Animals often occupy home ranges where they conduct daily activities. In many parrotfishes, large terminal phase (TP) males defend their diurnal (i.e., daytime) home ranges as intraspecific territories occupied by harems of initial phase (IP) females. However, we know relatively little about the exclusivity and spatial stability of these territories. We investigated diurnal home range behavior in several TPs and IPs of five common Caribbean parrotfish species on the fringing coral reefs of Bonaire, Caribbean Netherlands. We computed parrotfish home ranges to investigate differences in space use and then quantified spatial overlap of home ranges between spatially co-occurring TPs to investigate exclusivity. We also quantified spatial overlap of home ranges estimated from repeat tracks of a few TPs to investigate their spatial stability. We then discussed these results in the context of parrotfish social behavior. Home range sizes differed significantly among species. Spatial overlap between home ranges was lower for intraspecific than interspecific pairs of TPs. Focal TPs frequently engaged in agonistic interactions with intraspecific parrotfish and interacted longest with intraspecific TP parrotfish. This behavior suggests that exclusionary agonistic interactions may contribute to the observed patterns of low spatial overlap between home ranges. Spatial overlap of home ranges estimated from repeated tracks of several TPs of three study species was high, suggesting that home ranges were spatially stable for at least one month. Taken together, our results suggest that daytime parrotfish space use is constrained within fixed intraspecific territories in which territory holders have nearly exclusive access to resources. Grazing by parrotfishes maintains benthic reef substrates in early successional states that are conducive to coral larval settlement and recruitment. Behavioral constraints on parrotfish space use may drive spatial heterogeneity in grazing pressure and affect local patterns of benthic community assembly. A thorough understanding of the spatial ecology of parrotfishes is, therefore, necessary to elucidate their functional roles on coral reefs. 

Study species and sites

We conducted our research on Scarus iseri, Scarus taeniopterus, Scarus vetula, Sparisoma aurofrenatum, and Sparisoma viride at five fringing coral reefs along the leeward coast of Bonaire during January (Winter) and May-July (Summer) 2019: Angel City (12.10305º, -68.28852º), Aquarius (12.09824º, -68.28624º), Bachelor’s Beach (12.12605º, -68.28819º), Invisibles (12.07805º, -68.28175º), and The Lake (12.10618º, -68.29079º). The fringing coral reefs of Bonaire, Caribbean Netherlands have remained resilient despite multiple disturbances, and boast higher coral cover than most Caribbean coral reefs (Perry et al. 2013, Steneck et al. 2019). The abundance and biomass of different fish groups, including parrotfishes, is also much higher on Bonaire’s coral reefs compared to more heavily fished reefs in the Eastern Caribbean (Hawkins & Roberts 2003, 2004, Steneck et al. 2019). The benthic composition across our study sites was similar, with relatively high coral cover (~20%) and low macroalgal cover (< 3%; Manning & McCoy 2022). Additionally, our five focal parrotfish species comprised more than 96% of the parrotfish biomass at our study sites (Manning & McCoy 2022).

Parrotfish tracking

We conducted concurrent GPS tracking and behavioral observations of TP and IP parrotfishes between 1000–1600 hrs, peak foraging times for parrotfishes (Bruggemann et al. 1994b a). We identified focal parrotfish (TP or IP) haphazardly at ~10 m depth on SCUBA at each site. Each fish was then allowed to acclimate to diver presence for approximately 1–2 mins, during which time we visually estimated standard length (to the nearest cm) by measuring the distance between reference objects passed by the fish using a collapsible meter-stick. We then estimated body mass (g) from standard length using published length-weight relationships (Bohnsack & Harper 1988; Appendix: Table A1). We followed focal fish from ~2 m, and recorded their behavior in high resolution (4K) using a GoPro Hero 4 Silver (GoPro, Inc) attached to a ‘selfie-stick’.

Focal parrotfishes were tracked at the surface by a snorkeler carrying a handheld GPS receiver (Garmin GPSMap 78sc, United States of America) for 13.56 ± 0.19 mins (mean ± SE, n = 215 total tracks). The GPS receiver was set to record data as often as possible, resulting in a mean (± SE) relocation interval of 12.32 ± 0.21 s (mean ± SE, n = 215 total tracks). We ensured that we did not track the same individuals unintentionally by progressively moving north along the reef, using reference structures, until we identified another unique individual to observe. A few times, we unintentionally conducted repeat tracks of previously tracked individuals (on the same day or within a few days; confirmed as described below). In such cases, unintentional repeat tracks were excluded from our analyses. Because our interest was in the home range behavior of territorial fishes, we excluded non-territorial, transient TP fishes from our analyses. Transient, non-territorial TPs were not site attached and were frequently chased along the reef by territory holders.

We preliminarily tracked several territorial TP (hereafter, just TP) Sp. viride at two sites in Winter 2019. Then, during Summer 2019, we tracked several TP Sc. taeniopterus, Sc. vetula, and Sp. viride at all five sites, and TP Sc. iseri and Sp. aurofrenatum at two sites. To investigate differences in space use among ontogenetic phases, we also tracked IP Sc. taeniopterus, Sc. vetula, and Sp. viride at two sites during Summer 2019. Finally, to quantify short-term spatial stability of parrotfish home ranges (described below), we conducted planned repeat tracks of several TP Sc. taeniopterus, Sc. vetula, and Sp. viride at two sites during Summer 2019. Repeat tracks were obtained by tracking TPs along the same portions of the reef where they had been tracked ~1 month prior. Individual parrotfish are identifiable by unique color patterns and markings on their bodies (Dubin 1981, van Rooij et al. 1996). We compared color patterns and markings of each fish using stills taken from our video recorded behavioral observations to confirm that initial and repeat tracks were of the same fish and not different fish occupying the same areas (Figures A1-A3). We also obtained unplanned, repeat tracks of 4 TP Sp. viride at Angel City and Bachelor’s Beach (n = 2 per site) during Summer 2019 that were initially tracked in January 2019. Home ranges of the 4 fish were in the same locations in both tracking periods, and visual observation of color patterns and markings from video stills confirmed that they were the same fish. A full breakdown of our sampling effort is reported in the Appendix (Table A2).

Home range estimation

Visual analyses of stationarity confirmed that our GPS tracks were sufficiently long to capture home range behavior (Figure A4; Benhamou 2014). We used movement-based kernel density estimation (MKDE) to estimate utilization distributions from our tracks of individual parrotfishes (sensu Benhamou 2011), and define home range and core use areas as the areas underneath the 95% and 50% cumulative isopleths of each utilization distribution, respectively. Though we used MKDE estimates of home ranges for analyses of space use in our study, we also present home range sizes estimated using traditional location-based kernel density estimation (ad hoc bandwidth selection) and minimum convex polygons to facilitate comparisons of home range area with other studies (Table A3). All home ranges and core use areas were computed in the adehabitatHR package in R (Calenge 2006, R Core Team 2020).

Home range exclusivity and stability

To investigate the exclusivity of parrotfish HRs, we quantified spatial overlap between HRs of spatially co-occurring TP parrotfishes tracked during Summer 2019. This measure of spatial overlap estimates shared space use between neighboring fish that are sharing at least some space on the reef. We used only HRs estimated from the first GPS track of TPs for which we had replicate GPS tracks. Spatial overlap was estimated using Bhattacharyya’s Affinity (BA), a function of the product of two utilization distributions that ranges from 0 (no overlap) to 1 (perfect overlap) and is a strong metric of joint space use between animals, particularly when comparing utilization distributions estimated for the same animal at different times (Fieberg & Kochanny 2005). As such, we also used BA to quantify the spatial overlap of HRs estimated from repeat tracks of TP Sc. taeniopterus, Sc. vetula, and Sp. viride to determine the spatial stability of those HRs. To distinguish spatial overlap between different spatially co-occurring individuals and spatial overlap of home ranges estimated from repeat tracks of the same individuals, we use the terms spatial overlap and temporal overlap, respectively. Spatial and temporal overlaps of MKDE home ranges were computed in the adehabitatHR package in R (Calenge 2006).

Occupancy change

During our attempt to repeatedly track parrotfishes at Aquarius, we observed an occupancy change in a TP Sc. vetula home range. We analyzed this occupancy change as a separate case study (Figure A5). To investigate how a change in occupancy affected space use, we quantified spatial overlap between the home range of the new occupant and the home range of the old occupant. We also quantified spatial overlap between the home range of the new occupant and the home ranges of spatially co-occurring intraspecific and interspecific fishes from the initial tracking (Table A4).

Social behavior

We quantified the social behavior of the TP parrotfishes tracked in Summer 2019 (n = 128) by analyzing the video recordings of the initial tracks for each fish (when repeat tracks existed) in the behavioral software BORIS (v. 7.9.8; Friard and Gamba 2016). For each fish, we quantified the number of agonistic interactions they had with other parrotfishes, the identity (i.e., ontogenetic phase and species) of interacting parrotfishes, and interaction durations (s). In some cases, we were unable to determine the identity of the interacting parrotfish, and classified such interactions as ‘unknown’. Agonistic interactions between parrotfishes consisted of displays (i.e., extension of dorsal and pelvic fins) and more aggressive (and typically longer) chases that are characteristic of territorial behavior/aggression in parrotfishes (e.g., Buckman & Ogden 1973, Mumby & Wabnitz 2002). 

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