Graham E. Forrester

Marine reserves as linked social–ecological systems

Marine reserves are increasingly recognized as having linked social and ecological dynamics. This study investigates how the ecological performance of 56 marine reserves throughout the Philippines, Caribbean, and Western Indian Ocean (WIO) is related to both reserve design features and the socioeconomic characteristics in associated coastal communities. Ecological performance was measured as fish biomass in the reserve relative to nearby areas. Of the socioeconomic variables considered, human population density and compliance with reserve rules had the strongest effects on fish biomass, but the effects of these variables were region specific. Relationships between population density and the reserve effect on fish biomass were negative in the Caribbean, positive in the WIO, and not detectable in the Philippines. Differing associations between population density and reserve effectiveness defy simple explanation but may depend on human migration to effective reserves, depletion of fish stocks outside reserves, or other social factors that change with population density. Higher levels of compliance reported by resource users was related to higher fish biomass in reserves compared with outside, but this relationship was only statistically significant in the Caribbean. A heuristic model based on correlations between social, cultural, political, economic, and other contextual conditions in 127 marine reserves showed that high levels of compliance with reserve rules were related to complex social interactions rather than simply to enforcement of reserve rules. Comparative research of this type is important for uncovering the complexities surrounding human dimensions of marine reserves and improving reserve management.

FACTORS INFLUENCING THE JUVENILE DEMOGRAPHY OF A CORAL REEF FISH

A notable recent development in marine ecology has been the suggestion that the size of demersal populations is limited and that patterns in demersal abundance are determined by the settlement of pelagic larvae (recruitment). Here I examine some factors potentially limiting and determining population density in a small coral reef fish. An experimental study of the demography of juveniles of a small planktivorous damselfish, the humbug Dascyllus aruanus (Pomacentridae), was done at One Tree Reef, southern Great Barrier Reef, Australia. Recently settled fish were transplanted to replicate units of habitat at three lagoonal sites. The interactive effects of initial recruit density, the presence of adults, and supplemental feeding on the growth, survival, migration, and maturation of the recruits were examined over the following 10 mo. Migration was apparently rare and thus unimportant. Effects of the factors on the growth and survival of recruits were complex. Survival of recruits was generally inversely related to their density but enhanced in the presence of adults. Average survival also varied among locations. Mean growth was generally depressed at high recruit densities and in the presence of adults but was enhanced by supplemental feeding. Maturation was related to size and so was influenced by effects on growth. The results suggest that regulatory interactions and shortages of food may limit the size of adult populations via their effect on growth rates. Effects of the factors on survival were slight. Total abundance may thus be limited primarily by recruitment unless effects on growth are ultimately translated to effects on demersal mortality. As a result of the effects on survival, there was some modification of the initial pattern of recruit abundance. The importance of these effects in determining patterns of abundance in unmanipulated populations depends on variation in recruitment at similar scales. The number of recruits entering the adult population under different treatments was highly modified from the pattern of initial recruit density. The number of recruits that became mature by the end of the experiment was totally unrelated to their initial density. In addition, that number was reduced in the presence of adults, and was limited by the availability of food. The results suggest that complex effects of these factors on juvenile demography can modify recruitment patterns at small spatial scales. They also suggest that these factors may have different effects on the adult and total demersal populations. In general terms, both recruitment and postrecruitment processes influence patterns in abundance.

Recent Region-wide Declines in Caribbean Reef Fish Abundance

Profound ecological changes are occurring on coral reefs throughout the tropics, with marked coral cover losses and concomitant algal increases, particularly in the Caribbean region. Historical declines in the abundance of large Caribbean reef fishes likely reflect centuries of overexploitation. However, effects of drastic recent degradation of reef habitats on reef fish assemblages have yet to be established. By using meta-analysis, we analyzed time series of reef fish density obtained from 48 studies that include 318 reefs across the Caribbean and span the time period 1955-2007. Our analyses show that overall reef fish density has been declining significantly for more than a decade, at rates that are consistent across all subregions of the Caribbean basin (2.7% to 6.0% loss per year) and in three of six trophic groups. Changes in fish density over the past half-century are modest relative to concurrent changes in benthic cover on Caribbean reefs. However, the recent significant decline in overall fish abundance and its consistency across several trophic groups and among both fished and nonfished species indicate that Caribbean fishes have begun to respond negatively to habitat degradation.

Spatial patterns in abundance of a damselfish reflect availability of suitable habitat

Abstractu2002For species with metapopulation structures, variation in abundance among patches can arise from variation in the input rate of colonists. For reef fishes, variability in larval supply frequently is invoked as a major determinant of spatial patterns. We examined the extent to which spatial variation in the amount of suitable habitat predicted variation in the abundance of the damselfish Dascyllus aruanus, an abundant planktivore that occupies live, branched coral throughout the Indo-Pacific. Reef surveys established that size, branching structure and location (proximity to sand) of the coral colonies together determined the ”suitability” of microhabitats for different ontogenetic stages of D. aruanus. Once these criteria were known, patterns of habitat use were quantified within lagoons of five Pacific islands. Availability of suitable habitat generally was an excellent predictor of density, and patterns were qualitatively consistent at several spatial scales, including among different lagoons on the same island, among different islands and between the central (French Polynesia and Rarotonga) and western (Great Barrier Reef, Australia) South Pacific. A field experiment that varied the amount of suitable coral among local plots indicated that habitat for settlers accounted for almost all of the spatial variation in the number of D. aruanus that settled at that location, suggesting that spatial patterns of abundance can be established at settlement without spatial variation in larval supply. Surveys of four other species of reef-associated fish revealed that a substantial fraction of their spatial variation in density also was explained by availability of suitable reef habitat, suggesting that habitat may be a prevalent determinant of spatial patterns. The results underscore the critical need to identify accurately the resource requirements of different species and life stages when evaluating causes of spatial variation in abundance of reef fishes.

PREDATORS, PREY REFUGES, AND THE SPATIAL SCALING OF DENSITY-DEPENDENT PREY MORTALITY

We tested the biological cause of density-dependent mortality in the bridled goby (Coryphopterus glaucofraenum), a small coral reef fish, and evaluated whether this knowledge allowed us to detect density dependence at different spatial scales in natural habitats. To identify the biological cause of density dependence, we manipulated both population density and the availability of shelter (crevices used as refuges from predators) in small plots of continuous reef. We detected strong density-dependent mortality in plots with few refuges, but mortality was density independent in plots with abundant refuges, indicating that limited shelter causes density dependence. Predator density was unrelated to the density of gobies and refuges, suggesting that predators displayed a type III functional response in patches with few refuges. In a second experiment, we manipulated goby density within replicate plots of three sizes (4, 16, and 64 m2) that varied naturally in the availability of refuges. If refuge availability was ignored, mortality appeared to be density independent at all scales. If, however, plots were grouped by refuge availability, mortality was density dependent in plots with few refuges, but low and density independent in plots with many refuges at all spatial scales. Understanding the mechanism of density dependence (refuge shortage) was thus required to measure the strength of density dependence in natural, spatially variable, habitat. We suggest that density dependence was detectable in plots of different sizes because the relationships between the densities of gobies, refuges, and goby predators were similar across the spatial scales we studied. Our work demonstrates that identifying the biological interactions that cause density dependence, and characterizing the spatial domains at which those interactions operate, will be important to accurately assess the effects of density dependence on population dynamics.

Spatio-temporal and interspecific variation in otolith trace-elemental fingerprints in a temperate estuarine fish assemblage

Abstract We tested whether estuarine fishes have site-specific differences in the concentrations of trace elements in their otoliths that can be used as ‘fingerprints’ to identify them to their estuary of origin. To evaluate the robustness of this approach, we tested whether elemental fingerprints were consistent among individuals of five species that were collected in 1996 from three temperate estuaries in southern California. We also tested whether elemental fingerprints were consistent between spring and autumn 1996 for three species in one of the sites, Carpinteria Marsh. The species evaluated comprised a mid-water-dwelling smelt ( Atherinops affinis ), two benthic gobies ( Clevelandia ios and Ilypnus gilberti ), and two flatfish ( Paralichthys californicus and Hypsopsetta guttulata ). The concentrations of six elements (Mn, Cu, Zn, Sr, Ba, and Pb) were determined in the otoliths using inductively coupled plasma-mass spectrometry (ICP-MS). Within estuaries, the five species exhibited strong variation in elemental concentration, indicating substantial interspecific differences in otolith environmental history. When the five fish species were considered separately, multivariate (MANOVA) and univariate (ANOVA) analyses of variance indicated that the elemental composition of otoliths differed significantly among the estuaries in four of the five species. Based on linear discriminant function analyses (DFA), differences were strong enough that trace element composition could be used to accurately assign fish to their site of origin [mean (range): 93.5% (74–100%)]. However, elemental signatures within Carpinteria Marsh were not consistent between spring and autumn 1996, and this was reflected in a substantial reduction in the accuracy of assigning fish to their true site of origin. When we compared site differences between fish species (site×species interactions), the elemental fingerprints were most similar between closely related species (e.g. the two gobies and the two flatfish) and most dissimilar between distantly related species, both phylogenetically and ecologically. Among the six elements analyzed, Sr and Ba exhibited the most inconsistent pattern among species, with significant differences in 80 and 70% of the pairwise species comparisons, respectively. The remaining four elements showed ≥70% consistency in the pattern of variation among sites for the different species. Thus, while otolith elemental fingerprinting can be a useful tool for inferring estuarine residency, such fingerprints may be temporally variable and species specific.

Influences of predatory fish on the drift dispersal and local density of stream insects

Researchers have recently suggested that frequent prey dispersal into and out of an area can swamp the local effect of predation. Where prey are mobile, the extent of prey movement can therefore explain variation in the apparent effect of predators on local prey density. I compared the effect of the dominant predator in a temperate stream (brook charr, Salelinus fontinalis Mitchill) on five insect prey taxa (mayflies: Ephemer- optera), to test the prediction that the extent of density reduction should be less for prey taxa dispersing more frequently. The propensity of the mayflies to disperse by drifting downstream in the water column was measured in unmanipulated areas of the stream. Relative propensity to drift of the five mayflies was, from greatest to least: Baetis, Paraleptophlebia, Ephemerella, Eurylo- phella, Stenoneina. I then tested effects of charr on the mayflies by manipulating charr density in fenced 35-m sections of the stream. Charr densities were adjusted to zero, medium, and high levels relative to natural densities. Rates of predation by charr in stream sections did not vary among the five mayfly taxa. Charr caused a large reduction in the density of Baetis, had a smaller effect on Paraleptophlebia, but had no detectable effect on the density of the other mayflies. Drift dispersal into the stream sections did not differ among charr densities. Effects of charr on prey densities could thus have been caused by direct predation or by increases in emigration from areas containing charr. Charr caused increased drift dispersal of Baetis and of Paraleptophlebia, but had no influence on drift of the other three mayflies. The reduction in density of Baetis by charr was due more to the charr-induced increase in drifting of Baetis than to direct predation on Baetis. The hypothesis that frequent prey dispersal swamps the effects of predators assumes that predators influence prey density primarily by consuming prey. In this system charr also influenced prey densities by causing increases in prey drift rates, affecting the mayflies that drifted most frequently. For this reason, the mayflies drifting less frequently were not, as predicted, the ones most strongly affected by charr. Variation in the effect of predators on prey density may thus be partially explained by both (1) changes in prey dispersal not related to predators, as proposed in the initial hypothesis, and (2) influences of predators on prey dispersal.

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.

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 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.