Twilight migrators: factors determining larval vertical distribution in Nephrops norvegicus with implications for larval retention

The vertical distribution of pelagic marine larvae can greatly influence their dispersal due to depth-varying currents, which can determine larval retention or transport away from critical habitat. Vertical distribution of commercially important lobster Nephrops norvegicus larvae was examined over fishing grounds off the west and east coasts of Ireland. Larval vertical distribution for both grounds was significantly influenced by the temperature differential between the surface and 60 m depth, zooplankton biomass and, to a lesser extent, stratification, measured using the potential energy anomaly. Fixed station sampling was conducted over 3 d in the western Irish Sea (WIS) to investigate the occurrence and extent of a diel vertical migration (DVM). Larvae performed twilight DVM with an similar to 10 m ascent prior to sunset and sunrise and a descent at midnight and after sunrise. Particle-tracking model simulations were used to examine the effect of DVM behaviour on larval retention over mud habitat. The presence of a DVM actually reduced the likelihood of retention on both the Aran and WIS grounds. Predicted larval retention was unusually low over the Aran grounds in 2018, which is potentially significant in the context of historic stock fluctuations in this area. These findings suggest that understanding larval dynamics could be crucial in managing N. norvegicus stocks on fishing grounds, in particular those with variable interannual oceanography and a low rate of larval donation from other grounds.