First Field-Based Experiment Supporting the Meeting Point Hypothesis for Schooling in Pelagic Fish

Animal Behaviour 78 (2009) 1441–1446

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Animal Behaviour

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First ﬁeld-based experiment supporting the meeting point hypothesis for schooling in pelagic ﬁsh

Marc Soria a,*, Laurent Dagorn b,1, Gae¨l Potin c,2, Pierre Freón d,3 a UMR EME, Institut de Recherche pour le De´veloppement (IRD), La Reúnion b UMR EME, Institut de Recherche pour le De´veloppement (IRD), Seychelles c Laboratoire d’Ecologie Marine (ECOMAR), Universite´ de la Reúnion d UMR EME, Institut de Recherche pour le De´veloppement (IRD), Centre de Recherche Halieutique Me´diterraneénne et Tropicale article info Aggregations of fish around fish aggregation devices (FADs) have been widely described in the literature; Article history: most commercial catches of tuna by surface fisheries are performed around FADs, taking advantage of Received 17 May 2009 this behaviour. The meeting point hypothesis (MPH) suggests that fish could make use of FADs to Initial acceptance 2 July 2009 increase the chance of encounters between conspecifics, helping individuals to form larger schools. To Final acceptance 7 September 2009 attempt a validation of the MPH, we performed an experiment in the field to test the following Available online 25 October 2009 predictions: (1) fish spend more time at FADs than at any other random points and therefore aggregate MS. number: D-09-00317R1 around FADs, and (2) fish arrive at FADs as isolated individuals or in small groups and leave them in larger groups. Our investigation involved acoustic telemetry techniques commonly used to observe fish Keywords: at FADs. The study was carried out on a small pelagic fish species, the bigeye scad, Selar crumenoph- acoustic tagging thalmus, in Saint-Paul’s Bay (Reunion Island). Our results validated our two predictions: FADs acted as bigeye scad retention points, increasing the encounter rate of fish and enhancing schooling behaviour, thereby fish aggregation device meeting point hypothesis supporting the meeting point hypothesis. FADs could be beneficial to the fitness of the associated fish, schooling fish behaviour promote increased school size and hence confer the advantages of being in a larger group. The impact of Selar crumenophthalmus the deployment of large number of FADs in some ocean regions is reinterpreted in light of our results. Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

Group formation is a widespread phenomenon throughout the group formation and dispersion in the wild are poorly documented. animal kingdom. This social behaviour can be a permanent trait or Splitting or dispersion of a group can come from individual deci- occur only during some seasons or some parts of the life cycle. sions or result from processes linked to predation, foraging activity, Swarming insects, schooling fish, flocking birds and herd mammals loss of sensory contact or habitat features. The homogeneous and all illustrate this behaviour. Different reasons for grouping have expansive nature of certain habitats, such as the marine pelagic been proposed, including decreasing the predation risk, promoting domain, favours dispersion. Hence, for gregarious fish species some optimal foraging, increasing reproductive success and facilitating processes must enhance the gathering of individuals. The simplest migration and learning. There is a large body of literature on the way to meet is to go to the same place at the same time, which is dynamics of group size (including optimal group size) based on trivial for territorial species, but not so obvious for nomadic animals experimental observations and modelling (Mangel 1990; Giraldeau such as pelagic fish. This matter has led to studies of the impact of & Caraco 1993; Zemel & Lubin 1995; Parrish & Hamner 1997; fish aggregating devices (FADs) on fish behaviour and biology Bonabeau et al. 1999; Freón & Misund 1999; Krause & Ruxton (Gooding & Magnusson 1967; Castro et al. 2002; Dempster & 2002). However, the processes responsible for the dynamics of Taquet 2004). The use of FADs in tropical ocean fisheries is massive and, in the last 10 years, has been responsible for 50–70% of the total purse-seine catch of tuna in the western Indian Ocean

* Correspondence: M. Soria, IRD. Parc Technologique Universitaire, 2 rue Joseph (Fonteneau 2003). At any given time in this region, 2500 FADs are Wetzell, BP 172, 97492 Sainte Clotilde cedex, La Reúnion, France. estimated to be in use (Moreno et al. 2007). This led to the E-mail address: [email protected] (M. Soria). hypothesis that FADs may act as ecological traps (Marsac et al. 1 Laurent Dagorn is at IRD, BP 570, Victoria, Mahe´, Seychelles. 2000; Schlaepfer et al. 2002; Hallier & Gaertner 2008). Under this 2 Gae¨l Potin is at Universite´ de la Reúnion, 15 av. Rene´ Cassin, BP 7151, 97715 St hypothesis, the proliferation of FADs could cause fish to stay too Denis messag cedex 9, La Reúnion, France. 3 Pierre Freón is at Centre de Recherche Halieutique Me´diterraneénne et Tropi- long around FADs even when local conditions are unfavourable for cale, Avenue Jean Monnet, BP 171, 34203 Se`te Cedex, France. feeding, affecting their biology (e.g. growth).

0003-3472/$38.00 Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2009.09.025 1442 M. Soria et al. / Animal Behaviour 78 (2009) 1441–1446

Dagorn & Freón (1999) proposed the meeting point hypothesis (MPH) to explain how tropical tuna could enhance the aggregation St-Paul’s Bay of isolated individuals or small schools in vast environments. This –20.98 hypothesis suggests that fish could make use of floating objects to increase the chance of encounters between conspecifics, helping individuals to form larger schools. Freón & Dagorn (2000) gener- 1 alized this hypothesis to other marine species and other types of associations. However, thus far, no field data have been obtained to validate it. Here, we investigated the MPH through the study of the 100 m spatial dynamics of individual fish around FADs. Using a small c pelagic fish species, the bigeye scad, Selar crumenophthalmus,in 3 Saint-Paul’s Bay (Reunion Island), we aimed to test the following –21 2 predictions: (1) fish spend more time at FADs than at any other c 4 random points, and (2) fish arrive at FADs as isolated individuals or in small groups and leave them in larger groups. If fish spend more 40 m 6 time at FADs than around any random point, and if simultaneity of departure from a FAD is higher than that of arriving at a FAD, one 20 m 5 could consider that FADs increase the encounter rate of fish and 10 m enhance schooling behaviour. Our investigation involved acoustic 500 m receivers and coded transmitters, a technique commonly used for –21.02 observing associated fish (Klimley et al. 1988; Ohta & Kakuma 55.24 55.26 55.28 2005; Dagorn et al. 2007; Girard et al. 2007; Taquet et al. 2007). Figure 1. Map of Saint-Paul’s Bay with location of FADs (blue dots). The nine FADs METHODS equipped with VR2 Vemco receivers are identified (red disks) with their area of detection (outlined circles). The positions of the seven VR2 used as control listening stations (CSs) are indicated (green triangles) and fish release points are also shown Species and Study Site (+). The seventh CS was located far to the north and appears only in the upper box. The letter C indicates the position of aquaculture cages. The broken line represents Bigeye scad is a ubiquitous cosmopolitan species that occurs isobaths (m). Numbers adjacent to some of these FADs identify those used in Fig. 2. mostly in clear coastal waters in tropical and subtropical regions and naturally aggregates around coastal FADs (Roos et al. 2007). FADs located at a distance lower than the range of reliability, we This carangid is an obligate schooling fish species, travelling in deployed only one VR2 to monitor these neighbouring FADs. In this compact groups of hundreds or thousands of fish (Soria et al. 2007). case, to compensate for VR2s covering multiple FADs, we applied As such, this species appears to be an appropriate candidate for a weighted index to calculate the estimated residence time per FAD. investigating the MPH. This index took into account the number of FADs involved and the The study took place in Saint-Paul’s Bay, Reunion Island, an open theoretical detection areas which would have been covered by the 2 sandy bay with a surface area of about 8 km . Fifteen different VR2s, had they been deployed on each FAD. The estimated artificial structures were already present in the bay, anchored in continuous residence time CRTesti was of the general following water 15–50 m deep (Fig. 1). Some were small shallow-moored form: FADs specifically deployed for enhancing commercial fish capture; ! Xn Xn others were marker buoys delimiting a private aquaculture facility, CRT obs Anci þ CRT Aci a submerged shipwreck and two aquaculture cages, one with fishes n obs CRT ¼ i ¼ 1 i ¼ 1 (1) fed daily (Fig. 1), without any increased attractive effect on bigeye esti n scad (results not shown). The distance between adjacent structures ranged between 120 and 1020 m. where n is the number of multiple FADs, CRTobs is measured by the VR2, Anci is the percentage of the area around the FADi not covered Stationary Acoustic Array by the range of detection of the VR2 and Aci the percentage of the area around the FADi covered by the range of detection of the VR2. Acoustic receivers were deployed at FADs and at random points The residence time means of FADs and CSs were compared using away from the FADs, called control stations or CSs (Fig. 1). To a Student’s t test. monitor the behaviour of fish around FADs, we used coded acoustic tags and acoustic receivers (VR2) from Vemco (Vemco, Halifax, Capture, Tag Implantation and Releasing Strategy Canada). In situ range tests indicated that 80% of signals emitted during 1 h were detected at a range of 150 m. We therefore defined One hundred and five fish were captured on 19 May 2006 by this distance as the maximum range of reliability of the acoustic fishers around the aquaculture facility of Saint-Paul’s Bay using receivers. hand lines. Immediately after capture fish were transferred into To monitor all FADs, we deployed nine VR2s in total (seven oxygenated 60-litre buckets. Eighteen fish not big enough to be covering individual FADs and two covering groups, containing two tagged were released. A total of 87 fish were finally conveyed to the and five FADs, respectively). From here on, the term FAD is used to Aquarium of Reunion Island and housed in three holding tanks of refer to the floating structure(s) equipped with an acoustic receiver. 2.5 m3 each. A preventive treatment with methylene blue (3 mg/litre) We also deployed seven VR2s on CSs (Fig. 1). Receivers were fixed and copper sulphate (3 103 ml/litre) was applied. Fish were fed either to mooring ropes or on the bottom, mounted on anchored daily with a mixture of pieces of crustacean and fish flesh. On the 30 bases consisting of tyres filled with concrete (Taquet et al. 2006). and 31 May 2006, 46 fish were tagged with coded Vemco V7 Receivers on the bottom were installed 1 m above the seabed with transmitters (V7-2L-R04K, 69 kHz, random rate of transmission no surface structure to avoid creating new attraction devices. The every 70–140 s, rated battery life 130–150 days, length 18.5 mm, distance between adjacent FADs ranged from 310 to 1140 m. For diameter 7 mm, mass in water 0.75 g). The mean fish size SD, M. Soria et al. / Animal Behaviour 78 (2009) 1441–1446 1443 expressed in fork length was 18 1.2 cm (102 22 g). The weight of the transmitter expressed as a percentage of the animal’s weight 75 1 (N = 709) was 0.73% on average, ranging between 0.97% and 0.43% for the 2 (N = 717) 3 (N = 96) smallest and largest animal marked, respectively. Fish were 50 4 (N = 2628) anaesthetized in an aqueous solution of benzocaine (60 mg/litre) 5 (N = 1683) prior to tag implantation. Anaesthetized fish were placed dorsally 25 6 (N = 505) on a modelling clay pad covered with plastic cling film. During the entire operation, the head was submerged in oxygenated salt water, irrigating the gills and avoiding injuries to the eyes. All surgical tools were sterilized. A 1 cm long incision was made in the skin on 7.5 the ventral midline, 0.5–1 cm anterior to the anus. A tag was then 5 inserted in the peritoneal cavity and the wound closed with two independent monofilament nylon nonabsorbable sutures (Mono- sof). The fish was then placed in a recovery bath, after which it was 2.5 transferred into a new holding tank. A recovery period of 8–9 days Percentage of intervals > 1 was allowed before the fish were released. On 8 June 2006, 80 bigeye scads were released, including the 0.8 tagged fish. Fish were transported to the site in oxygenated buckets 0.5 and then brought down inside the buckets by divers in order to release the fish gently at three different points, in roughly equal 0 20 40 60 80 100 120 140 160 180 200 220 numbers (Fig. 1). The acoustic receiver array was operational during Interval t between visits (min) the entire 4-month experimental period. Of the 46 tagged fish, four were never detected by the array of receivers and four more were Figure 2. Log-survivor plots of percentages of intervals between successive fish removed from the analysis as they were only detected during the arrivals greater than time t over periods of 220 min at six representative FADs in Saint- Paul’s Bay. The numbering of FADs matches those indicated on Fig. 1. first 2 h after release. Therefore the analysis was conducted on 38 fish (83% of the total tagged fish). Detections were recorded by all VR2s in the bay (FADs and CSs). More than half of the remaining fish (22/38 ¼ 58%) were no longer detected by the VR2 array after day 6. the VR2. Therefore, in bad conditions, fish can be resident (near Five other fish disappeared from the array on day 24. No fish were the FADs) and not detected. To take into account this effect we detected after day 40 of the experiment (17 July 2006). defined a predetermined maximum blanking period (MBP) during which a tagged fish is not detected but is considered as present Ethical Note around a FAD. The MBP value must be higher than the minimum threshold (10 min, www.vemco.com) to avoid the nondetection of We took several procedures to minimize the welfare impact on tags caused by code collisions between multiple tags present around fish. The fishing operation was carried out by minimizing direct the same receiver at the same time. The MBP value affects the esti- handling of fish to avoid injuries and loss of scales that are often the mates of CRT, as high MBPs result in high CRTs, but it is possible to first cause of mortality. The use of barbless hooks greatly helped to choose the MBP to minimize these effects. One issue here is how to minimize handling since fish can free themselves or can be define a bout of time spent at the FAD, which includes movements unhooked easily. The method to convey fish to the experimental just outside the range of the FAD and back into it (Klimley & station avoided high mortality (less than 4%). The continuous sea Holloway 1999). We achieved this by plotting the distribution of the water flow in tanks maintained suitable temperature and oxygen intervals between successive arrivals semilogarithmically (Fagen & content (mean temperature SD was 26.3 0.5 C; mean oxygen Young 1978). On the graph of a log survivor function, the abscissa content SD was 5.9 0.02 mg/litre). The maximum density inside consists of the time t and the ordinate displays the log of the number the holding tanks was 11 fish/m3. During the tag implantation no of intervals greater than time t. A frequency distribution of randomly mortality occurred. Only five untagged fish were lost during the occurring events is described by a negative exponential distribution. period of acclimatization caused by fungal infections on the skin. The A log-survivor plot of intervals between arrivals is fitted by a straight fish were released in Saint-Paul’s Bay near their site of capture. We line, whose slope is proportional to the probability of an arrival were thus confident that the experiments did not have any signifi- occurring at any given time after the last event. Abrupt changes in cant effect on the population and did not cause undue distress to the the slope indicate a change in probability. We plotted the intervals subjects. Protocols were carried out with the authority of the between arrivals in this way and defined an MBP at the FAD by the National Veterinary School of Nantes (France) validating a certificate interval at which the slope of the curve first changes abruptly (Fig. 2). of training in animal experimentation and a degree in experimental First inflection points ranged between 30 and 100 min but inflec- surgery on fish. The fish experimental protocols were permitted tions at intervals greater than 60 min depend upon few returns (less under the Aquarium of Reunion Island animal care certificates than 5%) and are therefore probably not significant. As such, we delivered by the French Veterinary Medicine Directorate. adopted a conservative MBP value of 30 min. The residence time in the network (RTN) was the time period Data Analyses between the time of release and the last detection recorded by a VR2. The association rate (AR) was the ratio between the sum of We determined the CRT of fish around acoustic receivers (FADs CRT (either under FADs or CSs) and RTN. and CSs), using Ohta & Kakuma’s (2005) definition. The CRT corre- The hourly aggregation index was the average number of tagged sponded to the time during which a tagged fish was continuously fish aggregated around a receiver during each hour of the day over monitored around one FAD without any absence. However, the the first 6 days of the experiment. increase in the background noise, coming from the waves, the To study the simultaneity of arrivals and departures at the undertow or outboard motors, can interfere with the transmission of same VR2s, we first defined a batch as the period between the pings and lead to loss of detections or a reduction in the range of arrival of the first fish and the departure of the last fish. We 1444 M. Soria et al. / Animal Behaviour 78 (2009) 1441–1446 considered that a fish belonged to a batch until the time lag between two detections did not exceed an MBP of 720 min (12 h). 9 This large value for MBP was adopted to avoid detecting spurious arrivals or departures. Then we compiled an index of concentra- 8 tion, the normalized Herfindahl–Hirschmann Index (HHI, Hirschman 1964) as follows: 7

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 6 u uXn 2 pffiffiffiffiffiffiffiffi t xi 1=n X 5 i ¼ 1 HHI ¼ pffiffiffiffiffiffiffiffi (2) 1 1=n 4 P where X ¼ n xi, xi is the arrival or departure time of a fish in i ¼ 1 Hourly aggregation index 3 a batch, calculated from the same origin (60 min before the first fish of the batch) for all fish of the same batch, and n is the number of 2 fish (size of the batch). The HHI was computed for 19 batches composed of 2–11 tagged fish under five different FADs and for 17 1 batches composed of 2–10 fish around five different CSs. In addition we calculated the median time lag (TL) between two consecutive 0000 arrivals or departures. 0200 0400 0600 0800 1000 1200 1400 1600 1800 2000 2200 All the statistical tests used were two tailed. Night Day Night Dawn Dusk

RESULTS Figure 3. Dial patterns of the hourly aggregation index (mean SD) during the first 6 days for fish on FADs (solid lines) and on CSs (broken line). This index is simply the The mean CRT of individuals at FADs was significantly higher average number of tagged fish detected together under the same FAD or CS at the same period. than the mean CRT at CSs (Student’s t test: t1221 ¼ 6.52, P < 0.001; Table 1). This showed that fish remained nearly twice as long at a single FAD as at a single CS, once they had arrived near these features. Moreover, when the tagged fish were present in the study heterogeneity in the timing events occurring during a batch, area, the cumulative time they spent associated with FADs (AR) was confirmed the above results. The median TL value was lower for three times higher than the time spent with CSs (Wilcoxon signed- departure from a FAD than for arrival and the TL means were ranks test: T ¼ 155, N ¼ 37, P < 0.01; Table 1). significantly different (Mann–Whitney U test: U ¼ 1127, The aggregation under FADs, measured by the number of NArr ¼ NDep ¼ 55, P < 0.01; Table 2). In contrast, medianTL measured tagged fish detected together at the same time (Fig. 3), for CSs were significantly higher than for FADs (Mann–Whitney U showed a regular increase from dawn up to 1300–1400 hours test: U ¼ 3122, NFAD ¼ 110, NCS ¼ 104, P < 0.001; Table 2), and arrival (mean SD ¼ 4.7 3.3), after which a rapid decrease led to and departure time lags for CSs were not significantly different from a stable low value during the night (1.7 1.1). The average each other (Mann–Whitney U test: U ¼ 1238, NArr ¼ NDep ¼ 52, number of aggregated fish per h under FADs was significantly P ¼ 0.46). To conclude, HHI and TL values both showed that tagged higher during the day than at night (Mann–Whitney U test: individuals did not arrive at FADs or CSs together, and did not leave U ¼ 10 536, NDay ¼ 294, NNight ¼ 155, P < 0.0001). There was no CSs together, while they tended to leave FADs simultaneously. The evidence of aggregations forming at CSs at any time of the day two indices appeared to be correlated (R2 ¼ 0.87). (Fig. 3). Departures of fish from FADs occurred at all times of day, There was a high simultaneity of departure for fish leaving FADs, although there was a peak at dusk (32% of departures occurred while no simultaneity was observed for fish arriving at FADs or for between 1700 and 1900 hours; number of fish in batches leaving at 2 fish arriving at or leaving CSs (Table 2). Of the 19 batches identified dusk: chi-square test: c23 ¼ 82.3, P < 0.001; Fig. 4). This result under FADs, 17 (89%) showed a departure HHI significantly lower suggests that FAD departure simultaneity could not exclusively be than their arrival HHI, indicating a higher simultaneity at departure attributed to regular circadian cues. Conversely, tagged fish arrived (Mann–Whitney U test: U ¼ 63, NArr ¼ NDep ¼ 19, P < 0.001). In at FADs mostly at dawn (56% of arrivals occurred between 0500 and contrast, HHI values for fish arriving at and leaving CSs were not 0700 hours; Fig. 5) corroborating the dramatic increase in the significantly different, indicating no simultaneity at any point number of fish aggregating around FADs during this period (see (Mann–Whitney U test: U ¼ 109, NArr ¼ NDep ¼ 17, P ¼ 0.22). Fig. 3). Therefore, it appeared that most fish tended to arrive at FADs The median time lag (TL) between two consecutive arrivals or during the same period of the day (dawn), while departures did not departures, a variable less sensitive than the HHI index to any show a clear pattern (Fig. 5).

Table 1 Descriptors of the residence behaviour under FADs and CSs (control stations) Table 2 Mean values of HHI (Herﬁndahl–Hirschmann Index) and median of the time lag (TL) N MeanSD Median (range) P between consecutive events, either arrivals or departures of ﬁsh at FADs or CSs RTN (day) Total 38 11.411.6 5 (0.3–38.8) (control stations) CRT (min) FAD 779 60104 23 (1–733) 0.0001 N Arrival Departure P CS 443 3360 13 (1–526) AR (%) FAD 38 1820 9.1 (0.3–78.2) 0.003 Mean HHISD FAD 19 0.120.09 0.040.05 0.0006 CS 37 6.313.1 2.4 (0.1–71.3) CS 17 0.150.13 0.260.24 0.22 Median TL in min FAD 55 19.4 (0.4–164.4) 11.3 (0.0–215.8) 0.002 RTN: residence time in the network; CRT: continuous residence time; AR: associa- (range) CS 52 59.3 (0.6–654.1) 51.9 (0.9–1116.4) 0.46 tion rate. M. Soria et al. / Animal Behaviour 78 (2009) 1441–1446 1445

40 0000 Ndep = 75 8 N = 69 2200 0200 35 arr

30 6 2000 Number of batch 0400 25 4 20

15 2 1800 0600 Number of fish (%) 10 0 5 1600 0800

0 0500 0100 0300 0700 0900 1100 1300 1500 1700 1900 2100 2300 0400 1000 Time (hour) 1200 Figure 4. Percentage of numbers of fish and number of batches arriving at (black histogram and filled square, respectively) or departing from (grey histogram and open Time triangle, respectively) FADs by hour throughout the day. The time associated with batches is the median of the times of fish constituting the batch. These same batches of Figure 5. Diel distribution of arrivals (in orange) and departures (in blue) of 23 tagged fish were used to build the HHI index. fish in batches observed on FADs from the second to the 21st day. Green indicates cases where both arrivals and departures occurred during the time interval. Grey background indicates night-time. Each concentric ring corresponds to one tagged fish. On the clock diagram radial lines indicate hours (24 h). DISCUSSION

Small pelagic fish usually form schools during daytime, and are dispersed during the night (Blaxter & Holliday 1963; Freón et al. the observed simultaneity of fish leaving FADs indicated that fish 1993). The 38 bigeye scad tagged in Saint-Paul’s Bay showed not used FADs to form larger schools, supporting the MPH. Our results only this general pattern (Fig. 3), but also an agreement with the showed that fish tended to associate with FADs at dawn since the timing and shape of the aggregation process observed by Freón numbers of fish and of batches were greater during this period et al. (1996). These authors indicated that for a different pelagic (Fig. 5). In contrast to fish departure, we cannot disentangle species in a different area (Sardina pilchardus in the Mediterranean external triggers such as the presence of predators that could Sea), the pattern of aggregation begins with a sharp increase in induce fish to arrive at FADs at this specific period (supposing that aggregation at dawn, a plateau during daytime, a slower dispersion FADs provide a defensive advantage) from the MPH, linked to process starting before dusk and a full dispersion at night. There- a ‘meeting time’. Indeed, in addition to meeting points (such as fore, we considered that the general aggregative behaviour of our FADs), a common meeting time could considerably help fish to tagged fish was typical and not affected by the tagging operation. form schools. The synchronicity of return to FADs of different Our results showed that our two predictions were correct, groups of tagged tuna (Klimley & Holloway 1999) could be rein- which lends some strength to the MPH but whether fish are terpreted in this light (Freón & Dagorn 2000). Indeed, Klimley & purposively using FADs to aggregate, or whether the aggregation is Holloway (1999) observed a precise timing of arrival of yellowfin a by-product of the fish doing something else remains debatable. tuna, Thunnus albacares, at FADs where they usually remained We discuss this issue here. First, we found that fish spent more time a short time (<1 h) and the timing of arrival differed according to at FADs than around CSs, and that fish aggregated around FADs the monitored FAD. The authors interpreted their results only in the (mostly during daytime), demonstrating that FADs acted as reten- context of foraging behaviour, which is not incompatible with our tion points. Second, we found that the simultaneity (Table 2) of fish MPH interpretation. leaving a FAD was higher than the simultaneity of fish arriving at The absence of simultaneity in fish arrivals at both FADs and CSs a FAD, and that this pattern did not occur at CSs. The observed high suggested that the lifetime of free-swimming schools in the bay simultaneity of departures from FADs can be caused by either an was short, and shorter than the circadian period. This short lifetime individual reaction to some regular external trigger, or schooling could be caused by frequent attacks by predators or to foraging behaviour. The timing of major arrivals to or departures from FADs behaviour (Pitcher et al. 1988; Parrish & Hamner 1997), and also can help to disentangle the issue of fish motivation in their asso- low encounter rates of fish or schools away from FADs. Conse- ciation with FADs. Our results showed that departures of batches quently, we argue that the main way for bigeye scad to form schools from FADs did not happen at a precise time of day (Fig. 4, open was to encounter conspecifics around FADs. triangles), suggesting that departure from FADs is not primarily From our study, it might seem that FADs could be beneficial to linked to circadian cues. However, the number of fish leaving FADs the fitness of the associated fish. FADs contribute to increased was significantly higher at dusk (Fig. 4, grey histogram; Fig. 5). school size and hence confer the advantages of being in a larger Since we have few data at this period and no information on the group such as improving foraging and feeding (Pitcher & Parrish real size of schools, it is difficult to interpret this result as a prefer- 1993) and the survival of their members (reviewed in Parrish & ential time departure of fish caused by predator attacks or a starting Hamner 1997), which can help in environments where prey is time to forage outside the FADs. Consequently, we considered that scarce as in our tropical area of study. However, because of the 1446 M. Soria et al. / Animal Behaviour 78 (2009) 1441–1446 competition between FADs, high densities of FADs could actually Freón, P., Soria, M., Mullon, C. & Gerlotto, F. 1993. Diurnal variation in fish density not lead to increased free-swimming school sizes compared to estimates during acoustics surveys in relation to spatial distribution and avoidance reaction. Aquatic Living Resources, 6, 221–234. areas with no FADs, as too many potential meeting places would Freón, P., Gerlotto, F. & Soria, M. 1996. Diel variability of school structure with exist for fish to aggregate consistently. In such a situation, school special reference to transition periods. ICES Journal of Marine Science, 53, members could not fully benefit from advantages provided by 459–464. Giraldeau, L. A. & Caraco, T. 1993. Genetic relatedness and group size in an FADs. Further studies on the exact effects of different densities of aggregation economy. Evolutionary Ecology, 7, 429–438. FADs on school sizes (and other behavioural features) are therefore Girard, C., Dagorn, L., Taquet, M., Aumeeruddy, R., Peignon, C. & Benhamou, S. necessary to improve our assessment of the impacts of the large 2007. Homing abilities of dolphinfish (Coryphaena hippurus) displaced from fish aggregating devices (FAD) determined using acoustic telemetry. Aquatic Living numbers of FADs present in some ocean regions. Resources, 20, 313–321. Gooding, R. M. & Magnusson, J. J. 1967. Ecological significance of a drifting object Acknowledgments to pelagic fishes. Pacific Science, 21, 486–497. Hallier, J. P. & Gaertner, D. 2008. Drifting fish aggregation devices could act as an ecological trap for tropical tuna species. Marine Ecology Progress Series, 353, We are grateful to P. Durville, S. Durville and T. Mulocheau of the 255–264. Aquarium of Reunion Island. E. Tessier, P. Chabanet, P. Berthier and Hirschman, A. O.1964. The paternity of an index. American Economic Review, 54,761. Klimley, A. P. & Holloway, C. F. 1999. School fidelity and homing synchronicity of L. Berthier are thanked for their assistance in catching and releasing yellowfin tuna, Thunnus albacares. Marine Biology, 133, 307–317. fish and in deploying the acoustic system. P. Cotel and G. Lemartin Klimley, A. P., Butler, S. B., Nelson, D. R. & Stull, A. T. 1988. Diel movements of are thanked for their assistance during data acquisition and data scalloped hammerhead sharks, Sphyrna lewini Griffith and Smith, to and from a seamount in the Gulf of California. Journal of Fish Biology, 33, 751–761. processing. We are also grateful to J. Filmalter for improving the Krause, J. & Ruxton, G. D. 2002. Living in Groups. Oxford: Oxford University Press. English of this manuscript. We thank Professor P. Klimley for his Mangel, M. 1990. Dynamic information in uncertain and changing worlds. Journal of constructive review. This work is a contribution of the research Theoretical Biology, 146, 317–332. 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