Larval transport dynamics in Nephrops norvegicus
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Transport of meroplankton larvae in the ocean is a crucial process as it enables connectivity between populations and determines larval supply for species with narrow habitat requirements and sedentary adult stages. The Norway lobster (Nephrops norvegicus), Europe’s most important commercial crustacean, has a patchy distribution across the Northeast Atlantic Ocean and Mediterranean Sea. Adults inhabit areas of muddy substrate where they excavate and spend most of their time within burrows. The pelagic larval phase enables connectivity between populations separated by uninhabitable substrate. Larvae rely on settlement on suitable mud habitat for survival. Therefore, larval settlement, driven by local hydrography, may act as a constraint on recruitment. Biophysical models offer a method of simulating larval transport, which is extremely difficult to observe in-situ due to the inherent difficulties in tracking miniscule larvae in vast areas of the ocean. In the current study, a biophysical larval transport model was used to estimate larval retention, dispersal distance and connectivity for N. norvegicus grounds around Ireland. Models parameters were supported by empirical data in order to accurately represent the biological and behavioural processes of larvae. In Chapter 2, the vertical distribution and occurrence of a Diel Vertical Migration (DVM) in N. norvegicus larvae was examined. Larval vertical distribution was influenced by the vertical temperature differential in the water column, zooplankton biomass and the potential energy anomaly. A twilight DVM was identified and involved two ascents and two descents per day. In Chapter 3, historical zooplankton datasets were used to identify an earlier larval phenology shift in N. norvegicus by 19.1 days from 1982 - 1995 to 2000 - 2010. Ocean warming was identified as the most likely cause as increasing temperatures led to a contraction of the embryo incubation period and earlier hatching of larvae. The phenology shift appeared to have a limited effect on larval duration and transport. Only large variations in modelled larval retention and dispersal distance were observed between larvae released very early and very late in the season. In Chapter 4, a 20-year time series of modelled larval retention, dispersal distance and connectivity estimates for 6 N. norvegicus Functional Units (FUs) demonstrated their capacity to retain, import and export larvae. Smaller FUs had a decreasing trend in retention over the time series which appeared to be as a result of strengthening currents. On the Aran grounds, a link between modelled larval retention and dispersal distance and empirically observed burrow densities from underwater television with a 3-year lag was observed. The findings indicate that larval transport may act as a constraint on recruitment for N. norvegicus populations like the Aran grounds with low and variable larval retention and limited larval imports due to spatial isolation from other grounds. It demonstrates the potential of using larval transport estimates to identify instances of poor recruitment, due to low larval settlement, early in the life cycle before its effects manifest in the adult population. It can also be applied to similar species with defined habitat and planktonic life stages and may assist in limiting overexploitation for commercial species, particularly in the face of climate change and the likely impacts on oceanography.
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