Comparisons of catches of large leptocephali using an IKMT and a large pelagic trawl in the Sargasso Sea

Abstract

Leptocephali of several elopomorph families can reach sizes of 100–300 mm or larger, but it is questionable if these large eel or notacanth larvae are effectively sampled by small-mesh sampling gear in the open ocean because of net avoidance or insufficient fishing effort. A sampling survey in the Sargasso Sea using both an Isaacs-Kidd Midwater Trawl (IKMT) with fine mesh and a large pelagic trawl with large mesh sizes found that fewer species and individual large leptocephali >100 mm were collected by 25 IKMT tows compared to 4 trawl deployments. Net avoidance of the IKMT by the larger leptocephali appeared to occur at least during the day, and the large trawl did not catch any small leptocephali because of the large mesh size. A combination of net avoidance of the IKMT and greater sampling effort of the much larger mouth-opening trawl seems to have resulted in more large leptocephali being collected by the trawl. This indicates that IKMT surveys undersample large leptocephali species and that large trawls do not sample the whole assemblage of leptocephali. To fully understand the biodiversity and abundance of leptocephali in the world’s oceans, large fine-mesh sampling gear like the IKMT and very large trawls with smaller mesh will likely be needed. This may be important because leptocephali are probably more abundant in the open ocean than is realized, and their role in the ocean surface layer communities and carbon cycle is not understood.