A summary of target strength observations on fishes from the shelf off West Africa

Abstract

In many areas, and in tropical waters in particular, it may be difficult to fulfill the resolution criteria for high-quality target strength measurements both with respect to density and species mixing. Since 1985 target strength data have been collected periodically during cruises with R/V DR. FRIDTJOF NANSEN in West African waters. As a first attempt to establish a reasonably correct target strength for important species in the area, swimbladder morphology was studied and compared with species from the North Atlantic. After 1986, in situ target strength data was collected off Morocco and in Namibian waters using split-beam echo sounders. Data from the following species have been analyzed and discussed: Pilchard (Sardina Pilchardus), horse mackerel (Trachurus Capensis), hake (Merluccius Capensis), and myctophids. Only data which were considered to be of high quality have been analyzed. The calculated average target strength for pilchard are several decibels higher than the currently applied target strength of North Sea herring, but not very different from other reported target strength of sardine in other areas. Also, the target strength of horse mackerel is high compared to the presently applied target strength. For hake, the measured target strength are comparable to reported data for gadoids.