Annual egg production estimates of cod (Gadus morhua), plaice (Pleuronectes platessa) and haddock (Melanogrammus aeglefinus) in the Irish Sea: The effects of modelling choices and assumptions

The annual egg production (AEP) of cod (Gadus morhua), plaice (Pleuronectes platessa) and haddock (Melanogrammus aeglefinus) in the Irish Sea during the 2008 spawning season was estimated using generalized additive models (GAMs that further developed previous applications of the annual egg production method to Irish Sea stocks by including 2-D spatial smoothing, automatic selection of the degree of smoothing and precision estimates. The estimates of AEP from the GAMs were compared to those from a stratified mean method and the sensitivity of the estimates to different model specifications, outliers, boundary effects and prediction period was assessed. The influence of outlying large observations was notable but overall the estimates of AEP were robust to the factors studied, with effect sizes comparable to the estimates’ coefficients of variation.

► Egg production of cod, plaice and haddock in the Irish Sea in 2008 is estimated.
► Generalized additive models are compared to estimates based on the stratified mean.
► Sensitivity to model specification, outliers, boundaries and dates is assessed.
► The influence of outlying large observations was notable.
► Overall the estimates were robust to the factors studied.