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The damselfish Pomacentrus amboinensis is common on coral reefs of the Indo-Pacific. Males and females are strongly site attached and often live in a discrete group. P. amboinensis has a pelagic larval duration of 15–23 days and settles at 10.3–15.1 mm standard length with its juvenile body plan largely complete [33]. Although P. amboinensis settle to a wide variety of habitats on reefs of the northern Great Barrier Reef they are found in highest densities associated with mixed live coral, rubble and sand areas on the shallow reef base or reef slope. A tagging study of 295 newly settled fish on the continuous reef edge found that fish moved little over the first 3 months after settlement (mean = 0.63 m [34]). A recent study showed that P. amboinensis who had newly metamorphosed in light traps displayed a dramatic and statistically significant preference for live coral (Pocillopora damicornis) over bleached, dead coral or sand habitats in selection trials conducted in 500 l circular tanks [35]. Studies of this species also suggest high levels of mortality are typical within the first 24 h on the reef, with values of up to 98% mortality being recorded (mean ∼50%) [9], [10]. Interestingly, despite this high mortality, McCormick and Gagliano (in press [36]) showed that for P. amboinensis there is a relationship between the size of the otolith at hatching and who changes sex to become the dominant male later in life; that is, early life history somehow pro-rates subsequent individual success. This link stresses the importance of carry-over effects between life stages to the fundamental population dynamics of this species. Newly settled reef fishes tend to be site attached and are subject to an array of resident and transient predators [see 9,37]. Predators can be seen striking at and occasionally capturing recently settled and juvenile reef fishes during the summer recruitment period. A previous study that undertook behavioural observations on 10 fish over 20 consecutive 1 min periods found that a 3 min observation period was sufficient to obtain a representative quantification of behaviour for P. amboinensis (Mero and McCormick unpublished data; also see [38]).
This research was undertaken with approval of the James Cook University (JCU) animal ethics committee (JCU ethics permit: A112) under the JCU animal ethics guidelines.
The present study was conducted at the base of a shallow reef at Lizard Island (14°38′S, 145°28′E), northern Great Barrier Reef, Australia, during October to December 2008. In overview, the study involved three separate experiments. The first experiment measured size-related mortality trajectories on three different habitats. Large and small newly metamorphosed P. amboinensis were placed in pairs onto either live healthy Pocillopora damicornis (a bushy hard coral); thermally bleached P. damicornis (see protocol below); or dead P. damicornis. The dead coral was structurally intact and with small levels of algal growth and fouling invertebrates. Patches were spaced to preclude fish migration between patches and observations were conducted for up to 140 h. The second experiment examined movement between habitat patches and the influence of fish size. Here, the first experiment was repeated, but this time the three different habitat types were placed in close proximity to one another to allow fish migration. Fish were monitored for ∼48 h and their behaviour was quantified in detail. The third experiment examined whether high densities of fish on small patches of live coral promoted movement to nearby bleached or dead patches, and whether that movement was related to fish size. This experiment had the same spatial design of clusters of three habitat types in close proximity, but placed 6 tagged fish of a size range on the live coral patch. Movement and limited aspects of behaviour were recorded for 48 h. Experiment 1: Habitat and size related persistence: Light traps were used to collect P. amboinensis at the end of their larval phase. Fish were placed into an aquarium with aerated flowing seawater. Fish were kept for 24 h and fed newly hatched Artemia sp. twice per day ad libitum to allow recovery from (or acclimation to) the stress of capture, prior to sizing and tagging. There was minimal mortality during this time. Individual fish were placed into clip-seal plastic bags containing aerated seawater and measured with digital calipers (±0.1 mm). Fish were paired, such that one individual (‘large’ individual) was 0.8–1.0 mm greater in standard length than the other (‘small’) (see supporting information Figure S1). To enable individual identification fish were tagged through the plastic bag with either a red or blue subcutaneous fluorescent elastomer tattoo using a 27-gauge hypodermic needle (as per 39). This left a 1.5–2 mm long stripe of colour, which was visible under the scales. Colours were alternated between large and small fish among replicates to avoid the possible bias of predators selecting prey based on tag colour. Previous studies have found no evidence of this selection (T. Holmes unpublished data). Tagging with a single elastomer tattoo has been found to have no influence on the mortality or growth of this species [39]. Behaviour observations in the field previously [e.g. 9], [10], [12], [40], [41] and in the present study have indicated no size-dependent adverse affect of tagging. Size-paired fish were transported to the study site in 2 l plastic bags of aerated seawater, and then released onto small patches (20×10×15 cm) of one of the three substrata: healthy P. damicornis (a bushy hard coral); thermally bleached P. damicornis (see below); dead P. damicornis. Habitat patches were about 4 m from the hard reef edge on sand and organized in a row 5 m apart to prevent migration between patches. All fish and mobile invertebrates were removed from the substrata prior to the commencement of a trial. A small wire cage (∼30×30×30 cm, 6 mm mesh size) was placed over each patch for 30–40 min to allow the tagged fish to acclimate to their new surroundings while being protected from predators. Fish were released onto the reefs between 10:00 and 10:30 h. Survival of tagged fish was monitored 2–3 times per day (morning, mid-day, evening) by visual census (occasionally the mid-day census was not undertaken). When one or both individuals were missing the adjacent area (within 3 m of the release site) was searched to determine if the fish had simply migrated. During the study period, densities of up to six newly settled P. amboinensis were found to naturally occur on dead and live coral habitats of the size used in this study. Monitoring finished when both tagged individuals were lost from the sites, or was terminated due to the end of a field trip.
Clusters of the three different habitat types were constructed to examine the importance of movement between patches and the social dynamics that may underlie mortality of individuals within a group. Clusters were composed of one small coral head (20×10×15 cm) of each of three coral habitat types 0.4 m from one another: healthy Pocillopora damicornis, bleached P. damicornis, and dead P. damicornis. In a similar methodology to above, P. amboinensis caught in a light trap were kept for 24 h, measured with calipers, tagged with one of six different colours and size matched for a 0.8–1 mm difference in SL (as above). Pairs were then transported out to the field in a labeled plastic bag at 10:00–11:00 h. One pair was placed on each of the three substrata per cluster. Patches were visually obscured from one another by plastic barriers during a 30 min acclimation period, during which time fish were also enclosed within a 6 mm mesh cage to prevent predation (as above). After the acclimation period the barriers and then the cages were removed. Fifteen minutes after release the behaviour of all fish within the set of three patches was quantified in detail (3 min each; see below), and this was repeated over 48 h at ∼16:00 and ∼11:00 h. Mortality and movement between patches was also assessed at this time.
Experiment 3: Migration at high densities: To determine the likelihood of a fish migrating to a bleached or dead coral when the density of fish on a live coral patch is very high, fish were stocked in high density onto live coral patches and given the choice of dead or bleached coral patches nearby. The configuration and composition of patches was exactly the same as above (experiment 2), with the exception that only the healthy Pocillopora coral head was stocked with 6 newly settled P. amboinensis. Fish were once again tagged for individual recognition and they were measured as above prior to release. Acclimation procedure was the same as experiment 2 and the location of fish were monitored over 48 hours as above. Experiment 3 was conducted 2 weeks after experiment 2 and at the same location.
In experiment 2 the behaviour of fish on the three patch types within each cluster was assessed over 3 min periods. Behaviour of the fish was assessed by a scuba diver positioned 1.5 m away from the patch. A magnifying glass (4x) aided the assessment of bite rates and space use over the 3 min focal animal sampling period. Six aspects of activity and behaviour were assessed: a) total distance moved; b) distance ventured from the habitat patch (categorized as % of time spent within 0, 2, 5 or 10 cm away from the patch); c) height above substratum (categorized as % of the time spent within the bottom, middle or third of the patch); d) number of fin displays; e) the number of chases or bites; f) number of avoidance episodes in response to a conspecific; g) boldness (recorded as a variable on a scale from 0 to 3 at 0.5 increments, where: 0 is hiding in hole and seldom emerging; 1 is retreating to hole when scared and taking more than 5 sec to re-emerge, weakly or tentatively striking at food; 2 is shying to shelter of patch when scared but quickly emerging, purposeful strikes at food; and 3 is not hiding when scared, exploring around the coral patch, and striking aggressively at food). At the end of the 3 min observation period, the fish was approached with a finger and the fish's reaction and latency to emerge from shelter was taken into account in the assessment of boldness. Two additional variables were devised from these variables to summarise information and reduce the number of variables that were required in analyses. Relative height on the patch was summarized as a cumulative proportion of the time spent at varying heights over the 3 min observation period, with the top of the patch taken as height of 1, mid-patch a height of 0.5, and bottom a height of 0. An aggression index was also created by adding the number of displays to the product of three times the number of chases/bites and then subtracting the number of avoidance events. A weighting factor of three was used in conjunction with the chases/bites as the influence of this behaviour on the spatial distribution of the recipients appeared to be many times greater than their response to displays. In experiment 3, the dominance status of the individuals within each pair was also categorised from the ∼10 min observation period as dominant or subordinate, based on the number of displays, chases and avoidances.
Bleaching (the loss of zooxanthellae) was induced by placing Pocillopora damicornis colonies in 500 l seawater aquaria and raising the temperature incrementally over 48 h from ambient (28°C) to a sustained maximum of 32°C for 9 to 10 days. Aquaria were constantly aerated, flow maintained using 2 1220 l.hr−1 powerheads and heated with two 300 W aquarium baton heaters under very low light levels. After colonies were visibly bleached, water temperature was lowered to ambient incrementally over 48 h. This protocol resulted in a live coral with few or no zooxanthellae that would stay bleached in the field for up to 2 months. Many ended up regaining zooxanthellae over a 6 to 8 week period.
To examine the differences in behaviour between large and small fish among the three habitat treatments repeated measures ANOVAs (RMANOVAs) were conducted. In these analyses fish size (large or small) was used as the variable on which the repeated measure was undertaken (effectively pairing the observations; [42]). As there are only two levels of size the assumption of sphericity was irrelevant. Assumptions of normality and homogeneity of variance was examined using residual analysis. Tukey's HSD tests were undertaken after RMANOVA to explore the nature of any significant differences found among more than 2 means. Trends were tested for data collected 15–30 min after cage removal. To determine whether the spacing pattern between large and small fish differed among habitats after cage removal, a one-factor ANOVA was undertaken to test the equality of distances between fish. To estimate this variable the difference between the distance from the patch for the largest fish was subtracted from the smallest fish. Data were log10(x+1) transformed. Survival (up to 165 h) between large and small fish on the three substrata was compared using Survival Analysis (Statistica 8.0). Survival curves of each fish size and substrata were calculated and plotted using the Kaplan–Meier product–limit method. The Kaplan–Meier method is a non-parametric estimator of survival that incorporates incomplete observations, such as those cases where censuses had to be terminated on trials prior to their completion due to time limitations of a field trip. Projected survival were compared between the three substrata using a Chi-square statistic, while differences in survival between large and small fish were compared using a Cox-F statistic.
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