MEPS 284:279-291 (2004)  -  doi:10.3354/meps284279

Knowledge of the spatial and temporal distribution of spawning is essential for an increased understanding of the ecology of the early life-history of a species, and is also a key factor in the management of species that are heavily exploited. Conventionally, the abundance and distribution of spawning are determined for individual species by visual identification of early-stage eggs collected from ichtyoplankton surveys. The accuracy of this method remains contentious for commercial gadoids and pleuronectids, due to potential misidentification of eggs at early developmental stages. In this study, pelagic fish eggs collected on spring ichthyoplankton surveys of the Irish Sea in 2000 were identified using iso-electric focusing (IEF) and histochemical staining of isozymes. Comparison of isozyme patterns obtained from known adult muscle tissue samples or broodstock eggs enabled identification of eggs of ambiguous identity. Application of this technique to 4813 pelagic eggs in the size range 1 to 2 mm collected between late February and May 2000 showed that the following species were present: cod Gadus morhua, haddock Melanogrammus aeglefinus, plaice Pleuronectus platessa, whiting Merlangius merlangus, lemon sole Microstomus kitt and witch Glyptocephalus cynoglossus. The abundance and distribution of newly spawned eggs of cod, haddock, whiting and plaice in the Irish Sea were mapped using kriging as an interpolation method. Times and locations of spawning were considered in relation to the hydrographic characteristics of the region and were compared with distributions estimated based on the morphological identification of eggs. The modification of an established biochemical technique to suit the needs of the present study has enabled the distribution and abundance of the eggs of commercial gadoids in the Irish Sea to be determined on a species-specific basis. The results have a potential application in identifying areas within the Irish Sea that could be targeted for fishery management and in refining spawning-stock biomass estimates used in stock assessments of these species.

KEY WORDS: Spawning distributions · Abundance · Gadoids · Pleuronectids · Iso-electric focusing · Species identification