suggested that the larger the trawl, the larger the proportion of larger fishes in the length distribution, the longer the trawl duration the greater the likelihood of catching larger fish and the faster the trawl speed in relation to swimming speed, the high the chances of increased catches of larger fish. Thus, further work is needed to try and eliminate the bias in the trawl sampling of R/V Welwitschia, to ensure that the survey samples the whole population and thereby improving the accuracy of the biomass estimation