Wrasse

Abstract: The temporal and spatial patterns of larval settlement of the-bluehead wrasse, Thalas- soma bifasciatum, were documented in the San Blas Islands of Panama. Daily censuses indicated that larvae settle onto coral reefs in brief episodes that tended to occur around the new moon and peak in intensity between August and December of each year. The magnitude of settlement each day was unrelated to changes in the local population size but was found to be significantly correlated with the nightly catch of planktonic reef fish larvae in the waters over the reef. The spatial pattern of settlement was examined on three scales. On a large geographic scale, 24 reefs within 1000 km2, there was tremendous variability in settlement intensity that was very consistent from one year to the next. The best correlate of recruit density was exposure to the onshore current. It is suggested that some large- scale oceanographic process determines the intensity of settlement of bluehead wrasses on this scale. The densities of adults on these reefs directly reflected the densities of recruits. Counts of daily otolith increments indicated that those larvae that settled onto low-density reefs subsequently grew signifi- cantly faster. On an intermediate scale, sites within 1 km2, there was no consistency among sites or months in settlement intensity, suggesting that variance in settlement intensity on this scale may be the product of random processes. On the smallest scale, habitat selection within a patch reef, it was found that recruits had distinct preferences in their choices of settlement sites. The daily mortality schedule of bluehead wrasses indicated that mortality was extremely high for the first 3 d on the reef. Juvenile mortality was, furthermore, found to be independent of density. As a result, the patterns of larval settlement of bluehead wrasses persisted into the adult population relatively unchanged. It is therefore proposed that the distribution and abundance of bluehead wrasses in this region are more a product of the external processes controlling larval settlement than of any biological interactions within the reef assemblage.

Abstract: Demographically open communities are often viewed as stochastically structured assemblages because most colonizing juveniles arrive via unpredictable dispersal mechanisms. However, interactions between established residents and incoming juveniles may affect juvenile persistence in species-specific ways and could therefore impose a degree of determinism on future community structure. Using 16 spatially isolated communities of coral reef fishes, I conducted two experiments to determine how prior residency by two guilds of fishes affected juvenile recruitment. Each experiment factorially manipulated the presence and absence of two guilds: resident piscivores (groupers and moray eels) and interference competitors (territorial damselfishes). In the first experiment, guilds were manipulated via selective removals, and subsequent recruitment (larval settlement minus mortality) was monitored for 44 days. In the second experiment, guilds were placed within large cages to prevent direct resident–juvenile interactions, while allowing for any cues produced by enclosed fishes, thereby testing whether incoming larvae used resident-derived cues to select or reject settlement sites. Colonizing juveniles were collected from each reef over 42 days to prevent confounding resident- and recruit-derived cues. In the first experiment, piscivores inhibited recruitment of a damselfish (Pomacentridae) and a surgeonfish (Acanthuridae), and enhanced recruitment of a wrasse (Labridae). In contrast, territorial damselfishes inhibited recruitment of the damselfish and the wrasse, and enhanced recruitment of the surgeonfish. Observations of early recruitment patterns suggested that recruitment differences were established rapidly during the night or dawn periods shortly after settlement and before each daily census. In the second experiment, there was no evidence that larvae used resident-derived cues to select settlement sites, suggesting that recruitment differences in the first experiment resulted from differential mortality caused by direct resident–recruit interactions rather than differential larval settlement. These results demonstrate that interactions between established residents and newly arrived juveniles can have a strong influence on juvenile persistence, and that such interactions appear to be strongest within hours of larval settlement. Furthermore, because resident effects were species specific, the present composition of these communities may impose a previously undocumented degree of determinism on their future structure.

Abstract: Many heavily fished coral reefs have a high abundance of sea urchins that may suppress the recov- ery offish and coral populations once fishing effort is reduced or eliminated. Restoration of these reefs may be accelerated by intervening and reducing sea urchin populations-particularly those of long-lived species. We studied three Kenyan coral-reef lagoons to determine the influence of reductions in sea urchin popula- tions on coral, algae, and fish populations. Populations were monitored seven times over a 1-year period in 50 X 50 m unmanipulated controlplots and experimental sea urchin reduction plots where sea urchin popu- lations were reduced by about 85%. Census of the most-abundantfish families found the greatest positive pop- ulation responses to sea urchin reduction in plots protected from fishing. The wet weights offish nearly tri- pled, population density increased by 65%, and species richness increased by 30% compared with adjacent control plots. Parrolfish, wrasse, scavenger, and snapper families showed the greatest population and wet- weight increases. In the two fished reefs, fish populations also increased but to a lesser degree than in the un- fished reef-particularly when comparing the wet-weight estimates offish. Small-bodied species of the dam- selfish and wrasse families and juvenile parrolfish exhibited the largest population increases in these fished reefs. In fished reefs algae and seagrass cover exhibited the greatest increases following sea urchin reduction. Seagrass colonized bare sand, whereas fleshy brown algae colonized hard substrate (dead coral). The tall can- opy-forming alga Sargassum latifolium became the dominant alga in the fished reefs, whereas the more pros- trate genus Padina dominated in the unfished reef. Fleshy algae cover in the unfished reef was about half that of the two fished reefs-attributable to the greater abundance of parrotfish in the unfished reef. This suggests that greater herbivory in the unfished reef kept the algae from reaching an algalforest climax typified by Sar- gassum dominance. In the fished reefs the high S. latifolium cover and the increased thickness of the algal turf reduced hard coral cover by around 30%. In the unfished reef coral cover was reduced by around 13%, but by the end of the experiment coral cover and genera richness were the same in both the control and the sea ur- chin reduction plots. We conclude that, primarily, fishing and, secondarily and indirectly, high sea urchin abundance are reducing fish numbers and diversity in Kenyan reefs and that sea urchin reduction has the potential to increase reeffisheries production and recovery from overfishing. Sea urchin reduction is not rec- ommended, however, on moderately fished reefs due to the possible loss of coral cover and diversity. If reefs are either severely degraded or iffishing effort is reduced or eliminated, then sea urchin reduction is recom- mendedfor the restoration offish numbers, feeding importance, species diversity, andfisheries production.