Offshore larval fish assemblages in contrasting environmental zones around the Iberian Peninsula

Abstract

The changes in species composition, abundance and vertical distribution of larval fish assemblages were analysed in four offshore zones with contrasting oceanographic conditions, around the Iberian Peninsula (northeast Atlantic and western Mediterranean). In each zone, sampling was conducted at four stations using a MOCNESS-1 net. Larvae of mesopelagic species dominated the samples (70.4% of abundance), with a higher number in the Atlantic (41 taxa) than in the Mediterranean (28 taxa). The most frequent and abundant were the Gonostomatidae Cyclothone spp. and the Myctophidae Ceratoscopelus spp. Shelf species were frequent, albeit in low abundance, denoting larval transport from nearshore areas, but limited survival. Temperature and chlorophyll were identified as the main environmental drivers structuring larval fish assemblages. Vertical distributions showed that species within the same genus had similar vertical patterns across all zones, highlighting the relevance of phylogeny vs hydrography. Larvae of most taxa (i.e., Gonostomatidae, Myctophidae, Paralepididae and Vinciguerriidae) were concentrated in the upper 100 m, whereas Sternoptychidae occurred mainly between 100 and 300 m. In highly stratified zones, many species occurred both at the upper mixed and pycnocline layers, so the pycnocline did not appear to act as a barrier between these two hydrographically different layers. Instead, it seemed to act as a barrier mainly for larvae with deeper vertical distribution that, when vertical gradients were weak could reach near surface layers. The water column stability was used to discuss reasons to explain why the more productive and unstable zones exhibited lower larval abundances compared to the more oligotrophic and stratified zones.