journal of fish biology (2001) 59, 968-986 1coweeta.uga.edu/publications/1442.pdf · 3 of 30)....

Journal of Fish Biology (2001) 59, 968-986doi:10.1006/jfbi.2001.1711, available online at http://www.idealibrary.com on IDEj^L1

Intraspecific aggression in rosyside dace, a drift-feeding

stream cyprinid

P. A. RiNc6N* AND G. D. GROSSMAN

Daniel B. Warnell School of Forest Resources, University of Georgia, Athens, GA 30602,^ U.S.A.

(Received 8 August 2000, Accepted 12 July 2001)

Individual rosyside dace Clinostomus funduloides in a semi-natural, artificial streainslisplayedsubstantial differences in their aggressiveness and could be classified as: (1) non-aggressive (NA,18 of 30 rosyside dace), (2) moderately aggressive (MA, 9 of 30) and (3) highly aggressive (HA,3 of 30). Rosyside dace groups, however, did not exhibit linear dominance hierarchies and fishsize was only weakly correlated with the number of aggressive acts performed per individual.Small rosyside dace (<56 mm LF) were always non-aggressive, but larger fish were present in allthree aggression classes. The difference in size between the contestants was significantly,although not very strongly, correlated with the probability of winning an agonistic interaction(r2=0-39>.- Aggressive rosyside dace may have ultimately gained higher fitness than lessaggressive ones. HA individuals occupied the upstream-most position within foraging groupssignificantly more often than other rosyside dace. This location should be the most profitableone because its occupant will, be the first to encounter prey. HA rosyside dace also occupiedsignificantly higher focal velocities that were closer to energetic optima than MA and NA ones.They also had greater foraging rates and were less solitary than less aggressive fish, but thesedifferences only were significant at the P=0-066 and />=0-081 level, respectively. Finally, HAfish performed significantly more aggressive acts and feedings backwards than other individ-uals. Despite these differences, the effects of intraspecific aggression in rosyside dace appearedless substantial than those that have been observed in stream salmonids.

© 2001 The Fisheries Society of the British Isles

Key words: Rosyside dace; Clinostomus funduloides', intraspecific aggression; drift-feeding;stream cyprinids; dominance hierarchies; resource use.

INTRODUCTION

Mobile animals should engage iri inter- or intraspecific aggression when thebenefits accrued through this behaviour exceed its costs (Brown, 1964; MaynardSmith, 1982). This tenet has substantial empirical support because dominantindividuals typically have greater access to critical resources such as food, matesor refuges (Grossman, 1980; Huntingford & Turner, 1987; Katano, 1990;Blanckenhorn, 199la; Cuadrado, 1997; Hall & Fedigan, 1997). Hence, it followsthat intraspecific aggression should be strongly influenced by factors that affectthe economic defensibility (Brown, 1964) of a resource, a result demonstrated by-several researchers (Grant & Noakes, ,1988; Sneddon et al., 1998). Followingthis rationale, Barlow (1993) argued that the marked environmental instability offreshwater systems (Resh et al., 1988) made economic defensibility of trophic

* Author to whom correspondence should be addressed. Present address: Departamento de EcologiaEvolutiva, Museo Nacional de Ciencias Naturales, C.S.I.C., c/ Jose Gutierrez Abascal 2, E-28006 Madrid,Spain. Tel.: +34 91 411 13 28; Fax: +34 91 564 50 78; email: [email protected]

9680022-1112/01/100968 + 19 $35.00/0 © 2001 The Fisheries Society of the British Isles

INTRASPECIFIC AGGRESSION IN ROSYSIDE DACE 969

resources difficult and hence, may be responsible for the comparative rarity ofterritorially among freshwater fishes. Barlow (1993) also noted that streamsalmonids were a marked exception to this generalization, which may be aconsequence of the relative predictability of invertebrate drift, the primary preyof salmonids in streams (Elliot, 1967; Allan & Russek, 1985). Stream-dwellingsalmonids exhibit in fact positive inter- and intraspecific relationships betweenreliance on drift as food source and aggressiveness (Grant & Noakes, 1988;Hutchinson & Iwata, 1997).

Intraspecific aggression has profound consequences on the resource use,spatial distribution, life history, social structure and, ultimately, fitness of thesefishes (Jenkins, 1969; Li & Brocksen, 1977; Bachman, 1984; Fausch, 1984; Grant& Noakes, 1988; Grant, 1990; Hughes, 1992; Nakano & Furukawa-Tanaka,1994; Metcalfe et al, 1995; Nakano, 1995). Furthermore, intraspecrfic aggres-sion also may be a mechanism of density-dependent population regulation insalmonid fishes because subordinate individuals that are excluded from profit-able foraging sites typically emigrate or die (Elliott, 1990; Grant & Kramer, 1990).

Given the strong relationship between intraspecrfic aggression and access toboth trophic and spatial resources in drift-feeding salmonid fishes, it would beuseful to determine whether similar relationships are present in other drift-feeding fishes. In temperate streams, drift feeding cyprinids often dominate fishassemblages (Jenkins & Burkhead, 1994; Matthews, 1998; Moyle & Cech, 1999)and they are also common in both African and Asian streams and rivers(Matthes, 1964; Moyle & Senanayake, 1984; Skelton, 1993). Although there is ageneral lack of information on aggressive behaviour in cyprinids, there is someevidence that it may influence their use of trophic and spatial resources (Symons,1975; Reeves et al, 1987; Freeman & Grossman, 1992; Katano, 1996). Conse-quently, the dynamics of intraspecific aggression in rosyside dace Clinostomusfunduloides Girard, a drift-feeding stream cyprinid native to portions of theeastern U.S.A., was quantified.

In southern Appalachian streams, rosyside dace feed mostly on driftinginvertebrates. They forage both solitarily and in groups, and occasionallydisplay intraspecific aggression (Grossman & Freeman, 1987; Freeman &Grossman, 1992; Rincon & Grossman, 1998). In the present study, inter-individual, differences in aggressive behaviour were examined and the effects offish size on aggressiveness and the outcome of contests quantified. Based on thelatter, dominance relationships between pairs of rosyside dace were also estab-lished and whether these were consistent with a linear dominance hierarchy wastested. Next, it was ascertained whether intraspecific aggression affected the useof either spatial (microhabitat characteristics or position in a group) or trophicresources (i.e. feeding rate), and, therefore, could be considered adaptive.Finally, it was determined whether rosyside dace defended ' exclusive use'foraging areas (Grant, 1990) or positions relative to neighbouring conspecifics(Hall & Fedigan, 1997).

MATERIALS AND METHODS

EXPERIMENTAL PROCEDURESData were obtained for analyses from a set of experiments conducted to determine

the effects of rainbow trout Oncorhynchus mykiss (Walbaum) on microhabitat use

970, P. A. RINC6N AND G. D. GROSSMAN

by rosyside dace (Rincon & Grossman, 1998). Because these results indicatedthat rainbow trout did not have a significant effect on the use of spatial resourcesby rosyside dace the data were also utilized to examine intraspecific aggression in rosysidedace. Because the experimental apparatus and protocols have been extensively describedpreviously (Rincon & Grossman, 1998) they are only summarized here.

The artificial stream was a 10 m long ellipsoid, 50-75 cm wide, with pools at its curvedends (c. 50 cm deep) and a run-riffle (c. 10-30 cm deep) habitat between the two pools(Grossman & Boule, 1991; Grossman et ai., 1995). Experiments were run in 1994 (22October-16 November) and 1995 (29 April-23 May and 17 July-9 August). A thermo-statically controlled cooling system was used to maintain water temperatures at15-0 ±0-5° , . and an electronic timer was employed to regulate photoperiod at14-5L: 9-5D with 1 h transitional phases representing dusk and dawn. These conditionsresemble summer in Coweeta Creek, North Carolina, U.S.A. Attempts to performexperiments under spring and autumn conditions (i.e. 10° C) failed due to a lack ofactivity by rosyside dace; this behaviour has been observed in field populations inCoweeta Creek (Hill & Grossman, 1993). Fish were fed using two automatic feeders thatdispensed trout chow pellets (salmon starter No. 3, Ziegler Bros. Inc., 2^4 mm long,0-5-0-75 mm wide, 19037kj kg"1) at the surface of high velocity riffles "at irregularintervals (every 40-80 min, average 60 min) during dawn, daytime and dusk. The totaldaily ration in all experiments was c. 12 g of food (about 0-86 g per delivery) which wasat least a maintenance ration. Pellets quickly dispersed through the stream and circulatedthroughout the water column for 10-15 min before sinking. Rosyside dace readilyaccepted trout chow as food and either gained or maintained mass throughout exper-iments, a result identical to that obtained for rosyside dace by Grossman & Boule (1991).Despite the presence' of fixed feeders in the experiments, rosyside dace were not observedto aggregate under feeders.

Wild rosyside dace and rainbow trout were captured using dipnets and electrofishing inboth Coweeta Creek and its tributaries. After capture, rosyside dace were measured (forklength, Lf to the nearest mm) and weighed (to the nearest 0-01 g) using an electronicbalance. Rosyside dace were then either placed in the artificial stream for a 48 hacclimation period or were held in tanks to be used in the event of mortality amongexperimental subjects. However, there were only two mortalities at the beginning of thestudy. Two rosyside dace jumped out of the artificial stream, were replaced with newspecimens and the artificial stream was then covered with a sheet of transparent plasticplaced 50 cm above the stream surface.

The experiment was replicated three times. Each replicate consisted of three 5 daytreatment segments each separated by at least a 2 day acclimation period. Treatmentsconsisted of: (1) 10 rosyside dace (DD), (2) five rosyside dace and five rainbowtrout (DT), and (3) five rosyside dace (D) and were designed to test for the effectsof both intra- and interspecific competition on microhabitat use by rosyside dace(Rincon & Grossman, 1998). Each individual fish was only used in one treatmentsegment of one experimental replicate. In all treatments data were recorded onlyon five individually identified rosyside dace to maintain equal sample sizes and, as aconsequence, interactions among the five extra rosyside dace present in the DD trialsand their positions were not recorded. Therefore, data from DD treatments wereexcluded from further analyses of aggressive behaviour. Hence, the data base consists ofobservations from either 45 or 30 rosyside dace respectively (nine groups of five fish,with five extra rosyside dace present in three of them and rainbow trout present in theother three). In the first replicate rosyside dace were marked with subcutaneous injectionsof latex paint using the methods of Hill & Grossman (1987). However, in the second andthird replicates size and unique marks (scars, spots, colour pattern) were used to identifyindividual rosyside dace. Rosyside dace marked with acrylic paints have previously beenshown not to exhibit higher mortality or altered behaviour (Hill & Grossman, 1987).

The sizes of rosyside dace used in experiments ranged from 49 to 81 mm Lf dependingon the availability of wild specimens. Rosyside dace density in the trials (one or tworosyside dace m ~2) was within those reported from a variety of habitats in CoweetaCreek (Grossman & Freeman, 1987; Freeman & Grossman, 1993).


OBSERVATIONSBehavioural observations were made on rosyside dace three times daily (0800-1000,

1300-1500 and 1800-2000 hours) during each 5 day treatment. Each of the five dace waslocated and the positions they occupied recorded as X (longitudinal position, cm) Y(distance from the stream wall, cm) and Z (distance from the bottom, cm) coordinates.As the relationship between intraspecific aggression and access to food resources wasbeing investigated, behavioural observations began 3 min after a delivery of food into thestream. During the 3 min of observations on each specimen, the number of food pelletscaptured, the number of tunes the focal fish displaced another fish, the number of timesthe focal fish was displaced by another fish, the number of chases performed, and thenumber of chases received were recorded using the methods of Freeman & Grossman(1992). Depending on the particular analysis, behavioural events were expressed as eithernumber of events 3 min~ , or as total number of events per treatment. The number ofchases and the number of displacements were combined into a single variable (number ofone-sided aggressive acts) to increase sample sizes and reduce colinearity amongvariables. The location (X, Y, Z coordinates) of rosyside dace was also recorded at thebeginning and end of each aggressive act or food capture and the orientation and distancetravelled for these events calculated.

To quantify the areas within which individual rosyside foraged or performed aggres-sion, each fish was considered to be located at the centre of a circle. The locations at theend (in the horizontal plane) of all feeding and agonistic interactions were then used tocalculate the sizes of aggressive and foraging areas (cm2) using the minimum convexpolygon method (Nakano, 1995). An aggressive area of 0 cm2 was assigned to individ-uals with less than four aggressive acts per treatment because smaller sample sizesproduced erratic results. Using the criteria of Freeman & Grossman (1992) it was alsodetermined whether the individual was a solitary forager (i.e. located further than fivebody lengths from the nearest rosyside dace), or in a group. Finally, the positions thatgroup-foraging rosyside dace held within the group (i.e. anterior-most fish or not) werequantified.

When a treatment ended, the techniques of Grossman & Ratajczak (1998) and Rincon& Grossman (1998) were used, to quantify microhabitat use by rosyside dace. At eachposition, total depth (cm), distance from shelter (a structure capable of hiding at least50% of the fish's body, cm), focal-point velocity (cm s"1, using an electronic velocitymeter accurate to 0-01 cm s~ l) and average water column velocity (cm s"1) weremeasured and a visual estimate made of the percentage of bedrock, boulder, cobble,gravel, sand, silt and debris in the 20 x 20 cm quadrat under the fish. In addition,microhabitat availability was quantified by measuring total depth, mean velocity andsubstratum composition at 40 randomly selected points within the artificial stream at theend of each replicate. The number of focal positions occupied by each fish during atreatment was also determined. Focal positions were considered to be different if theywere >5 cm apart.

DATA ANALYSISInter-individual differences in aggressiveness and social structure

The null hypothesis was tested that all five rosyside dace in a treatment group exhibitedsimilar levels of aggressive behaviour with a partitioned goodness-of-fit ̂ test (Zar, 1996)on the number of one-sided aggressive acts performed per individual rosyside dace.Alpha levels were adjusted for multiple comparisons using the Dunn-Sidak procedure(Ury, 1976). Rosyside dace were then classified as either 'aggressive' (those whocontributed to significance in the goodness-of-fit test) or' non-aggressive' (those who didnot contribute to significance in the goodness-of-fit test) based on the partitioned testresults (Zar, 1996). The total number of one-sided aggressive acts per rosyside dace wasarranged in descending order and the plot inspected for gaps, which would be indicativeof the presence of groups of rosyside dace exhibiting similar levels of aggressiveness. Thisprocedure confirmed results from x2 goodness-of-fit tests and also enabled classificationof fish as (1) highly aggressive (HA, >27 aggressive acts per treatment), (2) moderately

972 P. A. RINC6N AND G. D. GROSSMAN

aggressive (MA, 6-17 aggressive acts per treatment), and (3) non-aggressive (NA, <fouraggressive acts per treatment, save for one fish that performed seven). Once all fish hadbeen assigned to an aggression class, tests were made for significant differences in theproportion of rosyside dace in each aggression class among treatments and replicatesusing log-linear analysis (Zar, 1996).

The effects of fish size on agonistic behaviour were examined in several ways. First,significant differences in mean size among aggression classes were tested for using athree-way ANOVA with replicate, treatment and aggression class as main factors,followed by tests for significant linear correlations between fish size and the number ofone-sided aggressions performed per rosyside dace. Finally, for the 27 dyads i.e. pairs ofrosyside dace that had three or more aggressive interactions, one member of the dyad waschosen randomly (so that each dyad was represented only once) and the difference in LF

between it and its opponent was calculated. Its probability of .winning an agonisticencounter was estimated as the number of one-sided aggressions performed divided by thenumber of total aggressive acts observed for the dyad and a logistic regression line wasfitted to the relationship between these two parameters (Metcalfe et al., 1995).

It was determined whether the six groups of five rosyside dace could bexarranged intolinear dominance hierarchies (Katano, 1996) using the randomization procedure of deVries (1995). A dominance hierarchy is completely linear if: (1) for every possible pair ofrosyside dace A and B either A dominates B or B dominates A, and (2) dominancerelationships are not circular; that is, for every three rosyside dace A, B and C, if Adominates B and B dominates C, then A dominates C (de Vries, 1995). First a. 5 * 5dominance matrix was constructed for each of the six groups with both row and columnentries for all five fish. According to Drews (1993), fish A (row entry) was considereddominant over fish B (column entry) if A performed more one-sided aggressive actstowards B than B towards A. Then, a one was placed in matrix cell XAB and a zero inmatrix cell XBA (de Vries, 1995). Both cells were given the value 0-5 if the individualswere tied, whereas an X was used to indicate unknown relationships (i.e. rosyside dacethat were not observed to interact). X values were used as markers and did not affect thecalculation of h.

Then, the original dominance matrix was submitted to two types of randommodifications. First, a 1/0 or 0/1 was assigned randomly to both cells in the interactionwith unknown dominance relationships. As defined by Landau (1951) h, was calculatedas:

where Vt is the sum of row / (i.e. the number of other individuals dominated by the rowindividual) and N is total number of individuals. Landau's h measures the degree oflinearity in a given matrix of dominance relationships, and is preferable to alternativeindices of linearity such as Kendall's K (de Vries, 1995). The value from the aforemen-tioned partially random matrix was termed h\. Values of h>Q-9 are typically consideredto be indicative of strong linearity (Martin & Bateson, 1993).

Next, 1/0 or 0/1 relationships were assigned randomly to all dyads in the originalmatrix and the value of Landau's h was recalculated. This value was termed H2. Then, hl

and h2 were compared using a counter to keep track of the number of times that h2>hl.This was repeated 10 000; times. The number of times h2>hl (i.e. the counter's value)divided by 10 000 represented the probability that the observed degree of linearity in theoriginal matrix could be explained by random processes alone, and was used as P valuefor linearity tests. The degree of linearity of the original matrix was estimated as themean of the 10 000 replicates of /z,, because this is an unbiased estimate of Landau's h(de Vries, 1995).

AGGRESSIVENESS AND RESOURCE USEThe experiments of Rincon & Grossman (1998) examined the effects of (1) replicate, (2)

treatment (i.e. inter- and intraspecific competition), (3) acclimation time and (4) time of


day, on microhabitat use by rosyside dace in the experimental stream. Rincon &Grossman (1998) quantified microhabitat use of rosyside dace using three parameters: (1)the scores of individual rosyside dace on principal component 1 (PCI was the dominanthabitat gradient in the experimental stream and reflected differences between riffles,shallow areas of fast water and coarse substratum, and pools, deeper areas of slow waterand fine substratum; (2) focal-point velocities; (3) distance from shelter of rosyside dace.To assess the effects offish aggressiveness on these microhabitat use parameters' rosysidedace aggression class' was added to this analysis and the data re-examined using afive-way (i.e. the four previous factors+rosyside dace aggression class) multivariateanalysis of variance with repeated measures (MANOVAR). MANOVAR was usedbecause the different levels of the factors acclimation and time of day included multiplemeasurements taken on the same individuals (von Ende, 1993). Because the effects ofreplicate, treatment (inter- and intraspecific competition), acclimation, and time of dayhave previously been discussed (Rincon & Grossman, 1998), the presentation of results isrestricted to aggression class and its significant interaction terms.

When a significant effect using MANOVAR was obtained, univariate repeated-measures ANOVAs were performed subsequently on the three microhabitat variablesusing the aforementioned factors as main effects (Scheiner, 1993). Huynh-Feldt's e"wasused to test for non-sphericity of the variance-covariance matrix (Winer et al., 1991). Ife<l, non-sphericity is present and the nominal degrees of freedom of factors withrepeated measures must be adjusted multiplying them by £ to avoid inflation of their/"-statistics (Winer et al., 1991). Rosyside dace from all aggression classes were notpresent in every treatment x replicate combination, therefore, several three-way andhigher order interaction terms could not be calculated. Nonetheless, results for thetwo-way interactions suggested that minimal information was lost through this problem.Due to a lack of normality, dependent variables were transformed to ranks in thisanalysis and the following one (Conover & Iman, 1981; Kepner & Robinson, 1988;Potvin et al., 1990) and test statistics and associated probabilities are for the rank-transformed data. However, means of the untransformed variables are presented whenthey are more informative than average ranks.

A three-way MANOVA was employed to determine whether replicate, competitiontreatment, or rosyside dace aggression class had a significant effect on the followingvariables: (1) total number of strikes at pellets per rosyside dace, (2) total number of timesrosyside dace occupied solitary foraging positions, (3) number of times rosyside daceoccupied the upstream-most position in a group, (4) number of focal positions occupiedby individual rosyside dace, (5) size of aggressive areas, and (6) size of foraging areas.

Finally, examination was made of whether agonistic and foraging behaviour wererandomly distributed in space for individuals in the three aggression classes. A focalindividual was considered to be located in the centre of a circle, and then aggressive actsand food captures were classified by circular quadrant (i.e. frontal, 45° to 135°; lateral,135° to 225° and 315° to 45°; or rear, 225° to 315°). Tests were then made, on whetheragonistic interactions and strikes at food were randomly distributed among quadrants foreach rosyside dace aggression class using partitioned x2 goodness-of-fit tests. PartitionedX2 tests for independence were also used to determine whether the spatial distributions ofthese acts differed among the three rosyside dace aggression classes.

RESULTS

INTER-INDIVIDUAL VARIATION IN AGGRESSIONTwo hundred and thirty-seven (209 displacements and 28 chases) one-sided

aggressive acts were observed and rosyside dace exhibited significant inter-individual differences in aggressive behaviour [Fig. l(a), all six partitioned x2

tests were significant at P<0-011]. Nonetheless, the majority of rosyside dace(60%) were non-aggressive [Fig. l(a)]. The number of aggressive rosyside daceper group ranged from one (one group) to three (one group), and most groups

974/ P. A. RINC6N AND G. D. GROSSMAN

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J..T TDl DTI D2 DT2 D3 DT3

Dace group

50

40

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np.

I10 20Aggressiveness rank

30

FIG. 1. (a) Distribution of aggressive interactions among individuals within groups of rosyside dace held inthe artificial stream, D and DT indicate groups without or with rainbow trout. The numbers indicateexperimental replicate, (b) Inter-individual differences in aggressiveness of the 30 observed rosysidedace. NA, Non-aggressive (•); MA, moderately-aggressive (D); HA, highly-aggressive (A).

(i.e. four) had two aggressive individuals [Fig. l(a)]. Using partitioned x2 tests 12rosyside dace were classified as ' aggressive ' and 18 as ' non-aggressive ' (NA).The arrangement of individual rosyside dace based on the number of one-sidedaggressive acts performed supported this classification [Fig. l(b)], but alsoindicated that three fish were substantially more aggressive than all otherrosyside dace. Therefore, aggressive rosyside dace were further divided into'highly aggressive' (HA, n=3), and 'moderately aggressive' (MA, n-9).Log-linear analyses detected no significant effects of replicate or competitiontreatment on the proportion of rosyside dace in each aggression class (maximumlikelihood x2=2-91,d.f.=6,P=0-820).


ao

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40

30

5 20

(a)'A

50 60 70.Fish size (mm)

80 90

-28 -24 -20 -16 -12 -8 -4 0 4Difference in Lp (mm)

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FIG. 2. Relationships between (a) aggressiveness and fish size •, NA; D, MA; A, HA. and (b) differencein LF and probability of winning an agonistic encounter A , MA-MA; •, MA-HA; D, HA-MA;O, HA-NA; O, MA-NA; •, NA-MA; +, NA-HA; *, NA-NA. Codes indicate the aggressionclass of the fish in the dyad with the probability of winning calculated for the first fish. Aggressionclass codes are as in Fig. 1 .

The observed differences in aggressive behaviour among rosy side daceappeared to be significantly, but weakly, related to LF. There was a signifi-cant, but not strong, positive correlation between rosyside dace size and thetotal number of aggressive acts performed (^=0-37, P=0-042) and meansize of rosyside dace increased with aggression class (mean ±9 5% CL:HA=70-0 ± 5-9 mm, MA=66-5 ± 3-8 mm, NA=60-6 ± 4-9 mm), although thesedifferences were significant only at the 0-088 level (three-way ANOVA with maineffects: replicate, treatment, aggression class; -F2,i5-2'88). The lack of strong sizeeffects reflected the fact that rosyside dace <56 mm always were NA, but largerindividuals (>64 mm) belonged to all three aggression classes [Fig. 2(a)]. Notsurprisingly, the size difference between rosyside dace involved in an agonisticinteraction was a significant, albeit modest predictor of its outcome (r2=0-39,


10

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I I I

0.4

0.3

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Aggression class x v-^..FIG. 3. Variations (meanis.E.) among rosyside aggression-classes in focal-point velocity (O) and

microhabitat use PCI score (•). Aggression class codes are as in Fig. 1.

Fl 25 =16-29, P<0-001,' 0-443 + 0-106

logistic regression equation y=a(l+a) , where*). Regardless of relative size, aggressive rosyside dace always won

50% or more of their encounters with NA rosyside dace [Fig. 2(b)].

AGGRESSION AND SOCIAL STRUCTUREThe social organization of rosyside dace was best characterized as a division

between aggressive and NA individuals, rather than as a linear dominancehierarchy. Dominance hierarchies observed in the six groups of rosyside dacewere never significantly more linear than those produced via randomizationprocedures (^ =0-713-0-625, P=0-086-0-498). Nonetheless, some individualsconsistently won their encounters with other fish [Fig. 2(b)]. Therefore, the lackof significant linearity in dominance hierarchies appeared to be a consequence ofthe apparent lack of dominance relationships among NA rosyside dace (60%of the total).

AGGRESSION AND RESOURCE USEAggressive rosyside dace occupied significantly different microhabitats in the

artificial stream than NA individuals (MANOVAR, Wilk's A=0-333, d.f. = 6,26,P=0-018). Univariate ANOVAs showed that those differences were due tosignificant differences in focal velocity (-F2jl5=6-34, P<0-0101). Focal-pointvelocities used by HA rosyside dace (mean ± 95% CI=9-2± 1-2 cms" ') weresignificantly higher than those used by NA rosyside dace (6-0 ±0-57 cm s ~ ' ,Tukey test, P=0-009, MA=7-1 ±0-71 cm s"1, P=0-078, Fig. 3). Focal-pointvelocities of HA rosyside dace were closer also to the optimal velocity for thisspecies (Hill & Grossman, 1993) than those utilized by other individuals. Inaccordance with these findings, there was a trend for highly aggressive dace tooccupy areas of faster water and coarser substrata (i.e. lower PCI scores, Fig. 3)than those used by the other groups (mean ±95% CL, HA=-0-03 ±0-15,MA=0-28 ± 0-09, NA=0-32 ± 0-07), but it was not statistically significant(F2,15=2-33, PO-1319). Finally, distance to shelter did not differ significantly


15

10

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(a)50

40

130 |o,en

20

10

NA MA HA

400

300

200

100

(b)

NA MA HAAggression class

FIG. 4. Variations (mean ± S.E.) among rosyside aggression classes in. (a) the number of times at theupstream-most position of a group, (A), number of pellets captured (O), number of times alone(*) and number of focal positions used (•) and in (b) the aggressive (O) and foraging (•) areas.Aggression class codes are as in Fig. 1.

among aggression classes (F2>is=0-99, P<0-391) nor any of the two-way inter-actions between aggression class and the other factors (replicate, competition,acclimation and time of the day) was significant for any of the three microhabitatuse parameters.

Rosyside dace in different aggression classes displayed significantly differentspatial distributions (MANOVA, Wilk's A=0-079, d.f. = 12, 20, P=0-002). Forexample, HA individuals were found more frequently in the upstream-mostposition within a group than either MA or NA fish [Fig. 4(a); Table I, Tukeytests P< 0-0096). Conversely, NA rosyside dace occupied significantly morefocal positions within the artificial stream than either MA or HA rosyside dace[Fig. 4(a); Table I; Tukey tests P<0-0002). Also, the number of pellets capturedincreased and the number of times a fish was observed alone decreased with fish


TABLE I. Results of ANOVA (F statistics and associated probability values) and Tukeytests for differences among dace aggression classes (HA, MA, NA, highly, moderatelyand non-aggressive fish, respectively) in (1) number of pellets captured, (2) number oftimes in the upstream-most position of a group, (3) number of focal positions used, (4)number of times alone, (5) size of the aggressive area, and (6) size of the foraging area.Mean values of rosyside dace aggression classes with the same superscript did not differ

significantly

F2.15 P Tukey test

Pellets capturedGroup headFocal positionsTimes aloneForaging areaAggression area

3-2810-7235-13

2-991-47

24-5

0-0660-0013

<0-00010-0810-261

<0-0001

NA1 MA1 HA2

NA1 MA2 HA2

——

NA1 MA1 HA2v-^.

aggressiveness [Fig. 4(a)], but differences among aggression classes for theseparameters were only significant at the 0-066 and 0-081 levels, respectively(Table I).

Highly aggressive rosyside dace also performed aggression over significantlylarger areas than the other two aggression classes [Table I, Fig. 4(b)], and similar,albeit, non-significant results for foraging area were observed [Table I; Fig. 4(b)].For all three aggression classes, the aggressive area was significantly smaller thanthe foraging area (Wilcoxon matched pairs test, Z=4-74, P<0-0001). Thesedifferences were similar for the three aggression classes [mean =164 cm2,NA=179, MA=159, HA =151 cm2; Fig. 4(b)], which suggests that intraspecificaggression was not used to maintain ' exclusive use' feeding territories. Thisconclusion is also supported by the observation that rosyside dace typically hadoverlapping foraging ranges and that they moved significantly shorter distancesto engage in aggressive interactions (mean ± 95% CI=5-3 ± 0-48 cm) than to feed(7-4 ± 0-37 cm).

The MANOVA detected one final significant difference, an effect of replicate(x=0-092, d.f. = 12,20, P=0-0038) on the number of captured pellets(F215= 11-73, PO-001); which is discussed in Rincon & Grossman (1998).

Correlation analysis involving LF and the number of aggressive acts performedversus variables included in the MANOVA confirmed the previous results (TableII). Because size and number of aggressive acts were correlated partial corre-lation analysis was used to remove the potentially confounding effect of eachvariable when examining the relationship of the other with the six parameters inthe MANOVA. When this was done, LF was not significantly correlated withany of them, whereas number of aggressive acts was significantly correlated withfour of six variables (Table II). These correlations were positive for the numberof times a fish occupied the anterior-most position of a group and the size ofthe aggressive area and negative for the number of focal positions used and thenumber of times a rosyside dace was solitary (Table II). Therefore, thedifferences detected by the MANOVA truly represented variations in the waydace of different aggressiveness used resources and were not a consequence of the


TABLE II. Partial cprrelation analysis of the number of aggressiveacts performed and fish size (LF) with: (1) number of pelletscaptured, (2) number of times in the upstream-most position of agroup, (3) number of focal positions used, (4) number of timesalone, (5) size of the aggressive area, and (6) size of the foraging

area

Aggresive acts LF

Pellets capturedGroup headFocal positionsTimes aloneForaging areaAggression area

0-170-73***

- 0-66***- 0-37*

0-290-79***

0-13-0-06-0-04

0-100-020-14

*=P<0-05, **=P<0-01, *** = P<0-001.

differences in size (even if statistically non-significant) among dace aggressionclasses.

SPATIAL DISTRIBUTION OF AGGRESSION AND FORAGINGThe spatial distribution of aggressive acts performed by rosyside dace differed

among aggression classes fr2=38-06, d.f.=4, P<0-0001; Fig. 5(a)]. This resultwas a consequence of differences between HA fish and the remaining two groups,whose values did not" differ significantly Of2=0-29, P>0-85). Highly aggressiverosyside dace directed a significantly greater proportion of aggressive interac-tions backward and a significantly lower proportion of aggressive interactionslaterally than either MA or NA individuals [Fig. 5(a)] These results probably areattributable to the fact that conspecifics generally occupied positions behind HArosyside dace, whereas most neighbours of MA and NA rosyside dace occupiedforward or lateral positions.

Rosyside dace aggression class also had a significant effect on the spatialdistribution of foraging Or2 =19-96, d.f.=4, P<0-0006), and differed from thespatial distribution of aggressive interactions [Fig. 5(a),(b)]. Highly aggressiverosyside dace made significantly more food captures in the rear quadrant thaneither MA or NA fishes, whereas the latter group made significantly more strikesin the frontal quadrant than either MA or HA individuals [Fig. 5(b)]. Rosysidedace of all aggression classes made significantly more foraging strikes in theforward quadrant than expected by chance alone, whereas, MA and NA rosysidedace also captured pellets in the rear quadrant with significantly lower frequencythan expected by chance [Fig. 5(b); /, HA=6-1, P=0-047; MA=20-72, P<0-0001; NA= 112-75, P<0-0001).

DISCUSSION

The present results support the contention that, although intraspecific aggres-sion is not a common behaviour in rosyside dace (Freeman & Grossman, 1992),it probably is adaptive because dominant individuals should have preferentialaccess to drifting prey. This is because HA rosyside dace consistently occupied


80

Cfl «rta 60o

g1 40twOc_oI 20

60

a.S 40

Frontal Lateral' Rear*

oo

1I

20

(b)

Frontal* LateralQuadrant

Rear

FIG. 5. Spatial distribution of (a) aggressive acts and (b) feedings in rosyside dace aggression classes. El,NA; 0, MA; •, HA; D; random. An asterisk after a quadrant name denotes significantamong-groups differences in that quadrant. Asterisks over histograms denote significant departuresfrom random expectations. Aggression class codes are as in Fig. I.

the upstream-most position within groups; a position that has been shown toproduce higher feeding rates in other drift-feeding, stream taxa (Blanckenhorn,19916; Nakano, 1995). Similar to the present findings for rosyside dace,dominant individuals in stream-dwelling, drift-feeding fishes and water stridershave been shown to place themselves upstream of conspecifics (Blanckenhorn,19916; Hughes, 1992; Nakano & Furukawa-Tanaka, 1994; Nakano, 1995;Katano, 1996).

The study suggest that rosyside dace also utilized intraspecific aggression fordefence of a particular position relative to neighbouring conspecifics rather thanan exclusive-use foraging area (i.e. all conspecifics excluded from the foragingrange). This conclusion is based on the fact that rosyside dace foraged over areasthat were significantly larger than their aggressive areas and utilized overlappingforaging areas. This behaviour contrasts with that of drift-feeding salmonids,which generally defend exclusive foraging territories (Grant el at., 1989; Hughes,1992; Nakano & Furukawa-Tanaka, 1994; Keeley & Grant, 1995; Nakano,1995). However, the present observations agree well with reports for other


group-forming animals in which dominants are generally found at the bestpositions within the group in terms of foraging benefits or predation risk (Blacket al, 1992; Hall & Fedigan, 1997).

Rosyside dace did not exhibit linear dominance hierarchies like those observedfor stream-dwelling salmonids (Bachman, 1984; Nakano & Furukawa-Tanaka,1994; Nakano, 1995), but rather a distinction between aggressive and non-aggressive fish. However, the overall results do not seem to derive exclusivelyfrom the presence of the three HA individuals because the smaller amounts ofaggression performed by MA rosyside dace seem to have, nonetheless, the samefunction as that in their more aggressive counterparts. This is supported by thefact that the correlation between the number of one-sided aggressive acts a fishperformed and the number of times it held the upstream-most position in agroup remained significant when the three HA rosyside dace were removed fromthe analysis (r=0-45, P=0-020, n=27) and even when it was restricted to the MAfish (r=0-85, P=0-004, n=9).

Besides its effect on occupation of the anterior-most position in a group,aggressiveness significantly affected other aspects of the spatial distribution ofrosyside dace. For example, an inverse relationship was detected between thenumber of one-sided aggressive acts performed and (1) number of focal-positionsoccupied, and (2) number of times a fish was solitary. These results probablywere produced by the exclusion of NA rosyside dace from the more profitableforaging positions within groups. This is concordant with the findings of Pulliam& Caraco (1984), who suggested that subordinates should benefit more fromeither moving within or abandoning a group than dominants. Previous behav-ioural observations on rosyside dace hi Coweeta Creek have shown thatnon-aggressive individuals were more likely to leave a group when it containedan aggressive individual, while aggressive individuals within these groups tendedto remain at their foraging sites (Freeman & Grossman, 1992), Similarly,Nakano (1995) found that subordinate masou salmon, Oncorhynchus masou(Brevoort), used more focal positions than dominants.

Although the aforementioned evidence is consistent with the hypothesis thatengaging in aggression to occupy the anterior-most position in a group isadaptive, the present results did not demonstrate this directly. For example, thedirect measure of foraging rate (the total number of pellets captured) did increasewith rosyside dace aggression class, but this relationship only reached signifi-cance at the 0-066 level. This lack of significance may have been due to smallsample size (only three HA fish) and the consequently wide confidence intervalsaround estimates. It is also likely that the prey delivery system tended to reducepotential differences in feeding rates among rosyside dace aggression classes.The feeders dropped groups of pellets into the stream atthe same time, and thus,increased temporal clumping of food arrival relative to field conditions (Freeman& Grossman, 1992). Such temporal clumping of prey can produce increasedforaging rates for subordinates, because dominants may not be able to monop-olize a large number of simultaneously arriving prey (Blanckenhorn, 19916;Grant & Kramer, 1992). Occupying the upstream-most position in a foraginggroup can also be adaptive because it ensures the first opportunity to capture thescarce, though energetically important, large prey present in the drift, assuggested by Freeman & Grossman (1992).

982; P. A. RINC6N AND G. D. GROSSMAN

The present experiments were conducted in an artificial stream, and conse-quently, may have been subject to artefacts. Nonetheless, previous studies haveshown that the artificial stream is a good approximation of the physicalconditions found in Coweeta Creek (Grossman & Boule, 1991; Grossman et al.,1995) and other comparisons suggest that the major findings were not signifi-cantly biased by experimental procedures. First, rates of intraspecific aggressiondisplayed by rosyside dace in the artificial stream were similar to those observedin Coweeta Creek (artificial stream, 0-91 aggressive interactions x rosysidedace"1 x 5-2min"1; Coweeta Creek, 1-0 aggressive interactions x rosysidedace"1 x 5-2min"1, Freeman & Grossman, 1992). Second, identical behav-ioural repertoires for rosyside dace were observed in the artificial stream and inCoweeta Creek (Freeman & Grossman, 1992). Third, fish densities used inexperiments were comparable to those observed in the Coweeta Creek drainage(Freeman & Grossman, 1993). Fourth, mortality of rosyside dace was limited totwo specimens that jumped out of the artificial stream prior to placing-a cover ofplastic sheeting over it. Finally, rosyside dace either gained or maintained massduring experiments. Taken in concert, these data suggest that experimentalprocedures did not impose significant artefacts on either the behaviour or healthof rosyside dace.

The findings suggest that intraspecific aggression by rosyside dace is anadaptive behaviour that increases the foraging rate of dominant individuals.However, this behaviour was displayed by <50% of the rosyside dace observedin both this study and the study of Freeman & Grossman (1992). This isdiiferent from reports on groups of individually identified salmonids, wheresubordinates show enough aggressive behaviour to be precisely positionedwithin linear dominance hierarchies (Bachman, 1984; Nakano & Furukawa-Tanaka, 1994; Nakano, 1995). Behavioural categories that differ greatlyin the amounts of aggression performed by their members have also beenreported for salmonids (Jenkins, 1969; Grant, 1990; Nielsen, 1992). However,such variation reflected, in fact, a divergence:, in the whole foragingstrategy between more aggressive individuals thai defend, foraging stationsand floaters (Jenkins, 1969; Nielsen, 1992). In contrast, there was no suchmarked distinction in the foraging tactics of aggressive and non-aggressiverosyside dace.

This difference may reflect the different benefits that rosyside dace anddrift-feeding salmonids derive from aggression. The former defend a position inrelation to conspecifics which are close, while the latter defend both an exclusiveforaging area around them and preferred positions (Hughes, 1992; Nakano,1995). In the first case, once the one or two upstream-most locations within aforaging group are taken, the value of the remaining ones may become too lowor too homogeneous to offset the costs of engaging in aggression to occupy aparticular position. Also, Metcalfe (1986) showed that often the most profitablestrategy for subordinate fish is to minimize energetic expenditure and, therefore,altogether avoid engaging in aggression. Additionally, these differences maysimply reflect phylogenetic constraints on aggressive behaviour (Hutchinson &Iwata, 1997).

Even if most rosyside dace are not aggressive, at the spatial scales used in thisstudy (several channel units) intraspecific aggression may significantly affect the


spatial distribution and use of trophic resources by rosyside dace. Nonetheless,Freeman & Grossman (1993) showed that spatial distribution of rosyside dace inCoweeta Creek was largely determined by physical habitat features (presence ofeddies) and Hill & Grossman (1993) demonstrated that rosyside dace utilize onaverage near-optimal focal-point velocities. Hence, the effects observed hereinmay not have strong consequences on spatial distribution at larger spatial scalesor population regulation, in contrast to the observations in several stream-dwelling salmonid fishes (Elliott, 1990; Hughes & Reynolds, 1994). Hence,population-level effects of intraspecific aggression in rosyside dace warrantfurther study.

In conclusion, although aggression in rosyside dace mediates access toimportant resources, its consequences appear to rest more at the level of theindividual rather than that of the population. Long-term population studiessuggest that environmental variation has a stronger effect on populationregulation for this species than intraspecific interactions (unpubl. data), however,it is possible that aggression increases individual fitness sufficiently to warrantretention within the behavioural repertoire of some members of the rosyside dacepopulation.

We are particularly grateful to R. Ratajczak for his constant help throughout thestudy. We would also like to thank the following people: S. Allison, S. Elrod, T. Petty,A. Thompson and the staff of Jittery Joe's in Five Points. The manuscript benefited fromthe comments of M. Freeman, M. Wagner and R. Ratajczak and two anonymousreviewers. The senior author's stay at the University of Georgia was supported byMEC/Fulbright scholarship No. fu93-05633453. Additional funds for this project wereprovided by the D. B. Warnell School of Forest Resources. The manuscript wascompleted while the senior author held a postdoctoral scholarship of the Consejeria deEducation of the Comunidad Autonoma de Madrid co-financed by the European SocialFund of the European Union.

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