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Are drifting FADs essential for testing the ecological trap hypothesis?
Fisheries Research 106 (2010) 60–63
Contents lists available at ScienceDirect
Fisheries Research journal homepage: www.elsevier.com/locate/fishres
Are drifting FADs essential for testing the ecological trap hypothesis? Laurent Dagorn a,∗ , Kim N. Holland b , John Filmalter a,c a
Institut de Recherche pour le Développement (IRD), UMR 212, P.O. Box 570, Victoria, Seychelles Hawaiian Institute of Marine Biology (HIMB),University of Hawaii, P.O. Box 1346, Kaneohe, HI 96744, USA c South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa b
a r t i c l e
i n f o
Article history: Received 11 May 2010 Received in revised form 25 June 2010 Accepted 12 July 2010 Keywords: FAD Ecological trap hypothesis Tunas
a b s t r a c t Because tropical tunas are known to aggregate around floating objects, it has been suggested that the large number of drifting fish aggregating devices (FADs) built and deployed by purse seiners could act as an ‘ecological trap’. This hypothesis states that these networks of drifting FADs could take fish to areas where they would not normally go or retain them in places that they would otherwise leave. Because the ecological trap hypothesis was first advanced for drifting FADs, some have argued that only studies using drifting FADs can test this hypothesis. However, because working with drifting FADs is difficult, accepting this precept would preclude the scientific community from providing urgently needed information to organizations charged with management of fisheries that exploit drifting FADs. We argue that because both anchored and drifting FADs alter the natural environment, the more easily accessible anchored FADs can be used to test the ecological trap hypothesis. Also, based on a comparative scientific approach, we argue that understanding the behaviour of tunas around anchored FADs can improve our general understanding of tunas around all types of floating objects and help design new, well focused studies for drifting FADs. As anchored FADs are easier to access and offer a greater potential for research, we encourage scientists to design and conduct studies (in particular on the behaviour of fish at FADs) around the moored structures. © 2010 Published by Elsevier B.V.
1. Introduction Floating objects are known to aggregate pelagic fish (Castro et al., 2002; Dempster and Taquet, 2004; Taquet et al., 2007). This behaviour is exploited by fishermen who look for natural floating objects (e.g. logs) or build and deploy artificial fish aggregating devices (FADs). In the past two decades, this phenomenon has become particularly important for industrial tuna purse seine fisheries which regularly deploy large numbers of drifting FADs in the tropical oceans (Fonteneau et al., 2000; Moreno et al., 2007). In addition to the large number of tunas caught this way (Fonteneau et al., 2000), some scientists have proposed that FADs could have an additional deleterious impact on populations of tunas by acting as ecological traps (Marsac et al., 2000; Hallier and Gaertner, 2008). That is, by modifying the pelagic environment (Fauvel et al., 2009), artificial drifting FADs could have major effects on the behaviour and biology of tunas. It is hypothesised that tunas could be trapped within networks of artificial drifting FADs and alter their natural movements towards feeding or spawning areas (Marsac et al., 2000). These networks of drifting FADs could take fish asso-
∗ Corresponding author. Tel.: +248 224742; fax: +248 224742. E-mail addresses: [email protected] (L. Dagorn), [email protected] (K.N. Holland), jdfi[email protected] (J. Filmalter). 0165-7836/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.fishres.2010.07.002
ciated with them to areas where they would not naturally go or retain them in places that they would otherwise leave. These areas could be biologically inappropriate and the biology (e.g. growth) of the tunas would then be negatively affected by the concomitant changes in migration route or habitat selection. Dedicated field research is clearly needed to investigate whether FADs act as ecological traps for fishes. Hallier and Gaertner (2008) analyzed data that were opportunistically collected during previous studies and their results supported the ecological trap hypothesis. However, we feel that more research is clearly needed with protocols specifically designed to address this question, particularly regarding the behaviour of these fishes. Working on offshore drifting FADs requires major financial cost and logistical support. This explains why most behavioural studies on FAD-associated fishes have focused on anchored FADs usually moored within a few miles of the coast (Dempster and Taquet, 2004) or sometimes in offshore waters (Schaefer and Fuller, 2005, 2007). It is only recently that scientists have started to investigate the behaviour of tunas around drifting FADs (Schaefer and Fuller, 2002; Matsumoto et al., 2006; Dagorn et al., 2007a; Moreno et al., 2007). Because the ecological trap hypothesis was first advanced for drifting FADs, some have argued that only studies using drifting FADs can test this hypothesis. However, because working with drifting FADs is difficult, accepting this precept would preclude the scientific community from providing urgently needed information
L. Dagorn et al. / Fisheries Research 106 (2010) 60–63
to organizations (such as RFMOs) in charge of the management of fisheries that exploit drifting FADs. Our objective here is to examine the theoretical aspects of the ecological trap hypothesis and assess objectively if and how studies on anchored FADs could help the investigation of this hypothesis. 2. The origin of the ecological trap hypothesis Among the 16 different hypotheses that Fréon and Dagorn (2000) reviewed to explain why some fishes associate with natural or artificial items, two hypotheses for tunas seem more credible to the scientific community (Hallier and Gaertner, 2008): the meeting point (Dagorn and Fréon, 1999) and the indicator-log (Hall, 1992) hypotheses. FADs as meeting points would provide social advantages such as enhancing schooling behaviour (which has recently been proven for a small pelagic fish species, the bigeye scad Selar crumenophthalmus, Soria et al., 2009) while floating objects as indicators of prey-rich waters would provide foraging advantages. These two hypotheses on the origins of the associative behaviour are not in conflict and could operate simultaneously (Fréon and Dagorn, 2000). The ecological trap hypothesis is not an hypothesis to explain why fishes associate to floating objects. Rather, it speculates about the possible consequences of changes in the natural environment on the behaviour of animals (with subsequent impacts on other aspects of their biology). The ecological trap is an ecological concept developed over 30 years ago (Dwernychuk and Boag, 1972). An ecological trap is a specific type of evolutionary trap in which cues for habitat choice become less reliable (Schlaepfer et al., 2002). In an ecological trap, animals make errors in habitat assessment by attending to inappropriate environmental cues (Battin, 2004). Often, but not always, inappropriate cues are a result of anthropogenic influences. In the case of tropical tunas and FADs, the ecological trap was invoked because of the thousands of drifting FADs regularly deployed on the open oceans by fishermen (Moreno et al., 2007) in locations where this number of floating objects would not naturally occur. These FADs modify the natural habitat of tunas (Fauvel et al., 2009). The hypothesis of FADs acting as an ecological trap was first proposed by Marsac et al. (2000) who postulated that the large numbers of drifting FADs in the equatorial zone may alter the natural movements of these fishes towards feeding grounds. If the influence of floating objects on tunas is sufficiently strong, anthropogenic changes in the spatio-temporal distribution of floating objects could modify their natural migrations. This, in turn, could have repercussions for the overall health of the population (Marsac et al., 2000; Hallier and Gaertner, 2008). Although not stated as such by Marsac et al. (2000), and later by Hallier and Gaertner (2008), the FAD ecological trap hypothesis (as they formulated it) is actually an extension of the indicator-log hypothesis (Hall, 1992) which is one of the two main hypotheses advanced to explain why tunas could have developed this associative behaviour. It stipulates that natural floating objects (such as logs) are often indicators of productive areas, either because most natural floating objects originate in rich areas (i.e. river mouth, mangrove swamps) and remain within these rich bodies of water as they drift offshore, or because they become entrained in rich frontal zones regardless of their original source. Because of a possible strong spatial correlation between natural floating objects and rich waters, tunas could have evolved to use those objects as cues to find or stay within high quality areas. If the indicator-log hypothesis is correct (it has not yet been validated), the ‘random’ release of artificial drifting FADs by fishing boats could obviously disrupt the spatial correlation between floating objects and rich waters. If tunas have developed this associative behaviour because it was providing them with a social benefit (meeting point hypoth-
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esis), the deployment of large numbers of FADs may have effects on school size (either increasing it by facilitating school formation or decreasing it by providing multiple alternative sites for school formation). In this case, an ecological trap would only occur if largescale movements are dependent on the sizes of tuna schools. This was not tackled by either Marsac et al. (2000) and Hallier and Gaertner (2008) and was never advanced by any author previously. Consequently, we will not study this type of ecological trap in this document. Of course, the possibility exists that regardless of the reason for associating with floating objects (e.g. ‘indicator log’ or ‘meeting’ point), this behaviour is abandoned when other motivations (such as spawning migrations or hunger due to poor feeding success) become dominant. Such a phenomenon was recently demonstrated in shark species that showed seasonal fluctuations in participating in anthropogenically induced associations with dive tourism operations (Meyer et al., 2009).
3. The ecological trap hypothesis and anchored FADs The ecological trap hypothesis (in its “indicator-log” form) was first advanced for drifting FADs. But any artificial floating object arriving in the ocean modifies the natural environment of tunas where only natural floating objects would traditionally be encountered. In this sense, anchored FADs also modify the natural environment of tunas. In the same way that Marsac et al. (2000) and Hallier and Gaertner (2008) wondered if drifting FADs deployed in the ocean would modify the natural large-scale movements of tunas, one can wonder if the deployment of anchored FADs would not also modify the natural large-scale movements of tunas. The generic question of whether FADs can act as ecological traps is not necessarily restricted to drifting FADs but can also be asked about anchored FADs. The issue is to know if any results obtained about whether anchored FADs act as ecological traps could be used to determine if drifting FADs also act as ecological traps (and vice versa). Although drifting FADs are the most abundant worldwide, a comparative analysis should take into account densities, not numbers.
4. Behaviour of tunas at FADs Both anchored and drifting FADs are known to aggregate tunas but some scientists propose that because anchored FADs are moored (i.e. have a mooring line) and drifting FADs drift (no mooring line), this difference may result in different phenomena underlying the formation of the aggregations of tunas associated with the different types of FAD. This associative behaviour can be expressed as the combination of two behaviours (Girard et al., 2004): an attraction behaviour and a retention behaviour. Considering that only one is necessary to obtain aggregations but both can also occur simultaneously, it is appropriate to compare the current scientific knowledge about tunas around anchored and drifting FADs. Using data from active tracking studies on yellowfin tuna (Holland et al., 1990; Marsac and Cayré, 1998; Brill et al., 1999; Dagorn et al., 2000) around anchored FADs, Girard et al. (2004) could demonstrate that yellowfin tuna are able to orient towards anchored FADs from a distance of up to 10 km (see Table 1). No similar active tracking study on tunas around drifting FADs has yet been conducted but without prior knowledge of the results on anchored FADs, skippers of tuna purse seiners gave the same value for the distance from which tunas could orient towards drifting FADs (Moreno et al., 2007). This congruence strongly suggests that there is no major difference in the distance of orientation of tunas to anchored or drifting FADs (Moreno et al., 2007).
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L. Dagorn et al. / Fisheries Research 106 (2010) 60–63
Table 1 Scientific evidence that tunas can be attracted to FADs, and can be retained at FADs.
Attraction behaviour
Retention behaviour
Anchored FADs
Drifting FADs
Girard et al. (2004) and all active tracking studies on which this study was based on (Holland et al., 1990; Marsac and Cayré, 1998; Brill et al., 1999; Dagorn et al., 2000) Ohta and Kakuma (2005), Dagorn et al. (2007b)
Moreno et al. (2007)
Schaefer and Fuller (2002), Matsumoto et al. (2006), Dagorn et al. (2007a)
Using passive telemetry, Ohta and Kakuma (2005) and Dagorn et al. (2007b) have shown that anchored FADs retain yellowfin and bigeye tunas for about a week in average (ranging from a few minutes up to 64 days). Values found for drifting FADs are inferior but of the same order of magnitude: average of 3 days for bigeye tuna in the Eastern Pacific Ocean (maximum of 7 days), maximum values up to 15 days in the Central Pacific Ocean (Matsumoto et al., 2006; Dagorn et al., 2007a), the average values being difficult to provide for these last two studies due to frequent interruptions of monitoring. In other words, both types of FADs can retain tunas (see Table 1). There are therefore no scientific results to indicate that the associative behaviour of anchored and drifting FADs results from different behavioural processes. An ecological trap is a consequence of a behaviour that has become maladapted. The study of the ecological trap therefore requires the analysis of empirical behavioural data (e.g. long term movements from conventional tagging in Hallier and Gaertner, 2008) or active and passive tracking (see above) or biological consequences of the behaviour (e.g. condition factors, see Hallier and Gaertner, 2008). A behaviour is a set of actions or reactions of an animal in response to external or internal stimuli. The study of the behaviour of tunas should therefore take into consideration the following factors (Dagorn et al., 2001): • Presence of FADs (e.g. densities of FADs). • Characteristics of the abiotic environment (mainly the oceanography of the area, e.g. water temperature, dissolved oxygen, etc.). • Characteristics of the biotic environment (prey, predators, competitors, conspecifics—e.g. schooling behaviour). • The internal state of the individuals. A comparative approach (Dagorn et al., 2001) would help to better understand the behaviour of tunas at both types of FADs. However, as a start, any understanding of the behavioural processes (in particular the respective roles of the different stimuli listed above) driving the association of tunas with anchored FADs would help the study of the behaviour at drifting FADs. Of course, this would not prove that the behavioural processes are identical in both types of FADs. However, as both types of FADs aggregate tunas and both attraction and retention behaviours have been observed at anchored and drifting FADs, it is difficult to imagine that the behavioural processes could be very different. At least, if any difference is observed, it is difficult to say whether the differences are due to the type of FADs or to any other stimuli (internal or external) that could play a role in this associative behaviour. In fact, studying the respective roles of these different stimuli on the behaviour of tunas at FADs would answer the question of the ecological trap hypothesis. A major behavioural parameter that is needed to elucidate if tunas can be trapped in a network of FADs is the motivation driving
a tuna to become associated with a floating object (or, the probability of joining a floating object when it is encountered). Hallier and Gaertner (2008) considered that this parameter is constant, which they expressed as “In this altered environment [increased number of FADs in the ocean], tunas leaving a FAD have a high probability of encountering other FADs”, although the debate is still open to know whether this parameter is indeed constant or not. For instance, Dagorn et al. (2000) proposed that the motivation for tunas to aggregate may vary with their internal state. We consider that studying this parameter for anchored FADs (such as measuring the condition factor of fish tracked around FADs) would help the scientific community to better assess what may be occurring at drifting FADs.
5. Conclusions and perspectives The ecological trap hypothesis is an hypothesis that speculates about the consequences of the addition of new floating objects (artificial ones called FADs) on the migratory behaviour of tunas, with possible consequences on their overall biology. This hypothesis, although initially advanced for drifting FADs, can be asked for any artificial floating objects deployed in the habitat of tunas, including anchored FADs. Different scientific studies have shown that both types of FADs (anchored and drifting FADs) can attract and retain tunas. The problem is to know if findings on the behaviour of tunas at anchored FADs can be used to understand the behaviour of tunas at drifting FADs. First, there is no reason to assume that the behavioural processes would be different as both attract and retain tunas with similar efficacy. However, even if they are different, understanding the behavioural processes that drive the behaviour of tunas at anchored FADs would certainly help in designing studies for drifting FADs. This corresponds to the comparative approach often effectively employed in ecology. Using anchored FADs to study the ecological trap hypothesis provides many advantages. Anchored FADs are easy to monitor, as they are close to land and a network of anchored FADs is spatially static. Residence times at FADs or within the network can be monitored over long periods (Ohta and Kakuma, 2005; Dagorn et al., 2007b). The array of anchored FADs around the island of Oahu (Hawaii, USA), for example, has been monitored continuously since 2002 (Itano, pers. comm.). Nearshore waters around islands with anchored FADs can therefore represent ideal open laboratories for investigating the effects of FADs or the environment on the behaviour of fish. The exact number of anchored FADs is also easy to monitor while this is a complicated and difficult task for drifting FADs. Precise data on environmental conditions can also be more easily collected in nearshore waters as they are more accessible, although less-precise remote sensing data can be collected anywhere. Tropical tuna purse seiners regularly deploy many drifting FADs throughout the world’s oceans and this trend raises concerns with the various RFMOs. Despite the fact that drifting FADs contribute to increased tuna catches and by-catches, it is of utmost importance to investigate whether FADs also act as ecological traps. However, working on drifting FADs usually requires major funding and resources, while studies on anchored FADs are generally easier to conduct. By showing that research on anchored FADs would also provide appropriate information to determine whether drifting FADs act as ecological traps, we aim at encouraging more scientists and funding agencies to invest in this topic. Restricting the investigations to only include drifting FADs would result in small sample sizes and studies with short time periods and would not serve the urgent need for science-based advice to RFMOs in charge of the management of drifting FAD-based fisheries.
L. Dagorn et al. / Fisheries Research 106 (2010) 60–63
References Battin, J., 2004. When good animals love bad habitats: ecological traps and conservation of animal populations. Conserv. Biol. 18 (6), 1482–1491. Brill, R.W., Block, B.A., Boggs, C.H., Bigelow, K.A., Freund, E.V., Marcinek, D.J., 1999. Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes. Mar. Biol. 133, 395–408. Castro, J.J., Santiago, J.A., Santana-Ortega, A.T., 2002. A general theory on fish aggregation to floating objects: an alternative to the meeting point hypothesis. Rev. Fish Biol. Fish. 11, 255–277. Dagorn, L., Fréon, P., 1999. Tropical tuna associated with floating objects: a simulation study of the meeting point hypothesis. Can. J. Fish. Aquat. Sci. 56 (6), 984–993. Dagorn, L., Josse, E., Bach, P., 2000. Individual differences in horizontal movements of yellowfin tuna (Thunnus albacares) in nearshore areas is French Polynesia, determined using ultrasonic telemetry. Aquat. Living Resour. 13, 193–202. Dagorn, L., Bertrand, A., Bach, P., Petit, M., Josse, E., 2001. Improving our understanding of tropical tuna movements from small to large scales. In: Sibert, J.R., Nielsen, J. (Eds.), Electronic Tagging and Tracking in Marine Fisheries. Kluwer Academic Publishers, The Netherlands, pp. 385–407. Dagorn, L., Pincock, D., Girard, C., Holland, K., Taquet, M., Sancho, G., Itano, D., Aumeeruddy, R., 2007a. Satelite-linked acoustic receivers to observe behaviour of fish in remote areas. Aquat. Living Resour. 20, 303–312. Dagorn, L., Holland, K.N., Itano, D.G., 2007b. Behaviour of yellowfin (Thunnus albacares) and bigeye (T. obesus) tuna in a network of fish aggregating devices (FADs). Mar. Biol. 151, 595–606. Dwernychuk, L.W., Boag, D.A., 1972. Ducks nesting in association with gulls: an ecological trap? Can. J. Zool. 50, 563–599. Dempster, T., Taquet, M., 2004. Fish aggregating device (FAD) research: gaps in current knowledge and future directions for ecological studies. Rev. Fish Biol. Fish. 14, 21–42. Fauvel, T., Bez, N., Walker, E., Delgado, A., Murua, H., Chavance, P., Dagorn, L., 2009. Comparative study of the distribution of natural versus artificial drifting. In: Fish Aggregating Devices (FADs) in the Western Indian Ocean, IOTC-2009-WPTT-19, 17 pp. Fonteneau, A., Ariz, J., Gaertner, D., Nordstrom, V., Pallares, P., 2000. Observed changes in the species composition of the tuna schools in the Gulf of Guinea between 1981 and 1999, in relation with the Fish Aggregating Device fishery. Aquat. Living Resour. 13, 253–257. Fréon, P., Dagorn, L., 2000. Review of fish associative behaviour: towards a generalisation of the meeting point hypothesis. Rev. Fish. Biol. Fish. 10, 183–207. Girard, C., Benhamou, S., Dagorn, 2004. FAD: fish aggregating device or fish attracting device? A new analysis of yellowfin tuna movements around floating objects. Anim. Behav. 67, 319–326. Hall, M., 1992. The association of tuna with floating objects and dolphins in the Eastern Pacific Ocean. VII. Some hypotheses on the mechanisms governing the
63
associations of tunas with floating objects and dolphins. In: Proceedings of the International workshop on Fishing for Tunas Associated with Floating Objects, La Jolla, CA, February 11–14, 6 pp. Hallier, J.P., Gaertner, D., 2008. Drifting fish aggregation devices could act as an ecological trap for tropical tuna species. Mar. Ecol. Prog. Ser. 353, 255–264. Holland, K.N., Brill, R.W., Chang, R.K.C., 1990. Horizontal and vertical movements of yellowfin and bigeye tunas associated with Fish Aggregating Devices. Fish. Bull. 88, 493–507. Marsac, F., Cayré, P., 1998. Telemetry applied to behaviour of yellowfin tuna (Thunnus albacares, Bonnaterre, 1788) movements in a network of fish aggregating devices. Hydrobiologia 371–372, 155–171. Marsac, F., Fonteneau, A., Ménard, F., 2000. Drifting FADs used in tuna fisheries: and ecological trap? In: Le Gall, J.Y., Cayré, P., Taquet, M. (Eds.), Pêche thonière et dispositifs de concentration de poisons. Actes Colloq. IFREMER 28, 537–552. Matsumoto, T., Okamoto, H., Toyonaga, M., 2006. Behavioural Study of Small Bigeye, Yellowfin and Skipjack Tunas Associated with Drifting FADs using Ultrasnoic Coded Transmitter in the Central Pacific Ocean. Western and Central Pacific Fisheries Commission, SC2-2006/FT IP-7, pp. 1–25. Meyer, C.G., Dale, J.J., Papastamatiou, Y.P., Whitney, N.M., Holland, K.N., 2009. Seasonal cycles and long-term trends in abundance and species composition of sharks associated with cage diving ecotourism activities. Environ. Cons. 36 (2), 1–8. Moreno, G., Dagorn, L., Sancho, G., Itano, D., 2007. Fish behaviour from fishers’ knowledge: the case study of tropical tuna around drifting fish aggregating devices (DFADs). Can. J. Fish. Aquat. Sci. 64, 1517–1528. Schaefer, K.M., Fuller, D.W., 2002. Movements, behavior, and habitat selection of bigeye tuna (Thunnus obesus) in the eastern equatorial Pacific, ascertained through archival tags. Fish. Bull. 100, 765–788. Schaefer, K.M., Fuller, D.W., 2005. Behaviour of bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis) tunas within aggregations associated with floating objects in the equatorial eastern Pacific. Mar. Biol. 146, 781–792. Schaefer, K.M., Fuller, D.W., 2007. Vertical movement patterns of skipjack tuna (Katsuwonus pelamis) in the eastern equatorial Pacific Ocean, as revealed with archival tags. Fish. Bull. 105, 379–389. Schlaepfer, M.A., Runge, M.C., Sherman, P.W., 2002. Ecological and evolutionary traps. Trends Ecol. Evol. 17 (10), 474–480. Ohta, I., Kakuma, S., 2005. Periodic behaviour and residence time of yellowfin and bigeye tuna associated with fish aggregating devices around Okinawa Islands, as identified with automated listening stations. Mar. Biol. 146, 581–594. Soria, M., Dagorn, L., Potin, G., Fréon, P., 2009. First field-based experiment supporting the meeting point hypothesis for schooling in pelagic fish. Anim. Behav. 78 (6), 1441–1446. Taquet, M., Sancho, G., Dagorn, L., Gaertner, J.C., Itano, D., Aumeeruddy, R., Wendling, B., Peignon, C., 2007. Characterizing fish communities associated with drifting fish aggregating devices (FADs) in the Western Indian Ocean using underwater visual surveys. Aquat. Living Resour. 30, 331–341.
Contents lists available at ScienceDirect
Fisheries Research journal homepage: www.elsevier.com/locate/fishres
Are drifting FADs essential for testing the ecological trap hypothesis? Laurent Dagorn a,∗ , Kim N. Holland b , John Filmalter a,c a
Institut de Recherche pour le Développement (IRD), UMR 212, P.O. Box 570, Victoria, Seychelles Hawaiian Institute of Marine Biology (HIMB),University of Hawaii, P.O. Box 1346, Kaneohe, HI 96744, USA c South African Institute for Aquatic Biodiversity (SAIAB), Private Bag 1015, Grahamstown 6140, South Africa b
a r t i c l e
i n f o
Article history: Received 11 May 2010 Received in revised form 25 June 2010 Accepted 12 July 2010 Keywords: FAD Ecological trap hypothesis Tunas
a b s t r a c t Because tropical tunas are known to aggregate around floating objects, it has been suggested that the large number of drifting fish aggregating devices (FADs) built and deployed by purse seiners could act as an ‘ecological trap’. This hypothesis states that these networks of drifting FADs could take fish to areas where they would not normally go or retain them in places that they would otherwise leave. Because the ecological trap hypothesis was first advanced for drifting FADs, some have argued that only studies using drifting FADs can test this hypothesis. However, because working with drifting FADs is difficult, accepting this precept would preclude the scientific community from providing urgently needed information to organizations charged with management of fisheries that exploit drifting FADs. We argue that because both anchored and drifting FADs alter the natural environment, the more easily accessible anchored FADs can be used to test the ecological trap hypothesis. Also, based on a comparative scientific approach, we argue that understanding the behaviour of tunas around anchored FADs can improve our general understanding of tunas around all types of floating objects and help design new, well focused studies for drifting FADs. As anchored FADs are easier to access and offer a greater potential for research, we encourage scientists to design and conduct studies (in particular on the behaviour of fish at FADs) around the moored structures. © 2010 Published by Elsevier B.V.
1. Introduction Floating objects are known to aggregate pelagic fish (Castro et al., 2002; Dempster and Taquet, 2004; Taquet et al., 2007). This behaviour is exploited by fishermen who look for natural floating objects (e.g. logs) or build and deploy artificial fish aggregating devices (FADs). In the past two decades, this phenomenon has become particularly important for industrial tuna purse seine fisheries which regularly deploy large numbers of drifting FADs in the tropical oceans (Fonteneau et al., 2000; Moreno et al., 2007). In addition to the large number of tunas caught this way (Fonteneau et al., 2000), some scientists have proposed that FADs could have an additional deleterious impact on populations of tunas by acting as ecological traps (Marsac et al., 2000; Hallier and Gaertner, 2008). That is, by modifying the pelagic environment (Fauvel et al., 2009), artificial drifting FADs could have major effects on the behaviour and biology of tunas. It is hypothesised that tunas could be trapped within networks of artificial drifting FADs and alter their natural movements towards feeding or spawning areas (Marsac et al., 2000). These networks of drifting FADs could take fish asso-
∗ Corresponding author. Tel.: +248 224742; fax: +248 224742. E-mail addresses: [email protected] (L. Dagorn), [email protected] (K.N. Holland), jdfi[email protected] (J. Filmalter). 0165-7836/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.fishres.2010.07.002
ciated with them to areas where they would not naturally go or retain them in places that they would otherwise leave. These areas could be biologically inappropriate and the biology (e.g. growth) of the tunas would then be negatively affected by the concomitant changes in migration route or habitat selection. Dedicated field research is clearly needed to investigate whether FADs act as ecological traps for fishes. Hallier and Gaertner (2008) analyzed data that were opportunistically collected during previous studies and their results supported the ecological trap hypothesis. However, we feel that more research is clearly needed with protocols specifically designed to address this question, particularly regarding the behaviour of these fishes. Working on offshore drifting FADs requires major financial cost and logistical support. This explains why most behavioural studies on FAD-associated fishes have focused on anchored FADs usually moored within a few miles of the coast (Dempster and Taquet, 2004) or sometimes in offshore waters (Schaefer and Fuller, 2005, 2007). It is only recently that scientists have started to investigate the behaviour of tunas around drifting FADs (Schaefer and Fuller, 2002; Matsumoto et al., 2006; Dagorn et al., 2007a; Moreno et al., 2007). Because the ecological trap hypothesis was first advanced for drifting FADs, some have argued that only studies using drifting FADs can test this hypothesis. However, because working with drifting FADs is difficult, accepting this precept would preclude the scientific community from providing urgently needed information
L. Dagorn et al. / Fisheries Research 106 (2010) 60–63
to organizations (such as RFMOs) in charge of the management of fisheries that exploit drifting FADs. Our objective here is to examine the theoretical aspects of the ecological trap hypothesis and assess objectively if and how studies on anchored FADs could help the investigation of this hypothesis. 2. The origin of the ecological trap hypothesis Among the 16 different hypotheses that Fréon and Dagorn (2000) reviewed to explain why some fishes associate with natural or artificial items, two hypotheses for tunas seem more credible to the scientific community (Hallier and Gaertner, 2008): the meeting point (Dagorn and Fréon, 1999) and the indicator-log (Hall, 1992) hypotheses. FADs as meeting points would provide social advantages such as enhancing schooling behaviour (which has recently been proven for a small pelagic fish species, the bigeye scad Selar crumenophthalmus, Soria et al., 2009) while floating objects as indicators of prey-rich waters would provide foraging advantages. These two hypotheses on the origins of the associative behaviour are not in conflict and could operate simultaneously (Fréon and Dagorn, 2000). The ecological trap hypothesis is not an hypothesis to explain why fishes associate to floating objects. Rather, it speculates about the possible consequences of changes in the natural environment on the behaviour of animals (with subsequent impacts on other aspects of their biology). The ecological trap is an ecological concept developed over 30 years ago (Dwernychuk and Boag, 1972). An ecological trap is a specific type of evolutionary trap in which cues for habitat choice become less reliable (Schlaepfer et al., 2002). In an ecological trap, animals make errors in habitat assessment by attending to inappropriate environmental cues (Battin, 2004). Often, but not always, inappropriate cues are a result of anthropogenic influences. In the case of tropical tunas and FADs, the ecological trap was invoked because of the thousands of drifting FADs regularly deployed on the open oceans by fishermen (Moreno et al., 2007) in locations where this number of floating objects would not naturally occur. These FADs modify the natural habitat of tunas (Fauvel et al., 2009). The hypothesis of FADs acting as an ecological trap was first proposed by Marsac et al. (2000) who postulated that the large numbers of drifting FADs in the equatorial zone may alter the natural movements of these fishes towards feeding grounds. If the influence of floating objects on tunas is sufficiently strong, anthropogenic changes in the spatio-temporal distribution of floating objects could modify their natural migrations. This, in turn, could have repercussions for the overall health of the population (Marsac et al., 2000; Hallier and Gaertner, 2008). Although not stated as such by Marsac et al. (2000), and later by Hallier and Gaertner (2008), the FAD ecological trap hypothesis (as they formulated it) is actually an extension of the indicator-log hypothesis (Hall, 1992) which is one of the two main hypotheses advanced to explain why tunas could have developed this associative behaviour. It stipulates that natural floating objects (such as logs) are often indicators of productive areas, either because most natural floating objects originate in rich areas (i.e. river mouth, mangrove swamps) and remain within these rich bodies of water as they drift offshore, or because they become entrained in rich frontal zones regardless of their original source. Because of a possible strong spatial correlation between natural floating objects and rich waters, tunas could have evolved to use those objects as cues to find or stay within high quality areas. If the indicator-log hypothesis is correct (it has not yet been validated), the ‘random’ release of artificial drifting FADs by fishing boats could obviously disrupt the spatial correlation between floating objects and rich waters. If tunas have developed this associative behaviour because it was providing them with a social benefit (meeting point hypoth-
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esis), the deployment of large numbers of FADs may have effects on school size (either increasing it by facilitating school formation or decreasing it by providing multiple alternative sites for school formation). In this case, an ecological trap would only occur if largescale movements are dependent on the sizes of tuna schools. This was not tackled by either Marsac et al. (2000) and Hallier and Gaertner (2008) and was never advanced by any author previously. Consequently, we will not study this type of ecological trap in this document. Of course, the possibility exists that regardless of the reason for associating with floating objects (e.g. ‘indicator log’ or ‘meeting’ point), this behaviour is abandoned when other motivations (such as spawning migrations or hunger due to poor feeding success) become dominant. Such a phenomenon was recently demonstrated in shark species that showed seasonal fluctuations in participating in anthropogenically induced associations with dive tourism operations (Meyer et al., 2009).
3. The ecological trap hypothesis and anchored FADs The ecological trap hypothesis (in its “indicator-log” form) was first advanced for drifting FADs. But any artificial floating object arriving in the ocean modifies the natural environment of tunas where only natural floating objects would traditionally be encountered. In this sense, anchored FADs also modify the natural environment of tunas. In the same way that Marsac et al. (2000) and Hallier and Gaertner (2008) wondered if drifting FADs deployed in the ocean would modify the natural large-scale movements of tunas, one can wonder if the deployment of anchored FADs would not also modify the natural large-scale movements of tunas. The generic question of whether FADs can act as ecological traps is not necessarily restricted to drifting FADs but can also be asked about anchored FADs. The issue is to know if any results obtained about whether anchored FADs act as ecological traps could be used to determine if drifting FADs also act as ecological traps (and vice versa). Although drifting FADs are the most abundant worldwide, a comparative analysis should take into account densities, not numbers.
4. Behaviour of tunas at FADs Both anchored and drifting FADs are known to aggregate tunas but some scientists propose that because anchored FADs are moored (i.e. have a mooring line) and drifting FADs drift (no mooring line), this difference may result in different phenomena underlying the formation of the aggregations of tunas associated with the different types of FAD. This associative behaviour can be expressed as the combination of two behaviours (Girard et al., 2004): an attraction behaviour and a retention behaviour. Considering that only one is necessary to obtain aggregations but both can also occur simultaneously, it is appropriate to compare the current scientific knowledge about tunas around anchored and drifting FADs. Using data from active tracking studies on yellowfin tuna (Holland et al., 1990; Marsac and Cayré, 1998; Brill et al., 1999; Dagorn et al., 2000) around anchored FADs, Girard et al. (2004) could demonstrate that yellowfin tuna are able to orient towards anchored FADs from a distance of up to 10 km (see Table 1). No similar active tracking study on tunas around drifting FADs has yet been conducted but without prior knowledge of the results on anchored FADs, skippers of tuna purse seiners gave the same value for the distance from which tunas could orient towards drifting FADs (Moreno et al., 2007). This congruence strongly suggests that there is no major difference in the distance of orientation of tunas to anchored or drifting FADs (Moreno et al., 2007).
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Table 1 Scientific evidence that tunas can be attracted to FADs, and can be retained at FADs.
Attraction behaviour
Retention behaviour
Anchored FADs
Drifting FADs
Girard et al. (2004) and all active tracking studies on which this study was based on (Holland et al., 1990; Marsac and Cayré, 1998; Brill et al., 1999; Dagorn et al., 2000) Ohta and Kakuma (2005), Dagorn et al. (2007b)
Moreno et al. (2007)
Schaefer and Fuller (2002), Matsumoto et al. (2006), Dagorn et al. (2007a)
Using passive telemetry, Ohta and Kakuma (2005) and Dagorn et al. (2007b) have shown that anchored FADs retain yellowfin and bigeye tunas for about a week in average (ranging from a few minutes up to 64 days). Values found for drifting FADs are inferior but of the same order of magnitude: average of 3 days for bigeye tuna in the Eastern Pacific Ocean (maximum of 7 days), maximum values up to 15 days in the Central Pacific Ocean (Matsumoto et al., 2006; Dagorn et al., 2007a), the average values being difficult to provide for these last two studies due to frequent interruptions of monitoring. In other words, both types of FADs can retain tunas (see Table 1). There are therefore no scientific results to indicate that the associative behaviour of anchored and drifting FADs results from different behavioural processes. An ecological trap is a consequence of a behaviour that has become maladapted. The study of the ecological trap therefore requires the analysis of empirical behavioural data (e.g. long term movements from conventional tagging in Hallier and Gaertner, 2008) or active and passive tracking (see above) or biological consequences of the behaviour (e.g. condition factors, see Hallier and Gaertner, 2008). A behaviour is a set of actions or reactions of an animal in response to external or internal stimuli. The study of the behaviour of tunas should therefore take into consideration the following factors (Dagorn et al., 2001): • Presence of FADs (e.g. densities of FADs). • Characteristics of the abiotic environment (mainly the oceanography of the area, e.g. water temperature, dissolved oxygen, etc.). • Characteristics of the biotic environment (prey, predators, competitors, conspecifics—e.g. schooling behaviour). • The internal state of the individuals. A comparative approach (Dagorn et al., 2001) would help to better understand the behaviour of tunas at both types of FADs. However, as a start, any understanding of the behavioural processes (in particular the respective roles of the different stimuli listed above) driving the association of tunas with anchored FADs would help the study of the behaviour at drifting FADs. Of course, this would not prove that the behavioural processes are identical in both types of FADs. However, as both types of FADs aggregate tunas and both attraction and retention behaviours have been observed at anchored and drifting FADs, it is difficult to imagine that the behavioural processes could be very different. At least, if any difference is observed, it is difficult to say whether the differences are due to the type of FADs or to any other stimuli (internal or external) that could play a role in this associative behaviour. In fact, studying the respective roles of these different stimuli on the behaviour of tunas at FADs would answer the question of the ecological trap hypothesis. A major behavioural parameter that is needed to elucidate if tunas can be trapped in a network of FADs is the motivation driving
a tuna to become associated with a floating object (or, the probability of joining a floating object when it is encountered). Hallier and Gaertner (2008) considered that this parameter is constant, which they expressed as “In this altered environment [increased number of FADs in the ocean], tunas leaving a FAD have a high probability of encountering other FADs”, although the debate is still open to know whether this parameter is indeed constant or not. For instance, Dagorn et al. (2000) proposed that the motivation for tunas to aggregate may vary with their internal state. We consider that studying this parameter for anchored FADs (such as measuring the condition factor of fish tracked around FADs) would help the scientific community to better assess what may be occurring at drifting FADs.
5. Conclusions and perspectives The ecological trap hypothesis is an hypothesis that speculates about the consequences of the addition of new floating objects (artificial ones called FADs) on the migratory behaviour of tunas, with possible consequences on their overall biology. This hypothesis, although initially advanced for drifting FADs, can be asked for any artificial floating objects deployed in the habitat of tunas, including anchored FADs. Different scientific studies have shown that both types of FADs (anchored and drifting FADs) can attract and retain tunas. The problem is to know if findings on the behaviour of tunas at anchored FADs can be used to understand the behaviour of tunas at drifting FADs. First, there is no reason to assume that the behavioural processes would be different as both attract and retain tunas with similar efficacy. However, even if they are different, understanding the behavioural processes that drive the behaviour of tunas at anchored FADs would certainly help in designing studies for drifting FADs. This corresponds to the comparative approach often effectively employed in ecology. Using anchored FADs to study the ecological trap hypothesis provides many advantages. Anchored FADs are easy to monitor, as they are close to land and a network of anchored FADs is spatially static. Residence times at FADs or within the network can be monitored over long periods (Ohta and Kakuma, 2005; Dagorn et al., 2007b). The array of anchored FADs around the island of Oahu (Hawaii, USA), for example, has been monitored continuously since 2002 (Itano, pers. comm.). Nearshore waters around islands with anchored FADs can therefore represent ideal open laboratories for investigating the effects of FADs or the environment on the behaviour of fish. The exact number of anchored FADs is also easy to monitor while this is a complicated and difficult task for drifting FADs. Precise data on environmental conditions can also be more easily collected in nearshore waters as they are more accessible, although less-precise remote sensing data can be collected anywhere. Tropical tuna purse seiners regularly deploy many drifting FADs throughout the world’s oceans and this trend raises concerns with the various RFMOs. Despite the fact that drifting FADs contribute to increased tuna catches and by-catches, it is of utmost importance to investigate whether FADs also act as ecological traps. However, working on drifting FADs usually requires major funding and resources, while studies on anchored FADs are generally easier to conduct. By showing that research on anchored FADs would also provide appropriate information to determine whether drifting FADs act as ecological traps, we aim at encouraging more scientists and funding agencies to invest in this topic. Restricting the investigations to only include drifting FADs would result in small sample sizes and studies with short time periods and would not serve the urgent need for science-based advice to RFMOs in charge of the management of drifting FAD-based fisheries.
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References Battin, J., 2004. When good animals love bad habitats: ecological traps and conservation of animal populations. Conserv. Biol. 18 (6), 1482–1491. Brill, R.W., Block, B.A., Boggs, C.H., Bigelow, K.A., Freund, E.V., Marcinek, D.J., 1999. Horizontal movements and depth distribution of large adult yellowfin tuna (Thunnus albacares) near the Hawaiian Islands, recorded using ultrasonic telemetry: implications for the physiological ecology of pelagic fishes. Mar. Biol. 133, 395–408. Castro, J.J., Santiago, J.A., Santana-Ortega, A.T., 2002. A general theory on fish aggregation to floating objects: an alternative to the meeting point hypothesis. Rev. Fish Biol. Fish. 11, 255–277. Dagorn, L., Fréon, P., 1999. Tropical tuna associated with floating objects: a simulation study of the meeting point hypothesis. Can. J. Fish. Aquat. Sci. 56 (6), 984–993. Dagorn, L., Josse, E., Bach, P., 2000. Individual differences in horizontal movements of yellowfin tuna (Thunnus albacares) in nearshore areas is French Polynesia, determined using ultrasonic telemetry. Aquat. Living Resour. 13, 193–202. Dagorn, L., Bertrand, A., Bach, P., Petit, M., Josse, E., 2001. Improving our understanding of tropical tuna movements from small to large scales. In: Sibert, J.R., Nielsen, J. (Eds.), Electronic Tagging and Tracking in Marine Fisheries. Kluwer Academic Publishers, The Netherlands, pp. 385–407. Dagorn, L., Pincock, D., Girard, C., Holland, K., Taquet, M., Sancho, G., Itano, D., Aumeeruddy, R., 2007a. Satelite-linked acoustic receivers to observe behaviour of fish in remote areas. Aquat. Living Resour. 20, 303–312. Dagorn, L., Holland, K.N., Itano, D.G., 2007b. Behaviour of yellowfin (Thunnus albacares) and bigeye (T. obesus) tuna in a network of fish aggregating devices (FADs). Mar. Biol. 151, 595–606. Dwernychuk, L.W., Boag, D.A., 1972. Ducks nesting in association with gulls: an ecological trap? Can. J. Zool. 50, 563–599. Dempster, T., Taquet, M., 2004. Fish aggregating device (FAD) research: gaps in current knowledge and future directions for ecological studies. Rev. Fish Biol. Fish. 14, 21–42. Fauvel, T., Bez, N., Walker, E., Delgado, A., Murua, H., Chavance, P., Dagorn, L., 2009. Comparative study of the distribution of natural versus artificial drifting. In: Fish Aggregating Devices (FADs) in the Western Indian Ocean, IOTC-2009-WPTT-19, 17 pp. Fonteneau, A., Ariz, J., Gaertner, D., Nordstrom, V., Pallares, P., 2000. Observed changes in the species composition of the tuna schools in the Gulf of Guinea between 1981 and 1999, in relation with the Fish Aggregating Device fishery. Aquat. Living Resour. 13, 253–257. Fréon, P., Dagorn, L., 2000. Review of fish associative behaviour: towards a generalisation of the meeting point hypothesis. Rev. Fish. Biol. Fish. 10, 183–207. Girard, C., Benhamou, S., Dagorn, 2004. FAD: fish aggregating device or fish attracting device? A new analysis of yellowfin tuna movements around floating objects. Anim. Behav. 67, 319–326. Hall, M., 1992. The association of tuna with floating objects and dolphins in the Eastern Pacific Ocean. VII. Some hypotheses on the mechanisms governing the
63
associations of tunas with floating objects and dolphins. In: Proceedings of the International workshop on Fishing for Tunas Associated with Floating Objects, La Jolla, CA, February 11–14, 6 pp. Hallier, J.P., Gaertner, D., 2008. Drifting fish aggregation devices could act as an ecological trap for tropical tuna species. Mar. Ecol. Prog. Ser. 353, 255–264. Holland, K.N., Brill, R.W., Chang, R.K.C., 1990. Horizontal and vertical movements of yellowfin and bigeye tunas associated with Fish Aggregating Devices. Fish. Bull. 88, 493–507. Marsac, F., Cayré, P., 1998. Telemetry applied to behaviour of yellowfin tuna (Thunnus albacares, Bonnaterre, 1788) movements in a network of fish aggregating devices. Hydrobiologia 371–372, 155–171. Marsac, F., Fonteneau, A., Ménard, F., 2000. Drifting FADs used in tuna fisheries: and ecological trap? In: Le Gall, J.Y., Cayré, P., Taquet, M. (Eds.), Pêche thonière et dispositifs de concentration de poisons. Actes Colloq. IFREMER 28, 537–552. Matsumoto, T., Okamoto, H., Toyonaga, M., 2006. Behavioural Study of Small Bigeye, Yellowfin and Skipjack Tunas Associated with Drifting FADs using Ultrasnoic Coded Transmitter in the Central Pacific Ocean. Western and Central Pacific Fisheries Commission, SC2-2006/FT IP-7, pp. 1–25. Meyer, C.G., Dale, J.J., Papastamatiou, Y.P., Whitney, N.M., Holland, K.N., 2009. Seasonal cycles and long-term trends in abundance and species composition of sharks associated with cage diving ecotourism activities. Environ. Cons. 36 (2), 1–8. Moreno, G., Dagorn, L., Sancho, G., Itano, D., 2007. Fish behaviour from fishers’ knowledge: the case study of tropical tuna around drifting fish aggregating devices (DFADs). Can. J. Fish. Aquat. Sci. 64, 1517–1528. Schaefer, K.M., Fuller, D.W., 2002. Movements, behavior, and habitat selection of bigeye tuna (Thunnus obesus) in the eastern equatorial Pacific, ascertained through archival tags. Fish. Bull. 100, 765–788. Schaefer, K.M., Fuller, D.W., 2005. Behaviour of bigeye (Thunnus obesus) and skipjack (Katsuwonus pelamis) tunas within aggregations associated with floating objects in the equatorial eastern Pacific. Mar. Biol. 146, 781–792. Schaefer, K.M., Fuller, D.W., 2007. Vertical movement patterns of skipjack tuna (Katsuwonus pelamis) in the eastern equatorial Pacific Ocean, as revealed with archival tags. Fish. Bull. 105, 379–389. Schlaepfer, M.A., Runge, M.C., Sherman, P.W., 2002. Ecological and evolutionary traps. Trends Ecol. Evol. 17 (10), 474–480. Ohta, I., Kakuma, S., 2005. Periodic behaviour and residence time of yellowfin and bigeye tuna associated with fish aggregating devices around Okinawa Islands, as identified with automated listening stations. Mar. Biol. 146, 581–594. Soria, M., Dagorn, L., Potin, G., Fréon, P., 2009. First field-based experiment supporting the meeting point hypothesis for schooling in pelagic fish. Anim. Behav. 78 (6), 1441–1446. Taquet, M., Sancho, G., Dagorn, L., Gaertner, J.C., Itano, D., Aumeeruddy, R., Wendling, B., Peignon, C., 2007. Characterizing fish communities associated with drifting fish aggregating devices (FADs) in the Western Indian Ocean using underwater visual surveys. Aquat. Living Resour. 30, 331–341.