Trawl sampling of small pelagic species off Angola. Effects of avoidance, towing speed, tow duration and time of day

Abstract

A series of pelagic trawl sampling trials on pelagic species distributed near surface off Angola were conducted by R/V «Dr. Fridtjof Nansen» operating a medium sized pelagic trawl with a vertical opening of about 20 m. The results have been analysed with respect to effects such as vessel and gear avoidance, time of day, towing speed and tow duration. Numerous trawls targeting on dense sardinella shoals failed to yield large catches. Shoals that were at the trawl depth as the vessel passed over were often observed far below the foot rope of the trawl, indicating vertical avoidance. Often shoals in the net mouth were recorded by the net sonde for long periods. It was assumed that these fast swimming fish were able to swim along with the trawl, and escaped out of the trawl when slowing down the towing speed during hauling. No correlations were, however, found between towing speed in the range 2.7 - 5.0 knots and the total catch of sardinella. Increasing the tow duration during the day to more than 120 minutes also failed to yield consistently higher catches. At night, there was a negative correlation between tow duration in the range 15 - 60 min and the catch size, simply because the trawl was towed for a longer time to obtain proper samples in areas with scattered recordings. During the day sardinella was caught both inshore and further offshore, while horse mackerel was caught further offshore only. Both species were well represented in the catches made at night. There were substantial day and night differences in the length distributions of sardinella with small sardinella being caught only at night.