Since our departure from Pemba, 10 pelagic trawls have been performed. Four during daytime hours and six during the night when the DSL (deep scattering layer) has finished its ascent. The DSL, is a sound reflecting layer in the water column that can be located with echo-sounders and is caused by aggregations of zooplankton or fish.
At daytime, the surface layer of the water (10m deep) and aggregations present between 200-250m have been trawled. Near the surface, catches consisted almost exclusively of juveniles of coastal coral-reef fish and of tunas. Such fish are often found in the stomach contents of top predators foraging at the surface (some birds and tunas). The deeper water aggregations trawled during the day typically consisted of mono-specific (i.e. single species) schools of mesopelagic fish such as the myctophid (lantern fish) Diaphus richardsoni.
A night, most of the trawls were conducted between 10 to 20m depth. They were charactherized by a high catch of flying squids (Ommastrepidae, essentially Sthenoteuthis oualaniensis) and cigarfish (Nomeidae, Cubiceps pauciradiatus). Myctophids were represented by several species. In deeper trawls (40m depth) the catch of flying squids decreases and the catch of myctophids increases in terms of diversity.
Cigar fish during night-time surface trawlsOne of the many coastal reef species caught during day-time surface trawls
Red flying squid that are prevalent during night-time trawls
written by: Michel Potier (IRD, France)
- At night, a stratification of the communities is observed, myctophids being deeper in the DSL than the flying squids and the cigarfish.
- Coral reef fish larvae and juveniles are observed all over the Mozambique channel even far away from the coasts. This is an interesting observation that suggests that reef-fish populations from the east coast of Africa and the islands of the western Indian Ocean may be connected.
- It must be noted than shrimps and euphausids are absent of the trawls.