An individual-based growth and transport model of the early life-history stages of mackerel ( Scomber scombrus) in the eastern North Atlantic

Abstract

A Bio-Physical Transport Model (BPTM) for the prediction of year-to-year survival during the early life-history stages of the north-east Atlantic stock of mackerel ( Scomber scombrus) is being developed within the EU funded SEAMAR (Shelf Edge Advection, Mortality and Recruitment) programme. The circulation model which provides the physical input data to the transport model is a geographical variant of the HAMSOM model (HAMburg Shelf Ocean Model). The transport model originally incorporated only advection and diffusion by the flow field as well as vertical migration of the larvae. The first step in the development of the BPTM focusses on incorporating an individual-based growth module within the transport model. To achieve this, an i-space configuration model is being used in which large numbers of individuals are followed as discrete entities. The information describing each individual are the i-states, e.g. age, length, spawning date and location and current geographical position. Initially, larval growth is modelled as a function of a restricted set of parameters in order to test the functioning of the growth module. This first-stage growth module is used to determine the combined effect of drift and temperature on the length distributions in various sub-areas of the model domain. The results of the simulation scenarios show an anti-cyclonic gyre over Porcupine Bank which retains eggs and larvae in this area and reduces their dispersal southwards into warmer water and northwards into colder water. Northerly drift away from Porcupine Bank towards sub-optimal temperatures, where growth is reduced, and southerly drift towards optimal temperatures, where growth is enhanced, results in a wide spread of length classes. Larvae dispersed in areas where drift routes show relatively little divergence and/or temperatures are close to the optimum temperature, have a narrow spread of length classes. The higher growth rates in areas of warmer water are associated with lower mortality rates and hence improved survival of the eggs, larvae and post-larvae during their planktonic drift phase.