Behavioral Ecology Vol. 11 No. 1: 13-18
© 2000 International Society for Behavioral Ecology
The effects of prior residence on behavior and growth rates in juvenile Atlantic salmon (Salmo salar)
Fish Biology Group, Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow, G12 8QQ, UK
Address correspondence to K. O'Connor. E-mail: k.oconnor{at}udcf.gla.ac.uk .
Received 16 January 1999; revised 13 April 1999; accepted 20 May 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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It is well documented that prior residence confers advantages in territorial disputes, but its impact on other aspects of behavior and fitness is less understood. We tested how prior residence influences the subsequent feeding behavior and growth performance of dispersing Atlantic salmon fry (Salmo salar) using experimental manipulations of residence in a seminatural stream tank. In replicated trials, groups of seven "primary" fish were released into the stream tank 3 days ahead of seven "secondary" fish. Standardized behavioral observations were made on each fish over the following 14 days, after which all fish were removed and measured. Primaries and secondaries were initially the same size and body condition and exhibited the same degree of site fidelity. However, primaries darted higher into the water column to intercept prey items, fed at a higher rate, and subsequently grew faster. Larger fish (in terms of body length) tended to be more dominant, and dominants grew faster than subordinates. However, there was no difference in dominance between primaries and secondaries. These results suggest that the well-documented advantage of early-emerging salmon fry over late-emerging fry cannot be completely attributed to intrinsic differences and that the advantage is partly mediated via a prior residence effect. Furthermore, prior residents gain foraging advantages without necessarily becoming more dominant.
Key words: Atlantic salmon, dispersal, emergence, growth rates, prior residence, Salmo salar.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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It is generally accepted that animals dispersing from their natal site incur a cost of emigration that will generally increase with distance traveled from their natal site (Plissner and Gowaty, 1996
). Dispersing individuals face increasing
mortality risks associated with unfamiliar habitat, passage through areas of
high predator density, and the physiological costs associated with extensive
movement (Plissner and Gowaty,
1996).
Environmental dispersal, "the movement that an animal makes away from
its birthplace in response to crowded conditions"
(Howard, 1960), is a common
phenomenon in nature, and it tends to occur when a population is living at or
close to its carrying capacity [see "density responsive
emigration" (Lidicker,
1962), "saturation dispersal"
(Lidicker, 1975)]. Dominant
individuals tend to be more sedentary and move only short distances provided
resources are available, whereas subordinate individuals disperse even in the
absence of severe population pressure because they are displaced or excluded
by dominants well before food resources become limited
(Gauthreaux, 1978). Given the
costs of dispersal, unequal competitive ability leading to unequal resource
partitioning will thus ultimately affect individual survival
(Elliott, 1994).
The Atlantic salmon (Salmo salar) is an ideal species for studying
the individual asymmetries that contribute to differential dispersal, as the
fry emerge from the nest (redd) in numbers that far exceed the limits of their
natal habitat's carrying capacity (Giles,
1994). Thus, the individual differences that exist between
individuals will become instrumental in determining their success in the
ensuing dispersal.
After hatching, the fry remain in the gravel substratum of their natal
stream for a variable period while obtaining nutrients from their yolk sacs
(Kalleberg, 1958). Once this
endogenous food supply is absorbed, they exhibit a normally distributed
temporal pattern of emergence and dispersal from the spawning ground
(Brännäs,
1987; Godin, 1982). The peak of emergence is highly synchronized
both in natural (Gustafson-Marjanen and
Dowse, 1983) and in artificial redds
(Brännäs,
1987; Godin,
1980). Competition for food and space is fierce
(Keenleyside and Yamamoto,
1962), and if a population exceeds the carrying capacity of the
habitat, some fry will be forced downstream by the aggressive behavior of
those that have already settled (Backiel
and LeCren, 1978; Chapman,
1962; Elliott,
1986; Mason and Chapman,
1965).
The prior residence effect, whereby interactions between residents and
intruders are more commonly won by the resident
(Leimar and Enquist, 1984;
Maynard Smith and Parker,
1976), has been well-documented in a variety of animals
(Archer, 1987;
Huntingford and Turner, 1987).
In seminatural stream channels, early-emerging salmon fry have a better chance
of establishing a feeding site than late-emerging fry
(Brännäs,
1995; Chandler and Bjornn,
1988; Chapman,
1962; Fausch and White,
1986; Mason and Chapman,
1965; Metcalfe and Thorpe,
1992). This apparent advantage of early first feeding could be the
result of an intrinsic quality common to early first-feeding fish. For
example, fish with higher standard metabolic rates (SMRs) will tend to use up
their yolk sacs sooner than those with low SMRs and so will emerge earlier.
Because fish with high SMRs tend to be dominant over fish with lower SMRs
(Metcalfe et al., 1992;
Yamamoto et al., 1998), the
true extent of the advantage of prior residence is difficult to assess.
Recent experimental work on older salmon suggests that territory ownership
per se may be more important than differences in resource holding potential
(RHP) between fish, as pilot data from a single trial indicated that
intrinsically dominant individuals were unable to displace, and consequently
grew slower than, subordinates that already held territories
(Huntingford and Garcia De Leaniz,
1997). In this study we tested whether this is also true at the
fry stage when there is greatest competition for resources. In a series of
replicated trials we examined how prior residence influences the behavior and
growth performance of salmon fry, using experimental manipulations of
residence to control for confounding effects.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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A batch of full-sibling eggs from sea-run salmon caught in the River Almond, Perthshire, UK, were incubated at the Scottish Office Agriculture Environment and Fisheries Department, Almondbank salmon-rearing unit. Because Atlantic salmon emerging from a redd in the wild would at least share the same mother, it was decided that full siblings should be used as this would best mimic the competitive situation faced by fry dispersing from redds in the wild. The egg fish were moved to the Glasgow University Field Station at Rowardennan on Loch Lomond on 8 November 1996. The fish were kept under ambient photoperiod and temperature conditions in 1-m square tanks. These tanks received a continual turn-over of water pumped from Loch Lomond and were therefore rich in zooplankton. Once the fishes yolk sacs were completely absorbed, the fish diet was supplemented with commercial salmon food (Fulmar feeds, BOCM Pauls Ltd.), which they received ad libitum from automatic feeders.
We carried out the experiment between 1 June and 30 July 1997 (i.e., 7-11
weeks after first feeding), in sections of an artificial stream tank that had
a glass side-wall to allow behavioral observations. Salmonids of this age are
in the critical period for survival which follows emergence, and they
experience intense competition for space and high mortality rates
(Elliott, 1994). This tank had
a continual turnover of water pumped from Loch Lomond and therefore was rich
in zoo-plankton, which provided ample food for the salmon fry. To ascertain
the exact levels of plankton in the water, we collected samples by placing
fine-mesh plankton nets in the water column at three different locations in
the stream tank for 20-min periods. These samples were collected on 4 June and
16 and 30 July at 1300 h. The contents of the nets were then dried and weighed
to the nearest microgram. Over the duration of the experiment the mass of
plankton collected during a 20-min period did not vary (one-way ANOVA, effect
of date: F2,6 = 0.18, p =.84).
Each section of the stream tank measured 100 cm long by 30 cm wide with a water depth of 15 cm. During the course of the experiment the average water temperature was 16.03°C (±0.41). The upstream and downstream barriers between neighboring sections were made of plastic mesh. A water pump created a flow through the sections [mean water velocity at the midpoint of the sections and 9 cm below the surface = 0.03/ms ± 0.01 (SD)]. Each section had a layer of gravel mixed with marble chips (Esmo, Rowebb) to produce a topographically natural but light-colored substratum against which the fish could easily be seen. Salmon parr in the wild will naturally come across patches of substrate of similar color to the marble chips used in this experiment. Sections were marked into ten 10-cm long zones along their length, with zone number 1 being the farthest upstream.
We used 14 fish randomly chosen from a stock tank in each of the 5
replicate trials. The fish were anesthetized with 5 ml/l solution of 10 g/l
benzocaine (ethyl-p-aminobenzoate) in 95% ethanol. Fish were given
unique combinations of marks with alcian blue dye on their pectoral, pelvic,
dorsal, anal, and/or caudal fins. This marking technique has no effect on the
behavior or growth of first-feeding salmon fry
(Metcalfe et al., 1992). On
day 1 of each trial we randomly selected 7 fish from each group of 14 and
introduced them into a single section of the stream tank. These fish were then
left to settle for 3 days and are referred to as "primaries"
because they were the first fish to be released into the stream tank. On the
morning of day 4 the primaries were removed, anesthetized, and their weights
(to the nearest 0.01 g) and fork lengths (to the nearest 0.1 mm) were
recorded. Although all of the fish recovered from the anesthetic and were able
to swim in a normal manner within 2-3 min of being placed in fresh aerated
water, they were left for an additional hour before they were returned to the
same section of the stream tank. In the evening of day 4 we anesthetized the
remaining seven fish and recorded their weights and lengths. Once these fish
had fully recovered they were also placed in the same section of the stream
tank. These fish are referred to as "secondaries." The resulting
high density of fish within each section (47 fish/m2) approximated
the wild situation where Atlantic salmon emerge from the redd in numbers that
far exceed the natural carrying capacity of the natal site.
We collected observational data on days 6, 8, 11, and 13. On each of these
days each fish was observed twice (for 2 min at each time), with the two
observations being approximately 1.5 h apart. During each 2-min observation we
recorded the number of feeding attempts made by the fish, along with its
location in the stream section (zone 1-10), mean height maintained off the
bottom, and the maximum height in the water column to which it swam in order
to intercept a prey item. Both height measurements were made to the nearest
0.5 cm using a 15-cm transparent Perspex ruler placed against the glass wall
of the tank. In addition, any aggressive interaction (i.e., lateral display,
charge or nip, as described by Keenleyside
and Yamamoto, 1962) with another fish during the 2 min was
recorded. On day 15 we removed all 14 fish from each section of the stream
tank, anesthetized them, and recorded their final weight and fork lengths.
Three primaries and two secondaries died during the course of the trials; the
data collected for these individuals were omitted from the analyses.
Individual specific growth rates for weight were calculated as 100[Ln(final
measurement) — Ln (initial measurement)]/duration of experiment in
days), following Ricker
(1979). To control for the
effect of body size on growth rates, these values were then regressed against
the initial weight of the fish and the equation of the resulting line was used
to calculate the expected specific growth rate for each fish, given its
initial weight. The difference between the expected and observed specific
growth rate (adjusted specific growth rate) was used in subsequent analysis of
growth rates. We used weight for these calculations because length did not
change greatly over the 2-week period. Using the weight (W) and fork
length (L) measurements, an index of each fish's condition (condition
factor, K) was also calculated as K = 105 (W/La),
where a is the slope of a regression of log10(weight) on
log10(length) following Bolger and Connolly
(1989).
Data from each fish have been used as individual experimental units in the statistical analysis.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Initial size and condition
Initial (i.e., day 4) weights and lengths of the fry varied significantly between trials because fish had grown larger by the onset of later trials. However, there was no overall difference in the initial size of primary [mean weight = 0.53 ± 0.05 g (SE), n = 32; fork length = 37.54 ± 1.37 mm, n = 32] and secondary fish (weight = 0.54 ± 0.05 g, n = 33; fork length = 37.35 ± 1.19 mm, n = 33; two-way ANOVA, effect of trial number on initial weight: F4,55 = 71.30, p <.001; primary/secondary residence: F1,55 = 0.22, p =.64; effect of trial number on initial fork length: F4,55 = 103.41, p <.001; primary/secondary residence: F1,55 = 1.51, p =.22).
The initial condition factor (K) of the fish also varied with trial but not with the resident category (two-way ANOVA, effect of resident category: F1,55 < 0.001, p =.97; trial: F4,55 = 4.47, p <.001), with fish in later trials having lower K values than those in earlier trials.
Spatial positions, feeding behavior, and aggression
The position of individual fish within the stream tank was relatively
stable, such that an average of 49.2 ± 2.5% (SE) of observations of a
given fish were from its most frequently occupied of the 10 zones. There was
no difference in this measurement of zone fidelity between primaries and
secondaries (Mann-Whitney U = 470.5, p =.89, mean for
primaries = 47.6 ± 3.2%, secondaries = 50.8 ± 4.0%). There was
also no difference in mean height maintained off the stream substrate (primary
fish = 1.2 ± 0.3 cm, n = 32, secondary fish = 1.0 ± 0.3
cm, n = 32; Mann-Whitney U = 432, p =.21). However,
when swimming upward to intercept prey items, primary fish tended to dart to a
greater maximum height above the substrate than secondary fish (primary fish =
5.1 ± 0.7 cm, n = 32, secondary fish = 3.0 ± 0.8 cm,
n = 32, Mann-Whitney U = 383.5, p =.05).
Although the feeding rates of the fish varied between the trials (with fish in later trials feeding more intensively), there was a consistent and significant trend within trials for primary fish to feed more frequently than secondary (primary fish = 2.71 ± 0.29 feeding attempts/min, n = 32, secondary fish = 1.52 ± 0.22, n = 32, two-way ANOVA, effect of resident category: F1,55 = 13.09, p <.001, trial: F4,55 = 4.10, p <.001; Figure 1).
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A dominance score was calculated as the number of successful aggressive interactions made by a focal fish (i.e., when the opponent was displaced) during the total observation time, minus the number of times it lost an interaction; positive values therefore indicate a more dominant fish. Primary fish did not have significantly different dominance scores compared to secondary fish (t test assuming equal variances: t63 = 1.00, p =.32, mean for primaries = 0.50 ± 0.62, secondaries = -0.48 ± 0.62).
There was no difference between primary and secondary fish in the total number of attacks they initiated (mean for primaries = 1.03 ± 0.33 attacks/min, secondaries = 0.61 ± 0.22, Mann-Whitney U = 482, p =.51). Nor was there a difference in the number of attacks they initiated against the opposite resident category (mean for primaries = 0.55 ± 0.21 attacks/min, secondaries = 0.29 ± 0.11, Mann-Whitney U = 498.5, p =.62) or the same resident category (mean for primaries = 0.47 ± 0.15, secondaries = 0.32 ± 0.13, Mann-Whitney U = 475.5, p =.41). There was also no difference in the number of successful attacks that a fish made between the resident categories (Mann-Whitney U = 106, p =.61). There was no evidence that overall aggression levels were related to food availability, as the mean rate of attacks in a trial did not correlate with the mean feeding rate (Spearman rank rs =.1, p =.87). Because fish weight and length varied between trials the two variables, standardized values (z scores) were calculated for each fish in relation to the size of the other fish in its trial. Dominance appeared to be dependent on both standardized length and weight, with relative length providing the strongest relationship (Figure 2; quadratic regression based on relative length: r2 =.19, N = 64, p <.001). However, relative weight had an additional minor effect explaining a further 2% of the variation (overall multiple regression, r =.21, N = 64, p <.05). To test whether this relationship was similar for both residence categories of fish, residual dominance scores were calculated as the difference between the observed dominance score and the score predicted from the quadratic equation (Figure 2). As these residuals did not differ significantly between primary and secondary residents (t test assuming equal variances: t63 = 1.313, p =.19), we can assume that the relationship between relative length and dominance is the same for both categories of fish.
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Growth rates
To investigate the effect of dominance status on growth, the fish were
split into two groups on the basis of their dominance scores
("dominant" fish, with dominance scores > 0, and
"subordinate" fish with dominance scores 
0). A three-way
ANOVA revealed that the trial to which a fish belonged had a strong effect on
adjusted specific growth rate (Table
1). This is probably due to fish being larger in later trials, so
that although the absolute level of food supply was stable (see Methods), the
relative food supply per unit weight of fish decreased through the season.
More interesting, dominance and residence category also affected adjusted
specific growth rate (Table 1):
dominant fish typically grew faster than subordinates
(Figure 3), and independent of
dominance, primaries grew faster than secondaries
(Figure 4).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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Despite the absence of any apparent differences in body size or condition, the first fish to establish themselves in the stream tank fed at a greater rate and subsequently had higher rates of growth than those introduced later. These differences in performance arose despite there being no significant differences between the residence categories in their spatial distribution or dominance status. It is possible that the primary fish may have suppressed the feeding activity of the later animals without there having been any major differences in their levels of aggression. Circumstantial evidence in support of a suppression of feeding activity in secondary fish comes from the fact that they did not dart as high into the water column when feeding as did the prior residents. However, this could reflect differences in the quality of their feeding stations, with prior residents having access to more drift food. This result indicates that the previously documented advantage of early over late emergence in salmon fry (Brännäs, 1995
; Chandler and Bjornn,
1988; Chapman,
1962; Fausch and White,
1986; Mason and Chapman,
1965) is partly due to a prior residence effect. The tendency for
the first animals to arrive at a territory to win any subsequent disputes over
the territory with intruders is well documented
(Alcock, 1993;
Archer, 1987;
Huntingford and Turner, 1987;
Krebs and Davies, 1987;
Leimar and Enquist, 1984;
Maynard Smith and Parker,
1976) and has been examined in a variety of animals including
juvenile salmonids (Cutts et al.,
1999; Johnsson et al.,
1999; Maynard Smith and
Reichert, 1984; Stamps and
Krishnan, 1995; Tobias,
1997). The present study shows that prior residents gain foraging
benefits even without excluding later arrivals from a territory.
Although this experiment controlled for any intrinsic differences between
residence categories, the first fish to emerge (and therefore become prior
residents) may be intrinsically more dominant. Metcalfe and Thorpe
(1992) found that the first
fish to emerge and begin feeding were dominant over later-feeding fry from the
same family in experiments that controlled for prior residence. It was later
found that this superiority was due to differences in the SMR, with
early-feeding fry having relatively higher SMR values, and the date of first
feeding having no influence on dominance interactions once SMR was taken into
account (Metcalfe et al.,
1995). The effect of date of first feeding arises because fish
with a relatively high SMRs will tend to have exhausted their yolk sacs sooner
than fish with low SMRs and will emerge from the redd and begin feeding
sooner. Thus, fish with a high SMR potentially have a double advantage: they
are intrinsically dominant and emerge earlier and so obtain a prior resident
advantage over later-feeding fry.
It should be noted that early emergence is not without its costs. Young
animals and those at vulnerable stages in life are more likely to avoid
predation if their period of dispersal or habitat shift is synchronized
(Pulliam and Caraco, 1984).
This is partly due to the dilution effect
(Daan and Tinbergen, 1979),
whereby the probability of predation decreases as the potential prey's group
size increases. The benefit of early emergence (i.e., a prior residence
advantage) may thus be counterbalanced by the risk of predation. Fry are
highly vulnerable to predation from piscivorous birds and other fish and stand
a better chance of survival if they emerge synchronously
(Brännäs,
1995; Godin, 1982). Later emergers are afforded protection by
large numbers; as prey number increases, predator satiation decreases the
individual risk of predation (Begon and
Mortimer, 1986; Peterman and
Gatto, 1978).
In a single trial using older salmon, Huntingford and Garcia De Leaniz
(1997) found that the
probability of settlement (as opposed to emigration downstream) and growth
rates following settlement were significantly higher in the first fish to be
released into a stream tank. They also found that the probability of a fish
settling decreased as its dominance rank increased. The similarity between the
scaling of metabolic rate (slope of regression line b = 0.87;
Steingrimsson and Grant, 1999)
and territory size (b = 0.86;
Grant and Kramer, 1990) on
body mass suggests that territory size may reflect metabolic requirements
(Steingrimsson and Grant,
1999). Therefore dominant fish (with a higher metabolic rate) may
require larger territories to meet their metabolic requirements. So the
reduced probability of settlement of dominant fish compared to fish of a lower
rank could be due to the low abundance of suitable territories for high SMR
fish. When coupled with early emergence, and hence prior residence, a high SMR
is an advantage, but this may become a disadvantage if uncoupled due to a
reduction in suitable territories.
It should be noted that although some fish lost weight in this experiment,
in nature competition for resources may lead to some of the fish dispersing
downstream. This downstream migration will tend to be by subordinate fish in
poor physical condition, who will frequently die
(Elliott, 1994). Fast early
growth will lead to fish establishing and maintaining a size advantage. This
in turn will increase the likelihood of smolting in the first year
(Thorpe et al., 1992). Thus
early growth rate is instrumental in determining salmon life-history
strategies.
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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This work was carried out while K.I.O'C. was in receipt of a Natural Environment Research Council studentship. We are grateful to Ian McCarthy, Vivien Cameron, and all the staff at the Glasgow University Field Station, Rowardennan, for their technical assistance and BOCM Pauls Ltd. for provision of fish feed, and we thank three referees for helpful comments.
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