Biology and ecology of flatfish species on west of Ireland nursery grounds

Abstract

Commercial flatfish captured in demersal fisheries along the west coast of Ireland include quota species such as plaice (Pleuronectes platessa L), in addition to high value non-quota species, turbot (Psetta maxima L.) and brill (Scophthalmus rhombus L.). Important non-commercial angling species of interest include flounder (Platichthys flesus L.). However, knowledge of the population dynamics of these flatfishes along the west coast of Ireland is lacking, and considered to be inadequate for the establishment of any population trends. In light of this, habitats along the west coast of Ireland serving as nursery grounds for the juveniles of these species were investigated (2000 - 2009). Correctly distinguishing between pairs of recently settled flatfish species, which are morphologically similar in appearance, was a prerequisite to this research. Counts of the meristic characters, fin rays, which are species specific, proved to be a reliable identification tool for juveniles. Plaice were the most abundant flatfish species present on nursery grounds, followed by turbot, and to a lesser extent brill. Assessment of nursery grounds over the eight year period, revealed both inter-annual and spatial variability in the relative abundance, growth, and condition of plaice, turbot and brill. Certain trends were revealed in these parameters, indicating the existence of both high and low quality nursery habitats. Abundances of both plaice and turbot on nursery grounds were negatively correlated with sea water temperatures during the pelagic stage. Baseline data on the timing of critical events, including hatching, larval durations, and settlement, in addition to growth experienced during early life, were established from the otolith microstructure of turbot. Post-settlement growth rate estimates of turbot also indicated certain locations as high quality nurseries. The feeding ecology of juvenile turbot and brill on nursery grounds was described, with temporal and resource partitioning revealed, implying that competition is unlikely to arise between these two morphologically similar species. A high incidence of prey was observed in the gut content of all turbot and brill, suggesting that food was not limited on Irish nursery grounds. Turbot fed on a much wider range of prey in contrast to brill, which fed almost exclusively on one prey organism. This observation may be one reason for the general sparse distribution of brill. The improved understanding of the early life history of plaice, turbot and brill may aid in the development and implementation of effective management strategies in the future; at the very least for the protection of important nursery grounds, as any loss, or decline in the quantity or quality of these areas, will undoubtedly have a negative impact on the recruitment of these species.