Are drifting FADs essential for testing the ecological trap hypothesis?
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 associated 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, Schaefer and Fuller, 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 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.
Section snippets
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
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
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.
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
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