Modelling the retention and survival of Browns Bank haddock larvae using an early life stage model
Browns Bank, the principal spawning ground for haddock on the south-west Nova Scotian shelf, is composed of two distinct production zones: the inshore Bay of Fundy (BoF) region and the offshore south-west Nova (SWN) bank region. Fish growing in the BoF are larger at age than those in the SWN region. Analysis of research vessel (RV) data shows that the majority of age-2 haddock have the size of south-west Nova Scotia fish, suggesting that the majority of surviving fish were retained in SWN. We used an early life stage (ELS) model to address two questions. First, we asked whether the length-at-age difference between Bof and SWN is evident at the larval stage. Using a temperature-based growth model, we found that predicted size differences for late larvae would be less than 0.5 mm. From consideration of the average growth curves to age-2 for the two regions, we showed that this difference was not the seed for the size difference in later life stage. The second question we addressed was how well the ELS model predicted partitioning of late larvae between SWN and the BoF corresponding to the ratio of SWN/BoF age-2 juveniles found in the RV data. We ran the model on a representative number of years between 1973 and 1992, and found a significant correlation between the model simulation and the data (cor=0.71, P=0.047). This indicates that the age-2 size distribution is a reflection of retention and survival occurring during the first two months of life. Model/data discrepancies are explained in terms of differential mortality between the two regions.