Larval distribution of commercial fish species in waters around Ireland

Abstract

In April 2000 a base line survey was conducted on the larval distribution of commercial fish species off the west, north and south coasts of Ireland. Ichthyoplankton samples and in situ CTD data were collected, whilst simultaneously capturing remote sensing images of chlorophyll and sea surface temperatures. The survey sampling area covered the Celtic Sea from the Irish south coast to 49 degree N, the western shelf including the Porcupine Bank and the northern shelf up to the Stanton Bank. The sample grid design was based on the international mackerel & horse mackerel egg survey with station spacings of 0.5 degree latitude and 0.5 degree longitude. Ichthyoplankton samples were collected with a Gulf III plankton sampler, which was deployed on oblique tows from the surface to within 5 metres of the bottom (200m max). A self-logging CTD sensor (Promonitor) was attached to the Gulf and recorded depth, temperature and salinity profiles for each deployment. Results from the Promonitor CTD showed that strong temperature and salinity gradients were encountered during the survey. Lowest temperatures coincided with lowest salinity in the North Channel of the Irish Sea while highest salinities and temperatures were found to the south west of Ireland.Thermal fronts were found in the eastern Celtic Sea and on the north west coast of Ireland.The AVHRR images showed a progressive increase in surface temperatures in the Celtic Sea and west of Ireland. Highest surface chlorophyll concentrations were associated with cooler less saline water in the Irish Sea and the coastal areas around Ireland. In the western Celtic Sea surface chlorophyll concentrations increased as the survey progressed to form a phytoplankton bloom towards the end of the survey. Larvae of interest showed distinct distribution patterns, with some species being confined to particular areas or spawning grounds while others were spread over the whole survey area. The survey identified two important larval hotspots: Cod larvae were concentrated in the eastern Celtic Sea, where other gadoid species such as haddock, whiting, pollack and saithe were also found in high numbers.This area is associated with the Celtic Sea front and shows increased primary productivity, which could present a favourable environment for successful larval survival. Stations in the southwest of Ireland sustained high concentrations of hake, megrim and mackerel larvae. The waters with high numbers of these three species stretched from shallow inshore stations to deeper ones along the continental shelf and were characterised by high temperatures and salinities. SeaWIFS satellite images suggest the formation of a phytoplankton bloom within this larval hotspot, which would provide the necessary resources for successful larval growth.