Seasonal changes in otolith increment width trajectories and the effect of temperature on the daily growth rate of young sardines

We studied the otolith microstructure and growth of sardine, Sardina pilchardus, in the North Aegean Sea (eastern Mediterranean Sea), using samples of larvae and juveniles that had hatched in winter (November–January) and winter–spring (February–May), respectively. The juveniles had developed during an extended period coinciding with marked pelagic ecosystem changes (from winter, mixed conditions to summer, stratified waters). To examine the relationship between environmental changes and the observed variability in their otolith increment–width trajectories (width-at-age), we summarized the shape of trajectories with a four-parameter set estimated from a growth model fit to each width trajectory. The individual parameter sets were then related to the potential oceanographic conditions that fish experienced during their development, derived from a hydrodynamic–biogeochemical model (POM-ERSEM), implemented in the sampling area. Substantial seasonal effects were demonstrated on the otolith microstructure (platykurtic versus leptokurtic trajectories in winter-mixed versus summer-stratified conditions), which were related to the progressive sea surface warming. In a subsequent step, in order to study the effect of oceanographic conditions on larval and juvenile daily growth rates, a GAM (Generalized Additive Model) analysis of otolith increment widths was carried out, using model-derived oceanographic parameters and taking into account the ‘inherent otolith growth’, expressed by the explanatory variables ‘previous increment width’ and ‘Age’. Results showed a strong and positive, linear effect of temperature on the growth rate of winter-caught larvae, whereas in juveniles, which had developed within a wide range of temperatures, an optimum temperature for growth was observed at around 24°C.Part of this work was supported by the European project SARDONE (Improving assessment and management of small pelagic species in the Mediterranean, FP6-44294). This work was also financially supported by the General Secretariat of Research and Technology, Greece through the project REPROdUCE (REcruitment PROcesses Using Coupled biophysical models of the pelagic Ecosystem) – ‘Strengthening the links between European marine fisheries science and fisheries management – MARIFISH’ (ERAC-CT-2006-025989) within the framework of the EU ERA-Net initiative (6th Framework Program), and the project SEAMAN (Spatially resolved Ecosystem Models and their Application to marine MANagement) – ‘Towards integrated European marine research strategy and programmes – SEAS-ERA’ (ERAC-CT2009-249552) within the framework of the EU ERA-Net initiative (7th Framework Program). M.P. received partial economic support from the research project REC2 (grant #CTM2011-23835, RECUMARE and Unidad Asociada LIMIA-IMEDEA).Peer Reviewe

Development, application and evaluation of a 1-D full life cycle anchovy and sardine model for the North Aegean Sea (Eastern Mediterranean).

A 1-D full-life-cycle, Individual-based model (IBM), two-way coupled with a hydrodynamic/biogeochemical model, is demonstrated for anchovy and sardine in the N. Aegean Sea (Eastern Mediterranean). The model is stage-specific and includes a 'Wisconsin' type bioenergetics, a diel vertical migration and a population dynamics module, with the incorporation of known differences in biological attributes between the anchovy and sardine stocks. A new energy allocation/egg production algorithm was developed, allowing for breeding pattern to move along the capital-income breeding continuum. Fish growth was calibrated against available size-at-age data by tuning food consumption (the half saturation coefficients) using a genetic algorithm. After a ten-years spin up, the model reproduced well the magnitude of population biomasses and spawning periods of the two species in the N. Aegean Sea. Surprisingly, model simulations revealed that anchovy depends primarily on stored energy for egg production (mostly capital breeder) whereas sardine depends heavily on direct food intake (income breeder). This is related to the peculiar phenology of plankton production in the area, with mesozooplankton concentration exhibiting a sharp decrease from early summer to autumn and a subsequent increase from winter to early summer. Monthly changes in somatic condition of fish collected on board the commercial purse seine fleet followed closely the simulated mesozooplankton concentration. Finally, model simulations showed that, when both the anchovy and sardine stocks are overexploited, the mesozooplankton concentration increases, which may open up ecological space for competing species. The importance of protecting the recruit spawners was highlighted with model simulations testing the effect of changing the timing of the existing 2.5-months closed period. Optimum timing for fishery closure is different for anchovy and sardine because of their opposite spawning and recruitment periods