PRE-SETTLEMENT MORTALITY OF CORAL REEF FISHES

Transcription

1 PRE-SETTLEMENT MORTALITY OF CORAL REEF FISHES Morgan Bond, Immanuel Hausig, Jennifer Lape Abstract Pre-settlement mortality is important in structuring populations in open systems. In this case, we used the damselfish (Dascyllus trimaculatus) as a model to demonstrate transit mortality of fish as they move across the reef crest to their settlement site. Sea anemones (Heteractis magnifica) to which D. trimaculatus exclusively settles were placed in two rows parallel to the reef crest in Moorea, French Polynesia. We measured transit mortality by looking at differences in settlement on anemones close to and far from the reef crest. The experiment was conducted between the third and first quarter moon phases, and the amount of moonlight during the settlement period had a strong impact on transit mortality. We also observed a lag effect, where high settlement on the crest was followed by a high settlement far from the crest approximately two days later, indicating that some fish may be moving slowly across the reef flat over a period of days. Introduction A fundamental question for reef fish population biology is; how much of an effect does mortality in the dispersal stage have on the resulting adult population? Most reef fish have open populations, where larvae disperse from the reef and enter the pelagic stage in the open ocean. It is thought that there is a 99 percent mortality of young during this dispersal stage (Jones 1991). This is the so called black box, because it is still unclear what the larvae are doing, and where they are going during the time they leave the reef until they return from the ocean. After a period of time the larvae return as settlers to be brought over the reef crest, and settle on their preffered substrate in the lagoon. Many studies have been conducted to determine what levels of mortality are occurring for these young fish immediately following settlement (Hixon & Carr, Schmitt & Holbrook). However, little work has been conducted to determine how much mortality is taking place prior to settlement. Much of this mortality may be taking place at the end of the dispersal stage, when settlers come over the reef crest in search of an adequate site for settlement. Rene Galzan recently determined that Naso unicornis larvae suffer a 60% pre-settlement mortality during this period (Galzan, 2000 unpublished). Larval fish may travel over large expanses of

2 reef before choosing to settle, and are generally at the mercy of currents and have few defenses against predation (Leis 1991) Our study sought to determine how much mortality occurs in settlers as they move through the lagoon prior to settlement. We used a pomacentrid, Dascyllus trimaculatus, to test this process of pre-settlement mortality because of the settlement specificity to the sea anemone, Heteractis magnifica. This unique feature of D. trimaculatus allowed us to detect differential settlement between areas near, and far from the reef crest, thus obtaining the mortality rate of the larvae as they move into the lagoon. Materials and Methods Study site Our field work was conducted near the west crest of Oponohu Bay on the northern shore of Papetoal reef (17 29 S W), Moorea, French Polynesia. The island of Moorea is surrounded by a barrier reef that forms a lagoon. Water enters the lagoon over the reef crest and mainly exits through Tareu pass, to the east. Our work was conducted in 1.5 to 2.5 meters of water on the reef flat, just behind the reef crest, an area of consistent coral cover. The site was chosen due to the abundance of predatory reef fishes, and a unidirectional current flow perpendicular to the reef crest. A complete survey of the area revealed no naturally occurring sea anemones of the species Heteractus magnifica. Study Species Dascyllus trimaculatus (three spot dascyllus, Pomacentridae) is a small greyish to black diurnal planktivore. Juveniles have a large blueish white to white stripe on either side of the upper middle dorsal area of their body, as well as a white spot on their forehead. These markings dissapear with age, and the adults may become all black (Fautin & Allen 1992). Juvenile D. trimaculatus enter the lagoon areas over the reef crest and settle to sea anemones (in Moorea, Heteractis magnifica) between 00:00 and 5:00am (Schmitt and Holbrook 1999). This settlement occurs within 3-5 days of the first and third quarter moon phases, and there is little to no settlement during the intervening days between quarter moon phases (Schmitt and Holbrook 1999). Schmitt and Holbrook also report that there is almost no migration between anemones, once the fish have settled (Schmitt and Holbrook 1999). The migration between anemones occurs more readily after the fish are a few weeks old. Larval D. trimaculatus

3 have black pigment which ends at the caudal peduncle. After the young settle on an anemone, the black pigment begins to fill in the tail area, and the tail turns black. The presence or absence of black pigment in the tail is an excellent method for determining how long the fish has been residing on an anemone. Heteractis magnifica is the most prominent species of anemone in Moorea, and the only one that D. trimaculatus settles on. It is a large pink to brown anemone that may grow to 1 meter in diameter, but is most commonly found in the cm size range (Fautin & Allen). H. magnifica has a high survivorship after transplantation, and survives well in high current areas, but is also mobile, and may move away from the transplanted area if suitable substrate is available. Experimental Design We transplanted sixteen sea anemones (H.magnifica) attached to dead corals by breaking off pieces of their substrate. The anemones were transported to the study site and attached to existing dead coral heads using Z-Spar Splash Zone Compound (Kop-Coat, Incorporated, Los Angeles, California) and cable ties. We placed anemones in two parallel rows (A "crest row" and a "channel row") consisting of eight anemones, each spaced between 10 and 18 meters apart. The crest row was 50 meters from the crest and the channel row was 100 meters from from the crest row. The rows were oriented so that the unidirectional movement of water coming over the crest was perpendicular to the array of anemones. No naturally occurring anemones were found in the total area of the field experiment. The diameter of individual anemones was measured to obtain the surface area of the oral disc. It is assumed that the total surface area of all anemones at the crest row was insignificant in comparison to the total area over which the experiment was conducted. Therefore the area of the oral surface did not deplete the available larvae for the channel row. Prior to the start of the experiment the anemones were cleared of all associated fish so that resident densities of fish would not inhibit settlement. We assumed that the larvae moved in a unidirectional flow over the crest through our study dsite It was assumed that the rate of immigration of settlers was equal to the rate of emigrating settlers, thus having no effect on the settlement rate of juveniles. Observational study to determine the distance from which a new recruit can stumble onto an anemone

4 A cleared anemone attached to a dead coral head was transported to a sand flat near the channel of the west crest of Opunohu Bay. At a distance of one meter from the anemone a meter tape was laid parallel to the reef crest. Eight new recruits were released at varying distances along the meter tape. We laid out five meters of tape running west to east. The 2.5 meter mark was positioned directly in front of the anemone. Fish one was released at 2.5m on the tape, fish two at 3.0m, fish three at 3.5m, fish four at 4.5m, fish five, six, and seven at 2.0m, and fish eight at 1.5m. Data Collection Data collection occurred from November 18 th through December 5 th. Collections of D. trimaculatus recruits were obtained daily. Fish that settled on the anemones during the previous night were counted and cleared with small aquarium nets, and a 1:10 mixture of clove oil and ethanol when the fish could not be removed with a net alone. Each fish caught was returned to the lab in plastic bags separating crest and channel individuals, and their total length was measured. After 5 days D. trimaculatus of various sizes were stocked in pairs on three anemones from each row to determine if a resident fishes facilitated the settlement of new recruits (Schmitt and Holbrook 1996). New recruits that settled to these anemones were counted but not removed following stocking. Results Settlement patterns of D. trimaculatus from the third quarter moon phase, through the new moon, until the first quarter phase After monitoring D. trimaculatus settlement for one half of a lunar phase, an increasing trend in the daily settlement was observed (Fig. 1). However, overall settlement was higher at the crest location then it was at the channel location. We observed a significant amount of settlement on both of our sites between the third and first quarter moon phases. Settlement at both the channel and crest locations was variable, but an overall cyclic increasing trend was observed (Fig. 1). This data revealed a lag effect in the pattern of

5 settlement at the two locations. A strong settlement pulse at the crest was usually followed by a strong settlement event at the channel location two to three days later. As the first quarter lunar phase approached, there was a marked increase in overall settlement, and the lag effect became less prominent. Little to no settlement was observed at either site in the 6 days immediately following the third quarter moon phase. The peak settlement yield from the 25 th to the 28 th of November was a little over 1/5 of the maximum yield for the crest site (observed on the 2 nd of December). Settlement in the channel during the same period was 7/10 of the maximum recruitment observed at that site. Maximum settlement at both sites occurred two days prior to the first quarter moon phase (Dec. 2). Density of settlers throughout half of the lunar cycle The density of settlers (settlers/cm 2 of anemone) from each site was compared to the day in the lunar cycle, and a distinct pattern emerged (Fig. 2). During the first 4 days of monitoring the density of new settlers on the crest was higher than the channel, but decreasing in number. Days 4 through 11 showed a higher density on the channel than the crest. After day 11, the density of settlers on the crest again became higher than on the channel, with the rate of crest settlement increasing each day. During this period, the density of settlers on the crest more than doubled. Density of settlers with increased lunar intensity There is a strong correlation between the density of settlers observed at each site, and the brightness of the moon (percentage of moon illuminated) (Fig. 3). Increased lunar intensity had a strong positive effect on the density of settlers on the crest. However, the lunar intensity had almost no effect on the density of settlers at the channel site (P = 0.02), and the density of settlers remained virtually static (0.3 settlers/cm 2 of anemone with a slope of almost 0 for the entire range of lunar intensities. The density of settlers increased from a low of 0 settlers/cm 2 of anemone when the sky was dark, to a high of ~0.5 settlers/cm 2 of anemone when the moon was bright. Effects of a previous cohort on settlement of D. trimaculatus There was a significant difference in settlement of new fish between anemones that were cleared of all previous D. trimacuatus, and those that were stocked with either a pair of larger fish (> 2.5cm), a pair of new recruits, or a pair containing one large fish and one small

6 fish. On anemones stocked with fish, the average number of settlers per day was The average number of fish on anemones withoout stocking was 1.04 (p=0.02). Observational study to determine the distance from which a new recruit can stumble onto an anemone Our observations revealed that D. trimaculatus does not seem to prefer anemones over other substrates for refuge. The majority of recruits released at varying distances from the anemones found refuge at the first substrate that they came across. Fish one slowly moved with the current toward the coral head and associated anemone. Fish two and three moved in a nearly linear fashion toward the coral head. Fish four swam along the meter tape for a distance of one meter before moving in the direction of the coral head. The fifth fish swam with the current away from the anemone towards no apparent substrate. Fish six swam towards the end of the meter tape that was secured by a weight and found it to be a suitable refuge. Fish seven swam up current approximately 5 meters to an entirely different coral head without an anemone. Fish eight swam towards the coral head and associated anemone. Migration of Heteractis magnifica during the experiment During the course of the experiment, 5 of the 16 original H. magnifica began to move off of their transplanted coral heads. These anemones moved no more than 1m during the 16 days of the experiment. There was no significant difference in D. trimaculatus settlement between anemones that remained on their transplanted coral heads, and those that migrated to other coral in the area. Discussion Pre-settlement mortality of larval reef fish must be significant because of the low numbers of settlers observed returning after such large quantities of larvae are released into the ocean. In our study, we sought to determine how much mortality occurs once larval fish have passed the reef crest on their way to settlement sites. Our results show that there is a significant amount of pre-settlement mortality of D. trimaculatus settling during times of high recruitment. This mortality is heavily influenced by the presence or absence of moonlight during the early morning settlement period.

7 Highest settlement of D. trimaculatus has been found to occur within 3-5 days of the quarter moon phases with little or no settlement between quarter moon phases (Schmitt and Holbrook 1997). However, in our study we found a fairly high number of settlers between the quarter moon phases of November 17 and December 4 (Fig. 1). Between November 24 and November 28 there were as many as seven settlers on the channel row for one night. This is a large number of settlers considering that at maximum settlement on the channel row there were only 10 settlers. The maximum number of settlers on the crest between November 24 and November 28 at the crest was four per night. This is a little more than 1/5 of the maximum settlement of nineteen that occurred on December 2 (highest crest settlement observed). Our study suggests that even between quarter moon phases a significant amount of settlement may occur. Oscillations of higher and lower settlement days were observed for both the crest and channel rows during our study period (Fig. 1). A lag effect occurred where settlement peaks at the crest corresponded to settlement peaks at the channel two days later. We propose that this lag effect of approximately 2 days corresponds to the time it takes for larvae to move from the crest row to the channel row. Two days of transit time might seem unlikely for larvae traveling 100 meters from the crest to the channel, but a couple of subexperiments hint that this may occur. When we released new recruits at different distances from an anemone attached to coral in a sand flat, we saw that new recruits look for just about any kind of substrate for refuge. Five of the recruits released at varying distances from the anemone swam toward the boulder that the anemone was attached to, hid in its crevices and after a few minutes moved on to the anemone. Three recruits that were released did not even swim to the anemone attached coral head. The first one swam to a further coral head in the opposite direction. The second individual took refuge in a weight that was used to hold down the meter tape and the third individual swam past the anemone and was not seen taking refuge in any substrate. The results of this sub-experiment suggest that D. trimaculatus larvae coming over the crest do not directly swim to their target anemones but spend a longer time in transit than was previously thought. Measurements of standard length of new settlers from the crest and channel rows revealed no significant differences in size, but future otolith work on these fish may show age differences. The data plotted in Figure 2 is divided into three distinct regions. Region 1 being the area where the density of settlers at the crest is higher than those at the channel. In the second regioin, the density at the channel overtakes the crest density, and the third the crest

8 density is again higher. In region 1, the initial density at the crest is higher than that of the channel due to transit mortality. As the young settlers move across the reef flat, predation causes the resulting density of fish that are able to settle on the channel row to be much lower than what initially crossed the crest. In the second region, the density of settlers at the crest falls below the density of the channel for a period of 7 days. Settlement during this time was low overall, and the preference for the lower current channel site becomes apparent during this period. It is unclear whether the preference for the lower current site is a behavioral choice of the young fish, or whether they are simply removed from the crest area by hydrodynamic forces before they can take up residence on an anemone. In the third region crest density is again higher suggesting that transit mortality becomes strong again due to the high levels of settlement overall, and strong lunar effects. The lunar phase was one of the continuous variables of our experimental design. Although we had not anticipated a pronounced effect from this environmental factor, considerable correlation occurred between fish density and the lunar cycle. The analysis of the data (Fig. 3) establishes that the lunar effect is only significant at the crest and not at the channel site. The new moon phase yields no lunar light and consequentially no transit mortality. During this period low densities of fish at the crest are explained by hydrodynamics and their settlement preference for low current areas. Higher densities of fish settle at the channel site due to this preference and the low transit mortality at the new moon phase. As the amount of light increases there is an increased risk of predation leading to an increased difference in settlement density between the crest and channel sites. We think that nocturnal predators may be more effective in perceiving prey at increased levels of moonlight. Stocking of anemones with new recruits revealed unexpected results. Settlement of D. trimaculatus has been shown to be facilitated by the presence of previous cohorts (Schmitt and Holbrook 1996). Our study showed that stocked anemones had less settlelment per day than anemones that were cleared each day. It is unclear why this relatioinship is observed, but the differences were significant. Our study has shown that transit mortality may play a significant role in determining reef fish populations, and this mortality may be variable. The pre-settlement mortality we observed was strongly affected by lunar cycles, and varying levels of light during settlement. We have also observed that larvae may not move direcly to their settlement sites, but may spend a significant amount of time on the reef prior to settling.

9 Acknowlegements We would like to thank Giacomo Bernardi, Pete Raimondi, Jonna Engel, Mark Readie, and Shauna Reisewitz for assistance in the field and for experimental imput. We especially want to the invaluable assistance of Pete Raimondi concerning experimental design and data analysis. We appreciate the logistical assistance and hospitality of staff of CRIOBE-EPHE especially Yannick Chancerelle. Thanks to the staff of the Biology Department of the University of California, Santa Cruz who supported the manifestion of the 2000 class Biology 162, especially Kay House for her dedication and funding of our supply of drinking water. Finally, our gratitude to all the students of the class of Biology 162 who made our stay in Moorea unforgetable. Works Cited Hixon, M.A., and M.H. Carr Synergistic predation, density dependence, and population regulation in marine fish. Science 227 : Fautin, D.G. and G.R. Allen Field guide to anemonefish ans their host anemones. Western Australain Museum, Perth. Galzin, R Personal correspondence. C.R.I.O.B.E. /E.P.H.E, Moorea, French Polynesia. Holbrook, S.J. and Schmitt R.J Settlement patterns and processes in a coral reef damselfish : in situ nocturnal observations using infared video. Proc 8 th Int. Coral Reef Symp. 2 : Jones G.P Postrecruitment processes in the ecology of coral reef fish populations : a multifactoral perspective. In Sale, P.F. (ed). The Ecology of Fishes on Coral Reefs. Academic Press, San Diego pp Leis, J.M The pelagic stage of reef fishes: The larval biology of coral reef fishes. In Sale, P.F. (ed). The Ecology of Fishes on Coral Reefs. Academic Press, San Diego pp Schmitt R.J. and S.J. Holbrook Local-scale patterns larval settlement in planktivorous damselfish-do they predict recruitment? Mar Freshwater Res 47 : Schmitt R.J. and S.J. Holbrook Mortality of juvenile damselfish : Implications for assessing processes that determine abundance. Ecology 80 :35-50.