Prespawning herring distribution in the Irish Celtic Sea between 2005 and 2012

Knowledge of the spatial distribution of species is fundamental to understanding trophic interactions and ecosystem structure. Intraspecies-specific dynamics and environmental factors shape species distribution within an ecosystem. Distribution patterns and the realized habitat of Atlantic herring (Clupea harengus), a key fish species in the Celtic Sea, were examined using distribution data collected during annual acoustic stock assessment surveys during 2005-2012. Distribution patterns during migration to spawning grounds were analysed using geostatistical methods, including modelling of variograms and comparing the centre of gravity (CG). Distribution patterns were further linked to oceanographic variables collected with the acoustic data to describe habitat. Herring density was greatest inshore along the spawning grounds and lower farther offshore. Herring shoals were clustered and randomly distributed during spawning migration. Variograms of fish densities described the global structure, with high local variability of the same order of magnitude as variability at a regional scale, indicating that no continuous structure can be found within the study area. The CG values showed that the average position of the population was located northeast. The realized habitat for herring encompassed a wide range of temperatures and salinities; therefore, oceanographic features were not a limiting factor for herring distribution during spawning migration. The present study changes the perception of the spatial distribution of the Celtic Sea herring stock from a more continuous distribution to a discrete model, with implications for trophic ecosystem modelling on local scales. Future studies of herring distribution and its influence within the Celtic Sea ecosystem would benefit from using the approach employed in the present study for analysing aggregation patterns.