Trawl sampling of small pelagic fish off Angola: effects of avoidance, towing speed, tow duration, and time of day

Pelagic fish distributed near the surface off Angola were sampled with a me dium-sized pelagic trawl having a vertical opening of about 20 m. Numerous trawls targeting on dense sardinella schools (Sardinella maderensis and Sar dinella aurita) failed to yield large catches. Schools that were at the tra wl depth as the vessel passed over them were often observed far below the f oot rope of the trawl, indicating vertical avoidance. The net sonde transdu cer often recorded schools in the net mouth for long periods: it was assume d that these fast-swimming fish were able to swim along with the trawl, and escaped when the towing speed was reduced during hauling. No correlations were found, however, between towing speed (2.7-5.0 knots) and the total cat ch of sardinella. Increasing the two duration during the day to more than 2 h also failed to yield consistently higher catches. A negative correlation at night between tow duration (15-60 min) and catch size arose simply beca use the trawl was towed for a longer time to obtain adequate samples in are as with scattered recordings. During the day, sardinella was caught both in shore and further offshore, while horse mackerel (Trachurus trecae and Trac hurus capensis) was caught further offshore only. Both species were well re presented in catches made at night. There were substantial day and night di fferences in the length distributions of sardinella, with small sardinella being caught only at night. (C) 1999 International Council for the Explorat ion of the Sea.