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

The main purpose of this paper is to provide the core description of the modelling exercise within the Shelf Edge Advection Mortality And Recruitment (SEAMAR) programme. An individual-based model (IBM) was developed for the prediction of year-to-year survival of the early life-history stages of mackerel (Scomber scombrus) in the eastern North Atlantic. The IBM is one of two components of the model system. The first component is a circulation model to provide physical input data for the IBM. The circulation model is a geographical variant of the HAMburg Shelf Ocean Model (HAMSOM). The second component is the IBM, which is an i-space configuration model in which large numbers of individuals are followed as discrete entities to simulate the transport, growth and mortality of mackerel eggs, larvae and post-larvae. Larval and post-larval growth is modelled as a function of length, temperature and food distribution; mortality is modelled as a function of length and absolute growth rate. Each particle is considered as a super-individual representing 10 super(6) eggs at the outset of the simulation, and then declining according to the mortality function. Simulations were carried out for the years 1998-2000. Results showed concentrations of particles at Porcupine Bank and the adjacent Irish shelf, along the Celtic Sea shelf-edge, and in the southern Bay of Biscay. High survival was observed only at Porcupine and the adjacent shelf areas, and, more patchily, around the coastal margin of Biscay. The low survival along the shelf-edge of the Celtic Sea was due to the consistently low estimates of food availability in that area.

Particle export during a bloom of Emiliania huxleyi in the North-West European continental margin

Coccolithophores, the dominant pelagic calcifiers in the oceans, play a key role in the marine carbon cycle through calcification, primary production and carbon export, the main drivers of the biological CO2 pump. In May 2002 a cruise was conducted on the outer shelf of the North-West European continental margin, from the north Bay of Biscay to the Celtic Sea (47.0 degrees-50.5 degrees N, 5.0 degrees-11.0 degrees W), an area where massive blooms of Emiliania huxleyi are observed annually. Biogeochemical variables including primary production, calcification, partial pressure of CO2 (pCO(2)), chlorophyll-a (Chl-a), particle load, particulate organic and inorganic carbon (POC, PIC) and Th-234, were measured in surface waters to assess particle dynamic and carbon export in relation to the development of a coccolithophore bloom. We observed a marked northward decrease in Chl-a concentration and calcification rates: the bloom exhibited lower values and may be less well developed in the Goban Spur area. The export fluxes of POC and PIC from the top 80 m, determined using the ratios of POC and PIC to Th-234 of particles, ranged from 81 to 323 mg C m(-2) d(-1) and from 30 to 84 mg C m(-2) d(-1), respectively. The highest fluxes were observed in waters presenting a well-developed coccolithophore bloom, as shown by high reflectance of surface waters. This experiment confirms that the occurrence of coccolithophores promotes efficient export of organic and inorganic carbon on the North-West European margin.