A Dynamic Energy Budget simulation approach to investigate the eco-physiological factors behind the two-stanza growth of yellowfin tuna (Thunnus albacares)
Introduction
Yellowfin tuna (Thunnus albacares; YFT) is an epipelagic species widely distributed in the tropical and subtropical waters of the world’s major oceans (Pecoraro et al., 2017). Owing to its high value as food source, YFT supports important commercial and recreational fisheries. With an annual global catch of about 1.3 million tonnes in the last decade, it ranks among the world’s ten most harvested marine species (FAO, 2018). At global scale, YFT has recently been assessed as Near Threatened following the Red list criteria of the International Union for Conservation of Nature (Collette et al., 2001), while the assessments of the status of its four oceanic stocks conducted within the tuna Regional Fisheries Management Organizations indicate that overfishing currently occurs in the Indian Ocean (IOTC, 2018).
For assessment and management purpose, growth of wild YFT has been the subject of considerable research efforts since the 1960s through analysis of mark-recapture data, size frequency distribution of commercial fishery catches and ageing from calcified and bony structures (Diaz, 1963, Schaefer, Chatwin, Broadhead, 1961, Wild, Foreman, 1980). YFT growth varies between ocean basins and regions and shows in some areas some complex pattern that departs from the traditional von Bertalanffy growth curve used for most fish stocks (Pecoraro et al., 2017). In particular, some growth studies conducted in the Atlantic, Indian and Western-Central Pacific Oceans support a two-stanza growth pattern with a sharp acceleration in growth rate at about 60–65 cm fork length (Dortel, Sardenne, Bousquet, Rivot, Million, Le Croizier, Chassot, 2015, Eveson, Million, Sardenne, Le Croizier, 2015, Gascuel, Fonteneau, Capisano, 1992, Lehodey, Leroy, 1999).
The complex growth pattern observed in some populations of YFT has been assumed to result from a combination of physiological, ecological and behavioral factors but the role and contribution of each of them have not been addressed yet (Bard, 1984, Dortel, Sardenne, Bousquet, Rivot, Million, Le Croizier, Chassot, 2015, Fonteneau, 1980, Gaertner, Pagavino, 1992). A better understanding of these mechanisms is essential to assess the plasticity of YFT growth and eventually strengthen the scientific advice on the stock status and improve the overall quality of current stock assessments.
We investigated the factors governing the growth of YFT with a bioenergetic model that covers the different post-metamorphic life stages of the Indian Ocean YFT within the framework of the Dynamic Energy Budget (DEB) theory (Kooijman, 2010). DEB theory has already proven successful to study growth and reproduction of a large range of marine taxa (Freitas, Cardoso, Lika, Peck, Campos, Kooijman, van der Veer, 2010, Pecquerie, Petitgas, Kooijman, 2009, Sousa Tnia, Domingos Tiago, Kooijman S.A.L.M, 2008, van der Veer, Cardoso, van der Meer, 2006) and more recently to study the dynamics of populations and ecosystems (Maury, 2017, Maury, Poggiale, 2013, Pethybridge, Roos, Loizeau, Pecquerie, Bacher, 2013). Other bioenergetic approaches are available but they are generally specific to a particular life stage and unsuitable for modelling the changes between distinct life stages (Hansen, Boisclair, Brandt, Hewett, Kitchell, Lucas, Ney, 1993, Ney, 1993). In contrast, the DEB theory provides a powerful conceptual framework to describe the lifelong growth in relation to other physiological requirements, such as metabolism and body internal maintenance, development and reproduction, as a function of environmental food conditions and organism state (Freitas, Cardoso, Lika, Peck, Campos, Kooijman, van der Veer, 2010, Kooijman, 2010).
Section snippets
Data sources
We collected data sets from the literature for both wild and farmed YFT to compare the outputs of the bioenergetic model with morphometric, ageing and reproduction observations.
Validation of a reference DEB model for yellowfin tuna
The reference DEB model successfully reproduced the growth in length and the length-weight relationship observed for farmed Pacific YFT as described in the study of Wexler et al. (2003). Furthermore, the observed values of FDR and FCR were well within the simulated range (Fig. 2). Thus, the set of parameters, some of which were derived from PBT (Jusup et al., 2011), appeared suitable to assess the effects of variations in the quantity and quality of food on YFT growth (Table 1).
From the
Discussion
We formulated a bioenergetic model covering the juvenile and adult stages of YFT in the context of Dynamic Energy Budget theory. Some essential parameters of the model were estimated with Bayesian analysis from morphometric, age, and reproduction data sets collected from YFT sampled in the Indian Ocean while other parameters were derived from the DEB model developed for PBT (Jusup et al., 2011). Our results show that both lower food availability in the juvenile stage and ontogenetic changes in
Conclusion & perspectives
Building on the Dynamic Energy Budget modelling framework that mechanistically links the biology of individual organisms to abiotic drivers (Kooijman, 2010), our model suggests that the unique growth pattern observed in wild YFT could stem from a major change in intensity of schooling and associative behavior with size, which would substantially modify both the quantity and quality of food available for maintenance, reproduction and growth. This change would be triggered by the acquisition of
Declaration of Competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was funded by the Data Collection Framework of the European Union (Reg.199/2008, 665/2008 and SI2.604453), the French Ministry of Agriculture and Fisheries (301629/00), ORTHONGEL, the Institut de Recherche pour le Developpement, and the French ANR funded project EMOTION (ANR 11JSV7 007 01). We wish to acknowledge the contributions of all the people who have been involved in the Regional Tuna Tagging Project of the Indian Ocean, funded under the 9th European Development Fund
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