Distribution and Taxonomic Composition of Leptocephali and Hydrographic Structure From New Zealand to the South Equatorial Current

Abstract

In October-November 2024, an interdisciplinary sampling survey of anguilliform leptocephalus larvae was conducted in the western South Pacific Ocean. Hydrographic measurements and net sampling using an Isaacs–Kidd Midwater Trawl (IKMT), a Rectangular Midwater Trawl (RMT) and a Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS), were conducted across a broad region north and northwest of New Zealand, extending to the South Equatorial Current region. The hydrographic structure reflected the typical pattern of warmer low-latitude water with a low-salinity surface layer (Fresh Pool). Net deployments at 39 sampling stations (34 IKMT, 17 RMT, 24 MOCNESS) collected 367 leptocephali and six juvenile eels of ∼99 species from 13 families of eels and notacanths. The family Congridae was the most diverse, with ∼25 species including seven Ariosoma species-types. Conger leptocephali were widely present in the New Caledonia region, and recruitment-stage Gnathophis larvae were collected over the continental slope of northern New Zealand. There were also ∼21 species of Muraenidae leptocephali caught in the north, along with some larvae of other shallow water eel families (Ophichthidae, Chlopsidae, Moringuidae). The leptocephali of the mesopelagic eel families (Serrivomeridae, Nemichthyidae, Derichthyidae) were widely present at northern latitudes. Various rare types of leptocephali were also collected ( Scalanago lateralis , Congriscus , Type I) and importantly, nine Anguilla leptocephali were found exclusively at stations within the warmer, lower-salinity Fresh Pool. Together, these results provide a broad overview of leptocephalus diversity and distribution in the western South Pacific and demonstrate the value of combining multiple trawl types for sampling across size ranges.