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. Received: 26 November 1996 / Accepted: 5 April 1997     

Population dynamics of coral-reef fishes: Controversial concepts and hypotheses

Abstract Knowledge of processes that drive the local population dynamics of coral-reef fishes is important for managing reef fisheries, and for using these species as models for understanding the ecology of demersal marine fishes in general. However, the reef-fish literature is replete with poorly defined concepts and vague hypotheses regarding the issue of population dynamics. Dichotomous arguments, such as whether or not recruitment drives population dynamics, are misdirected because they fail to incorporate several important concepts. First, changes in local population size are driven by four demographic rates (birth, death, immigration and emigration), all of which must be studied to understand population dynamics. Second, all populations that persist do so because at least one of these demographic rates operates in a density-dependent way that is both sufficiently strong and appropriately time-lagged. Therefore, identifying the source(s) of direct density dependence is critical for understanding the limits to variation in population size (i.e. population regulation). Third, regulation does not imply a simple point equilibrium in population size; density dependence in populations of reef fishes is bound to lie within a field of stochastic variation, and thus be difficult to detect. Since its formal origin in 1981, the ‘recruitment limitation’ hypothesis for explaining local population dynamics in reef fishes has undergone ambiguous changes in definition that threaten its usefulness. ‘Recruitment, ‘originally defined as the appearance of newly settled fish on a reef, more recently is often measured months after settlement, thus confounding pre- and post-settlement processes. ‘Limitation, ‘ which originally referred to recruitment being so low as to preclude local populations from reaching densities where resources were limiting, is more recently defined as an absence of any form of density dependence after settlement. The most effective means of testing whether post-settlement mortality is in fact density-independent is to examine patterns of mortality directly, rather than indirectly by interpreting the shape of the relationship between initial recruit density and subsequent adult density within a cohort (the recruit-adult function). Understanding the population dynamics of coral-reef fishes will require a more equitable focus on all four demographic rates, be they density dependent or not, as well as greater attention to identifying sources of density dependence. Such a pluralistic focus necessitates integrated studies of both pre- and post-settlement processes conducted at multiple spatial and temporal scales. For example, recent evidence suggests that density-dependent pre-dation on new recruits that have settled among reefs at different densities may prove to be an important source of local population regulation, especially via the aggregative response of transient piscivores.     

Bottlenecks to coral recovery in the Seychelles

Processes that affect recovery of coral assemblages require investigation because coral reefs are experiencing a diverse array of more frequent disturbances. Potential bottlenecks to coral recovery include limited larval supply, low rates of settlement, and high mortality of new recruits or juvenile corals. We investigated spatial variation in local abundance of scleractinian corals in the Seychelles at three distinct life history stages (recruits, juveniles, and adults) on reefs with differing benthic conditions. Following widespread coral loss due to the 1998 bleaching event, some reefs are recovering (i.e., relatively high scleractinian coral cover: ‘coral-dominated’), some reefs have low cover of living macrobenthos and unconsolidated rubble substrates (‘rubble-dominated’), and some reefs have high cover of macroalgae (‘macroalgal-dominated’). Rates of coral recruitment to artificial settlement tiles were similar across all reef conditions, suggesting that larval supply does not explain differential coral recovery across the three reef types. However, acroporid recruits were absent on macroalgal-dominated reefs (0.0 ± 0.0 recruits tile^?1) in comparison to coral-dominated reefs (5.2 ± 1.6 recruits tile^?1). Juvenile coral colony density was significantly lower on macroalgal-dominated reefs (2.4 ± 1.1 colonies m^?2), compared to coral-dominated reefs (16.8 ± 2.4 m^?2) and rubble-dominated reefs (33.1 ± 7.3 m^?2), suggesting that macroalgal-dominated reefs have either a bottleneck to successful settlement on the natural substrates or a high post-settlement mortality bottleneck. Rubble-dominated reefs had very low cover of adult corals (10.0 ± 1.7 %) compared to coral-dominated reefs (33.4 ± 3.6 %) despite no statistical difference in their juvenile coral densities. A bottleneck caused by low juvenile colony survivorship on unconsolidated rubble-dominated reefs is possible, or alternatively, recruitment to rubble-dominated reefs has only recently begun. This study identified bottlenecks to recovery of coral assemblages that varied depending on post-disturbance habitat condition.     

Spatial variation in juvenile and adult Ecklonia radiata (Laminariales) sporophytes

Juvenile and adult sporophytes of Ecklonia radiata were counted on in-shore and off-shore reefs at Marmion (Perth, Western Australia), across spatial scales of kilometres, 10s of metres and metres. The position on in-shore reefs vs. off-shore reefs did not influence recruit and adult kelp density. There was considerable site-to-site (kilometres) variation in densities of recruits but not adults. The majority of variation in both recruit and adult densities was found between quadrats separated by a few metres. There was no relationship between abundance of recruits and abundance of adult sporophytes at any of the spatial levels of investigation, suggesting a decoupling of recruitment and canopy processes. The results emphasise the patchy nature of kelp canopy distribution and suggest that relatively small-scale processes, acting on scales of a few metres, are of primary importance in generating heterogeneity in the canopy of these kelps.     

Patterns of recruitment of coral reef fishes in a monsoonal environment

The abundance of newly settled recruits of coral reef fishes was monitored at a total of 11 sites at two islands and two coastal locations in the central Philippines for a 20-month period (February 2008 to September 2009) that included two monsoon cycles. Recruitment occurred throughout the year. Most of the abundant species exhibited protracted recruitment seasons. This confirms the expectation of extended breeding of reef fishes at lower latitudes. The annual pattern of recruitment of reef fishes as a group was predictable. Annual fluctuations of sea surface temperature and wind strength largely explained the pattern. Rainfall, however, did not significantly influence the pattern of recruitment. Peaks in density and species richness of recruits occurred during the southwest monsoon and the second inter-monsoonal period of the year (July to October) when temperatures were highest and when most of the sites were sheltered from winds or when winds were weak. Conversely, lowest density and species richness were observed during the northeast monsoon (November to March) when temperatures were lowest and most sites were exposed to winds. The same pattern could also be seen in the recruitment of both damselfishes (Pomacentridae) and wrasses (Labridae), notwithstanding a tenfold difference in abundance of recruits between the two families. The pattern was fairly consistent across most sites, among most of the species that were examined, and between the 2 years that were sampled. This study is one of the few to provide insights into the influence of environmental factors on the recruitment patterns of fishes on Indo-Pacific coral reefs situated at lower latitudes.     

Effects of a Marine Reserve on Recruitment of Grunts (Pisces: Haemulidae) at Barbados, West Indies

The effects of a non-extractive marine reserve on the recruitment dynamics of haemulid fishes and their predators on Barbados coral reefs were studied using visual census and mark-recapture methods. Size and abundance of piscivores (including large adult grunts) known to prey on grunts were greater within the reserve than on adjacent reefs, whereas size and abundance of older juvenile grunts did not differ between protected and exploited reefs. Recruitment and early juvenile abundance were lower within the reserve and were inversely related to predator density (including adult conspecifics). Patterns in density of new recruits may also have been influenced by oceanographic patterns of supply of larvae. Thus, although protection has a significant positive effect on the size and abundance of large carnivorous fishes, higher predation pressure within a reserve may serve to reduce juvenile recruitment within the reserve. At some size/age, cumulative recruitment due to lower size-specific predation mortality results in higher density within the reserve. This increased density is maintained by the absence of fishing mortality within the reserve. Despite maintaining high spawning biomass of several large, commercially exploited species that may export larvae to downstream areas, the Barbados Marine Reserve appears to be a local sink for juvenile grunts.     

Predators reduce abundance and species richness of coral reef fish recruits via non-selective predation

Predators have important effects on coral reef fish populations, but their effects on community structure have only recently been investigated and are not yet well understood. Here, the effect of predation on the diversity and abundance of young coral reef fishes was experimentally examined in Moorea, French Polynesia. Effects of predators were quantified by monitoring recruitment of fishes onto standardized patch reefs in predator-exclosure cages or uncaged reefs. At the end of the 54-day experiment, recruits were 74% less abundant on reefs exposed to predators than on caged ones, and species richness was 42% lower on reefs exposed to predators. Effects of predators varied somewhat among families, however, rarefaction analysis indicated that predators foraged non-selectively among species. These results indicate that predation can alter diversity of reef fish communities by indiscriminately reducing the abundance of fishes soon after settlement, thereby reducing the number of species present on reefs.     

The influence of adult density on larval settlement in a coral reef fish Coryphopterus glaucofraenum

 For marine species with open populations, patterns of larval settlement can have important consequences for performance and abundance at later life-stages. In this study, I tested whether larvae of a reef-dwelling goby (Coryphopterus glaucofraenum) settled differentially to reefs occupied by varying numbers of adults. I monitored settlement daily to reefs on which the density of adult gobies varied naturally, or was manipulated experimentally. Rates of settlement were constant across a broad range of adult densities, suggesting that larvae do not choose settlement sites based on the number of adults in their immediate vicinity. Accepted: 30 October 1998     

Recruitment pattern of Diadema antillarum in La Parguera, Puerto Rico

Recruitment success is one of the factors that may be influencing the slow population recovery of Diadema antillarum at many locations in the Caribbean. D. antillarum recruitment was measured with recruitment plates at monthly intervals from September 2005 to September 2006 at three reefs along an inshore-offshore gradient in La Parguera, Puerto Rico. A total of 275 recruits of D. antillarum were collected at the shelf-edge reef during this 13-month study. Two recruits were collected at the mid-shelf reef and no recruits were collected at the inner shelf reef. Recruitment varied among months at the shelf-edge, with the highest value (1,067 ind/m²) occurring in July 2006. Previous benthic surveys in the La Parguera region have demonstrated higher densities of adult D. antillarum at inshore reefs. The higher recruitment at the shelf-edge suggests that recruitment is not a major determinant of the spatial distribution of the adult population. Recruitment occurring in this study is indicative that sources of larvae were available upstream and larval survival was occurring.     

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres.     

Presence of cleaner wrasse increases the recruitment of damselfishes to coral reefs

Mutualisms affect the biodiversity, distribution and abundance of biological communities. However, ecological processes that drive mutualism-related shifts in population structure are often unclear and must be examined to elucidate how complex, multi-species mutualistic networks are formed and structured. In this study, we investigated how the presence of key marine mutualistic partners can drive the organisation of local communities on coral reefs. The cleaner wrasse, Labroides dimidiatus, removes ectoparasites and reduces stress hormones for multiple reef fish species, and their presence on coral reefs increases fish abundance and diversity. Such changes in population structure could be driven by increased recruitment of larval fish at settlement, or by post-settlement processes such as modified levels of migration or predation. We conducted a controlled field experiment to examine the effect of cleaners on recruitment processes of a common group of reef fishes, and showed that small patch reefs (61–285 m²) with cleaner wrasse had higher abundances of damselfish recruits than reefs from which cleaner wrasse had been removed over a 12-year period. However, the presence of cleaner wrasse did not affect species diversity of damselfish recruits. Our study provides evidence of the ecological processes that underpin changes in local population structure in the presence of a key mutualistic partner.     

Density dependent recruitment in the reef fish Chaetodon miliaris

Synopsis Recruitment of reef fish has generally been found to be unrelated to or positively related to adult densities. This paper reports an inverse relationship between the number of recruits of Chaetodon miliaris and the density of conspecific adults. C. miliaris are non-territorial, planktivorous butterflyfish. The study populations occurred on the flanks of patch reefs of about 30 m in diameter. At their peak densities populations consisted of 300–400 C. miliaris. Recruitment (the appearance in the stock of fish less than 3.5cm in total length) occurred primarily from April to June and corresponded to the new moon period in these months. Populations generally showed a steady decline in numbers during months of the year when recruitment was not occurring: the finite rate of decline was approximately 30% per month. Populations varied in the density of adults at the start of the spring-summer period of recruitment, and the magnitude of recruitment to a reef was inversely related to the density of these conspecific adults. Reduction of the densities of adult C. miliaris through trap fishing, resulted in increased recruitment to the fished stock. These results provide evidence that in some. species of reef fish, benthic processes may play an important role in determining the magnitude of recruitment to an adult stock, in contrast to the widely held view that recruitment is in large part a result of chance events in the plankton or that recruitment is directly related to adult densities.     

Small-scale recruitment variation in a temperate fish: the roles of macrophytes and food supply

Synopsis The availability of reef-related resources, particularly food and shelter can play a significant role in determining the distribution and abundance of reef fishes. Much of the structure on temperate reefs is provided by macroalgae, and variability in the density of temperate reef fishes at large spatial scales (100's of meters) can often be explained by variation in macroalgal cover or density. In this study I investigated the role of macrophytes and associated food resources on the recruitment of a temperate fish, Tautogolabrus adspersus, at a small spatial scale (0.25 m²). No relationship between the density of new recruits and the percent cover of kelp, foliose or filamentous algae was observed. Multiple regressions revealed that less than 8% of variability in recruitment could be explained by variability in macroalgal cover. However, recruits were found in higher abundance in patches containing many functional forms of seaweeds than in patches dominated by a single form. A wide variety of prey were available for use by cunner recruits; however, crustaceans and mussels were the only common components of their diet, and crustaceans were clearly the most preferred prey. The prey composition in patches where fish were present was compared to randomly selected patches. Significantly greater numbers of isopods, amphipods and newly settled mussels were present in patches where fish were present than in randomly selected patches. The data presented in this study contradict previous work that has shown algal structure to be important in determining patterns of abundance and food supply to be of little significance. A conceptual model is proposed suggesting that settling fish select habitats in a hierarchical manner largely based on their dispersal tendencies. Hierarchical selection of habitats results in different attributes of the habitat being selected during different life-history intervals.     

SMURFs: standard monitoring units for the recruitment of temperate reef fishes

I evaluated a standard monitoring unit for the recruitment of reef fishes (SMURF) as a tool for ascertaining spatial and temporal patterns of reef fish recruitment in central California, USA. SMURFs consisted of a 1.0×0.35 m dia. cylinder of fine mesh plastic grid that contained a folded section of larger mesh plastic grid. SMURFs collected new recruits of 20 species of fish with 92% of the individuals collected from 10 species, mostly rockfish (genus Sebastes). An experiment varying depth of SMURFs in the water column (surface, mid-depth, or bottom) showed that surface SMURFs collected the greatest diversity of species and significantly greater abundance for eight species, with two species having significantly greater abundance on mid-depth SMURFs and three species having significantly greater abundance on bottom SMURFs. A comparison of cumulated recruitment from SMURFs that varied in sampling frequency (removal of new recruits every 1-3, 7, or 28 days) suggested that increasing the time between sampling caused a significant decrease in recruitment estimates for some species but not for others. To determine how well temporal patterns of recruitment to SMURFs reflected patterns to nearby reefs, I compared within season temporal patterns of recruitment to SMURFs with that at nearby reefs, estimated by visual transect surveys conducted on scuba. Temporal patterns of recruitment to SMURFs were significantly and positively related to early recruitment on reefs for one group of benthic-algal associated rockfish species when diver surveys were lagged by 30 days (r=0.87) and for another group of canopy-algal associated rockfish species when lagged by 5 days (r=0.72). SMURFs appeared to be an effective and efficient method for indexing relative rates of delivery of competent juveniles for many temperate nearshore reef fishes.     

Responses of coral and fish assemblages to a severe but short-lived tropical cyclone on the Great Barrier Reef,Australia

Changes in reef assemblages of corals and fishes following a tropical cyclone were assessed using data sets from (1) manta tow surveys of entire reef perimeters and (2) intensive surveys of specific sites, across 11 reefs lying close to the cyclone's track. Only one of the reefs experienced an obvious and immediate decline in mean coral cover (from 24 to 8%) due to the cyclone. The abundance and species richness of adult damselfish assemblages on intensive survey sites at this reef were not affected in the short term (6-8 weeks), despite the removal of 48% of living hard coral. Assemblages of adult fishes showed a similar lack of response at three other reefs where no significant habitat changes had occurred. Eleven to twelve months later, the total abundance of damselfishes had decreased substantially at eight of the sampled reefs, while the abundance of larger mobile fishes remained stable. We infer that the effects on coral assemblages reflect the short duration and orientation of the cyclone, the history of exposure to wave energy (influencing life-form structure and therefore degree of fragility), and the degree of consolidation of the reef matrix. The lack of short-term effects of the cyclone on adult fishes shows that these fishes can endure periods of intense underwater turbulence. The lack of change in damselfish assemblages weeks after loss of coral cover implies that this resource was not limiting adult fish densities. The reasons for widespread decreases in damselfish numbers 11-12 months after the cyclone are unknown.     

Evidence for positive density-dependent effects in recovering Diadema antillarum populations

Recovering populations may experience positive density-dependent feedbacks that contribute to population increases. Diadema antillarum, a keystone herbivore on Caribbean coral reefs, suffered a well-documented mass mortality in 1983-84. High densities of adults of this long-spined urchin could provide effective refuge from predation for juveniles under a spine canopy, as has been suggested for other urchin species. We evaluated the effect of adult density on juvenile persistence of D. antillarum experimentally, and examined size-frequency distributions of recovering local populations for evidence of positively density-dependent juvenile persistence at St. Croix, U.S. Virgin Islands. Juvenile persistence was significantly higher in high adult density treatments, and bimodal population size distributions also suggest potential positive effects of adult density on juveniles. This positive feedback could accelerate the recovery of this important coral reef grazer.     

Food ration and condition affect early survival of the coral reef damselfish, Stegastes partitus

The supply of larvae is a major determinant of population and community structure in coral reef fishes. However, spatial and temporal variation in condition (i.e. quality) of potential recruits, as well as their density (i.e. quantity), may influence survival and growth of juveniles. We conducted an experiment to test whether recent feeding history could affect growth, condition and post-recruitment survival in a Caribbean damselfish, Stegastes partitus. Fish were collected soon after settlement, and fed either low or high rations in aquaria for 7 days. Fish fed the high ration grew faster in aquaria and were in a better condition (higher total lipids and Fulton’s condition factor) at the end of the feeding period. Subsequently, we released 50 fish in 25 pairs (one fish subjected to low rations, the other to high rations) on a Bahamian coral reef and monitored survival for 10 days. Survivorship of high-ration fish was double that of low-ration fish (80 vs 40% over 10 days). However, low-ration fish that survived 10 days were of similar condition and grew at similar rates to high-ration fish, suggesting that short-term ration differences may not persist in surviving fish. Laboratory experiments showed that low-ration fish were taken by piscivorous fishes before high-ration fish, indicating that differential predation may account for survival differences. This study highlights the potential of feeding history and condition to affect the relationship between patterns of larval arrival at reefs, and subsequent juvenile and adult population densities. Received: 1 March 1999 / Accepted: 15 July 1999     

Stochastic density effects on adult fish survival and implications for population fluctuations

The degree to which population fluctuations arise from variable adult survival relative to variable recruitment has been debated widely for marine organisms. Disentangling these effects remains challenging because data generally are not sufficient to evaluate if and how adult survival rates are regulated by stochasticity and/or population density. Using unique time series for a largely unexploited reef fish, we found both population density and stochastic food supply impacted adult survival. The estimated effect of variable survival on adult abundance (both mean and variability) rivalled that of variable recruitment. Moreover, we show density‐dependent adult survival can dampen impacts of stochastic recruitment. Thus, food variability may alter population fluctuations by simultaneously regulating recruitment and compensatory adult survival. These results provide an additional mechanism for why intensified density‐independent mortality (via harvest or other means) amplifies population fluctuations and emphasises need for research evaluating the causes and consequences of variability in adult survival.     

Consequences of extreme life history traits on population persistence: do short-lived gobies face demographic bottlenecks?

The majority of coral reef goby species are short-lived, with some highly abundant species living less than 100 d. To understand the role and consequences of this extreme life history in shaping coral reef fish populations, we quantitatively documented the structure of small reef fish populations over a 26-month period (>14 short-lived fish generations) at an inshore reef on the Great Barrier Reef, Australia. Most species with life spans >1 yr, such as pomacentrids, exhibited a peak in recruitment during the austral summer, driving seasonal changes in the small fish community composition. In contrast, there were no clear changes in goby community composition, despite the abundance of short-lived, high turnover species. Species of Eviota, the most abundant gobiid genus observed, showed remarkably similar demographic profiles year-round, with consistent densities of adults as well as recently recruited juveniles. Our results demonstrate ongoing recruitment of these small cryptic fishes, which appears to compensate for an exceptionally short life span on the reef. Our results suggest that gobiid populations are able to overcome demographic limitations, and by maintaining reproduction, larval survival and recruitment throughout the year, they may avoid population bottlenecks. These findings also underline the potential trophodynamic importance of these small species; because of this constant turnover, Eviota species and other short-lived fishes may be particularly valuable contributors to the flow of energy on coral reefs, underpinning the year-round trophic structure.     

Interactions between year classes in the benthic amphipod Monoporeia affinis: effects on juvenile survival and growth

Summary The effect of 1-year-old Monoporeia (=Pontoporeia) affinis on the survival and growth of young of the year was studied in laboratory experiments. Amphipods were kept in jars with sediment and a continuous supply of cooled water for 3 months. Adults or juveniles were added to a control density of juveniles. Juvenile mortality increased at higher total densities, and was greater when adults were present. The mean length and dry weight of juveniles decreased at higher densities, but adults did not have a greater effect than juveniles. These density-dependent effects are probably caused by competition for food within and between year classes. Thus, a high abundance of juveniles or adults could reduce the recruitment of juveniles into the adult year-class the following year, and the suppression of juvenile growth could lower fecundity. The results of this experiment support the hypothesis, inferred from field data from the Baltic Sea and Swedish lakes, that intraspecific interactions contribute to fluctuations in the abundance of populations of M. affinis.