Influence of fish aggregating devices (FADs) on anti-predator behaviour within experimental mesocosms
Introduction
In open ocean environments, many pelagic species are attracted to physical structures such as drifting algal mats (Ida et al., 1967, Casazza and Ross, 2008), animal carcasses (Castro et al., 2002), jellyfish (Masuda et al., 2008, Masuda, 2009), pillars (Hunter and Mitchell, 1967, Kingsford, 1993, Fréon and Dagorn, 2000), petroleum platforms (Hastings et al., 1976, Franks, 2000), discarded fishing gear (Carr, 1987), rafts (Shomura and Matsumoto, 1982), and trash (Riera et al., 1999). These structures, commonly referred to as fish aggregating devices (FADs), facilitate spatially heterogenetic aggregations of marine species in otherwise oligotrophic waters. Such aggregations have been well documented for hundreds of fish species throughout the world's oceans (Castro et al., 2002).
It is commonly believed that aggregative behaviour in fish, either obligatory or facultative, likely evolved to reduce individual predation risk (Hamilton, 1971), enhance food detection (Stephens and Krebs, 1987), or increase mating success (Pitcher and Parrish, 1993). Schooling behaviour is defined by further organization of the aggregate into synchronized groups whereby swimming speed and direction uniformly change to increase hydrodynamic efficiency (Weihs, 1973) or reduce predation risk (Brock and Riffenburgh, 1960). For small aggregative fish, FADs provide either direct protection from predation (Hunter and Mitchell, 1967, Rountree, 1989, Castro et al., 2002) or they facilitate the implementation of mimicry or camouflage techniques that reduce a predator's ability to detect and capture its prey (Hunter and Mitchell, 1967, Kingsford, 1993, Fréon and Dagorn, 2000).
In the southern Mediterranean Sea, the number of permanently moored offshore FADs has consistently increased in recent decades (Morales-Nin et al., 2000). Moored FADs are often placed at great depths (800–1000 m) by commercial fishers, and, despite their obvious differences, possess a similar function and attractive capacity compared to man-made drifting FADs employed by purse seine fisheries in tropical waters (Dempster and Taquet, 2004, Dagorn et al., 2013). In general, FADs exploit aggregative behaviour of both demersal juvenile and adult pelagic species (Andaloro et al., 2007, Sinopoli et al., 2011). Of these, greater amberjack Seriola dumerili (Risso, 1810) and blue runner Caranx crysos (Mitchill, 1815) are two of the most common FAD-aggregated species in the Mediterranean Sea during their juvenile and semi-adult stages (Andaloro et al., 2007, Sinopoli et al., 2011). In these early life stages, both species are considered opportunistic predators that consume a variety of crustacean and teleost prey (Badalamenti et al., 1995, Sley et al., 2009), including the consumption of C. crysos by S. dumerili (Stergiou and Karpouzi, 2002, Auster et al., 2009). As adults, however, both species establish themselves as demersal reef-associated piscivores (Cervigón et al., 1992, Riede, 2004) with little direct predatory interaction (Andaloro and Pipitone, 1997).
Often, large-scale quasi-natural experimentation is hindered because of physical constraints such as the inaccessibility (i.e. depth) to offshore FADs (Andaloro et al., 2007). As a result, predator-prey experiments that observed the defensive strategies of prey in relation to physical structure have largely been carried out in laboratory reconstruction systems that try to mirror the environment of natural habitats (Masuda and Tsukamoto, 2000, Shoji et al., 2007, Masuda, 2009). Similar to large-scale experiments, predator-prey interactions observed under laboratory settings are also subject to limitations such as the size of the prey, the size of the predator, and physical structure implemented in the mesocosm (Masuda, 2009). Consequently, the functional basis of aggregative behaviour in fish is currently based upon empirical observations and unsubstantiated hypotheses (Capello et al., 2012, Dagorn et al., 2013, Robert et al., 2013). Among these, the “shelter from predator (SfP)” hypothesis (Hunter and Mitchell, 1967, Castro et al., 2002) attempts to disentangle the function of these behaviours in small aggregative fish.
The SfP hypothesis describes anti-predator behaviour of small fish when they are aggregated near FADs in open ocean environments (Fréon and Dagorn, 2000). Castro et al. (2002) proposed that post-larval and juvenile fish might aggregate with FADs in order to increase their dispersal capabilities and likelihood of survival during phases when they are most vulnerable to predation. Despite the importance of understanding the behavioural aspects implicated in SfP, studies that examined predator-prey interactions in relation to physical structure in situ are limited. Furthermore, if one considers the escapement potential by smaller fish from the gear commonly used in the commercial FAD purse seine fishery (Sinopoli et al., 2012), the SfP hypothesis could play an important role affecting the mechanisms that underlie fish population structure and dynamics. Therefore, additional predator-prey experiments using large-scale mesocosms under quasi-natural conditions are required to examine the role of SfP on aggregative behaviour in fish.
This study examined the two primary traits implicated in SfP behaviour (i.e. schooling vs. aggregative) during predator-prey interactions between S. dumerili (predator) and C. crysos (prey) held within offshore mesocosms. Three hypotheses related to SfP behaviour were examined using a variety of treatments (e.g., the presence of physical structure and/or predators). First, C. crysos would display a greater tendency to form schools as a main defensive strategy in the absence of FADs when predators were present. Second, C. crysos would show a significant tendency to aggregate if provided with available structure when predators were present. Finally, C. crysos would display randomized swimming behaviour similar to fish associated with FADs under natural conditions when predators were not present. The results of this study are necessary for implementing proper management strategies under conditions of increasing FAD use in the southern Mediterranean Sea.
Section snippets
Study area
This study was conducted from August–October, 2010 in the Gulf of Castellammare, on the northern coast of Sicily (LAT 38°02′31″ N; LONG 12°55′28″ E). Experimental treatments were performed in both quasi-natural mesocosms (i.e. large aquaculture cages; 3000 m3, diameter 12 m, depth 6 m, mesh size of 12 mm) and open-sea environments. Mesocosms were positioned approximately 1.2 km off the coastline in the eastern Gulf of Castellammare, moored to the bottom at a depth of 32 m. Open-sea treatments
Schooling formation (Sc-Deg)
In the presence of predators, C. crysos displayed an immediate schooling behavioural response. Significant differences in Sc-Deg were observed between the treatments, time periods, and interaction of the two factors. The SNK test analysing treatments within periods showed a fast response by C. crysos to predator presence. During T1, prey in the NFP treatment showed a significantly higher Sc-Deg than prey confined within a mesocosm with available structure (i.e. FP; Table 2; Fig. 3). However,
Disentangling behavioural effects
Compared with natural conditions, C. crysos displayed similar Sc-Deg and Ag-Deg in our experimental mesocosms. Although differences in environmental conditions between coastal and offshore systems can influence fish behaviour, for the purposes of the present study, we assumed that behaviour and abundance of C. crysos and S. dumerili were not dependent upon the distance between the FADs and the coastline (Sinopoli et al., 2010, Sinopoli et al., 2011). This allowed us to exclude potential
Acknowledgements
This study was partially supported by POR Regione Siciliana research project to GS (contract number: POR 4.17 - 199.IT.16.1.PO.0.11/4.17b/8.3.7/00.53). We are grateful to local commercial fishers for their help collecting fish used in this study. We also thank Andrea Di Maria and Antonio Di Maria for collecting fish and helping with videography during our experimental treatments.
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