Patterns in the spawning of cod (Gadus morhua L.), sole (Solea solea L.) and plaice (Pleuronectes platessa L.) in the Irish Sea as determined by generalized additive modelling

Eleven ichthyoplankton cruises were undertaken covering most of the Irish S ea during the period February to June, 1995. To identify spawning localitie s and investigate temporal trends in egg production, the data on stage 1 A egg distributions of cod (Gadus morhua), plaice (Pleuronectes platessa) and sole (Solea solea) have been modelled using generalized additive models (G AMs). A two-stage approach was adopted where presence/absence was firstly m odelled as a binary process and a GAM surface subsequently fitted to egg pr oduction (conditional on presence). We demonstrate that this approach can b e used to model egg production both in space and in time. The spawning site s for cod, plaice and sole in the Irish Sea were defined in terms of the pr obability of egg occurrence. For cod, we demonstrate that by integrating un der predicted egg production surfaces, a cumulative production curve can be generated and used to define percentiles of production and thus delimit th e extent of the spawning season. However, for plaice and sole, the surveys did not fully cover the spawning season and the limitations that this impos es on GAM modelling of these data are discussed. Comparison of the spawning sites in 1995 with historical data suggests that the locations of cod, pla ice and sole egg production in the Irish Sea have probably remained relativ ely constant over the last 30 years.