Does social behavior influence the dynamics of aggregations formed by tropical tunas around floating objects? An experimental approach
Highlights
► Aggregated biomass distributes asymmetrically under two close and identical FADs. ► Social behavior underlies aggregation process of tuna with floating objects. ► Difference between observation and theory decreases with decreasing number of units. ► Indices of schooling behavior around FADs are required to further our knowledge.
Introduction
Aggregation patterns can be defined as the gathering of individuals at a specific point in space leading to a local density higher than that observed nearby (Camazine et al., 2001). This phenomenon has been observed in a wide range of species, ranging from bacteria to vertebrates (Ben-Jacob et al., 1998, Chowhury et al., 2004, Couzin and Krause, 2003). The ultimate causes of aggregation can be very diverse, ranging from reproduction, feeding, and sheltering, to defense against predators (Camazine et al., 2001, Krause and Ruxton, 2002, Parrish and Edelstein-Keshet, 1999, Sumpter, 2010). Aggregations can result from two mechanisms, namely attraction and/or retention processes. The inflow of individuals can essentially produce large aggregations at a specific point in space despite the individual's short residence time. In contrast, retention of individuals leads to an increase in population density even if the degree of inflow is low (Ame et al., 2006, Girard et al., 2004). Two main factors can influence these two processes. Aggregation can either be an epiphenomenon resulting from individual and independent responses to environmental stimuli, or be emergent properties resulting from mutual attraction between individuals (Ame et al., 2004, Fretwell and Lucas, 1970, Sumpter, 2006). In the case of social species, several lines of evidence indicate that the formation of aggregations usually depends on both environmental factors and mutual attraction (Bayard and Elphick, 2010, Jeanson and Deneubourg, 2009). Different vectors may be involved in the detection of heterogeneity and the information transfer among individuals (Sumpter et al., 2008) in the form of chemical (Wertheim et al., 2005), visual (Goth and Evans, 2004), or mechanical (Faucher et al., 2010, Krause and Tegeder, 1994) cues.
One example, which is the focus of this study, is the large aggregations of tropical tunas and other marine fish species below objects floating at the surface of the ocean (Dagorn et al., 2012, Hunter and Mitchell, 1967, Ritz et al., 2011). While the ultimate causes of the associative behavior of pelagic fish with floating objects have received great attention [for review, see (Castro et al., 2002, Freon and Dagorn, 2000)], relatively little research has focused on the mechanisms driving aggregation. Answering this question has gained increased importance as fishers take advantage of this evolutionary aggregative behavior by deploying man-made floating objects, also called fish aggregating devices (FADs), to increase their catch. Currently, tuna catches employing these natural and artificial floating structures account for 40% of the world tropical tuna catches (Dagorn et al., 2012, Miyake et al., 2010).
Two main hypotheses have been proposed to explain why tuna associate with floating objects. The first one stipulates that natural floating objects might help tuna to stay in contact with rich feeding areas, as logs originate from river flows and drift within these rich water masses or concentrate in rich oceanic frontal zones [i.e., the indicator log hypothesis, (Hall, 1992)]. The second hypothesis states that such surface heterogeneities may constitute important features that enhance the encounter rate among fishes (or fish schools) and contribute to the fusion process between schools [i.e., the meeting point hypothesis, (Dagorn and Freon, 1999, Freon and Dagorn, 2000)]. These two hypotheses are not mutually exclusive.
The interplay between social behavior and response to external stimuli in the aggregation of fish with floating objects has been shown in a small pelagic fish species (Capello et al., 2011), but has not been demonstrated in tropical tunas, which are the main species exploited around FADs across the world. Although tropical tunas are known to form schools (a well known form of social behavior), in particular around floating objects (Doray et al., 2007, Moreno et al., 2007), this does not necessarily mean that social behavior plays a major role in their dynamics when they are associated with one floating object.
The objective of this study was to determine whether social behavior is involved in the dynamics of tuna association with floating objects. Such an objective can only be achieved through an experimental approach, which is usually a challenge in the pelagic realm. The binary choice approach is an experimental design used in behavioral ecology to identify the extent to which individual decisions of movement are influenced by the presence of conspecifics (Jeanson and Deneubourg, 2009). Several studies using the binary choice approach have employed arthropods (Canonge et al., 2011, Deneubourg and Goss, 1989, Dussutour et al., 2004, Jeanson et al., 2004, Seeley, 1995), and also fish, mainly from laboratory experiments on small species to investigate habitat selection strategies, influences of informed individuals on group decisions, accuracy of group decisions with increasing group size, and other phenomena in which group size and characteristics might be involved (Dill et al., 2003, Gomez-Laplaza, 2006, Krause and Godin, 1994, Svensson et al., 2000, Viscido et al., 2004, Webster et al., 2008).
Section snippets
Experimental design
The experimental setup consisted of two identical anchored FADs separated by less than three nautical miles (5 km). Two pairs of FADs were used, one on the southeast coast of D'Arros Island and one on the north coast of Desroches Islands, in the Amirantes archipelago of the Seychelles (Western Indian Ocean). Those two pairs of FADs were separated by 30 km. Table 1 indicates the geographical position, the anchored depth, and the date of the FAD deployment for each setup. All FADs had the same
Results
Maximum daily aggregated biomasses ranged from 1 to 36 t of tuna (with a mean and standard deviation of 13.5 ± 11.1 t for the D'Arros setup and 12.1 ± 9.7 t for the Desroches setup). Fig. 1 shows the histogram of the distribution of FX1, the fraction of the total aggregated biomass under one of the two FADs [X1 / (X1 + X2)], for each setup and for both setups analyzed jointly. The histogram of FX1 at the D'Arros setup has a mode of approximately 0.4, indicating frequent situations in which both FADs
Discussion
Among the hypotheses formulated to explain why fish associate with floating objects, the meeting point hypothesis involves social behavior (Freon and Dagorn, 2000). The existence of social behavior in the aggregation process of fish with floating structures has been experimentally validated for the big-eye scad (Selar crumenophthalmus) using acoustic tagging and modeling (Capello et al., 2011, Soria et al., 2009). However, the influence of social interactions in the aggregation of tuna with
Acknowledgments
The authors are deeply indebted to John Filmalter, Fabien Forget, and the staff of the Seychelles Fishing Authority for their help in the construction and deployment of the experimental setups, and to the D'Arros Center of Research and the Desroches Hotel for logistical support. Data analysis were carried out with financial support from the Commission of the European Communities, specifically the RTD program of Framework Programme 7, “Theme 2-Food, Agriculture, Fisheries and Biotechnology,”
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