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E nvironm ental B iology of Fishes 49: 239–246, 1997.  1997 K luwer A cadem ic Publishers. Printed in the N etherlands. The natural history of a monogamous coral-reef fish, Valenciennea strigata (Gobiidae): 1. abundance, growth, survival and predation R obert H . R eavis M useum of Vertebrate Z oology and D epartm ent of Integrative B iology, University of California, B erk eley, CA 94720, U.S.A . Current address: D epartm ent of L ife Sciences, A riz ona State University West, 4701 W. T hunderbird R d., Phoenix, A Z 85069–7100, U.S.A . R eceived 27.7.1995 A ccepted 27.5.1996 Key words: population ecology, equilibrium/nonequilibrium processes, recruitment, monogamy, mating system, mate guarding Synopsis The population dynamics of a monogamous coral-reef fish were examined to test hypotheses of recruitment limitation, predation, and postrecruitment processes, and to determine their affects on the mating system. Valenciennea strigata are monogamous gobies that live in sand and rubble zones throughout the Indo-Pacific. Seasonal abundance was recorded in the summer and winter over 2.5 years. A subset of this population was tagged (n = 256) and followed to determine mortality and mobility. Valenciennea strigata were more abundant in summer than in winter, suggesting that a pulse of recruitment in the spring set the maximum population density. G rowth rates derived from tagged fish support the hypothesis that recruitment peaked in the spring. Tagged fish experienced 88% mortality within six months; the annual mortality rate approached 100% . E vidence of predation, antipredatory behavior and strong site fidelity implicate predation as the primary source of mortality. Competition for space was not observed between adults, but may affect settlement and recruitment. D espite the lack of adult competition for space, both sexes guarded their mates and courted individuals of the opposite sex. Thus, although population size appears to be determined by nonequilibrium processes, the mating system is affected by competition for mates. Successful mate guarding by both sexes enforced monogamy. Introduction E lton’s (1927) model of community ecology suggests that most communities are structured by equilibrium processes. A lthough this model has dominated ecological theory, recent evidence supports a nonequilibrium view of the natural world (reviewed by R eice 1994). Studies of coral-reef fishes have been instrumental in this change of perspective (e.g. the lottery hypothesis, Sale 1978; the recruitment-limitation hypothesis, D oherty 1983). These and other studies suggested that populations of coral-reef fishes are determined primarily by oceanic processes that affect recruitment, while equilibrium processes have little effect (D oherty 1991, Victor 1991). Similarly, predation can maintain populations at levels that minimize competition (H ixon 1991). A lthough the emerging paradigm suggests that coral-reef fish populations are limited by nonequi- 240 Figure 1. Study sites on Moorea Island. librium factors, equilibrium processes may still affect these fishes (Jones 1991). In particular, competition for mates can occur even at low densities, especially if resources are clumped, affecting both reproductive output and the mating system (E mlen & O ring 1977). Thus, studying the mating systems of coral-reef fishes should provide evidence useful in distinguishing postrecruitment processes. For example, monogamy in coral-reef fishes has been explained by both equilibrium and nonequilibrium models (reviewed by Barlow 1984, Barlow 1986). Butterflyfishes appear to be monogamous because the competition for coral prevents males from sequestering multiple mates (H ourigan 1987). Conversely, a tilefish in the R ed Sea exists at such low densities that mates remain together because encounters with potential mates are rare (Clark & Pohle 1992). Valenciennea strigata (Broussonet) is one of the fish whose behavior was reviewed by Barlow. Little additional information had been published on V. strigata or their relatives until the recent review of the genus by H oese & Larson (1994). They described 15 species, ecologically similar in their use of sand and rubble habitats. Monogamy appears common to the genus; conspecifics are typically observed in pairs sharing a burrow that they construct themselves (e.g. Barlow 1984, H oese & Larson 1994). H ere I examine the population dynamics of V. strigata to determine whether their population is influenced primarily by equilibrium or nonequilibrium factors. My companion paper (R eavis 1997) addresses the behavior and mating system of the same population. Together, these data suggest that the population is limited by nonequilibrium processes, and that the low population density allows both sexes to successfully guard a monogamous mate. 241 Methods I followed members of a population of V. strigata over 2.5 years to determine the changes in population and the potential factors that influence these changes. Individually marked fish were observed daily to determine their survival and movement. R ecaptures of marked fish provided growth data. Finally, I observed the burrows of V. strigata, their anti-predator function, and territorial interactions. R esearch was conducted along the north shore of Moorea, Society Islands, French Polynesia. A ll work was done using snorkel gear over the course of five seasons: 27 D ecember 1991–17 A pril 1992; 24 June–15 August 1992; 20 January–18 A pril 1993; 23 June–6 July 1993, and 11–20 January 1994, for a total of 1046 research hours in the water. Work was concentrated at three sites: the Bali H ai (BH ), in the shallow backreef offshore of the H otel Bali H ai; the Moorea Lagon (ML), an area of shallow fringing reef that runs continuously into the back reef, beginning 100 m offshore and just east of the H otel Moorea Lagon, and the West Backreef (WB), a deeper backreef area 150–200 m seaward of the channel west of Cook’s Bay (Figure 1). These sites are similar in size: BH = 13 500 m 2, ML = 13 400 m 2, and WB = 14 800 m 2. They varied in depth between 1.5–4 m over a predominantly sand and rubble bottom with scattered coral heads. Sites were marked with floating bottles, and weighted plastic flags were placed near at least one burrow per territory. I measured the distance between these flags to construct maps of the study sites. The abundance of V. strigata at each site was recorded by swimming over the site in tandem with a dive buddy in 15-wide strips (after H elfman 1983). A ll V. strigata were counted and identified as adults or juveniles, and single or paired. The accuracy of these surveys was immediately verified by a search for tagged fish. A ll abundance surveys agreed exactly with the tagged fish surveys. A bundance was measured in the summer and winter of 1992 and 1993, and the summer of 1994. H owever, in the first season (summer 1992) abundance was not measured until early fall. Because the population declined from summer to fall, I have substituted data taken from daily surveys made earlier in the sum- mer. These surveys focused on tagged fish and probably underestimated the actual population size. A t the start of each of the first two seasons, and opportunistically thereafter, I tagged most of the V. strigata at each of the study sites. In the summer of 1993 tagging was concentrated only in certain areas of the study sites because of the high density of fish that season. Fish were captured by chasing them into their burrows, covering the entrances with nets, and then injecting a solution of quinaldine into the burrows. Most fish swam into the nets immediately after the quinaldine was applied; others became anesthetized and were collected by digging into the burrow. Pairs often went into a burrow together, and both were usually captured on the same day. Captured fish were placed into a plastic bag for handling and brought to a boat where I sexed them by genital papilla (214 of 256 tagged fish), measured their standard length (SL) to the nearest mm, and tagged them. Male genital papillae taper sharply from the base and become pointed at the distal end; female genital papillae are broad throughout (this sexual pattern is common in gobies, Miller 1984). Sex was later verified by behavior. I arbitrarily remeasured some fish at the original time of capture; all measurements agreed within 0–3 mm. A t each study site, individuals received a unique combination of colored dyes injected under the skin. These marks remained clear throughout the study. Finally, each fish was scored as either mated or single. Pairs seen or captured together were assumed to be mates. To determine growth rates I recaptured and remeasured 22 tagged fish. The longest lived female was remeasured in three seasons. The analysis of growth rates presented here treats each growth interval for this female as a separate datum, providing a total of 24 growth rates. A lternative analyses were not significantly different. I surveyed all sites daily to relocate each tagged fish during the summer and winter of 1992 and summer 1993 (27–89 days per season). Natural history and behavior were recorded opportunistically throughout the course of the study (for behavioral data see R eavis 1997). I also observed the burrows created by V. strigata, and noted evidence of preda- 242 Figure 2. Mean summer abundance of V. strigata is greater than mean winter abundance (one-way A NOVA , p < 0.05; n = 3 study sites, error bars = 1 SE ). * Summer 1992 surveys were conducted late in the season and underestimate mid-summer abundance. Figure 3. Survival of tagged V. strigata. Starred bars represent the number of fish tagged that season. Plain bars represent the number of tagged fish of that cohort resighted in subsequent surveys. # No tagged fish were found in January 1994. tion. Finally, I determined the diel pattern of activity from paired dusk-dawn observations at each site in the summer and winter of 1992. differ by sex (A NOVA , p > 0.10). H owever, pairs were positively assorted for size (simple regression, r 2 = 0.67, p < 0.01, n = 91), and males were 5.0 mm longer than their mates (paired t-test, p < 0.01, n = 91). D uring daily surveys, I usually found tagged fish on their original territories and with the same mate (for mate fidelity see R eavis 1997). Fish that changed territories over the course of the study typically moved to a neighboring territory after the loss of a mate. The longest recorded movements were 80 m (3 of 256 fish). H owever, the majority of tagged fish disappeared in the same season that they were tagged (Figure 3). O nly 31 (12.1% ) survived from one season to the next (5–7 months); two of these females survived at least one year, and one was seen 17 months after tagging. No tagged fish could be found in the summer of 1994, 24 months after the start of tagging. Survivorship did not vary by sex; 15 of 106 males and 13 of 108 females survived until a second season (chi-square, p > 0.50). A lthough some fish disappeared immediately after tagging, tags did not generally affect survival. O f the 245 fish counted in the summer 1993 surveys, 57 (23.2% ) had been tagged. E ight tagged fish survived to the winter survey, out of a total of 42 fish in that survey (19% ; chi-square, p > 0.50). G rowth rates decelerated with increasing size (Figure 4). From these data it could not be determined whether this change in rate was linear (sim- Results and discussion Population dynam ics Fish abundance did not differ among the three study sites over the course of the study (A NOVA , p > 0.10). A cross all sites, V. strigata were 4.7 times more abundant in summer than winter (A NOVA , p < 0.05, Figure 2). This difference was primarily due to the change in mean (± SD ) adult abundance between seasons: summer = 68.6 (± 43.9) adults per site, winter = 11.5 (± 7.1) adults per site (A NOVA , p < 0.05). Juvenile abundance did not differ significantly between seasons: summer = 8.9 (± 15.0) juveniles per site, winter = 5.0 (± 7.6) juveniles per site (A NOVA , p > 0.50). A dults occurred only in pairs or as singletons, except during agonistic interactions. In the summer, 10.8% of fish were singletons; 19.2% were alone in the winter (chi-square, p < 0.05). The mean (± SD ) SL of all tagged fish = 110.0 mm (± 25.4, range: 30–152, n = 256). O f these, 214 were of known sex. Mean male SL = 118.4 mm (± 15.4; range: 70–152, n = 106); mean female SL = 114.8 mm (± 17.1, range: 59–144, n = 108). O verall, size did not 243 Figure 4. G rowth rates of V. strigata decelerate with increasing body size (polynomial regression, r 2 = 0.387, p < 0.05; n = 24). ple regression, r 2 = 0.365, p < 0.01) or exponential (polynomial regression, r 2 = 0.387, p < 0.01). The largest growth rates occurred in small adults over short intervals. These small but reproductively active individuals grew up to 26 mm in 45 days, an increase of 25% . Males grew at a mean rate of 0.16 (± 0.15) mm per day (n = 12); females grew at a mean rate of 0.11 (± 0.09) mm per day (n = 12; A NO VA , p > 0.10). A bundance and survival data suggest that V. strigata recruit primarily in the spring with few individuals surviving a full year. A lternatively, higher summer abundance could be due to adult immigration into study sites in the spring and emigration from study sites in the fall. D ata from the tagged fish suggest that emigration from study sites was unlikely. Most fish remained on their original territories, and none were found greater than 80 m from where they were tagged, despite my searches over the entire study sites (13 400–14 800 m 2). Thus, fish that disappeared probably died. Similarly, the site fidelity of tagged fish suggests that increased summer densities were due to the settlement of juveniles, not immigration of untagged adults. A lthough V. strigata were not observed in the spring, recruitment of other fishes in Moorea peaks in spring and summer (Planes et al. 1993). A dditionally, size at settlement and growth data support the hypothesis that most of the summer population recruited in the spring. The smallest V. strigata tagged was 30 mm SL and the smallest untagged fish were estimated to be 25 mm SL. Thus, settlement occurs at about 25 mm SL. This size at settlement is large for gobies, although larger presettlement juveniles have been recorded in this family (Leis & R ennis 1983). If recruitment in the spring accounts for most of the summer population, young V. strigata grow about 75 mm in approximately 100 days (0.75 mm per day) to reach the 100 mm SL commonly seen in January. This rate is consistent with the growth curves derived here. The predicted growth rate of a 25 mm SL fish is either 0.62 mm per day (linear model) or 1.31 mm per day (exponential model). These growth rates vary by a factor of two and come from measurements of adults, therefore they provide only a rough estimate of juvenile growth rates. H owever, most fishes do experience increasing growth rates (exponential model) prior to reaching reproductive age (R icker 1975), and one reproductive male (102 mm SL) grew at a rate of 0.58 mm per day. Similarly, Thresher (1983) found growth rates approaching 0.70 mm per day in juvenile A canthochrom is polyacanthus, a monogamous damselfish. Thus, it seems reasonable that juveniles recruited in the spring could reach 100 mm SL by the end of January when most fish were tagged. Mortality did not differ by sex in V. strigata. This pattern is common in monogamous coral-reef fishes (e.g. A . polyacanthus, Thresher 1983; Paragobiodon echinocephalus, Kuwamura et al. 1994), and other monogamous vertebrates (e.g. Promislow 1992). By contrast, polygamy is associated with differential survival between the sexes (Promislow 1992). Predation and territoriality Potential predators were common. Muraenids, serranids and octopuses were seen in and around the burrows of V. strigata. Those burrows were avoided by V. strigata. Several burrows appeared dug up by octopuses. The resident V. strigata were usually missing on days when burrows were found opened, and some never returned. I only witnessed one act of predation, when I chased a male off his territory and he was swallowed by a serranid (E pinephelus m erra) stationed beneath a coral head. O ther fish were 244 seen with wounds on their bodies suggesting predation attempts. Burrows were dug under coral pavement and rubble. They were used as both a refuge and a nest site. Fish maintained several burrows per territory (range: 4–10) and remained in close proximity to a burrow. Burrows typically had two or more entrances, but only one remained open. The other entrance was covered by coral rubble, sand and algae. When capturing fish in their burrows, they were as likely to go out a closed entrance as an open entrance. Many burrows were maintained over multiple seasons, by either the original or subsequent inhabitants. D espite this investment in burrows, interactions between territory residents and intruders appeared to be over mates, not territories. Fish of both sexes attacked intruders of the same sex, but courted intruders of the opposite sex. Juveniles also maintained burrows and defended mates. H owever, in early summer 1993, when fish density was the highest recorded, multiple fish were found in close proximity at one site. I observed many agonistic bouts at this time with associated changes in mates and burrow use. Territories were not defended interspecifically. O ther fishes used V. strigata burrows, particularly small gobies (e.g. G natholepis anjerensis) and juveniles of other families (e.g. chaetodontids, labroids, and mullids). Valenciennea strigata rarely reacted to those fishes, except to chase them briefly. Many grazers and browsers fed in V. strigata territories, as well as goatfishes that probed within the substrate. Valenciennea strigata were diurnally active. Pairs returned to their burrows at least one hour before dark and typically closed the entrance behind them with algae (see also H iatt & Strasburg 1960). Most diurnal fishes were still active at this time (e.g. cleaner wrasses, L abroides spp). Pairs emerged from their burrows more than an hour after sunrise, when other fishes were already active. They were never seen out of their burrows at night. A lthough measuring predation is difficult, predation may have strong effects on populations of coral-reef fishes (reviewed by H ixon 1991). Sweatman (1984) estimated that a single species of lizardfish caused 65% annual mortality in a prey population. Thresher (1983) found 20–75% mortality in juve- nile A . polyacanthus in their first 30 days, and adult mortality was higher in the presence of a serranid. Predation probably accounts for the significant drop in abundance of V. strigata from summer to winter, and the high rate of loss in tagged fish throughout the year. A lternatively, fish may have died due to age or parasites, or emigrated from study sites. H owever, some fish lived over a year, there were no indications of parasites externally or internally (R eavis unpublished data) and tagged fish showed strong site fidelity. These data do not support the alternative hypotheses. A dditionally, predators and evidence of predation attempts were common (e.g. wounded fish, opened burrows). Besides the predators I observed, R . Caldwell (personal communication) witnessed a mantis shrimp, Lysiosquilla sulcata, capture V. strigata near my study sites. A t least two species of piscivorous mantis shrimp co-occur with V. strigata in Moorea and regularly prey on gobioid fishes (R . Caldwell personal communication). Valenciennea strigata also exhibited several forms of anti-predator behavior. They maintained multiple burrows and generally remained in close proximity to a burrow (quantified by R eavis 1997). The extra burrow entrances provided an escape route from predators. A lthough these additional entrances could also facilitate water movement through burrows for aerating eggs, all burrows had these extra entrances, but only one burrow was used at any time for egg care. The diel-activity pattern of V. strigata also suggests predator avoidance; they limited their twilight exposure to predators more than other fishes (e.g. wrasses, H elfman 1993). Finally, their strong site fidelity may be selected for by predators. R emaining on a known territory ensures knowledge of the location of refugia (i.e. burrows, H inde 1956). E quilibrium or nonequilibrium ? effects on the m ating system Juvenile settlement and recruitment were not addressed directly in this study; however, competition between juveniles was observed in the early summer of 1993. Thus, competition for space may affect recruitment in V. strigata. Predation apparently re- 245 duces adult competition over resources. The dramatic drop in seasonal abundance of V. strigata suggests that equilibrium processes on the population are negligible in the fall and winter. E ven in the summer, all adults maintained territories with multiple burrows. Burrows provide two resources that limit other populations of fishes: nest sites (Breder & R osen 1966) and refugia (Smith & Tyler 1975, Shulman 1984). The abundance of burrows suggests they are not limiting for V. strigata during most of the year. A dditionally, many territories were bordered by unused habitat indistinguishable from territories (R eavis & Barlow unpublished data). Finally, territory intrusions occurred infrequently, and these intrusions appeared to be over mates rather than resources (see also R eavis 1997). E mlen & O ring’s (1977) model of mating systems suggested that the distribution of limiting resources determines the distribution of females, and affects the ability of males to encounter and monopolize females. For V. strigata, however, resources appeared to be abundant, and these fish were broadly distributed across my study sites. Further, the high mortality and high fecundity (R eavis 1997) in these fish suggest that they may be relatively r-selected. Thus, E mlen & O ring’s (p. 215, 1977) model may not be applicable. Predation also affects the distribution of animals and their mating systems (e.g. Jarman 1974). H ere, predation probably selects for site fidelity, and limits the potential pool of mates to those within a radius of 80 m. A low probability of finding potential mates can select for strong pair bonds and monogamy (e.g. Clark & Pohle 1992). A lthough many pairs of V. strigata persisted over a study season, intruders of the opposite sex were courted (see also R eavis 1997). The relatively weak pair bonds exhibited suggest that a lack of potential mates does not explain monogamy in these fish. Proximally, both sexes enforced monogamy by mate guarding. Mate guarding should be selectively advantageous when the costs are low relative to the benefits. Mate-guarding costs for V. strigata appear to be relatively low. The adult stock was determined by nonequilibrium factors, suggesting that resources are abundant and not monopolizable by a few males. In particular for gobioid fishes, an abun- dance of burrows allows all males to defend a nest site (Barlow 1986). If all males can breed, the cost of mate guarding for females is reduced, and a guarding female becomes herself easily guarded because of her proximity to the male. A ll male V. strigata in this study maintained burrows (i.e. nest sites). Moreover, R eavis (1997) found that both sexes prefer big mates and that big mates may provide greater benefits. Thus, monogamy in V. strigata appears to be affected by both the nonequilibrium processes that limit the population, the resulting opportunities for mate guarding by both sexes, and competition for large mates. Because many populations of coralreef fishes appear to be recruit-limited, similar examples of monogamy may be found in these fishes. A cknowledgements I am indebted to Leilani E . Wright for her enthusiasm and hard work in the field, including hundreds of hours watching V. strigata. E van R aymond also participated in much of the water work. Thanks to G eorge W. Barlow who first introduced me to this problem and provided advice and encouragement along the way. I am also grateful to R . Caldwell, V. R esh, R . Swenson, C. St. Mary, R . Coleman and two anonymous reviewers for their helpful comments made on earlier drafts of this manuscript. This work was supported by an NSF Fellowship, two Sigma Xi grants, a Lerner-G ray grant, and the Museum of Vertebrate Z oology and D epartment of Integrative Biology, U niversity of California, Berkeley. The R ichard G ump South Pacific Biological R esearch Station provided additional support and assistance; thanks to the director W. Loher, managers R . Steger and F. Murphy, and to the many others I had the pleasure to work with, particularly M. G leason. This is contribution number 33 of the G ump Biological R esearch Station. Thanks to the government and people of French Polynesia for their gracious hospitality. A lec and A nne Langbridge, and John and Tehea McInnis provided occasional living space and diversion from field work, Mahu’uru’uru R oa! 246 References cited Barlow, G .W. 1984. Patterns of monogamy among teleost fishes. A rch. FischWiss. 35: 75–123. Barlow, G .W. 1986. A comparison of monogamy among freshwater and coral-reef fishes. pp. 767–775. In: Second Int. 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