A brief survey for mapping of the fish resources on the Tunisian shelf was made during 11 days in late March 1981.
The fish recordings were clearly off bottom and occurred rather patchy. Very little demersal fish were recorded. Pelagic fish tended to dive and stay close to the bottom during trawling operations. Therefore bottom trawl usually gave better catches than pelagic trawl. It is believed that large and fast-swimming fishes are seriously under-represented in the trawl catches due to avoidance reactions.
From integrated echo intensities, a total fish biomass of 610,000 tonnes was estimated. This estimate is based on a average target strength of -10 logL -22 dB (ref. 1 kg/m2) and an average fish length (L) of 15 cm. It is not known how representative this target strength is for the fish in the area. It has to be taken as a reference value which has been used earlier during surveys with the R/V “Dr. Fridtjof Nansen”. The biomass estimate may therefore be revised if better information on fish target strength becomes available. RIJAVEC & al. (1977) estimated a biomass of 580,000 tonnes from an acoustic survey during June-July 1973. This may indicate a rather stable biomass. More information about seasonal and annual fluctuations are, however, needed to give an evaluation of the potential yield for fisheries.
