first field-based experiment supporting the meeting point hypothesis for schooling in pelagic fish
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Animal Behaviour xxx (2009) 1–6
Contents lists avai
Animal Behaviour
journal homepage: www.elsevier .com/locate/anbehav
First field-based experiment supporting the meeting point hypothesisfor schooling in pelagic fish
Marc Soria a,*, Laurent Dagorn b,1, Gael Potin c,2, Pierre Freon d,3
a UMR EME, Institut de Recherche pour le Developpement (IRD), La Reunionb UMR EME, Institut de Recherche pour le Developpement (IRD), Seychellesc Laboratoire d’Ecologie Marine (ECOMAR), Universite de la Reuniond UMR EME, Institut de Recherche pour le Developpement (IRD), Centre de Recherche Halieutique Mediterraneenne et Tropicale
a r t i c l e i n f o
Article history:Received 17 May 2009Initial acceptance 2 July 2009Final acceptance 7 September 2009Available online xxxMS. number: D-09-00317R1
Keywords:acoustic taggingbigeye scadfish aggregation devicemeeting point hypothesisschooling fish behaviourSelar crumenophthalmus
* Correspondence: M. Soria, IRD. Parc TechnologiquWetzell, BP 172, 97492 Sainte Clotilde cedex, La Reun
E-mail address: [email protected] (M. Soria).1 Laurent Dagorn is at IRD, BP 570, Victoria, Mahe,2 Gael Potin is at Universite de la Reunion, 15 av. R
Denis messag cedex 9, La Reunion, France.3 Pierre Freon is at Centre de Recherche Halieutiqu
cale, Avenue Jean Monnet, BP 171, 34203 Sete Cedex,
0003-3472/$38.00 � 2009 The Association for the Studoi:10.1016/j.anbehav.2009.09.025
Please cite this article in press as: Soria, M., efish, Animal Behaviour (2009), doi:10.1016/j
Aggregations of fish around fish aggregation devices (FADs) have been widely described in the literature;most commercial catches of tuna by surface fisheries are performed around FADs, taking advantage ofthis behaviour. The meeting point hypothesis (MPH) suggests that fish could make use of FADs toincrease the chance of encounters between conspecifics, helping individuals to form larger schools. Toattempt a validation of the MPH, we performed an experiment in the field to test the followingpredictions: (1) fish spend more time at FADs than at any other random points and therefore aggregatearound FADs, and (2) fish arrive at FADs as isolated individuals or in small groups and leave them inlarger groups. Our investigation involved acoustic telemetry techniques commonly used to observe fishat FADs. The study was carried out on a small pelagic fish species, the bigeye scad, Selar crumenoph-thalmus, in Saint-Paul’s Bay (Reunion Island). Our results validated our two predictions: FADs acted asretention points, increasing the encounter rate of fish and enhancing schooling behaviour, therebysupporting the meeting point hypothesis. FADs could be beneficial to the fitness of the associated fish,promote increased school size and hence confer the advantages of being in a larger group. The impact ofthe deployment of large number of FADs in some ocean regions is reinterpreted in light of our results.� 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Group formation is a widespread phenomenon throughout theanimal kingdom. This social behaviour can be a permanent trait oroccur only during some seasons or some parts of the life cycle.Swarming insects, schooling fish, flocking birds and herd mammalsall illustrate this behaviour. Different reasons for grouping havebeen proposed, including decreasing the predation risk, promotingoptimal foraging, increasing reproductive success and facilitatingmigration and learning. There is a large body of literature on thedynamics of group size (including optimal group size) based onexperimental observations and modelling (Mangel 1990; Giraldeau& Caraco 1993; Zemel & Lubin 1995; Parrish & Hamner 1997;Bonabeau et al. 1999; Freon & Misund 1999; Krause & Ruxton2002). However, the processes responsible for the dynamics of
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group formation and dispersion in the wild are poorly documented.Splitting or dispersion of a group can come from individual deci-sions or result from processes linked to predation, foraging activity,loss of sensory contact or habitat features. The homogeneous andexpansive nature of certain habitats, such as the marine pelagicdomain, favours dispersion. Hence, for gregarious fish species someprocesses must enhance the gathering of individuals. The simplestway to meet is to go to the same place at the same time, which istrivial for territorial species, but not so obvious for nomadic animalssuch as pelagic fish. This matter has led to studies of the impact offish aggregating devices (FADs) on fish behaviour and biology(Gooding & Magnusson 1967; Castro et al. 2002; Dempster &Taquet 2004). The use of FADs in tropical ocean fisheries is massiveand, in the last 10 years, has been responsible for 50–70% of thetotal purse-seine catch of tuna in the western Indian Ocean(Fonteneau 2003). At any given time in this region, 2500 FADs areestimated to be in use (Moreno et al. 2007). This led to thehypothesis that FADs may act as ecological traps (Marsac et al.2000; Schlaepfer et al. 2002; Hallier & Gaertner 2008). Under thishypothesis, the proliferation of FADs could cause fish to stay toolong around FADs even when local conditions are unfavourable forfeeding, affecting their biology (e.g. growth).
d by Elsevier Ltd. All rights reserved.
ent supporting the meeting point hypothesis for schooling in pelagic
St-Paul’s Bay
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Figure 1. Map of Saint-Paul’s Bay with location of FADs (blue dots). The nine FADsequipped with VR2 Vemco receivers are identified (red disks) with their area ofdetection (outlined circles). The positions of the seven VR2 used as control listeningstations (CSs) are indicated (green triangles) and fish release points are also shown(+). 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 representsisobaths (m). Numbers adjacent to some of these FADs identify those used in Fig. 2.
M. Soria et al. / Animal Behaviour xxx (2009) 1–62
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Dagorn & Freon (1999) proposed the meeting point hypothesis(MPH) to explain how tropical tuna could enhance the aggregationof isolated individuals or small schools in vast environments. Thishypothesis suggests that fish could make use of floating objects toincrease the chance of encounters between conspecifics, helpingindividuals to form larger schools. Freon & Dagorn (2000) gener-alized this hypothesis to other marine species and other types ofassociations. However, thus far, no field data have been obtained tovalidate it. Here, we investigated the MPH through the study of thespatial dynamics of individual fish around FADs. Using a smallpelagic fish species, the bigeye scad, Selar crumenophthalmus, inSaint-Paul’s Bay (Reunion Island), we aimed to test the followingpredictions: (1) fish spend more time at FADs than at any otherrandom points, and (2) fish arrive at FADs as isolated individuals orin small groups and leave them in larger groups. If fish spend moretime at FADs than around any random point, and if simultaneity ofdeparture from a FAD is higher than that of arriving at a FAD, onecould consider that FADs increase the encounter rate of fish andenhance schooling behaviour. Our investigation involved acousticreceivers and coded transmitters, a technique commonly used forobserving associated fish (Klimley et al. 1988; Ohta & Kakuma2005; Dagorn et al. 2007; Girard et al. 2007; Taquet et al. 2007).
METHODS
Species and Study Site
Bigeye scad is a ubiquitous cosmopolitan species that occursmostly in clear coastal waters in tropical and subtropical regionsand naturally aggregates around coastal FADs (Roos et al. 2007).This carangid is an obligate schooling fish species, travelling incompact groups of hundreds or thousands of fish (Soria et al. 2007).As such, this species appears to be an appropriate candidate forinvestigating the MPH.
The study took place in Saint-Paul’s Bay, Reunion Island, an opensandy bay with a surface area of about 8 km2. Fifteen differentartificial structures were already present in the bay, anchored inwater 15–50 m deep (Fig. 1). Some were small shallow-mooredFADs specifically deployed for enhancing commercial fish capture;others were marker buoys delimiting a private aquaculture facility,a submerged shipwreck and two aquaculture cages, one with fishesfed daily (Fig. 1), without any increased attractive effect on bigeyescad (results not shown). The distance between adjacent structuresranged between 120 and 1020 m.
Stationary Acoustic Array
Acoustic receivers were deployed at FADs and at random pointsaway from the FADs, called control stations or CSs (Fig. 1). Tomonitor the behaviour of fish around FADs, we used coded acoustictags and acoustic receivers (VR2) from Vemco (Vemco, Halifax,Canada). In situ range tests indicated that 80% of signals emittedduring 1 h were detected at a range of 150 m. We therefore definedthis distance as the maximum range of reliability of the acousticreceivers.
To monitor all FADs, we deployed nine VR2s in total (sevencovering individual FADs and two covering groups, containing twoand five FADs, respectively). From here on, the term FAD is used torefer to the floating structure(s) equipped with an acoustic receiver.We also deployed seven VR2s on CSs (Fig. 1). Receivers were fixedeither to mooring ropes or on the bottom, mounted on anchoredbases consisting of tyres filled with concrete (Taquet et al. 2006).Receivers on the bottom were installed 1 m above the seabed withno surface structure to avoid creating new attraction devices. Thedistance between adjacent FADs ranged from 310 to 1140 m. For
Please cite this article in press as: Soria, M., et al., First field-based experimfish, Animal Behaviour (2009), doi:10.1016/j.anbehav.2009.09.025
FADs located at a distance lower than the range of reliability, wedeployed only one VR2 to monitor these neighbouring FADs. In thiscase, to compensate for VR2s covering multiple FADs, we applieda weighted index to calculate the estimated residence time per FAD.This index took into account the number of FADs involved and thetheoretical detection areas which would have been covered by theVR2s, had they been deployed on each FAD. The estimatedcontinuous residence time CRTesti was of the general followingform:
CRTesti ¼
Xn
i¼1
CRTobs
n� Anci
!þXn
i¼1
�CRTobs � Aci
�n
(1)
where n is the number of multiple FADs, CRTobs is measured by theVR2, Anci is the percentage of the area around the FADi not coveredby the range of detection of the VR2 and Aci the percentage of thearea around the FADi covered by the range of detection of the VR2.
The residence time means of FADs and CSs were compared usinga Student’s t test.
Capture, Tag Implantation and Releasing Strategy
One hundred and five fish were captured on 19 May 2006 byfishers around the aquaculture facility of Saint-Paul’s Bay usinghand lines. Immediately after capture fish were transferred intooxygenated 60-litre buckets. Eighteen fish not big enough to betagged were released. A total of 87 fish were finally conveyed to theAquarium of Reunion Island and housed in three holding tanks of2.5 m3 each. A preventive treatment with methylene blue (3 mg/litre)and copper sulphate (3 � 10�3 ml/litre) was applied. Fish were feddaily with a mixture of pieces of crustacean and fish flesh. On the 30and 31 May 2006, 46 fish were tagged with coded Vemco V7transmitters (V7-2L-R04K, 69 kHz, random rate of transmissionevery 70–140 s, rated battery life 130–150 days, length 18.5 mm,diameter 7 mm, mass in water 0.75 g). The mean fish size � SD,
ent supporting the meeting point hypothesis for schooling in pelagic
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Figure 2. Log-survivor plots of percentages of intervals between successive fisharrivals 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.
M. Soria et al. / Animal Behaviour xxx (2009) 1–6 3
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expressed in fork length was 18 � 1.2 cm (102 � 22 g). The weightof the transmitter expressed as a percentage of the animal’s weightwas 0.73% on average, ranging between 0.97% and 0.43% for thesmallest and largest animal marked, respectively. Fish wereanaesthetized in an aqueous solution of benzocaine (60 mg/litre)prior to tag implantation. Anaesthetized fish were placed dorsallyon a modelling clay pad covered with plastic cling film. During theentire operation, the head was submerged in oxygenated salt water,irrigating the gills and avoiding injuries to the eyes. All surgicaltools were sterilized. A 1 cm long incision was made in the skin onthe ventral midline, 0.5–1 cm anterior to the anus. A tag was theninserted in the peritoneal cavity and the wound closed with twoindependent monofilament nylon nonabsorbable sutures (Mono-sof). The fish was then placed in a recovery bath, after which it wastransferred into a new holding tank. A recovery period of 8–9 dayswas allowed before the fish were released.
On 8 June 2006, 80 bigeye scads were released, including thetagged fish. Fish were transported to the site in oxygenated bucketsand then brought down inside the buckets by divers in order torelease the fish gently at three different points, in roughly equalnumbers (Fig. 1). The acoustic receiver array was operational duringthe entire 4-month experimental period. Of the 46 tagged fish, fourwere never detected by the array of receivers and four more wereremoved from the analysis as they were only detected during thefirst 2 h after release. Therefore the analysis was conducted on 38fish (83% of the total tagged fish). Detections were recorded by allVR2s 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.Five other fish disappeared from the array on day 24. No fish weredetected after day 40 of the experiment (17 July 2006).
Ethical Note
We took several procedures to minimize the welfare impact onfish. The fishing operation was carried out by minimizing directhandling of fish to avoid injuries and loss of scales that are often thefirst cause of mortality. The use of barbless hooks greatly helped tominimize handling since fish can free themselves or can beunhooked easily. The method to convey fish to the experimentalstation avoided high mortality (less than 4%). The continuous seawater flow in tanks maintained suitable temperature and oxygencontent (mean temperature � SD was 26.3 � 0.5 �C; mean oxygencontent � SD was 5.9 � 0.02 mg/litre). The maximum density insidethe holding tanks was 11 fish/m3. During the tag implantation nomortality occurred. Only five untagged fish were lost during theperiod of acclimatization caused by fungal infections on the skin. Thefish were released in Saint-Paul’s Bay near their site of capture. Wewere thus confident that the experiments did not have any signifi-cant effect on the population and did not cause undue distress to thesubjects. Protocols were carried out with the authority of theNational Veterinary School of Nantes (France) validating a certificateof training in animal experimentation and a degree in experimentalsurgery on fish. The fish experimental protocols were permittedunder the Aquarium of Reunion Island animal care certificatesdelivered by the French Veterinary Medicine Directorate.
Data Analyses
We determined the CRT of fish around acoustic receivers (FADsand CSs), using Ohta & Kakuma’s (2005) definition. The CRT corre-sponded to the time during which a tagged fish was continuouslymonitored around one FAD without any absence. However, theincrease in the background noise, coming from the waves, theundertow or outboard motors, can interfere with the transmission ofpings and lead to loss of detections or a reduction in the range of
Please cite this article in press as: Soria, M., et al., First field-based experimfish, Animal Behaviour (2009), doi:10.1016/j.anbehav.2009.09.025
the VR2. Therefore, in bad conditions, fish can be resident (nearthe FADs) and not detected. To take into account this effect wedefined a predetermined maximum blanking period (MBP) duringwhich a tagged fish is not detected but is considered as presentaround a FAD. The MBP value must be higher than the minimumthreshold (10 min, www.vemco.com) to avoid the nondetection oftags caused by code collisions between multiple tags present aroundthe same receiver at the same time. The MBP value affects the esti-mates of CRT, as high MBPs result in high CRTs, but it is possible tochoose the MBP to minimize these effects. One issue here is how todefine a bout of time spent at the FAD, which includes movementsjust outside the range of the FAD and back into it (Klimley &Holloway 1999). We achieved this by plotting the distribution of theintervals between successive arrivals semilogarithmically (Fagen &Young 1978). On the graph of a log survivor function, the abscissaconsists of the time t and the ordinate displays the log of the numberof intervals greater than time t. A frequency distribution of randomlyoccurring events is described by a negative exponential distribution.A log-survivor plot of intervals between arrivals is fitted by a straightline, whose slope is proportional to the probability of an arrivaloccurring at any given time after the last event. Abrupt changes inthe slope indicate a change in probability. We plotted the intervalsbetween arrivals in this way and defined an MBP at the FAD by theinterval at which the slope of the curve first changes abruptly (Fig. 2).First inflection points ranged between 30 and 100 min but inflec-tions at intervals greater than 60 min depend upon few returns (lessthan 5%) and are therefore probably not significant. As such, weadopted a conservative MBP value of 30 min.
The residence time in the network (RTN) was the time periodbetween the time of release and the last detection recorded bya VR2. The association rate (AR) was the ratio between the sum ofCRT (either under FADs or CSs) and RTN.
The hourly aggregation index was the average number of taggedfish aggregated around a receiver during each hour of the day overthe first 6 days of the experiment.
To study the simultaneity of arrivals and departures at thesame VR2s, we first defined a batch as the period between thearrival of the first fish and the departure of the last fish. We
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considered that a fish belonged to a batch until the time lagbetween two detections did not exceed an MBP of 720 min (12 h).This large value for MBP was adopted to avoid detecting spuriousarrivals or departures. Then we compiled an index of concentra-tion, the normalized Herfindahl–Hirschmann Index (HHI,Hirschman 1964) as follows:
HHI ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiXn
i¼1
�xiX
�2vuut �
ffiffiffiffiffiffiffiffi1=n
p1�
ffiffiffiffiffiffiffiffi1=n
p (2)
where X ¼Pn
i¼1 xi, xi is the arrival or departure time of a fish ina batch, calculated from the same origin (60 min before the first fishof the batch) for all fish of the same batch, and n is the number offish (size of the batch). The HHI was computed for 19 batchescomposed of 2–11 tagged fish under five different FADs and for 17batches composed of 2–10 fish around five different CSs. In additionwe calculated the median time lag (TL) between two consecutivearrivals or departures.
All the statistical tests used were two tailed.
Dawn DuskFigure 3. Dial patterns of the hourly aggregation index (mean � SD) during the first 6days for fish on FADs (solid lines) and on CSs (broken line). This index is simply theaverage number of tagged fish detected together under the same FAD or CS at the sameperiod.
RESULTS
The mean CRT of individuals at FADs was significantly higherthan 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 ata single FAD as at a single CS, once they had arrived near thesefeatures. Moreover, when the tagged fish were present in the studyarea, the cumulative time they spent associated with FADs (AR) wasthree times higher than the time spent with CSs (Wilcoxon signed-ranks test: T ¼ 155, N ¼ 37, P < 0.01; Table 1).
The aggregation under FADs, measured by the number oftagged fish detected together at the same time (Fig. 3),showed a regular increase from dawn up to 1300–1400 hours(mean � SD ¼ 4.7 � 3.3), after which a rapid decrease led toa stable low value during the night (1.7 � 1.1). The averagenumber of aggregated fish per h under FADs was significantlyhigher during the day than at night (Mann–Whitney U test:U ¼ 10 536, NDay ¼ 294, NNight ¼ 155, P < 0.0001). There was noevidence of aggregations forming at CSs at any time of the day(Fig. 3).
There was a high simultaneity of departure for fish leaving FADs,while no simultaneity was observed for fish arriving at FADs or forfish arriving at or leaving CSs (Table 2). Of the 19 batches identifiedunder FADs, 17 (89%) showed a departure HHI significantly lowerthan their arrival HHI, indicating a higher simultaneity at departure(Mann–Whitney U test: U ¼ 63, NArr ¼ NDep ¼ 19, P < 0.001). Incontrast, HHI values for fish arriving at and leaving CSs were notsignificantly different, indicating no simultaneity at any point(Mann–Whitney U test: U ¼ 109, NArr ¼ NDep ¼ 17, P ¼ 0.22).
The median time lag (TL) between two consecutive arrivals ordepartures, a variable less sensitive than the HHI index to any
Table 1Descriptors of the residence behaviour under FADs and CSs (control stations)
N Mean�SD Median (range) P
RTN (day) Total 38 11.4�11.6 5 (0.3–38.8)CRT (min) FAD 779 60�104 23 (1–733) 0.0001
CS 443 33�60 13 (1–526)AR (%) FAD 38 18�20 9.1 (0.3–78.2) 0.003
CS 37 6.3�13.1 2.4 (0.1–71.3)
RTN: residence time in the network; CRT: continuous residence time; AR: associa-tion rate.
Please cite this article in press as: Soria, M., et al., First field-based experimfish, Animal Behaviour (2009), doi:10.1016/j.anbehav.2009.09.025
heterogeneity in the timing events occurring during a batch,confirmed the above results. The median TL value was lower fordeparture from a FAD than for arrival and the TL means weresignificantly different (Mann–Whitney U test: U ¼ 1127,NArr¼NDep¼ 55, P< 0.01; Table 2). In contrast, medianTL measuredfor CSs were significantly higher than for FADs (Mann–Whitney Utest: U ¼ 3122, NFAD ¼ 110, NCS ¼ 104, P < 0.001; Table 2), and arrivaland departure time lags for CSs were not significantly different fromeach other (Mann–Whitney U test: U ¼ 1238, NArr ¼ NDep ¼ 52,P ¼ 0.46). To conclude, HHI and TL values both showed that taggedindividuals did not arrive at FADs or CSs together, and did not leaveCSs together, while they tended to leave FADs simultaneously. Thetwo indices appeared to be correlated (R2 ¼ 0.87).
Departures of fish from FADs occurred at all times of day,although there was a peak at dusk (32% of departures occurredbetween 1700 and 1900 hours; number of fish in batches leaving atdusk: chi-square test: c23
2 ¼ 82.3, P < 0.001; Fig. 4). This resultsuggests that FAD departure simultaneity could not exclusively beattributed to regular circadian cues. Conversely, tagged fish arrivedat FADs mostly at dawn (56% of arrivals occurred between 0500 and0700 hours; Fig. 5) corroborating the dramatic increase in thenumber of fish aggregating around FADs during this period (seeFig. 3). Therefore, it appeared that most fish tended to arrive at FADsduring the same period of the day (dawn), while departures did notshow a clear pattern (Fig. 5).
Table 2Mean values of HHI (Herfindahl–Hirschmann Index) and median of the time lag (TL)between consecutive events, either arrivals or departures of fish at FADs or CSs(control stations)
N Arrival Departure P
Mean HHI�SD FAD 19 0.12�0.09 0.04�0.05 0.0006CS 17 0.15�0.13 0.26�0.24 0.22
Median TL in min(range)
FAD 55 19.4 (0.4–164.4) 11.3 (0.0–215.8) 0.002CS 52 59.3 (0.6–654.1) 51.9 (0.9–1116.4) 0.46
ent supporting the meeting point hypothesis for schooling in pelagic
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Figure 4. Percentage of numbers of fish and number of batches arriving at (blackhistogram and filled square, respectively) or departing from (grey histogram and opentriangle, respectively) FADs by hour throughout the day. The time associated withbatches is the median of the times of fish constituting the batch. These same batches offish were used to build the HHI index.
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Figure 5. Diel distribution of arrivals (in orange) and departures (in blue) of 23 taggedfish in batches observed on FADs from the second to the 21st day. Green indicates caseswhere both arrivals and departures occurred during the time interval. Grey back-ground indicates night-time. Each concentric ring corresponds to one tagged fish. Onthe clock diagram radial lines indicate hours (24 h).
M. Soria et al. / Animal Behaviour xxx (2009) 1–6 5
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DISCUSSION
Small pelagic fish usually form schools during daytime, and aredispersed during the night (Blaxter & Holliday 1963; Freon et al.1993). The 38 bigeye scad tagged in Saint-Paul’s Bay showed notonly this general pattern (Fig. 3), but also an agreement with thetiming and shape of the aggregation process observed by Freonet al. (1996). These authors indicated that for a different pelagicspecies in a different area (Sardina pilchardus in the MediterraneanSea), the pattern of aggregation begins with a sharp increase inaggregation at dawn, a plateau during daytime, a slower dispersionprocess starting before dusk and a full dispersion at night. There-fore, we considered that the general aggregative behaviour of ourtagged fish was typical and not affected by the tagging operation.
Our results showed that our two predictions were correct,which lends some strength to the MPH but whether fish arepurposively using FADs to aggregate, or whether the aggregation isa by-product of the fish doing something else remains debatable.We discuss this issue here. First, we found that fish spent more timeat FADs than around CSs, and that fish aggregated around FADs(mostly during daytime), demonstrating that FADs acted as reten-tion points. Second, we found that the simultaneity (Table 2) of fishleaving a FAD was higher than the simultaneity of fish arriving ata FAD, and that this pattern did not occur at CSs. The observed highsimultaneity of departures from FADs can be caused by either anindividual reaction to some regular external trigger, or schoolingbehaviour. The timing of major arrivals to or departures from FADscan help to disentangle the issue of fish motivation in their asso-ciation with FADs. Our results showed that departures of batchesfrom FADs did not happen at a precise time of day (Fig. 4, opentriangles), suggesting that departure from FADs is not primarilylinked to circadian cues. However, the number of fish leaving FADswas significantly higher at dusk (Fig. 4, grey histogram; Fig. 5).Since we have few data at this period and no information on thereal size of schools, it is difficult to interpret this result as a prefer-ential time departure of fish caused by predator attacks or a startingtime to forage outside the FADs. Consequently, we considered that
Please cite this article in press as: Soria, M., et al., First field-based experimfish, Animal Behaviour (2009), doi:10.1016/j.anbehav.2009.09.025
the observed simultaneity of fish leaving FADs indicated that fishused FADs to form larger schools, supporting the MPH. Our resultsshowed that fish tended to associate with FADs at dawn since thenumbers of fish and of batches were greater during this period(Fig. 5). In contrast to fish departure, we cannot disentangleexternal triggers such as the presence of predators that couldinduce fish to arrive at FADs at this specific period (supposing thatFADs provide a defensive advantage) from the MPH, linked toa ‘meeting time’. Indeed, in addition to meeting points (such asFADs), a common meeting time could considerably help fish toform schools. The synchronicity of return to FADs of differentgroups of tagged tuna (Klimley & Holloway 1999) could be rein-terpreted in this light (Freon & Dagorn 2000). Indeed, Klimley &Holloway (1999) observed a precise timing of arrival of yellowfintuna, Thunnus albacares, at FADs where they usually remaineda short time (<1 h) and the timing of arrival differed according tothe monitored FAD. The authors interpreted their results only in thecontext of foraging behaviour, which is not incompatible with ourMPH interpretation.
The absence of simultaneity in fish arrivals at both FADs and CSssuggested that the lifetime of free-swimming schools in the baywas short, and shorter than the circadian period. This short lifetimecould be caused by frequent attacks by predators or to foragingbehaviour (Pitcher et al. 1988; Parrish & Hamner 1997), and alsolow encounter rates of fish or schools away from FADs. Conse-quently, we argue that the main way for bigeye scad to form schoolswas to encounter conspecifics around FADs.
From our study, it might seem that FADs could be beneficial tothe fitness of the associated fish. FADs contribute to increasedschool size and hence confer the advantages of being in a largergroup such as improving foraging and feeding (Pitcher & Parrish1993) and the survival of their members (reviewed in Parrish &Hamner 1997), which can help in environments where prey isscarce as in our tropical area of study. However, because of the
ent supporting the meeting point hypothesis for schooling in pelagic
M. Soria et al. / Animal Behaviour xxx (2009) 1–66
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competition between FADs, high densities of FADs could actuallynot lead to increased free-swimming school sizes compared toareas with no FADs, as too many potential meeting places wouldexist for fish to aggregate consistently. In such a situation, schoolmembers could not fully benefit from advantages provided byFADs. Further studies on the exact effects of different densities ofFADs on school sizes (and other behavioural features) are thereforenecessary to improve our assessment of the impacts of the largenumbers of FADs present in some ocean regions.
Acknowledgments
We are grateful to P. Durville, S. Durville and T. Mulocheau of theAquarium of Reunion Island. E. Tessier, P. Chabanet, P. Berthier andL. Berthier are thanked for their assistance in catching and releasingfish and in deploying the acoustic system. P. Cotel and G. Lemartinare thanked for their assistance during data acquisition and dataprocessing. We are also grateful to J. Filmalter for improving theEnglish of this manuscript. We thank Professor P. Klimley for hisconstructive review. This work is a contribution of the researchunits R109 THETIS and R097 ECO-UP of IRD. This study was fundedby the Social European Fund (UE-IFOP) and the Regional Council ofReunion Island.
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