Depth distribution of nearshore temperate fish larval assemblages near rocky substrates

In this study, we compare the composition, abundance and structure of a temperate fish larval assemblage at different depth intervals (0-4, 4-8 and 8-12 m) in the extreme nearshore environment. We used a plankton net attached to an underwater scooter to sample in close proximity to the rocky substrate (< 50 cm). A total of 868 larvae from 27 taxa in 13 families were caught. The majority of larvae belonged to benthic reef-associated species (Blenniidae, Gobiidae, Gobiesocidae and Tripterygiidae), the four most abundant comprising 76% of the total larvae caught. A non-metric multidimensional scaling analysis (MDS) showed that there was a single multispecific larval patch near the substrate in the extreme nearshore up to 12 m depth. Nonetheless, distinct larval abundances were found in this relatively small depth range, with the majority of species being more abundant at the deepest interval, particularly Pomatoschistus pictus and Gobius xanthocephalus. Tripterygion delaisi was an exception being more abundant at the shallowest depth as young larvae. The density of pre-flexion larvae was not significantly different across depth intervals, but post-flexion larval density increased with depth. The full size range (from hatching to settlement) of P. pictus was present at the extreme nearshore. The innovative sampling technique used here revealed high densities of larvae close to the bottom, and depth was found to be an important factor influencing the distribution of several taxa and ontogenetic stages. The nearshore component of coastal fish larval assemblages near rocky substrates has been poorly studied, and our results suggest that the high densities of larvae found to aggregate in these environments must be taken into account when studying distribution and functional aspects of these assemblages.