Schooling affects the feeding success of Australian salmon (Arripis trutta) when preying on mysid swarms (Paramesopodopsis rufa)

Journal of Experimental Marine Biology and EcologyŽ .261 2001 93–106

www.elsevier.nlrlocaterjembe

Schooling affects the feeding success of Australianž /salmon Arripis trutta when preying on mysid

ž /swarms Paramesopodopsis rufa

Elizabeth G. Foster a,), David A. Ritz b, Jon E. Osborn c,Kerrie M. Swadling b

a IASOS, UniÕersity of Tasmania, GPO Box 252-77, Hobart, 7001 Tasmania, Australiab School of Zoology, UniÕersity of Tasmania, GPO Box 252-05, Hobart, 7001 Tasmania, Australia

c School of Geography and EnÕironmental Studies, UniÕersity of Tasmania, GPO Box 252-76, Hobart,7001 Tasmania, Australia

Received 16 October 2000; received in revised form 29 March 2001; accepted 10 April 2001

Abstract

Ž . ŽWhen feeding on mysid swarms Paramesopodopsis rufa , juvenile Australian salmon Arripis. Žtrutta had higher rates of successful attacks when foraging in a group of six fish 55% total

. Ž .advances than when foraging alone 39% total advances . Six schooling fish had lower approachŽ .rates than solitary fish 25% and 37% of total advances, respectively . This result indicated that

schooling fish were better at reducing the confusion effect of swarming prey, resulting in moreŽefficient feeding. In larger areas, schools achieved higher rates of successful attacks 19 preyrfish

.in the large tank, compared with 11 preyrfish in the smaller tank . There was no influence on thefeeding success of individual fish when changes were made to the number of prey presented toeach fish. Nearest neighbour distances were smallest in the absence of prey, and increased withthe introduction of prey and again in an attack sequence. Six fish schooled more cohesively thanthree fish, indicating increased benefits of schooling in larger groups that contribute to advancedvigilance and foraging techniques. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Predatory fish schools; Invertebrate prey swarms; Australian salmon; Mysids

1. Introduction

Schooling in fish is primarily dictated by two opposing forces, vigilance and foragingŽ . Ž .Ryer and Olla, 1991 . The Õigilance sharing hypothesis Ranta and Kaitala, 1991

) Corresponding author. Fax: q61-3-6226-2973.Ž .E-mail address: liz [email protected] E.G. Foster .–

0022-0981r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0022-0981 01 00265-9

( )E.G. Foster et al.rJ. Exp. Mar. Biol. Ecol. 261 2001 93–10694

states that due to the increase in surveillance by a school, the time spent by anyindividual being alert for predators can be shared among the schooling members, leaving

Ž .more time available for finding mates and foraging Schoener, 1971 . Vigilance isenhanced by social facilitation within the school, by which individuals rapidly pass on

Ž .information to one another by visual or other behavioural cues Ryer and Olla, 1991 .This information benefits the school by enhancing awareness of the presence ofpredators and aiding advanced predatory avoidance techniques. These benefits outweighthe costs of competition for food and mates and, therefore, it is more beneficial for

Ž .individuals to school with conspecifics than to be alone Baird et al., 1991 .Predatory feeding strategies of schooling fish primarily rely on the location of prey.

This strategy is particularly important when preying on motile aggregations that mayŽ .only be present for brief periods Baird et al., 1991 . Schooling by fish also enhances

protection of individuals within the school by creating a confusion effect in the field ofview of their potential predators, causing disorientation and allowing time for the

Ž .schooling fish to formulate more advanced escape responses Ritz, 1997 . For thesepredatory schooling fish, in periods of low predatory threat by their own predators,intraschool competition for food resources increases, forcing individuals to increase their

Ž .feeding rates Ranta and Kaitala, 1991 .Few studies have been done on schools of predatory fish preying on a natural

Ž .aggregation of invertebrate prey Ringler, 1983 . Those that have endeavoured toquantify the benefits of schooling on the feeding success of fish have generally

Ž . Žexamined predation on fish schools Major, 1978 , or on stationary or novel prey Baird.et al., 1991; Ranta and Kaitala, 1991; Ryer and Olla, 1991 .

This study looks at the feeding benefits to individuals in a fish school when preyingon a naturally occurring aggregated prey. Specifically, the success of the feeding

Ž .strategy adopted by the Australian salmon Arripis trutta when preying on mysidŽ . Ž .swarms Paramesopodopsis rufa is examined. Ritz et al 1997 and Flynn and Ritz

Ž .1999 have studied this relationship primarily in terms of the benefits of swarming tothe mysids as a risk-sensitive trade-off between foraging and vigilance, when preyedupon by predatory fish schools and benefits to individual predators. This study willfurther this research to encompass the benefits of schooling to the fish in terms ofindividual feeding success with increasing group numbers.

As an obligate schooler, the Australian salmon has a relatively poor memory because,Žunlike facultative schoolers, they do not have to learn purely from self-experience Croy

.and Hughes, 1991b . Their poor memory makes them ideal for laboratory studies as theyhave minimal capacity to modify their behaviour as a result of previous encountersŽ .Steven, 1961; Radakov, 1973 . The same reasoning is applicable to the mysids used.

Ž .The juvenile -100 mm , as opposed to adult, Australian salmon was chosen becausetheir diet changes from a dominance of crustacean swarms to fish and cephalopods at

Ž .around 100 mm Baker, 1971 .

2. Methods

Mysids were collected by towing a small hand-net of approximately 1-mm mesh sizeŽ X X . Ž X X .offshore at Kingston Beach 42859 N, 147819 E and Pirates Bay 43802 N, 147856 E ,

( )E.G. Foster et al.rJ. Exp. Mar. Biol. Ecol. 261 2001 93–106 95

southeast Tasmania. Swarms were located approximately 2–10 m offshore at depths of2–5 m. They were transported to the laboratory in 20-l plastic buckets, where they were

Ž .transferred to 60-l holding tanks. Juvenile Australian salmon ;60–90 mm in lengthŽ X X .were collected using a beach seine at South Arm 42881 N, 147842 E , southeast

Tasmania. They were also transported to the laboratory in 20-l plastic buckets, number-ing no more than six fish per bucket to minimise stress. Once in the laboratory, the

Ž .salmon were transferred to a 120-l 80=30=50 cm holding tank and fed daily tosatiation on trout pellets or abundant mysids.

Ž .Rectangular 60-l 30=60=35 cm experimental aquaria were lined, on all sidesexcept the front, with off-white painted PVC, to which fine sand had been glued. Thisprocedure enhanced clarity of the video footage used for analysis, and reduced stress and

Ž .confusion to the fish that could be induced by tank-wall reflections Ritz et al., 1997 .Natural lighting was eliminated to exclude the possibility of diurnal variations affectingthe salmon or mysid behaviour. Two fluorescent lights above the tank and a halogenlight on either side of the tank replaced natural lighting. To achieve the three-dimen-sional imagery required for accurate analysis, a mirror was placed at a 458 angle abovethe experimental aquarium. Temperature in the tanks was held at 14.5"0.58C and thesalinity at 31.5"1.

2.1. Feeding experiments

To standardise behavioural variation due to hunger, the experimental mysids andŽsalmon were starved for a period of 24 h prior to each trial Baird et al., 1991; Kaiser et

.al., 1992 . Food deprivation increases feeding rates, reaction distances and the size rangeŽ .of prey eaten Croy and Hughes, 1991a . These experiments were necessarily performed

under accelerated feeding conditions due to the lack of prey choice and restricted preyescape area. Salmon and mysids were initially partitioned off from one another in theexperimental tank. Experiments were run with: 1 salmon and 50 mysids; 3 salmon and

Ž .150 mysids; and 6 salmon and 300 mysids Table 1 . Each of these experimentalregimes was replicated 10 times. The fish were allowed a minimum 2-h acclimationperiod, and the mysids a minimum 20-min acclimation period in the experimental tankprior to each run. During this period, disturbance by external sound and movement waskept to a minimum.

Ž .Each experimental run was filmed using a video camera Panasonic , which wasattached to a tripod, and placed approximately 2.5 m back from the tank to minimiserefraction at the air–glass–water interface. Graduated scales in the 60-l aquarium were

Ž . Ž .set along the horizontal x-axis , the vertical y-axis and the z-axis to indicate scale onŽ .the recorded image Osborn and Ritz, unpublished . In the 120-l aquarium, the scales

were positioned at the two extremes of the tank to facilitate scaling in the video image.Mysids were stained with a 0.05% solution of neutral red to enhance their visibility.Previous experiments showed no effect of staining on the swarming behaviour of the

Žmysids nor on the reactive distance between the salmon and mysids Confer et al., 1978;.Ritz et al., 1997; Flynn and Ritz, 1999 .

Following the acclimation period, the flexible opaque partition separating the salmonand mysids was removed. Filming began 2 min prior to the removal of the partition, andcontinued for 10 min after the partition was removed. At the end of the 10-min feeding


Table 1Summary of experiments and analysis carried out in this study

Ž .Tank size l Mysid count Behavioural MOCHA

Feeding experiments1 Salmonq50 mysids 60 Ø Ø na3 Salmonq150 mysids 60 Ø Ø Ø

6 Salmonq300 mysids 60 Ø Ø Ø

Validation experimentsRatio variation1 Salmonq300 mysids 60 Ø – na1 Salmonq100 mysids 60 Ø – na

Tank variation6 Salmonq300 mysids 120 Ø Ø Ø

1 Salmonq300 mysids 120 Ø – na

na: Not applicable.Ø : analysis was carried out.–: analysis was not carried out.

run, the salmon were removed and returned to the holding tank. The maximum numberof fish caught and held in the laboratory at any one time was 18 fish, hence animals hadto be used in subsequent trials. However, a minimum 24-h settling period for the fishwas allowed after each trial to reduce stress on the individuals and to ensure that thesame animals were not used in consecutive trials. Remaining mysids were counted andreturned to a separate holding tank.

2.2. Mysid count analysis

The numbers of mysids at the beginning and end of each trial were recorded anddifferences were compared using t-tests for comparison of means.

2.3. BehaÕioural analysis

The video footage was analysed to determine the following behavioural patterns ofthe individual schooling fish:

1. the number of approaches—when a fish accelerated towards a mysid but abruptlyŽceased its chase before nearing the capture vicinity of its target Croy and Hughes,

.1991b ;2. the number of successful attacks—when a fish accelerated towards a mysid and

Žprotruded its jaw to capture its target prey successfully Major, 1978; Croy and.Hughes, 1991b; Flynn and Ritz, 1999 ;

3. the number of unsuccessful attacks—when a fish accelerated towards a mysid andprotruded its jaw, but missed its target prey due to misguided aiming or an

Ž .advanced escape response of the target Meyer, 1986; Flynn and Ritz, 1999 .

Each of the data sets was then analysed using ANOVA or Student’s t-test at the 95%confidence level to determine significant differences between groups. Tukey’s post hoccomparison of means was applied whenever ANOVA revealed significant differences.


2.4. MOCHA analysis

The video footage from experiments with 3 and 6 salmon were analysed usingŽ .MOCHA Jandel Scientific image processing software to calculate nearest neighbourŽ . Ž .distances NND between feeding salmon Ritz et al., 1997 . Video footage was

digitised using a digital time base corrector and captured as a still image using an IBMŽ .compatible frame grabber Osborn and Ritz, unpublished .

NND of salmon were calculated from still frames taken from the video footage ofeach trial. Two frames, one each minute, were extracted from before the introduction ofprey. Five frames, one every 2 min, were taken over the 10-min trial period. A series offrames, taken 1 s apart, were taken over the period of a randomly chosen successfulattack sequence. Frames were selected by time intervals to eliminate selectivity by theanalyst.

The spatial distribution of schooling fish was determined using the coordinates of thesnout in the side and plan views of the digitised image. The information was analysedusing the statistical tests described above.

2.5. Validation experiments

Experiments were run to validate the results obtained in the feeding experiments. Theeffect of varying salmonrmysid ratios was tested to ensure that variability in the feedingsuccess of salmon was not due to the number of mysids presented to each fish. Theeffect of varying tank size was tested to determine if there were any edge effects thatcontributed to the change in feeding efficiency of the salmon.

Firstly, experiments were run to analyse the effects of variable ratios of salmonrmys-Ž .ids on the feeding success of the fish Table 1 . The original ratio used in the feeding

experiments was 1 salmon to every 50 mysids. Experiments were also performed usingŽ .ratios of 1:100 and 1:300 salmonrmysids . The mysid count analysis was used alone,

unless significant results were obtained from the ANOVA, in which case the predatorybehaviour was examined and quantified.

Secondly, a 120-l aquarium was used to test the effect of the tank size on theŽ .predatory feeding behaviour and consequent feeding success of the salmon Table 1 . To

obtain a video image of the entire 120-l tank, the camera was moved back a further 20cm. However, due to the limitations imposed by the size and clarity of video footage, theresults of the large tank were limited to mysid count analysis unless significantdifferences between successful attack rates of like feeding regimes in the small and largetanks were observed. Then, where possible, the behavioural and MOCHA analyses wereexamined.

3. Results

3.1. Feeding experiments

ŽPredatory behaviour is displayed as the percentage of total advances approachesq. Ž .successful attacksqunsuccessful attacks made towards the prey Fig. 1a . This was

done to standardise the results so direct comparisons could be made between experi-ments.


Ž .Fig. 1. a Behavioural analysis of feeding experiments in terms of percent of total advances made towards theŽ . Ž .prey APsapproaches; SAssuccessful attacks; UAsunsuccessful attacks . b Average NND between

Ž .schooling fish in the feeding experiments average BLs75 mm .


3.1.1. Within experimental regimesWithin all feeding regimes, significant differences were found between predatory

Žbehaviours using one-way ANOVA F s4.859 for solitary salmon; F s16.6262,27 2,27.for three salmon; and F s47.837 for six salmon; p-0.05 for all feeding regimes .2,27

For solitary salmon, a post hoc comparison of means showed that the significance wasŽ .caused by the difference between rates of successful and unsuccessful attack Fig. 1a .

Three feeding fish followed the same trend as the solitary salmon, with the significantdifferences being between rates of successful and unsuccessful attack and rates of

Ž .unsuccessful attack and approach Fig. 1a . The rate of successful attack for six salmonŽ .was higher than both the rates of approach and unsuccessful attack Fig. 1a .

3.1.2. Between experimental regimesOne-way ANOVA showed a significant difference in rates of approach and success-

Žful attack between fish in the three feeding experiments F s5.695, ps0.009;2,27.F s5.350, ps0.011, respectively . Six fish had lower rates of approach and2,27

captured prey more successfully than either the solitary salmon or salmon feeding in aŽ .group of three Fig. 1a . There was no significant difference in the rates of unsuccessful

Ž .attack between fish in the three feeding experiments F s0.372, ps0.693 .2,27

3.2. Nearest neighbour distances

Ž .Based on an average fish length of 75 mm, nearest neighbour distances NNDŽ .ranged from 0.67 body lengths BL to 2.12 BL. The smallest NND were recorded

between fish in the absence of prey, and the largest NND were recorded between fishduring an attack sequence.

3.2.1. Within experimental regimesFig. 1b shows that NND for three and six fish increase from that in the absence of

prey to that in the presence of prey and again during an attack sequence. There was asignificant difference in NND within both the three and six salmon experimentsŽ .F s13.458, p-0.001; F s39.00, p-0.001, respectively . A post hoc com-2,181 2,382

parison of means showed this difference to be attributable to larger NND during anŽ .attack sequence for the three salmon Fig. 1b . There were significant differences in

Ž .NND for all comparisons within the six salmon experiments Fig. 1b .

3.2.2. Between experimental regimesSix salmon had smaller NND than three salmon in the absence of prey, in the

Žpresence of prey and during an attack sequence t s4.309, t s7.420, t s7.247,95 193 275.respectively; p-0.001 for all three tests .

3.3. Validation experiments

There was no significant variation in successful attack rates between solitary salmonŽ .feeding on 50, 100 or 300 mysids in the 60-l tank F s0.0983, ps0.907 .2,27

Inspection of Fig. 2a suggests there was a substantial effect of tank size on thecapture success of a solitary salmon feeding on 300 mysids in the 60-l tank compared


Ž .Fig. 2. a Average number of successful attacks per feeding fish in experiments with 1 salmonq300 mysidsŽ .and 6 salmonq300 mysids repeated in the 60-l tank then the 120-l tank. b Behavioural analysis of A6

salmonq300 mysidsB experiments in the 60-l and 120-l tanks in terms of percent of total advances madeŽ . Ž .towards the prey APsapproaches; SAssuccessful attacks; UAsunsuccessful attacks c Average NND

Žbetween schooling fish in the 60-l and 120-l tanks in the A6 salmonq300 mysidsB experiments average.BLs75 mm .


with the 120-l tank. However, statistical analysis showed no significant differenceŽ .t s1.926, ps0.070 . This lack of significance could be explained by the particularly18

Ž .variable nature of the data 60 l: data ranges0–23; 120 l: data ranges0–49 .Six salmon feeding on 300 mysids had a higher successful attack rate in the 120-l

Ž .tank than in the 60-l tank t s3.176, ps0.007; Fig. 2a . In the 120-l tank, the salmon13Žhad higher rates of approach and successful attack t s2.752, ps0.016; t s3.176,13 13

.ps0.007, respectively . However, in terms of percent of total advances made, theforaging behaviour did not appear to change significantly between six salmon feeding on

Ž .300 mysids in the 60-l tank and those in the 120-l tank Fig. 2b .One-way ANOVA showed that there was a significant difference in NND when

comparing the foraging behaviour of six salmon feeding on 300 mysids in the 120-l tankŽ .F s9.873, p-0.001 . In the absence of prey, six salmon in the 120-l tank had2,332

smaller NND than salmon in either the presence of prey or during an attack sequenceŽ .Fig. 2c . Six salmon in the 60-l tank had lower NND than six salmon in the 120-l tankin the absence of prey, in the presence of prey and during an attack sequenceŽ .t s7.797; t s8.309; t s6.706, respectively, p-0.001 for all tests; Fig. 2c .148 248 318

4. Discussion

Individuals in a school of six fish had higher predatory success than solitary feedingsalmon or individuals feeding in a group of three fish. The lower rates of approach madeby salmon in a school of six fish, compared with solitary salmon or fish in a group ofthree, indicate the utilisation of the school to enhance prey capture efficiency bybreaking up mysid swarms. This observation effectively reduces confusion in the field ofview of the individual salmon, enabling the fish to target specific prey. An increase inarea allowed for foraging increased the prey capture efficiency of the foraging fish.There were no significant effects of varying salmonrmysid ratios nor of time spent in

Žthe laboratory on the prey capture efficiency of the fish i.e. the number of successfulattacks by fish after 40 days in the laboratory often fell below, or were equivalent to, the

.number recorded for fish in the first few days of experimentation .Increased prey capture efficiency meant that the fish expended less energy on

unrewarding advances towards prey swarms, and thereby conserved energy for reproduc-tion and vigilance. Results from experiments with groups of three fish showed astatistical difference to the predatory behaviour of six fish, but no statistical difference tothe behaviour observed for solitary salmon. These results distinctly separate the preda-tory behaviour of fish in a group of six and individuals foraging alone or in a group ofthree fish, yet indicate similarities between the predatory behaviour of individuals in a

Ž .group of three fish and solitary salmon. This evidence contradicts Partridge’s 1982definition of a school as one comprising three or more fish. As we were unable toascertain any certain behavioural differences between solitary foragers and those in agroup of three, we were unable to conclude whether three fish did in fact forage as aschooling unit.

The benefits of schooling on the feeding success of salmon are attributable to aŽ .number of factors. Primarily, in accordance with Ryer and Olla’s 1991 local enhance-


ment theory, the schooling fish spends less time foraging for productive food patches orwatching out for potential predators due to the increase in visual field that a school

Ž .provides Schoener, 1971 . This theory means that the time between feeding bouts mayŽbe reduced, and the time to exploit ephemeral food patches may be maximised Baird et

.al., 1991; Flynn and Ritz, 1999 .Although the exact nature and development of the salmon’s learning capabilities was

not determined in this study, the salmon were observed to follow each other’s lead. Thisobservation was particularly evident when non-feeding salmon closely followed attack-ing salmon into the mysid swarms. Generally, a lead salmon penetrated the densestportion of the prey swarm from below to scatter the mysids and reduce visual field

Ž .confusion, a general approach also observed by Thetmeyer and Kils 1995 for predatoryfish schools. Secondary salmon followed regardless of their intent to attack. Thisbehaviour illustrates the cohesive nature of schooling attributable to social facilitation of

Žnon-feeding parameters, such as group protection and reproduction Ryer and Olla,.1991 . Those that followed to feed took the initiative from the lead fish. By working

together to break up the mysid swarms, the salmon could overcome the confusion effectŽof the prey aggregations and target specific prey more readily Radakov, 1973; Major,

.1978; Flynn and Ritz, 1999 . This accuracy requires a lot more energy and time if thesame feeding success is to be attained by a solitary salmon.

Variation in results of solitary salmon experiments could have been due to thevariable responses to the stress of transfer into a new environment, or to the solitary

Ž .state itself Meyer, 1986; Gomez-Laplaza and Morgan, 1993 . For a natural schooling´species such as the Australian salmon, the stress of isolation from other school members

Žis sufficient to suppress normal feeding activity significantly Gomez-Laplaza and Mor-´.gan, 1993; Ryer and Olla, 1991 . Solitary salmon that showed signs of stress or

suppressed feeding activity were not used in this study.Ž .Morgan and Ritz 1984 predicted that juvenile Australian salmon will forego the

vigilant benefits of strong school cohesion in place of the weaker school cohesionrequired for successful foraging. This theory was supported by the findings of this study,where the school was most cohesive in the absence of prey, thereby maximisingpredatory protection and reducing superfluous energy expenditure in foraging for food.The introduction of prey to the salmon foraging area results in a relaxation of the schoolcohesion as the hunger-state of the individual fish begins to outweigh the demand for theprotective benefits of the school. Upon pursuit of prey, the salmon move to anindependent mode of survival, in which the acquisition of food gains priority. At thesame time, they continue to reap the benefits of schooling that facilitate the break-up ofprey swarms. These benefits were reflected by the results of this study that indicatedsignificantly smaller nearest neighbour distances, combined with a more successfulfeeding rate for individuals in a foraging group of six, compared to the distances andfeeding rates of individuals in a group of three.

Once the prey was captured, school cohesion tightened and fish began to forage forŽ .further prey. Similarly, Hunter 1966 found that fish only briefly leave the school to

feed and then quickly rejoin the school again. In support of the theory of expansion ofŽ .the fish school when foraging, Eggers 1976 stated that a reduction in foraging success

appears much greater for larger school sizes and for those schools with smaller distances


to their nearest neighbours. Primarily, this is due to a greater visual field overlap andŽ .prey removal by forward members Eggers, 1976 . The fact that schools did not

completely lose cohesion during an attack sequence in this study, supports the hypothe-sis that it is more beneficial for Australian salmon to school with conspecifics than toremain solitary.

4.1. Validation of results

Fish predation is dependent, among other things, on the density of prey on which theŽ .fish forages Cushing and Harden Jones, 1968; Zaret, 1980 . Most fish are able to

recognise high or low prey densities and can modify their behaviour accordingly toŽ .enhance their feeding success Potts, 1983 . High prey densities often lead to increased

Žspecialisation by the fish to select for certain prey sizes or specific prey species Werner.and Hall, 1974; Wankowski, 1981 . Low prey densities can lead to a relaxation of this

specialisation, as prey are pursued when they are encountered rather than includedŽselectively in the predator’s diet Schoener, 1971; Werner and Hall, 1974; Confer et al.,

.1978 .The original ratio of salmonrmysids used in this study was 1:50 in the 60-l tank, as

this allowed for the mysid swarm to move freely about during the feeding tests andŽprovided a sufficient swarm size for successful foraging by the salmon Ritz et al.,

.1997 . We then examined the effects of prey density on predation to determine ifdifferent ratios of salmonrmysids would affect the overall feeding efficiency of thesalmon.

Predatory fish schools overcome the confusion effects of swarming prey by penetrat-ing the swarm and isolating target individuals. In this study, solitary salmon broke upmysid swarms and fed at the same rate, regardless of mysid numbers. This indicated thatthere was a threshold to the number of mysids a solitary salmon could handle in thegiven period, and the absolute number of mysids, once exceeding 50 mysids for every 1salmon, did not affect the feeding rate. It is possible that salmon either reached satiationbefore the end of the 10-min period, after which time they did not feed, or that the fishcould not distinguish individual prey until they were separated from the swarmregardless of swarm size.

Ž .Ritz et al. 1997 found no significant difference in the patterns of contraction andexpansion in volume of mysid swarms in response to feeding in larger tanks. Thisinformation was extrapolated and applied to mysid swarms being preyed upon in thisstudy. It was thus assumed that there were no significant variations in mysid swarmingattributes that may have influenced the salmon feeding success.

The relative tank effects were tested because there was a potential for environmentalŽvariation between tanks that may have influenced the salmon feeding behaviour Speare

.et al., 1995 . Coordinated group foraging can be limited by the clear space available topredatory fish schools. This theory was supported by the results indicating that a groupof six salmon feeding in the 120-l tank had a higher successful attack rate than sixsalmon feeding in the 60-l tank. The significantly greater number of approaches andsuccessful attacks made by fish in the 120-l tank indicated a greater level of activity bythese fish. This enhanced activity could have been a result of variation between fish, or


it could have been a result of greater foraging space that enabled the fish to coordinatetheir activities better and to capture their prey successfully. Although the fish in the120-l tank appeared to have similar activity in terms of the total advances made, there

Žwere significant behavioural differences compared to fish feeding in the 60-l tank Fig..2b . The greater distances that were observed between fish in the 120-l tank indicated a

more efficient use of the given area that led to an increase in the salmon’s feedingefficiency.

Ž .Croy and Hughes 1991b found that the frequency of unsuccessful attacks decreasedin later trials, as fish became more experienced with the prey. This finding contradicts

Ž .Meyer 1986 , who found that the number of unsuccessful attacks by fish did not changeŽ .with increased experience with the given prey. However, Meyer 1986 also reported a

significant increase in the feeding efficiency of fish that was associated with an increasein experience with the experimental set up. This trend was not evident in the presentstudy, although the time required for familiarisation and learning from other schoolersmay not have been attained in the short period of confinement in the laboratory. Thenumber of successful attacks made by experimental fish in the first few days and the last

Ž .few days of experimentation up to 60 days were compared for any effects ofexperience on the experimental fish. There were no significant differences, indicatingthat the salmon’s feeding rate neither improved nor declined with increasing time in thelaboratory.

Hunger differs between individuals primarily because of the different responsivenessŽ .to the gut stretch receptors and blood sugar levels of each fish Ringler, 1983 . This

study attempted to overcome this variable by setting a constant time of food deprivationfor all fish. Fluctuations in feeding remain, however, because of an individual’s response

Ž .to the allocated period of food deprivation Thomas, 1977 .In this study, smaller fish were the more active feeders, a result also observed by

Ž .Meyer 1986 . As our results indicate, this was probably not a result of acceleratedlearning or the effects of experience, but rather due to the fact that smaller fish need toeat more regularly and they experience the effects of hunger more acutely than larger

Ž .fish Meyer, 1986 . If the variable hunger levels were not considered as an importantinfluence on the feeding rates of fish, the fluctuating feeding success of individual fishcould easily be confused with the effects of increased experience.

This study contributes to our understanding of the benefits of schooling by predatoryfish when targeting invertebrate prey aggregations. Juvenile Australian salmon hadhigher prey capture efficiencies when foraging in a school compared to when foragingalone. This is indicative of the coordinated predatory behaviour adopted by fish schools,such as Australian salmon, in breaking up prey aggregations, such as mysid swarms, toreduce the confusion effect of multiple prey in the fish’s field of view, hence increasingprey capture efficiency.

Acknowledgements

We would like to thank Rowena Scott and Adam Stephens for their help with samplecollections. We would also like to thank members of the Zoology Department at the


University of Tasmania for their assistance with experimental design and provision of[ ]laboratory space. RW

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Documents

Schooling affects the feeding success of Australian salmon (Arripis trutta) when preying on mysid swarms (Paramesopodopsis rufa)