Blackeye Goby Research Articles

Experimental evidence of resilience to size-selective harvesting in a protogynous hermaphroditic reef fish, Rhinogobiops nicholsii

Most fishing methods are inherently size-selective, and many fisheries have minimum size limits, resulting in removal of the largest, most fecund size-classes of harvested species. Some modeling and correlative studies suggest protogynous (female-to-male sex changing) fishes may be particularly strongly impacted by removal of large individuals, but controlled experiments are necessary to test this expectation. By manipulating the density and size structure of populations of the blackeye goby (Rhinogobiops nicholsii) in the field via size-selective removals, this experiment aimed to identify the effects of size-selective harvesting on the (1) growth, (2) maturation, (3) sex change, and (4) reproductive output of a protogynous hermaphroditic reef fish that uses exogenous cues for sex change. Removals of ~ 25% of fish established in populations on standardized artificial reefs simulated a pulse of fishing mortality with minimum size limits, slot size limits, and maximum size limits, which are used in some fisheries. Size-selective removals had no statistically significant effects on the growth, maturation, sex change, or reproduction in blackeye gobies, indicating that this protogynous species is resilient to moderately intense size-selective removals. Some trends consistent with predicted effects of size-selective removals were present, but we had low power to detect some effects. Specifically, the rate of maturation was highest in the slot size limit treatment, and the rate of sex change was highest where large fish had been removed. Regardless of simulated fishing type, populations in all treatments exhibited similar size- and sex-structure by the end of the experiment, nearly two months after population structure had been manipulated. These results suggest that sequentially hermaphroditic fishes with exogenous cues for sex change may be able to quickly replace individuals with high reproductive value after experiencing moderately intense size-selective harvesting. However, experiments incorporating additional sex-changing species, particularly with different modes of reproduction and cues for sex change, as well as higher fishing intensities or longer durations of fishing are still necessary to reconcile differences in results reported herein and those from modeling and correlative studies.

Environmental Factors Influencing Reproduction in a Temperate Marine Reef Goby,Rhinogobiops nicholsii, and Associated Behaviors

Abstract The blackeye goby is a protogynous reef fish common to the northeastern Pacific Ocean. While this ubiquitous species has been the focus of numerous studies, there are several aspects of its reproductive ecology that are unknown. By directly quantifying reproduction from digital photographs of blackeye goby nests in the field, this study aimed to determine whether reproductive patterns were linked to 1) lunar phase or 2) ambient water temperature; and 3) whether the behavior of gobies changed when a nearby conspecific had eggs in his nest. At Santa Catalina Island, California, twenty 2.25-m2 artificial reefs were established and stocked with similar numbers and size-distributions of blackeye gobies during the summers of 2012 and 2013. Photographs of nests were taken weekly for ∼3 months each summer. Through analysis of photographs, incubation time was found to be more than 7 days but less than 14 days. Nests, each guarded by one male, contained an average of 8664 eggs, in an area of 43.8 cm2, with...

Sex differences in the energetic content of reproductive tissues in the Blackeye Goby, Rhinogobiops nicholsii

Differential energetic investment in reproduction between the sexes has been a driving a force of life history theory and sexual selection. However, reproductive costs between the sexes have often been based on morphology, such as gonad mass and gonadosomatic indices (GSI), and few have directly measured the energy content of gonadal tissues in relation to GSI. Using the blackeye goby, Rhinogobiops nicholsii, we measured the energetic content of whole gonadal tissues, specifically testes, ovaries and associated reproductive tissues using oxygen bomb calorimetry. The energy content per gram unit of gonadal tissues was generally predictive of GSI, indicating that GSI is a reasonable measure of energetic costs. Interestingly, although females had greater gonadal mass, GSI and energy content per gram than males, the sex difference in energy content per mass unit was only 13 %, suggesting that gross indices such as gonadal mass or GSI may overestimate energetic costs where instead the cost difference in a unit gram of gonadal tissues between the sexes is smaller than often predicted. This study also demonstrates that although the cost of ovaries is greater than testes, males’ investment in reproductive tissue can be considerable, which is consistent with the often inflated reproductive success for males in haremic mating systems.

Abundance, size and habitat relation of reef fish on biogenic structures (structure-forming invertebrates) at Anacapa Island, southern California

A biogenic structure ecosystem of extensive worm tubes of Chaetopterus sp. harbouring different fish species was investigated at Anacapa Island, southern California to determine the numerical abundance, species composition, body size, and seasonality of the ichthyofauna associated, and their relation with the worm patch size. Bimonthly, a 30-m swimming transect and a quadrat-based survey estimated the length of each fish, the type of habitat it occupied, and the worm tube patch size-class. Larger worm mats harboured higher densities of fish, mainly the dominants Rhinogobiops nicholsii and Chaenopsis alepidota, which live inside the worm tube patches. Fish population density varied between the ‘inside’ and ‘edge’ of Chaetopterus beds as smaller and younger individuals were hindered from reaching the middle of the patch by larger, older and territorial individuals. The prevalence of positive and significant correlations between the abundance of smaller individuals of R. nicholsii and specific habitat features (e.g. edge) suggests that the abundance and distribution of juveniles might be habitat-dependent besides intraspecific competition between older and younger individuals. There was a decreasing density of R. nicholsii at greater than 20 cm away from worm tubes and there were no individuals beyond 6 m away from structure. Chaenopsis alepidota was not recorded when the patch size was less than 50% tube coverage. Our results revealed that complex habitat structure had a positive influence on the abundance of juvenile and adult of C. alepidota and R. nicholsii, and suggest that these structured areas are preferentially utilized, which contributes to its patchy distribution pattern. The presence of biological structures in low-relief sedimentary habitats can have critical functional significance even for reef fish. These important habitat features may need to be identified and protected.

Interactive effects of nutritional condition and refuge availability on survival of a temperate reef goby

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 407:257-269 (2010) - DOI: https://doi.org/10.3354/meps08562 Interactive effects of nutritional condition and refuge availability on survival of a temperate reef goby Emily Y. Floyd1,2,3,*, Todd W. Anderson1 1Department of Biology and Coastal and Marine Institute, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA 2Graduate Group in Ecology, 2148 Wickson Hall, University of California, Davis, California 95616, USA 3Present address: ENTRIX Inc., 2300 Clayton Road, Suite 200, Concord, California 94520, USA *Email: emfloyd@yahoo.com ABSTRACT: Variation in individual performance and condition of marine organisms can influence larval supply and the outcome of post-settlement processes. The broad applicability of these patterns among systems and under different environmental conditions, however, remains relatively unknown. We determined whether nutritional condition would affect the survival of recently settled recruits of a temperate reef fish, the blackeye goby, and whether refuge availability would modify the importance of condition. Recruits were fed high or low food rations in the laboratory for 15 d, after which pairs of high- and low-ration fish were marked and placed on standardized plots of rock rubble in the field. Plots were monitored daily to determine mortality over 12 d. To examine the combined effects of condition and refuge availability, pairs of fish were placed on plots of high, medium, or low amounts of refuge. Laboratory experiments examined the effect of condition on burst swimming speed to elucidate the mechanisms underlying the observed patterns of survival. On standardized plots, fish fed low rations experienced higher mortality than fish fed high rations. When refuge was manipulated, low-ration fish experienced higher mortality than high-ration fish at higher levels of refuge. Surprisingly, higher mortality was observed for high-ration fish on low-refuge plots, possibly due to differences in risk-prone behavior. A reduction in the overall probability of mortality resulted in an unanticipated dampening of the importance of condition for survival. Our results indicate that the ecology of individuals, mediated by environmental conditions, is an important constituent in the population demography of reef fishes. KEY WORDS: Fishes · Energy reserves · Lipid · Habitat · Population demography · Rocky reefs Full text in pdf format PreviousNextCite this article as: Floyd EY, Anderson TW (2010) Interactive effects of nutritional condition and refuge availability on survival of a temperate reef goby. Mar Ecol Prog Ser 407:257-269. https://doi.org/10.3354/meps08562 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 407. Online publication date: May 20, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Demersal fish and habitat associations from visual surveys on the central California shelf

In 2004, we surveyed demersal fishes and habitats on the continental shelf off central California (65–110 m depth) using the occupied submersible Delta. Our objectives were to estimate the relative abundance of habitats and to examine demersal fish species composition, diversity, density, and sizes relative to these habitats. A total of 112 transects were completed covering 32 km of seafloor. A higher density of fishes was estimated in boulder and cobble habitats than in mud and brachiopod beds. More than 80% of the fishes were small, measuring 20 cm or less in total length. Species with small maximum size (primarily pygmy rockfish, Sebastes wilsoni, and blackeye gobies, Rhinogobiops nicholsii) accounted for nearly half (49%) of the total number of 12,441 fishes. Most fishes were immature, with only 4 of 20 harvested species having more than 50% of the individuals larger than the size at first maturity. Our study area on the continental shelf may be an ontogenetic transition zone for immature fishes before they move to their adult habitat on the slope. Alternatively, historical fishing pressure may have contributed to the lack of large, mature fishes in the survey area. Understanding the importance of these habitats to fishes at various life stages will improve our ability to assess these deepwater fish stocks and effectively manage these living resources on an ecosystem basis.

Demersal fish assemblages in the Southern California Bight based on visual surveys in deep water

Between 1995 and 2006, manned submers- ible fish surveys were conducted on the sea floor throughout the Southern California Bight. A total of 401 dives (comprising 1,015 transects and 14,373 habitat patches) were made in waters between 19 and 365 m deep. All natural habitat types were included, although both soft sea floors and rocky reefs were surveyed more than any other type. A total of 717,526 fishes, representing a minimum of 137 species and 47 families, were observed. Rockfishes (genus Sebastes), with a minimum of 50 species and 647,495 individ- uals (90.2% of all fishes observed), dominated most of the habitats. The most abundant species, squarespot (Sebastes hopkinsi), halfbanded (Sebastes semicinc- tus), shortbelly (Sebastes jordani), and pygmy rock- fishes (Sebastes wilsoni), are dwarf taxa that either school or aggregate. The most abundant non-rockfish species was the benthic and territorial blackeye goby (Rhinogobiops nicholsii). Both species richness and overall fish densities were highest in the shallowest sites. Most of the fishes in all habitats were small (≤20 cm TL long) and economically important species were generally uncommon. Forty-four species were found to be characteristic of the study area (occurring in at least 5% of the transects) and these species formed three faunal associations centered around depths of 62, 105, and 168 m. Based on size frequency distributions, at least 18 of the characteristic species exhibited ontogenetic movements, with young-of-the-year and older juveniles living in relatively shallow waters and larger individuals generally in deeper depths. In this study, the abundance of juvenile widow rockfish (Sebastes entomelas), and the virtual absence of adults, in southern California waters may demonstrate an ontogenetic northward movement of this species. This research implies that substantial harvesting of larger species by commercial and recreational fishers has helped alter some fish assemblages, allowing small and "weedy" species to thrive.

The Reef Fish Assemblage of the Outer Los Angeles Federal Breakwater, 2002–2003

The conspicuous and cryptic fish assemblage of the Los Angeles Federal Breakwater was assessed from 2002 to 2003. Thirty-five species were observed or collected during the study period. The assemblage of cryptic fishes was composed primarily of a mix of Oregonian and San Diegan, species including snubnose sculpin (Orthonopias triacis), coralline sculpin (Artedius corallinus) and blackeye goby (Rhinogobiops nicholsii). The species composition of conspicuous fishes was approximately equal between taxa from these two provinces. Blacksmith (Chromis punctipinnis), black perch (Embiotoca jacksoni) and kelp bass (Paralabrax clathratus) dominated the assemblage of conspicuous fishes. Species composition reflects the localized cool temperature regime of the area and the high relief kelp forest habitat.

Habitat-dependent recruitment of two temperate reef fishes at multiple spatial scales

The distribution and abundance of reef fishes has often been attributed to several pro- cesses that result in some measure of recruitment success (settlement and survival of young individ- uals). We employed a large-scale experimental rocky reef system to examine patterns of recruitment of 2 fishes, the California sheephead Semicossyphus pulcher and the blackeye goby Rhinogobiops nicholsii. We quantified recruitment over a 2 yr period on 21 experimental reefs (each 1600 m 2 ) that represented low, medium, and high treatments of habitat coverage. Recruitment of California sheep- head was higher on reefs of medium habitat coverage than on reefs with other coverages, while the blackeye goby exhibited lower recruitment on reefs of low coverage than on reefs with other cover- ages. Within reefs, recruitment to 'edge' habitat was lower than 'inside' the reef for each species. At the smallest scale, several measures of habitat structure were quantified within 1 m 2 quadrats to iden- tify microhabitat characteristics that might explain the distribution of recruits. Rugosity was an important characteristic for predicting the presence of recruits for each species at this small scale. The densities of recruits of California sheephead among treatments of habitat corresponded to the densi- ties of Age 1+ (individuals which recruited during the previous year) california sheephead among treatments the following year, suggesting that spatial patterns of abundance may be established early in life. The abundance of predators did not appear to influence the patterns of recruitment observed for either species. Low densities of recruits for each species may have led to the patterns of recruitment observed through the processes of habitat selection at settlement and density- independent mortality. Longer temporal studies covering variable recruitment are needed to determine the importance of habitat structure relative to other processes.

EARLY POSTSETTLEMENT PREDATION ON THREE REEF FISHES: EFFECTS ON SPATIAL PATTERNS OF RECRUITMENT

Marine organisms suffer extensive mortality just after settling from the plankton, yet, little is known about the role that predators play in causing this mortality. We estimated the rates of predation in the first 24 h, and the first week, after settlement in three species of reef fish. To estimate these rates we compared the accumulation of recent settlers on plots from which predators were excluded (by caging) to settlement on unmanipulated control plots. The magnitude of predation varied greatly among our three focal species, even though they are ecologically similar (all are small gobies that inhabit the reef/sand interface). Within 24 h of settlement, predators apparently killed 92% of settled blackeye gobies, 26% of bridled gobies, and 6% of goldspot gobies. Within a week, predation had significantly reduced recruitment of all three species and was the main cause of death during this period. We also tested whether predation on new settlers affected spatial patterns of abundance at two scales. At sm...

Variation in the relative importance of sublethal effects of predators and competitors on growth of a temperate reef fish

Sublethal effects of predators on the growth of their prey have been little studied in marine systems and consequently their importance relative to other factors is poorly known. Using a set of field experiments conducted on small artificial patch reefs, we measured the relative impor- tance of the effects of predators and conspecific competitors on the growth of a temperate reef fish, the blackeye goby Coryphopterus nicholsii, and we measured how the importance of the 2 processes varied over time. We found that predators and competitors reduced the growth of the gobies at some times, but not others, and that the relative importance of the 2 processes varied among time periods. Moreover, the importance of the 2 processes varied as a function of body size, with large, fast-grow- ing individuals suffering relatively more from the effects of predators than small, slow-growing indi- viduals, which felt the effects of competition more keenly. Competition increased the variation (CV) in growth rates within populations, but exposure to predators had no effect on growth variability. The strength of both the competitive and predatory effects on growth rates declined from summer to win- ter, but the effect of competition on growth variability did not change appreciably over time. The decline in the effects of competitors and predators on growth rates corresponded to predictable seasonal declines in water temperature and predator abundance. The relative importance of the sublethal effects of predators was greatest during the summer, when predators were most abundant. This study highlights the potential importance of sublethal effects of predators in marine systems and it suggests that the relative importance of predatory and competitive effects may vary in predictable seasonal and ontogenetic ways.

VARIATION IN THE PRESENCE AND CAUSE OF DENSITY-DEPENDENT MORTALITY IN THREE SPECIES OF REEF FISHES

Determining the mechanisms by which natural populations are regulated is a key issue in ecology. Identifying the biological causes of density dependence has, however, proved difficult in many systems. In this study we tested whether adults of three species of reef fish (all gobies) suffered density-dependent mortality, and whether the density-dependent component of mortality was caused by predation. We used field experiments to test for density dependence in each prey species, manipulating the presence of predators and prey density in a factorial design. Prey were stocked on replicate patches of reef constructed of natural materials, with each reef receiving a different density of gobies. Predatory fishes were excluded from half of the reefs using a combination of removals and exclusion cages. Survival of the first species, Lythrypnus dalli, was high and density-independent on reefs free of predators, but declined rapidly with increasing density on reefs to which predators had access. Density dependence in L. dalli was thus a result of mortality inflicted by predatory fishes. In the second species, Coryphopterus nicholsii, predators caused a large reduction in the survival in one experiment but had a negligible effect in a second experiment. More importantly, though, survival of C. nicholsii was always independent of its density, regardless of predator presence. In the final species, Coryphopterus glaucofraenum, two separate experiments showed that natural changes in adult abundance (experiment 1) and survival of stocked adults (experiment 2) were density-dependent regardless of predator presence. Both experiments thus indicated that the density-dependent component of loss in C. glaucofraenum was caused, at least in part, by an interaction other than predation. The presence, intensity, and biological cause of density-dependent survival were thus strikingly different for each of these three fishes, despite the fact that they are taxonomically closely related and ecologically similar. These findings suggest a need for further studies aimed at predicting under what circumstances different mechanisms of population regulation will operate.

Variation in the Presence and Cause of Density-Dependent Mortality in Three Species of Reef Fishes

Determining the mechanisms by which natural populations are regulated is a key issue in ecology. Identifying the biological causes of density dependence has, however, proved difficult in many systems. In this study we tested whether adults of three species of reef fish (all gobies) suffered density-dependent mortality, and whether the density-dependent component of mortality was caused by predation. We used field experiments to test for density dependence in each prey species, manipulating the presence of predators and prey density in a factorial design. Prey were stocked on replicate patches of reef constructed of natural materials, with each reef receiving a different density of gobies. Predatory fishes were excluded from half of the reefs using a combination of removals and exclusion cages. Survival of the first species, Lythrypnus dalli, was high and density-independent on reefs free of predators, but declined rapidly with increasing density on reefs to which predators had access. Density dependence in L. dalli was thus a result of mortality inflicted by predatory fishes. In the second species, Coryphopterus nicholsii, predators caused a large reduction in the survival in one experiment but had a negligible effect in a second experiment. More importantly, though, survival of C. nicholsii was always independent of its density, regardless of predator presence. In the final species, Coryphopterus glaucofraenum, two separate experiments showed that natural changes in adult abundance (experiment 1) and survival of stocked adults (experiment 2) were density-dependent regardless of predator presence. Both experiments thus indicated that the density-dependent component of loss in C. glaucofraenum was caused, at least in part, by an interaction other than predation. The presence, intensity, and biological cause of density-dependent survival were thus strikingly different for each of these three fishes, despite the fact that they are taxonomically closely related and ecologically similar. These findings suggest a need for further studies aimed at predicting under what circumstances different mechanisms of population regulation will operate.

The Role of Steroid Hormones in Protogynous Sex Change in the Blackeye Goby, Coryphopterus nicholsii (Teleostei: Gobiidae)

To detect and describe correlations between steroid concentrations and sexual function in the protogynous Coryphopterus nicholsii, we examined whole-body concentrations of the gonadal steroids testosterone (T), 11-ketotestosterone (11-KT), and 17β-estradiol (17β-E2). Our results show that T, 11-KT, and 17β-E2 are naturally occurring steroids in both female and male C. nicholsii. Further, mean whole-body concentrations of T and 11-KT in males were approximately twice those in females, whereas mean whole-body concentration of 17β-E2 in females was almost twice that in males. Second, we tested hypotheses on the causation of sex change arising from these correlations between steroid concentrations and sexual function. We examined the effects of administration of 11-KT, 11-KA, 17α-methyltestosterone, and an aromatase inhibitor (Fadrozole) on gonadal structures of mature females. Complete sex change was induced by 11-KT, 11-KA, and Fadrozole. Our results strongly suggest that an increase in 11-KT concentration and perhaps also a decrease in 17β-E2 concentration are involved in mediating natural sex change in C. nicholsii.

Social Organisation and Competition for Refuges and Nest Sites in Coryphopterus nicholsii (Gobiidae), a Temperate Protogynous Reef Fish

The ability of larger males to control resources and monopolise spawnings is thought to explain the benefits of protogyny in some animals. Here we describe the breeding and non-breeding social organisations of a temperate protogynous goby, Coryphopterus nicholsii, in the light of this prediction. Our results confirm that C. nicholsii has a stable social organisation, based on the year-round defence of a territory by both females and males. As predicted, larger individuals of both sexes were more likely to defend territories than smaller ones. These territories always included one or more shelter rocks, which were used as refuges by both sexes throughout the year, and also as nest sites by males in the breeding season. We then use experimental manipulation of shelter rock availability to examine the influence of intraspecific competition on territory establishment by smaller males. Following the removal of resident, non-nesting males, territories were quickly taken over. In addition, artificial reefs and nest sites were quickly colonised. Therefore, it appears that suitable shelter rocks are limited. Furthermore, intraspecific competition is strongly size-specific in C. nicholsii and determines which individuals gain access to shelter rocks. The presence of these social conditions is consistent with the prediction that the ability of larger males to monopolise more than one mate affects the occurrence of protogyny in some animals, and may clarify the occurrence of protogyny in C. nicholsii.

Effects of subcutaneous microtags on the growth, survival, and vulnerability to predation of small reef fishes

Marking reef fish with small, subcutaneous tags has proven to be a useful technique in ecological studies in both temperate and tropical systems. However, such tags may have adverse impacts on tagged individuals, possibly biasing estimates of demographic rates. We used field experiments on natural and artificial patch reefs to test for effects of subcutaneous acrylic paint and visual implant (VI) tags on the growth and mortality of the tropical goby Coryphopterus glaucofraenum (Gill). Growth of small ( ∼35 mm TL) fish was unaffected by tagging method. Neither acrylic paint, nor VI tags, influenced the mortality of C. glaucofraenum. We also conducted experiments in large outdoor tanks to test whether visual implant fluorescent elastomer (VIE) tags increased the susceptibility to predation of two temperate gobies, Coryphopterus nicholsii (Bean) and Lythrypnus dalli (Gilbert). For both goby species, tagged fish were no more susceptible to predatory kelp bass, Paralabrax clathratus (Girard) than untagged fish. Overall, with the exception of the slight reduction in growth of small C. glaucofraenum caused by acrylic paint tags, our results indicate that internal microtags can provide an effective means of marking small reef fish without introducing significant artifacts.

Effects of shelter and predators on reef fishes

The abundance of reef fishes is often influenced by the availability of potential shelter sites. The most prevalent hypothesis explaining this relationship between fish density and shelter is that post-settlement predation causes greater mortality in areas with little shelter vs. areas with abundant shelter. This hypothesis was tested with a set of field experiments, in which the availability of shelter and exposure to predators were manipulated orthogonally. The effects of shelter and predators were measured for recruitment and survival of two temperate reef fishes: the bluebanded goby ( Lythrypnus dalli Gilbert) and the blackeye goby ( Coryphopterus nicholsii Bean). Shelter was manipulated by creating isolated 1-m 2 reefs that were composed of three densities of rocks (16, 32 and 64 rocks/m 2), which both species of goby used for shelter. A preliminary set of experiments done on reefs exposed to predators revealed that recruitment and survival of both fishes were positively affected by the density of rocks. The main set of experiments revealed the proximate causes of these effects of shelter. Replicate reefs of all three shelter treatments were either kept free of predators (enclosed in predator-exclosure cages) or exposed to predators (enclosed in partial cages). In Coryphopterus, survival and recruitment varied among shelter treatments on reefs exposed to predators, but not on reefs free of predators. This result indicates that shelter influenced the density of Coryphopterus by altering the impact of predators, thus supporting the prevailing hypothesis. In contrast, the effects of shelter on Lythrypnus were not caused solely by altering the impact of predation: survival and recruitment both varied among shelter treatments even when predators were absent. For survival of Lythrypnus, differences among shelter treatments were larger on reefs exposed to predators than on reefs free of predators, indicating that shelter did also modify the impact of predation. However, recruitment of Lythrypnus was affected similarly by shelter treatments regardless of whether predators were present or absent. Therefore, the effects of shelter were not caused by altering the impact of predators, and instead, it appeared that recruits responded directly to the abundance of shelter. These results indicate that for some reef fishes, positive relationships between abundance and shelter may not be caused by lower rates of predation in areas with abundant shelter. Instead, such relationships between abundance and shelter may be driven by prey preferences (exerted at or after the time of settlement) for areas with abundant shelter, or by use of shelter sites for purposes other than refuge from predators. Therefore, in the absence of any other evidence supporting a role of predation, positive relationships between the density of reef fishes and the abundance of shelter should not be construed as proof of the importance of predation in determining the abundance of reef fishes.

The relative importance of predation and competition in two reef fishes.

Competition and predation may both strongly influence populations of reef fishes, but the importance of these processes relative to one another is poorly understood. I quantified the effects of predation and competition on the growth and survival of two temperate reef fishes, Lythrypnus dalli and Coryphopterus nicholsii, in field experiments in which I manipulated the densities of the two species and the abundance of predators (using exclosure cages) on small replicate patch reefs. I also evaluated the influence of predators on the behavior of the two species to help interpret the mechanisms of any predatory influences on growth or survival. Predation was much more important than competition (inter- or intraspecific) in Lythrypnus. For Coryphopterus, neither competition nor predation were particularly important. Behaviorally, both species responded to predators by reducing foraging rate and hiding. This altered behavior, however, had no repercussions for growth or survival of Coryphopterus. In contrast, Lythrypnus grew more slowly and suffered greater mortality when exposed to predators. Interspecific competition did not significantly influence either species. Intraspecific competition did not affect the growth of Coryphopterus, but survival tended to be lower at high densities. Growth of Lythrypnus was depressed by intraspecific competition, but survival was not, except that, in the presence of predators, survival was density dependent. In contrast to the historical emphasis placed on the role of competition, this study indicates that predation can be more important than competition in determining patterns of abundance of some reef fishes. For example, predators not only influenced foraging of both Lythrypnus and Coryphopterus, but they also reduced growth and survival of Lythrypnus, and therefore appear to help maintain the marked habitat segregation between the two species.

Influences of predators and conspecifics on recruitment of a tropical and a temperate reef fish

An active debate has focused on whether patterns in the abundance of reef fishes are primarily determined by the supply of larvae or by subsequent interactions occurring on the reef. By manipulating the presence of predators and the density of older conspecifics on small standardized reefs, we tested the influences of these 2 factors--and interactions between them--on recruitment of reef fishes. To assess the generality of our findings, we conducted similar experiments on 2 closely related species in 2 different systems: 1 tropical and 1 temperate. At Santa Catalina Island (a temperate site in southern California, USA) we worked with the blackeye goby Coryphopterus nicholsii and at Lee Stocking Island (a tropical site in the Bahamas) we studied the bridled goby Coryphopterus glaucofraenum. Predators reduced recruitment of blackeye gobies, but in contrast, in one experiment, recruitment of bridled gobies was positively affected by 1 class of predators (reef residents) and unaffected by transient predators. In another experiment, recruitment of bridled gobies was unaffected by either class of predators; however, there was little statistical power to detect a similar positive effect of predators. Older conspecifics (adults and subadults) did not significantly influence recruitment of blackeye gobies, but recruitment of bridled gobies was negatively related to density of adult conspecifics. For both species, the presence of predators did not influence the relationship between recruitment and the density of older conspecifics. Our results suggest that patterns of abundance among local populations of reef fishes can be decoupled from patterns of larval supply by reef-based biotic processes (namely predation and intraspecific interactions). However, the influences of older conspecifics and predators varied widely between the 2 quite similar species that we studied. This underscores the need to understand the specific reasons for such differences in order to make predictions regarding the relative importance of these processes in novel circumstances.

Population regulation by post-settlement mortality in two temperate reef fishes.

Input of individuals dispersing into open populations can be highly variable, yet the consequences of such variation for subsequent population densities are not well understood. I explored the influence of variable input ("supply") on subsequent densities of juveniles and adults in open local populations of two temperate reef fishes, the bluebanded goby (Lythrypnus dalli) and the blackeye goby (Coryphopterus nicholsii). Variable recruitment was simulated by stocking a natural range of densities of young fishes on replicate patch reefs. Density and mortality of the stocked cohorts were followed over time, until the fishes reached maturity. Over the first day of the experiments, mortality of both species was significantly density-dependent; however, there was still a very strong relationship between density on day 1 and density on day 0 (i.e., simulated recruitment was still an excellent predictor of population density). At this point in the study, the main effects of density-dependent mortality were to reduce mean densities and variation about the mean. Over the period from the start of the experiments until the time when maturity was reached by each species (about 1 and 3 months for Lythrypnus and Coryphopterus, respectively), mortality was strongly density-dependent. Such strong density-dependent mortality virtually eliminated any linear relationship between adult density and "recruit" density. However, for both species, the relationship between these two variables was well fit by an asymptotic curve, with the asymptotic density of adults equal to c. 3/m2 for Coryphopterus, and c. 10/m2 for Lythrypnus. Natural recruitment (via settlement of larvae) to the reefs over the period of the study (9 months) was above the asymptotic densities of adults for the two species, even though the study did not encompass the periods of peak annual recruitment of either species. This suggests that adult populations of these two gobies may often be limited, and regulated, by post-settlement processes, rather than by input of settlers. Other studies have shown that mortality of the two species is density-independent, or only weakly density-dependent, on reefs from which predators have been excluded. Hence, it appears that predators cause density-dependent mortality in these fishes.