Focusing the Recruitment of Juvenile Fishes on Coral Reefs

BULLETIN OF MARINE SCIENCE. 55(2-3): 623-630. 1994

FOCUSING THE RECRUITMENT OF JUVENILE FISHES ON CORAL REEFS

Richard E. Brock and Alan K. H. Kam

ABSTRACT This study quantitatively examined the role that aspect ratio plays on the recruitment of fishes on coral reefs. High aspect ratio reefs have significantly greater numbers of fishes recruiting to them than do low aspect ratio reefs. A low-cost method of increasing reef aspect ratio is through the use of small, tautline moored, midwater fish aggregation devices. Utilizing this technology we demonstrated that the recruitment of juvenile fishes to the demersal habitat may be focused to specific sites. Once in the demersal habitat, predation may be high so the design and scale of the benthic habitat beneath the mid water fish aggregation devices must be considered. Benthic habitat covering areas much greater than 2 m2 of substratum probably allows demersal predators to take up residence and impact the new recruits. The use of midwater fish aggregation devices that serve to focus the recruitment may attract and aggre- gate coastal pelagic species that also may be predators on the new recruits. Thus the technology suggests possibilities for the improvement of recruitment, but the scale and dispersion of the artificial habitat must be given careful consideration. If correctly applied, the technology may be useful in the restoration of coral reef fish resources.

Most coral reef fish have a pelagic larval phase (Leis and Miller, 1976; Sale, 1980). After a period of time in the plankton, these larval fish return and recruit to the adult habitat. Recruitment is defined as the process whereby juvenile fish join the adult habitat, and settlement is defined as the movement of larval fishes from the pelagic zone to the reef. Numerous studies have shown that the recruitment of juvenile fish to reef habitats is highly variable (Russell et a\., 1977; Luckhurst and Luckhurst, 1977; Talbot et aI., 1978; Eckert, 1984; Walsh, 1984; Schroeder, 1985). This variability is probably related to both the temporal and spatial scales at which the ecologist works but the constraints due to funding, equipment and time hamper definition of appropriate sampling scales. Causal factors to this variability in recruitment as suggested in the literature include variation in the (1) production of larvae, (2) survivorship of larvae, (3) survivorship of new recruits, (4) current patterns transporting larvae to and from the reef habitat, and (5) precise microhabitat requirements of recruits (see discussion in Sale et a\., 1984 and McFarland and Ogden, 1985). Sale et al. (1984) noted that 45 to 75% of all recruited juvenile fishes in their Great Banier Reef study were lost within 5 days of settlement in the adult habitat, suggesting that predation is high. If newly recruited juvenile fishes utilize benthic habitats for shelter and to escape predation, then the addition of appropriate shelter could potentially reduce juvenile mortality. Shulman (1984) experimentally demonstrated that recruitment and/or early survivorship of a number of coral reef fish species is strongly related to the number of refuges present. Sale et a\. (1984) found that juvenile fish will, to a degree, select settlement sites, but measurable differences in these sites are small. Eckert (1985), however, showed that some coral reef fish larvae exhibit predictable selection for certain substrates upon settlement. These data all suggest that the presence of appropriate habitat may be important to the survival of many coral reef fish at the time of and just after settlement. Perhaps an optimally designed habitat, one that has abundant refuge space will enhance subsequent survival of the new recruits. The degree of habitat complexity is of importance to the distribution, diversity, and abundance of adult

623 624 BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994 coral reef fish (Smith and Tyler, 1975; Luckhurst and Luckhurst, 1978; Brock and Norris, 1989). Spatial heterogeneity may be utilized by fishes as refuges from predation (Sale and Dybdahl, 1975; Sale, 1977, 1978). Many authors have hy- pothesized that the number of larval fishes that survive planktonic life and find their way back to the coral reef actually forms the limit to the adult population of fishes on the reef (Doherty, 1983; Victor, 1983; Sale and Douglas, 1984). Artificial reefs provide added spatial complexity to a given habitat. Spatial heterogeneity has been considered by ecologists to be important in controlling the distribution and diversity of organisms within their environment. Two categories of ecological effects of habitat complexity have been identified: (I) those allowing the co-existence of competing species through the use of separate microhabitats within a complex environment and (2) those increasing population abundances and species diversity of prey by providing refuges from predation. Both effects probably operate on coral and artificial reefs. The Japanese reef program is one of the most advanced of any in the world (Stone et aI., 1991). Besides constructing artificial reefs for adult fish, they have built nursery reefs which enhance juvenile recruitment (Grove and Sanu, 1985; Grove et aI., 1989). Little quantitative documentation exists as to the effectiveness of these nursery reefs or their design criteria. If the recruitment of juvenile fish to the adult habitat is a major bottleneck to the further growth of coral reef fish populations, then the development of artificial reef habitats designed with a high number of refuges may help to alleviate this problem and increase the productivity of inshore waters. Preliminary trials with small tautline moored midwater fish aggregation devices (MWFADs) deployed on a larger Japanese style concrete cube reef in Hawaiian waters resulted in a disproporationately greater recruitment of juvenile fishes from the water column to those parts of the reef directly beneath the MWFADs (Brock and Grace, 1987). Survivorship of the recruits was low and this was attributed to the lack of appropriately-scaled shelter on the reef. Subsequent experimental work by Beets (1989) demonstrated that small benthic artificial reefs deployed in conjunction with MWFADs offshore of the U.S. Virgin Islands led to greater fish community development than found around either benthic artificial reefs or MWFADs deployed separately. Similarly, the recruitment of juvenile fishes was greater on the combination MWFADs and benthic reefs. These preliminary studies lend support to the ideas that (1) high aspect ratio reefs (i.e., such as those outfitted with MWFADs) appear to be more attractive in the recruitment process and (2) the addition of benthic shelter at appropriate spatial scales may differentially enhance the survivorship of juvenile fishes. This study was undertaken to quantitatively ascertain the role of aspect ratio on artificial reefs in enhancing the recruitment of juvenile fishes from the water column to the benthos.

MATERIALS AND METHODS

To test the hypothesis that aspect ratio is important to the recruitment of juvenile fishes, we estab- lished an experimental array of three tautline moored MWFADs with small benthic artificial reefs and three benthic reefs without MWFADs offshore of Honolulu in 14 m of water in August 1989, The MWFADs were comprised of an anchor (six links of heavy ship chain weighing approximately 135 kg), 7 m of 9,5 mm galvanized steel cable as a mooring Iinc, a 30 cm hard plastic buoy to provide lift to the tautline mooring, and two commercially available McIntosh "Sea Kites" or parasols placed at 4,2 and 6.1 m above the seafloor on the steel cable, Where the steel cable made contact with other metal, i.e" as with the anchor or eyebolts of the sea kites, plastic tubing was placed over the cable to minimize wear and electrolysis, The site selected for this experiment has a substratum dominated by sand and rubble; few areas exist with a small amount of emergent limestone on scales from 2 to 30 BROCK AND KAM: FOCUSING JUVENILE FISH RECRUITMENT 625

Figure I. Photograph of the benthic reef design used in the experiment. Eighteen decorative concrete (12" X 12" X 4") tiles were arrangcd on end in a rough circle. Note the taut mooring line in the center of the reef.

m2 with some coral coverage (>2%). These hard substratum areas are spaced from 10 to 80 m apart. The experimental array was deployed in an area of sand and rubble. Benthic reefs were constructed of decorative concrete tile with outside dimensions of ]2" X 12" X 4" (Fig. I). Eighteen blocks were arranged in an approximate circular fashion around the anchor to form the benthic reef. The three benthic reefs without MWFADs were constructed in a similar fashion except that two additional decorative concrete tiles were substituted in place of the anchor. Previous preliminary studies with these small midwater devices suggested that if the MWFADs + benthic reef combinations were placed within visual range of each other or to artificial or other natural benthic features, juvcnile fishes newly recruited to the MWFAD + benthic reef combinations would often migrate between these features thus making interpretation of results difficult. To alleviate this problem all treatments were spaced approximately 50 m away from one another or local topographical features (outside of visual range of one another). Besides the treatments, we marked a lOX 10 m area of natural substratum having some topographical relief (i.e., hard substratum with I% coral coverage) as a control area. Treatments covered approximately I m2 of substratum; each of these as well as the control area were censused using visual census techniques. Censuses covered all fishes in a circle approximately 3 m in diameter centered on the treatment. All fishes in the watercolumn above the census area were also tallied thus including fishes associated with the midwater parasols. Parameters noted included the numbcr of individuals of each species of fish seen in the census area as well as their individual estimated lengths. Similarly, all fishes seen in the 100 m2 control area were censused. All censuses were carried out using SCUBA and efforts focused on the identification and tallying of newly settled juvenile fishes. These new recruits arc arbitarily defined as fishes that have settled in approximately the past 48 hours as determined by size, morphology, color and behavior characteristics. The estimated lengths of each species were used to compute standing crop (g·m 2) estimates using length-weight regression techniques (Ricker, 1975). Regression parameters for the individual species are from unpublished personal and other data sources (Hawaii State Division of Aquatic Resources, etc.). Problems associated with the use of the visual census method for the assessment of fishes have been dealt with elsewhere (Sale and Douglas, 1981; Brock, 1982). Despite its drawbacks, the visual census method remains one of the best non-destructive survey techniques available for the assessment of diurnally active coral reef fishes (Brock, ] 982). Census work continued for 217 days until an off-course tug and barge dropped a cable in the 626 BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994

10 25

o I'~OI. + 6[N.'IliIC .1 1lE:r.m-tIC • CONTROL (f> w <3

(f>~ 6 u. o '"~ 4 :> z~

100 100 TIME (doys) TIME (doys)

Figure 2 (left). The mean number of species encountered per census on the three treatments (MWFADs + benthic reef, benthic reef and natural substratum control) examined in this study over a 217 -day period. Figure 3 (right). The mean number of individual recruits encountered per census on the three treatments (MWFADs + benthic reef, benthic reef and natural substratum control) examined in this study over a 217-day period. process of overhauling. The cable was drug through the study site and scattered the experimental materials. To avoid assumptions of normality, the non-parametric Mann-Whitney V-test (Siegel, 1956) was used to discern differences in the data.

RESULTS The treatments were deployed over a 2-day period commencing on 17 August 1989. Materials were placed on site on the first day and the reefs were assembled and completed on the second day. Census work was carried out on the following dates (days after initiation): 17 August (day 0), 23 August (day 6), 30 August (day 13), 6 September (day 20), 20 September (day 34), 6 October (day 50), 9 November 1989 (day 84), 9 January (day 145) and 22 March 1990 (day 217). For purposes of this study, the fishes colonizing the treatments and control area were divided into two categories, new and post-recruits. The post-recruits include all fishes estimated to have been in the adult (demersal) habitat for a period greater than 48 h. In total 66 species of common reef fishes were censused among the two treatments and control area; 54 species were found associated with the MWFADs + benthic reef combinations, 40 species with the benthic reefs and 21 species in the control area. Approximately 21.2 m2 of substratum was sampled under each of the two treatments and 100 m2 in the control area, highlighting the attractiveness of the MWFADs + benthic reef combinations over the benthic reefs or control area. Fourteen species were encountered in the treatments and control as new recruits; in order of abundance these were Dascyllus albisella, Adioryx xantherythrus, Apogon taeniopterus, Aulostomus chinensis, Canthigaster jactator, Myripristes amaenus, Thalassoma duperrey, Ostracion meleagris, Pseudojuloides cerasinus, Chromis vanderbilti, Anampses chrysocephalus, Priacanthus cruenta- tus, Labroides phthirophagus and Dendrochirus barberi. Other than two Aulos- tomus chinensis all new recruits were associated with the benthic (concrete tile) portion of the reef. Figure 2 depicts the mean number of species of new recruits per census on each treatment and control area through the period of this study. These data indicate that the greatest number of species of new recruits is found on MWFADs + benthic reef combinations. Similarly, Figure 3 shows the mean number of recruits censused on each treatment by sample period. The number of recruits in BROCK AND KAM: FOCUSING JUVENILE FISH RECRUITMENT 627

Table 1. Results of eight visual censuses of fish community development over a 2J7-day period on three treatments examined in this study (MWFADs + benthic reefs, benthic reefs alone and natural substratum or control). In the body of the table are given the grand means for each treatment cate- gorized into two groups by the estimated time of settlement, i.e., newly settled recruits (within 48 h) as differentiated from all other fishes. The asterisks indicate where the Mann-Whitney V-test shows that for a given parameter, a treatment effect differs signifieant]y (P > 0.05) from the others.

MWFADs plus Parameter benthic reef Benthic reef Control

Newly settled recruits Mean no. species 7 4 3 Mean no. individuals 25* 13 I] All other fishes Mean no. species ]3* 9 5 Mean no. individuals 70* 23 ] I Mean biomass (g/m2) ]88* 73 16 Mean no. predators 6 5 ]

any treatment never exceeded 20 individuals but relative to the control, there appears to be a treatment effect. Table 1 summarizes these results; given are the grand means for each parameter (number of species, number of individuals, estimated biomass and the number of piscivorous predators) under each treatment (MWFADs + benthic reefs, benthic reefs and natural substratum control). Con- sidering newly settled recruits, there is a statistically significant greater mean number of individual fishes on the MWFADs + benthic reef combination treatment over either the benthic reef treatment or natural substratum (Mann-Whitney V-test, P < 0.05). Similarly, differences are also evident with the larger fishes; the MWFADs + benthic reef combinations had a statistically greater number of species, individuals and estimated biomass than either the benthic reef or natural substratum controls. There are no statistically significant differences between treatments and number of piscivorous predators at each (Table 1). Presumably the predation on each treatment is approximately the same. Piscivorous predators are defined as species that are known to feed on juvenile fishes and are large enough to do so. Thus the predaceous trumpetfish (Aulostomus chinensis) was present in the experiment but only as a newly settled recruit, thus probably did not pose an immediate threat to the other recruits. Species censused that were defined as piscivorous predators include Scorpaenopsis cacopsis, S. diabolus, Taenianotus tricanthus, Gymnotho- rax undulatus, G. meleagris, G. pete/Ii, G. jiavimarginatus, Synodus binotatus and in the water column Decapterus macarellus.

D]SCUSSION Studies conducted on coral reefs in Hawaii and elsewhere have estimated fish standing crops to range from 2 to 200 g·m-2 (Brock et aI., 1979; Goldman and Talbot, 1976). In this study, the mean standing crop of post-recruit (usually larger than 2-3 em) fish was estimated to be 188 g·m-2 on the MWFADs + benthic reef combinations; this biomass estimate is high because of (1) the relative lack of shelter at the appropriate scales in the surrounding habitat hence the clustering of larger fishes around these individual reefs as well as (2) encountering several 2 200 g fish in the relatively small sample area for each treatment (about 7 m ) results in an unduly high biomass estimate. The spatial scale at which one samples the fish community structure is central to the results and subsequent interpretation 628 BULLETIN OF MARINE SCIENCE, VOL. 55. NO. 2-3, 1994 of the data (Alevizon et aI., 1985). Biomass estimates were not made on the newly settling fishes because of their small size. The small size of the benthic reefs did not afford adequate shelter for many larger predaceous fishes; moray eels and scorpaenids were often seen resting on the outer perimeter of the reefs, Hixon and Beets (1989) demonstrated that small- hole reefs maintain a greater number of small fishes than do larger-hole reefs. Despite inadequate cover for many of these piscivorous predators, they persisted at the experimental sites probably because of the presence of local shelter in an area otherwise near devoid of it as well as the presence of food resources in the form of settling fishes. Larvae of many Hawaiian fish species are particularly abundant in the spring; other peaks of abundance are in the summer and fall (Watson and Leis, 1974). Recruitment to the adult habitat occurs from spring through fall often in pulses (Walsh, 1984). The present experiment commenced in August terminated in March thus covering some of the period of usual high recruitment. The numbers of recruits encountered during a census was never great, but observations on the sizes of fishes present suggest that many of the larger fishes in the visual censuses had settled at the site at some previous time (between censuses) and had grown beyond the "new" recruit stage. Some of these larger fishes were similar in size indicating that they may have settled together suggesting that recruitment may be somewhat episodic. As these "cohorts" grew in size, their numbers dwindled probably due to predation and migration. Although not the focus of this study, we noted that mackeral scad (Decapterus macarellus) quickly took up "residence" around the MWFADs and remained in the vicinity of the experimental site for the duration of the 217-day study. Similar behavior by this species has been reported from other localities, e.g., Caribbean (Workman et aI., 1985; Beets, 1989). Since mackeral scad are planktivorous they may serve as predators on the recruiting juvenile fishes thus posing a dilemma: the MWFADs which appear to be important to the recruitment process are like- wise important in the attraction and probable retention of a predator species on the juveniles. Beets (1989) demonstrated that enhanced settlement of fishes could be achieved with MWFADs + benthic reef combinations over either MWFADs or benthic reefs alone. This study has made the same finding. The apparent enhancement of juvenile recruitment is probably nothing more than a focusing of the settlement of larval forms passing in the water column within visual range of the MWFAD. Whether these larvae would be successful in settling elsewhere is unknown. The ability to some extent dictate where settlement occurs provides a mechanism to enhance the survivorship of juveniles by putting these fishes in close proximity to appropriate shelter thus lowering predation. We call this focusing phenomenon due to the presence of midwater structures the "stairway from heaven" effect. Beets (1989) suggested that appropriately designed structures could be used to enhance recruitment but that in order to have a meaningful impact to the adult populations, shelter must be developed that will service fishes as they grow to adulthood. Our observations suggest that if coverage of the substrate with shelter is very large (i.e., greater than 1-2 m2), the potential for concurrent enhancement of demersal piscivorous predators may negate the positive aspects of enhanced juvenile recruitment. Perhaps reefs developed to enhance the recruitment process should be spatially removed from the habitat constructed for the later life stages. The use of MWFADs in conjunction with artificial reefs designed for adult fishes leads to the rapid elimination of most recruiting juveniles due to predation (Brock and Grace, 1987). Despite these problems, the use of tautline moored MWFADs BROCK AND KAM: FOCUSING JUVENILE FISI1 RECRUITMENT 629

in shallow water (>30 m) provides a relatively long-term (at least 14 months without maintenance), low-cost means of focusing the recruitment of juvenile fishes to specific benthic sites. With development of appropriately scaled benthic shelter such as used in this study, the possibility exists for improving the survivorship of these recruits. This technology may have useful application in the restoration of reef resources.

ACKNOWLEDGMENTS

Funds for this work were from the University of Hawaii Sea Grant Program (contract no. NA- 89AA-D-SG063). The authors wish to thank Dr. J. Brock and J. Naughton for assistance in the field as well as J. Norris for help in the early phases of this study. SOEST contribution no. 3598.

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DATE ACCEPTED: July 29, 1993.

ADDRESS: University of Hawaii Sea Grant Program, 1000 Pope Road, MSB 204, HonoLuLu, Hawaii 96822.