A new hypothesis concerning the nature of small pelagic fish clusters: An individual-based modelling study of Sardinella aurita dynamics off West Africa |
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| Authors: | Jean Le Fur Patrick Simon |
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| Affiliation: | 1. Sorbonne Universités, UPMC Univ Paris 06, CNRS-INSU, Laboratoire d''Océanographie de Villefranche, 181 chemin du Lazaret, F-06230 Villefranche-sur-mer, France;2. Department of Ecology, Center for Scientific Research and Higher Education of Ensenada, Postal 22 860 Ensenada, Mexico;3. CNRS, UMR 7144, Station Biologique de Roscoff, Place Georges Teissier, Roscoff Cedex 29688, France;4. Laboratoire Adaptation et Diversité en Milieu Marin, Sorbonne Universités, UPMC Univ. Paris 6, Station Biologique de Roscoff, Place Georges Teissier, Roscoff Cedex 29688, France;5. Institute for Sustainable Development and International Relations, Sciences Po, 27 rue Saint Guillaume, F-75007 Paris, France;1. School of Biological Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia;2. Biodiversity Program, Queensland Museum, P.O. Box 3300, South Brisbane, Queensland 4101, Australia;3. Department of Life Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK;4. USR 3278 CNRS EPHE, Centre de Recherches Insulaires et Observatoire de l''Environnement (CRIOBE), BP 1013, Papetoai, Moorea, French Polynesia;5. Laboratoire d''Excellence CORAIL, French Polynesia;1. Laboratoire de Géologie de Lyon, CNRS UMR 5276, Université Lyon 1, 69622 Villeurbanne, France;2. Institut Universitaire de France, France |
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| Abstract: | Coastal populations of small pelagic fish display nested aggregation levels. Above the level of the school structure, clusters are observed the nature of which has not been definitively determined. We hypothesized that these clusters corresponded to a materialisation of the microcohorts originating from successive spawnings of fish populations in their vital domain.A candidate individual-based model was developed to investigate this hypothesis. This model is based on pattern-oriented modelling of a concrete documented case: the dynamics of the round sardinella (Sardinella aurita) population living off the West African coasts and subject to environmental fluctuations caused by seasonal upwelling. The simulated agents were round sardinella microcohorts situated and moving in a discretised physical environment. The combined effects of environmental forcing (temperature, wind, retention) and inner biological dynamics (reproduction, growth and mortality, competition) condition the dynamics of this population.The modelled behaviour generated realistic dynamic patterns (population distribution, spawning zones, periods and plasticity, biomass fluctuations), which were obtained simultaneously and successfully compared with observations. The steady-state number of microcohorts obtained after simulation convergence was similar to the number of clusters observed in situ in this area for this population.The realism and diversity of the patterns simultaneously simulated suggested the cluster-microcohort equivalence hypothesis as a candidate framework accounting for the origin of the clusters observed in situ. Within this preliminary exploration, we discuss the consistency of the hypothesis and the accuracy of the model. If the correspondence between clusters and microcohorts proves to be real, it may be transient and progressively modified by other environmental factors. If stable over time, as simulated in the model, the number of observed clusters should be related to the number of spawning events in the species’ lifetime. |
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