Fjord retention and dispersion of shrimp larvae (Pandalus borealis) in Northern Norway

Larval drift connects sub-populations within metapopulations of species with long lasting pelagic larval phases, like northern shrimp ( Pandalus borealis ), which inhabits patches of soft bottom along the Norwegian coast. The key objective of this study was to investigate whether northern shrimp in Northern Norwegian fjords constitute distinct fjord-based sub-populations, or if they form a coherent coastal population due to advection of larvae along the coast and between fjords. Shrimp larval fluxes within and out of a Northern Norwegian fjord, Kvænangen, were studied with a particle tracking model combined with a high-resolution hydrodynamic model, and we estimated the retention of shrimp larvae released within the fjord and the export of larvae with the currents to neighbouring fjords. Kvænangen is an open fjord with a deep sill dividing the inner and outer fjord. The model particles, representing shrimp larvae, were given a temperature dependent development rate and a diurnal vertical swimming behaviour dependent on development stage. Larval drift was modelled for the years 2018–2021 to capture interannual variability of oceanographic conditions. The dispersal of shrimp larvae varied from local settlement within the fjord, to long range advection (>250 km) to other fjords downstream or farther to the Barents Sea. Within Kvænangen, more larvae settled in the inner fjord basins behind the sill than in the outer fjord. Shrimp in the inner fjord seem to constitute a self-sustained sub-population, while the population outside the sill lost most of its spawning products offshore and therefore relies on import from the inner fjord and upstream populations for recruits. These results have implications for the management of shrimp fisheries and how the management units of shrimp should be defined. Our findings are also relevant for management of the salmon aquaculture industry; since shrimp larvae disperse throughout Kvænangen, there is no place in the fjord where chemicals from delousing baths can be released without potentially harming shrimp larvae. • Larval dispersal varied from local retainment to long range advection (>250 km). • Currents, wind and fjord-coast water exchange determined retainment and advection. • Connectivity between shrimp fields in Troms and western Finnmark is high. • The retention of larvae in inner Kvænangen is sufficiently large for possibly maintaining a self-sustained population.