Parrotfish

This report characterizes the state of Bonaire’s reefs as of March 2005. We pay particular attention to structural and functional attributes of reefs that have changed in so many other Caribbean reefs. We characterize coral reefs by their resident organisms and the forces regulating their distribution and abundance. Thus, corals, algae and fish define the “structure” of coral reefs but climate changes, diseases, hurricanes, overfishing, sedimentation and excess nutrients may affect how they “function”. Recent unfavorable changes in the structural and functional attributes of reefs have caused “the coral reef crisis” (Bellwood et al. 2004). In Caribbean coral reefs the most alarming changes have been the declines in the abundance of corals, sea urchins and reef fishes and the accompanying increases in large harmful seaweeds (called “macroalgae”). The decline in coral and increase in macroalgae, called a “phase shift”, represents a significant change in the structure of coral reef ecosystems that could lower its resilience.

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In March of 2005, a team of graduate students from the University of Maine revisited six study reefs on Bonaire to determine the status of those reefs and to detect if any change has occurred since March of 2003 when the last such survey was conducted. The study sites established in 2003 from north to south are: Karpata, Barcadera, Reef Scientifico, Forest on Klein Bonaire, Plaza and Windsock. Bonaire’s shallow (10 m) reefs remain in good condition. Coral cover averaged 47% in 2005 compared to 46% in 2003 (no change). Turf algae have increased and coralline algae have declined slightly over the past two years. Harmful seaweed “macroalgae” abundance remains low (2% in 2005 and 5% in 2003; see Steneck in this report) at the 10 m depth we studied. At depths below 20 m, macroalgae are now and have been (for at least the past 30 years) much more abundant (e.g. Van den Hoek et al. 1975) The absence of macroalgae in Bonaire most likely relates to the abundance of seaweedeating species or “herbivores”. Caribbean-wide, harmful macroalgal seaweed abundance corresponds inversely with the abundance of grazing fish such as parrotfish and tangs (Fig. 1). No comparable plot exists for seaweed abundance and any other measured factor on reefs.

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Changes over the past two decades
Comparisons between the status of reefs over a few years tell us little about long-term changes. For example, today there is a distinct demarcation between where Bonaire’s fringing reefs begin at 5 to 10 m depth and the shore. This region today is largely coralfree and dominated by rubble and sediment laden turf algae. However, this may not have always been the case. Prior to whiteband disease that killed nearly 90% of the elkhorn and staghorn corals in the Caribbean (i.e. Acropora palmata and A. cervicornis) (Aronson et al. 1998, Aronson and Precht 2001), most of the near shore zone was coral-dominated.
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Coral cover in the near shore zone surrounding Bonaire has declined dramatically and is now dominated by dead coral rubble where once elkhorn and staghorn corals had formed near monocultures prior to white band disease. Five of our six study sites have changed dramatically over the past 20 years except for Karpata. The decline of the Acropora species may have allowed competitively inferior species such as lettuce, pencil, finger and fire corals (Agaricia spp, Madracis spp, Porites porities and Millepora complanata) to expand since all have increased in abundance since the Van Duyl study (1985). Corals are not the only group to have changed dramatically since the 1980s. Diadema antillarum, the dominant grazing sea urchins was abundant in the near shore zone until it succumbed to the mass mortality of the mid 1980s. Today, more than 20 years later it remains below detectable levels at most of the sites we studied (Smith and Malek this report, Steneck this report). These changes, along with the significant declines in large predator finfish (see Bonaire Report 2003) indicate that several key players for the resilience of coral reefs (e.g. Fig. 3) have declined in abundance.

Widespread use of minimally selective fish traps has contributed to the overfishing of Caribbean coral reefs. Traps typically target high-value fish such as groupers (Serranidae and Epinephelidae) and snappers (Lutjanidae), but they also have high bycatch of ecologically important herbivores (parrotfish (Scaridae) and surgeonfish (Acanthuridae)) and non-target species. One strategy for reducing this bycatch is to retrofit traps with rectangular escape gaps that allow juveniles and narrow-bodied species to escape; yet the effectiveness of these gaps has not been thoroughly tested. On the shallow reefs of Curaçao, Netherlands Antilles, I compared the catch of traditional Antillean chevron traps (the control) to the catch of traps with short escape gaps (20 × 2.5 cm), traps with tall escape gaps (40 × 2.5 cm), and traps with a panel of large aperture mesh. With data from 190 24-h trap sets, the mean number of fish caught was 11.84 in control traps, 4.88 in short gap traps, 4.43 in tall gap traps, and 0.34 in large mesh traps. Compared to controls, traps with short or tall gaps caught significantly fewer bycatch fish (–74 and –80% respectively), key herbivores (–58 and –50% respectively), and butterflyfish (Chaetodontidae; –90 and –98% respectively). The mean length of captured fish was significantly greater in gap traps because juveniles were able to escape via the gaps. Escape gaps reduce neither the catch of high-value fish, nor the total market value of the catch. Therefore, using escape gaps could make trap fishing more sustainable without reducing fishermen’s revenues.

Bonaire’s reefs remain among the best in the Caribbean. However, our monitoring has revealed some potentially troubling trends that may require management action. In 2005, we reported to the Bonaire Marine National Park on the status of Bonaire’s coral reefs, and we suggested a strategy for monitoring trends among four key reef attributes we believe track the health and resilience of Bonaire’s reefs (Steneck and McClanahan 2005). Here we report the results of monitoring studies conducted 2003, 2005 and now 2007 at each site. Where appropriate, we drew from Bonaire’s first AGRRA assessment conducted in February 1999 (Kramer and Bischof 2003) to extend temporal trends over a period of eight years. 
Troubling trends
We see three troubling trends of increased macroalgae, declining herbivory from parrotfish, and increases in damselfish populations. Of these, the first two are most serious (see Chapters 1, 2 and 3). Secondary trends of concern, increases in damselfish populations (Chapter 4) and declines in coralline algae (Chapter 1), could lead to reduced recruitment of reef corals (Chapter 7), but to date this is not evident (Chapter 7). Importantly, coral cover remains relatively high (Chapter 1). The monitored group of carnivorous fishes, the lutjanid snappers, are holding constant but we remain concerned about the past (Steneck and McClanahan 2003) and continued loss of other larger bodied reef carnivores such as groupers and barracuda. The positive ecological role of parrotfish is well documented (e.g. Mumby et al. 2006) so their decline is troubling. It is unclear exactly why their population densities are declining. While parrotfish are not currently a widely sought group of reef fish (Chapter 8), fishing pressure on them is growing. It is possible they are vulnerable to even modest fishing pressure, particularly from fish traps. Accordingly, we recommend that the capture and killing of parrotfish be stopped because of their key ecological role on Bonaire’s coral reefs. Further, other groups of grazing herbivores such as the longspined sea urchin (Diadema antillarum) are increasing but too slowly to effectively replace the functional role of parrotfish (Chapter 1). We suggest continued monitoring of key drivers of reef health (coral cover, algal abundance, herbivory and coral recruitment). Some standard protocols such as the Atlantic and Gulf Rapid Reef Assessment (AGRRA) are entirely commensurable with the data presented in our reports in 2003, 2005 and 2007 (this report). A streamlined monitoring protocol is likely to be most useful to managers to alert them as a potential problem is growing and, perhaps more importantly, to show improvement when it occurs.
 
 

The rainbow parrotfish Scarus guacamaia has an obligate dependence on man- groves at juvenile stages, and, as the largest herbivorous fish in the Caribbean region, its distribution has important implications for coral reefs. The effect of connectivity with mangroves on relative density, biomass and size of S. guacamaia was assessed from over 65 km of visual sur- veys from Bonaire, Caribbean Netherlands. In addition, an individual-based, age-structured, mechanistic model (IBM) was developed to explain dispersal patterns from nurseries for S. guaca- maia. In the IBM, mortality was constant, growth was determined by a von Bertalanffy growth equation, and movement was modeled through a random walk process. Using the IBM, simula- tions were run to generate patterns of density, biomass, and size with distance from nurseries. Rainbow parrotfish were observed as far as 42 km away from the nearest mangroves on Bonaire. Relative density and biomass showed significant exponential declines with distance from the pri- mary mangrove nursery and were significantly higher in high versus low complexity non-man- grove habitats. Mean size increased linearly with distance (r2 = 0.74), reflecting an absence of smaller individuals with greater distance. These results were closely mirrored by the simulation study: density and biomass declined exponentially with distance from nurseries, and size and age increased following saturating functions. The results suggest that mangroves may have the poten- tial to supply individuals much further than previously thought. Both the empirical and simulation studies reaffirm calls to prioritise protection of reef habitats close to nurseries as well as the nurseries themselves. 

Abstract:

On coral reefs, herbivorous fishes consume benthic primary producers and regulate competition between fleshy algae and reef-building corals. Many of these species are also important fishery targets, yet little is known about their global status. Using a large-scale synthesis of peer-reviewed and unpub- lished data, we examine variability in abundance and biomass of herbivorous reef fishes and explore evidence for fishing impacts globally and within regions. We show that biomass is more than twice as high in locations not accessible to fisheries relative to fisheries-accessible locations. Although there are large biogeographic differences in total biomass, the effects of fishing are consistent in nearly all regions. We also show that exposure to fishing alters the structure of the herbivore community by disproportionately reducing bio- mass of large-bodied functional groups (scraper/excavators, browsers, grazer/ detritivores), while increasing biomass and abundance of territorial algal- farming damselfishes (Pomacentridae). The browser functional group that consumes macroalgae and can help to prevent coral–macroalgal phase shifts appears to be most susceptible to fishing. This fishing down the herbivore guild probably alters the effectiveness of these fishes in regulating algal abun- dance on reefs. Finally, data from remote and unfished locations provide important baselines for setting management and conservation targets for this important group of fishes. 

Abstract:

Most animals are active either during the day, night, or twilight, and transition periods between these times exhibit interesting behavior. Actions may be related to avoiding predators, seeking shelter, defending territory, feeding, or other interests. Herbivorous fishes on coral reefs, such as parrotfishes, forage constantly throughout daylight periods due to inefficient feeding and reliance on light. At sunset, parrotfishes seek cover under which to rest at night, to conserve energy and avoid predation. To do so, parrotfish decrease feeding and increasing migration and aggression to do so. This study compared how initial phase (IP) and terminal phase (TP) princess parrotfish (Scarus taeniopterus) allocate time between daylight and sunset periods, specifically regarding time spent feeding and being aggressive. Observations were performed using SCUBA at Yellow Sub dive site on Bonaire, Dutch Caribbean. Individuals of S. taeniopterus were followed for 1 min (to allow for acclimatization), followed by 5 min of behavioral observation. Percent time spent on each behavior was calculated and averaged across each category (e.g. IP, daylight), and mean percent time spent feeding and being aggressive were tested using a two- way analysis of variance (ANOVA), with phase and time of day as factors. Both IP and TP fish had a higher mean percent time feeding and a lower mean percent time being aggressive in the morning than at sunset, and time of day and phase were both significant factors affecting variation in both behaviors. The results of this study give insight into the adaptations parrotfish have developed to increase survival. 

Abstract:

Most animals are active either during the day, night, or twilight, and transition periods between these times exhibit interesting behavior. Actions may be related to avoiding predators, seeking shelter, defending territory, feeding, or other interests. Herbivorous fishes on coral reefs, such as parrotfishes, forage constantly throughout daylight periods due to inefficient feeding and reliance on light. At sunset, parrotfishes seek cover under which to rest at night, to conserve energy and avoid predation. To do so, parrotfish decrease feeding and increasing migration and aggression to do so. This study compared how initial phase (IP) and terminal phase (TP) princess parrotfish (Scarus taeniopterus) allocate time between daylight and sunset periods, specifically regarding time spent feeding and being aggressive. Observations were performed using SCUBA at Yellow Sub dive site on Bonaire, Dutch Caribbean. Individuals of S. taeniopterus were followed for 1 min (to allow for acclimatization), followed by 5 min of behavioral observation. Percent time spent on each behavior was calculated and averaged across each category (e.g. IP, daylight), and mean percent time spent feeding and being aggressive were tested using a two- way analysis of variance (ANOVA), with phase and time of day as factors. Both IP and TP fish had a higher mean percent time feeding and a lower mean percent time being aggressive in the morning than at sunset, and time of day and phase were both significant factors affecting variation in both behaviors. The results of this study give insight into the adaptations parrotfish have developed to increase survival. 

The parrotfish Sparisoma viride is an abundant and ecologically important member of the tropical NW Atlantic reef fish fauna. Sagittal otoliths of 417 individuals were analysed to estimate age-based demographic variables at 4 localities (Lee Stocking Island, Barbados, Los Roques Archipelago and the San Blas Archipelago) spanning 14° of latitude. The sampling localities ranged from an area protected from trap- and net-based reef fisheries (Los Roques) to an area supporting a dense human population and sustained trapping and spearing for reef fishes including S. viride (Barbados). Examination of sectioned sagittal otoliths from each locality revealed regular increments in the sagittal matrix. A preliminary validation at San Blas was consistent with these increments being annual check marks. These increments provided estimates of age structure, maximum longevities and mortality rates for the 4 study populations of S. viride. Von Bertalanffy growth functions fitted to each size-at-age plot generated similar growth curves from 3 of these 4 localities. The exception was Lee Stocking, where fish grew faster and reached a substantially larger size than those from the other 3 localities. Further analysis of the growth curves demonstrated that the differences between Lee Stocking and the other localities were attributable to more rapid growth over the first 4 yr of life. Age-based growth curves derived from the Los Roques population were very similar to a size-based curve generated by an independent study on S. viride carried out in Bonaire, adjacent to Los Roques. Maximum longevities for all 4 of our populations varied from 7 to 9 yr. Mortality rates generated from catch curve analysis were also similar among localities and suggest that maximum life spans do not exceed 12 yr. This result differs from that obtained at Bonaire, where repeated censuses of tagged fish suggest 30 yr maximum longevity. Abundances of S. viride varied 3-fold among localities, being highest at Los Roques (protected from reef fishing), lowest at Barbados (high fishing) and Lee Stocking (low fishing). Thus our age-based study suggests that S. viride is a relatively short-lived fish with consistent demographic parameters over a range of localities, latitudes and fishing intensities.