Multiple processes regulate long-term population dynamics of sea urchins on Mediterranean rocky reefs

We annually monitored the abundance and size structure of herbivorous sea urchin populations (Paracentrotus lividus and Arbacia lixula) inside and outside a marine reserve in the Northwestern Mediterranean on two distinct habitats (boulders and vertical walls) over a period of 20 years, with the aim of analyzing changes at different temporal scales in relation to biotic and abiotic drivers. P. lividus exhibited significant variability in density over time on boulder bottoms but not on vertical walls, and temporal trends were not significantly different between the protection levels. Differences in densities were caused primarily by variance in recruitment, which was less pronounced inside the MPA and was correlated with adult density, indicating density-dependent recruitment under high predation pressure, as well as some positive feedback mechanisms that may facilitate higher urchin abundances despite higher predator abundance. Populations within the reserve were less variable in abundance and did not exhibit the hyper-abundances observed outside the reserve, suggesting that predation effects maybe more subtle than simply lowering the numbers of urchins in reserves. A. lixula densities were an order of magnitude lower than P. lividus densities and varied within sites and over time on boulder bottoms but did not differ between protection levels. In December 2008, an exceptionally violent storm reduced sea urchin densities drastically (by 50% to 80%) on boulder substrates, resulting in the lowest values observed over the entire study period, which remained at that level for at least two years (up to the present). Our results also showed great variability in the biological and physical processes acting at different temporal scales. This study highlights the need for appropriate temporal scales for studies to fully understand ecosystem functioning, the concepts of which are fundamental to successful conservation and management.     

Herbivory on coral reefs: community structure following mass mortalities of sea urchins

The community structure of Jamaican coral reefs has undergone drastic change since mass mortalities of the long-spined black sea urchin Diadema antillarum Philippi occurred in 1983. In the absence of Diadema, algal abundance has increased enormously, up to a mean of 95% cover or 4.6 kg wet weight · m ⁻². Coral cover, which was already low on some reefs following Hurricane Allen in 1980, has been further reduced by as much as 60% since 1983 by competition with algae. Densities of D. antillarum at 10 sites in 1986 ranged from 0 to 12% of pre-1983 levels. Other echinoids, which might potentially compensate for the lack of herbivory from D. antillarum, have not increased significantly in density. Numbers of herbivorous scarids and acanthurids also remain at relatively low levels, because of overfishing. In the absence of high densities of fish and sea urchins, it is likely that recent changes in community structure will continue, resulting in further replacement of corals by algae in shallow water. The impact of the urchin mass mortalities is qualitatively similar to previous experimental removals of this species. In both cases, removal of echinoids resulted in substantial increases in macroalgae. However, quantitatively, the responses of algal and coral communities to the natural die-off were significantly greater, probably due to wide differences in spatial and temporal scales of the respective perturbations.     

Relationships between the spread of Caulerpa filiformis and fish communities on temperate rocky reefs

The previously sub‐dominant native marine macrophyte Caulerpa filiformis is now dominant on many sub‐tidal rocky reefs in New South Wales (NSW), Australia and is expanding its distribution. As C. filiformis is highly chemically defended and structurally different to co‐occurring habitat‐forming macrophytes, two key attributes that govern fish assemblages, we hypothesized that fish assemblages, particularly herbivorous fishes, would be different at sites where C. filiformis occurred from where it was previously absent and within sites, fish community structure would be correlated to the cover of C. filiformis. We investigated these hypotheses by determining reef‐associated fish assemblage attributes (assemblage structure, species richness, total abundance, Shannon‐Weiner diversity, abundance of herbivorous species) along transects within sites where C. filiformis was present and absent. Surprisingly, despite large patches and very high densities of C. filiformis on the reefs we sampled, at larger spatial scales (i.e., among sites) no fish assemblage metrics differed between sites with large stands of C. filiformis and sites without the alga. Moreover the abundance of one dominant herbivore, the rock cale Aplodactylus lophodon, was greater at sites within large beds of C. filiformis. At smaller spatial scales, however, i.e. within sites where C. filiformis was present, fish assemblages did vary as a function of C. filiformis cover along transects, although this was not consistent across sampling times. Overall, our results suggest that the potential effects of the spread of this alga on faunal communities warrants further investigation.     

Oceanographic and lunar forcing affects nearshore larval fish assemblages from temperate rocky reefs

The influence of oceanographic features and moon phases on ichthyoplankton assemblages in a temperate nearshore rocky reef off El Quisco Bay, central Chile, was assessed during austral spring–summer 2015–2016 using Bongo nets. Wind direction was predominantly south-west, and ocean temperature increased gradually during the study period, fluctuating between 11.6°C and 17.7°C. A relatively cold period (from late September to early December, 12.42?±?0.64°C) was distinguished from a relatively warmer phase (from mid-December to February, 13.56?±?1.08°C). Nearshore ichthyoplankton was composed of 13,700 individuals, belonging to 43 taxa. Larval Strangomera bentincki (Clupeidae) were collected in high numbers between late September and late October with peaks during full moon and first quarter (maximum?=?734 ind. 100?m^?3); larval Engraulis ringens (Engraulidae) was most abundant between late October and late December 2015, with peaks during the third quarter and full moon. Principal Component Analysis of ichthyoplankton data explained more than 44% of total variance and showed the influence of cold/warm periods in the structuring of larval fish assemblages. Water temperature had more influence than lunar phase in the structure and composition of nearshore fish larvae off central Chile. We conclude that larval fish assemblages found in nearshore waters change on a seasonal scale by differences in the reproductive activity among species, and that lunar phase exerts a low, but significant effect on the abundance of fish larvae, but this variability is species-specific.     

Spatial variation in sea urchins, fish predators, and bioerosion rates on coral reefs of Belize

Although sea urchins are critical for controlling macroalgae on heavily fished coral reefs, high densities threaten reefs, as urchins are also prodigous bioeroders. This study examined urchin population characteristics, bioerosion rates, their fish predators (Labridae), and potential competitors (Scaridae) on unprotected reefs and a reef within a marine protected area (MPA) in the lagoonal regions off Belize. Urchin density (<1 m⁻²) and bioerosion rates (∼0.2 kg CaCO₃ m⁻² year⁻¹) were lowest and members of the Labridae were the highest (∼20 fish 200 m⁻³) within the MPA, while several unprotected reefs had higher (∼18–40 m⁻²) urchin densities, lower Labridae abundances (1–3 fish 200 m⁻³), and bioerosion rates ranging from ∼0.3–2.6 kg CaCO₃ m⁻² year⁻¹. Urchin abundances were inversely related to Labridae (wrasses and hogfish) densities; however, on reef ridges, low algal cover (∼15%), small urchin size (∼14 mm), and low proportion of organic material in urchin guts suggested food limitation. Both top–down (predation) and bottom–up factors (food limitation) likely contribute to the control of urchins, predominantly Echinometra viridis, off Belize, thereby potentially diminishing the negative impacts of bioerosion activities by urchins.     

Ecology of marine algae on rocky shores and subtidal reefs in temperate Australia

Experiments in intertidal and subtidal rocky marine habitats in temperate Australia have identified the effects of various biological and physical factors on algal assemblages. In intertidal habitats, these involve micro- and macro-algae and grazing by gastropods. In subtidal habitats, interactions among micro- and macro-algae, echinoids, gastropods, micro-invertebrates and sessile invertebrates have been studied. Experimental studies on physical disturbances of algal assemblages have focussed on the effects of desiccation and storms. Most studies have not considered more than one spatial or temporal scale. Few have been concerned with seasonal influences and fewer have been concerned with variation from year to year. Most of the work lacks applicability to biogeographic comparisons. More experimental work across a variety of spatial and temporal scales is required to determine significant biological and physical processes affecting structure of algal assemblages across broad areas of temperate Australia.     

Response of fish-based metrics to anthropogenic pressures in temperate rocky reefs

The increasing degradation of marine ecosystems as a result of increasing impact caused by anthropogenic pressures, urges for well-founded knowledge to develop efficient tools to appraise the quality status of fish assemblages, as required by the “Marine Strategy Framework Directive”. This study analyzed the structural and functional response of rocky fish assemblages to several pressures on the Portuguese coast, i.e. fishing, sewage discharges, port activities and thermal effluent, by selecting fish-based metrics that best distinguished disturbed from control areas. One of the novel aspects of this research is the integrated assessment made through the analysis of several metrics representing numerous attributes of fish assemblages (namely diversity, abundance, trophic structure, mobility, resilience, habitat association, nursery function), which contrasts with the most commonly used approaches that in general focus on fish species/families. PERMANOVA results showed significant differences on metrics composition for all pressures with the exception of the thermal effluent. Moreover, two major patterns of stress were identified: (1) selective pressure, which affects differentially the fish assemblages (fishing); (2) broad-range pressure, which affects the entire fish assemblage with metrics of several attributes (e.g. structure, resilience, trophic guilds, nursery function) responding to its presence (sewage discharges, port activities). Taking into account the sensitivity results (discriminant analysis and Mann–Whitney test), biological meaning and redundancy with other metrics (Spearman correlations), the following metrics were selected as the most suitable to detect changes on temperate reef fish assemblages: “density of generalist individuals”, “density of territorial individuals”, “density of large individuals with medium to high commercial value (>20 cm)”, “density of juveniles” and metrics relative to trophic guild (except zooplanktivores). Since metrics grouped species that have some degree of functional overlap, the present approach was useful to understand human-induced changes at the assemblage level, contributing for the future use of marine fishes as biological indicators.     

Predator diversity and density affect levels of predation upon strongly interactive species in temperate rocky reefs

Indirect effects of predators in the classic trophic cascade theory involve the effects of basal species (e.g. primary producers) mediated by predation upon strongly interactive consumers (e.g. grazers). The diversity and density of predators, and the way in which they interact, determine whether and how the effects of different predators on prey combine. Intraguild predation, for instance, was observed to dampen the effects of predators on prey in many ecosystems. In marine systems, species at high trophic levels are particularly susceptible to extinction (at least functionally). The loss of such species, which is mainly attributed to human activities (mostly fishing), is presently decreasing the diversity of marine predators in many areas of the world. Experimental studies that manipulate predator diversity and investigate the effects of this on strongly interactive consumers (i.e. those potentially capable of causing community-wide effects) in marine systems are scant, especially in the rocky sublittoral. I established an experiment that utilised cage enclosures to test whether the diversity and density of fish predators (two sea breams and two wrasses) would affect predation upon juvenile and adult sea urchins, the most important grazers in Mediterranean sublittoral rocky reefs. Changes in species identity (with sea breams producing major effects) and density of predators affected predation upon sea urchins more than changes in species richness per se. Predation upon adult sea urchins decreased in the presence of multiple predators, probably due to interference competition between sea breams and wrasses. This study suggests that factors that influence both fish predator diversity and density in Mediterranean rocky reefs (e.g. fishing and climate change) may have the potential to affect the predators' ability to control sea urchin population density, with possible repercussions for the whole benthic community structure.     

Seasonal and interannual variation in fish assemblages of northern temperate rocky subtidal habitats

Fish assemblages on two inshore rocky subtidal sites on the west coast of Scotland, were studied using diver visual surveys on a monthly basis between September 1995 and December 1999. A total of 17 689 fishes and 26 species were recorded from the two sites, Saulmore Point (056°27'N; 005°24'W) near Oban and Davy's Rock (055°46'N; 004°53'W) on the Isle of Great Cumbrae. The gobiid Thorogobius eppiphiatus , dominated the Saulmore Point site; six fish species accounted for >93% of total abundance at that site. At Davy's Rock four species contributed at least 93% of total fish abundance, and the dominant species was the labrid Ctenolabrus rupestris . Total abundance of the dominant species displayed a clear seasonal trend, and this was significantly related to recorded daily average seawater temperature. A maximum abundance of 4.9 fishes m⁻² was recorded in November 1998 at Davy's Rock and 2.5 fishes m⁻² at Saulmore Point in October 1998. Multivariate analysis indicated a degree of variation in assemblage structure between winter and summer at both sites. A number of species showed some degree of interannual variation, in particular the gobiid Gobiusculus flavescens whose abundance increased by over 300 times over a 5 month period in 1998. Correlation analysis showed that variation in annual winter seawater temperature could act as an indicator of interannual variation in abundance of some of the dominant species utilizing rocky subtidal habitats.     

Precision and accuracy of visual census technique for fish assemblages on coral patch reefs

Synopsis A visual census technique is described in which the results of three separate enumerations of fish at a site are combined to produce a best estimate of the fish fauna present. Its precision and accuracy are examined, and compared to those of censuses obtained by modifications of the technique. Visual censuses can display high repeatability, but they seldom (if ever) completely sample the fish present at a site. Accuracy varies with technique used. In our tests, the preferred method yielded 82% of species and 75% of individuals known to be present and potentially censurable at the time the observations were made. Visual censuses are of comparable accuracy to ichthyocide collections of unenclosed sites, but the two methods sample different components of the total fish fauna. It is important when using visual censuses to remember that their accuracy is not 100%.     

The effects of food-web management on fish assemblage dynamics in a north temperate lake

There was an intensive trawl fishery in the Enonselkä basin in Lake Vesijärvi (southern Finland) in 1989–1993 and thereafter the fishing intensity declined. The fish assemblage dynamics were studied both during the years of intensive trawling and for 3 years afterwards. Roach Rutilus rutilus dominated the fish assemblage before the mass removal. The intensive trawling effectively diminished the roach stock and the present fishing intensity has been sufficient to prevent its recovery. There were significant decreasing trends in the gillnet cpue (catch per unit effort) of bream Abramis brama (L.) and bleak Alburnus alburnus (L.) while increasing trend was observed in cpue of ruffe Gymnocephalus cernuus (L.) and vendace Coregonus albula (L.). Perch Perca fluviatilis L. and pikeperch Stizostedion lucioperca (L.) showed variations in cpue but no trends existed. The proportion of cyprinids decreased while that of percids and coregonids increased in the catches. The changes in the fish assemblage were induced by the intensive fishing and by the concomitant decline in the ecosystem productivity.     

Kenyan coral reef lagoon fish: effects of fishing,substrate complexity,and sea urchins

Population density, number of species, diversity, and species-area relationships of fish species in eight common coral reef-associated families were studied in three marine parks receiving total protection from fishing, four sites with unregulated fishing, and one reef which recently received protection from fishing (referred to as a transition reef). Data on coral cover, reef topographic complexity, and sea urchin abundance were collected and correlated with fish abundance and species richness. The most striking result of this survey is a consistent and large reduction in the population density and species richness of 5 families (surgeonfish, triggerfish, butterflyfish, angelfish, and parrotfish). Poor recovery of parrotfish in the transition reef, relative to other fish families, is interpreted as evidence for competitive exclusion of parrotfish by sea urchins. Reef substrate complexity is significantly associated with fish abundance and diversity, but data suggest different responses for protected versus fished reefs, protected reefs having higher species richness and numbers of individuals than unprotected reefs for the same reef complexity. Sea urchin abundance is negatively associated with numbers of fish and fish species but the interrelationship between sea urchins, substrate complexity, coral cover, and management make it difficult to attribute a set percent of variance to each factor-although fishing versus no fishing appears to be the strongest variable in predicting numbers of individuals and species of fish, and their community similarity. Localized species extirpation is evident for many species on fished reefs (for the sampled area of 1.0 ha). Fifty-two of 110 species found on protected reefs were not found on unprotected reefs.     

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.     

Invasion dynamics on a temperate rocky shore: from early invasion to establishment of a marine invader

Populations of invading Asian shore crabs (Hemigrapsus sanguineus) and resident crabs (European green crab Carcinus maenas or species in the family Panopeidae) were monitored for up to 12 years along a south to north orientation on the open coast of Massachusetts and within the Narragansett Bay estuary, Rhode Island. At all sites, densities of resident crabs declined as H. sanguineus increased in abundance. Population dynamics were divided into 3 stages of the invasion (early, mid, late) based on statistically different densities of invading and resident crabs. Early in the invasion on the open coast, the relatively few H. sanguineus present had a wide range of individual sizes. By late in the invasion, relatively small crabs [<10 mm carapace width (CW)] constituted half of the population. Few seasonal differences occurred. Carcinus maenas, dominant at the coastal sites early in the invasion, showed strong recruitment of small individuals in the fall early in the invasion. Overall numbers of C. maenas declined as the H. sanguineus invasion progressed, and very few crabs >10 mm CW were present late in the invasion. In Narragansett Bay, a recruitment peak of H. sanguineus occurred shortly before it surged in population size, and large crabs were abundant late in the invasion. Geographic comparisons of H. sanguineus populations in southern New England showed similar growth trajectories. Monitoring populations of resident and invading species at multiple locations from early invasion through clear establishment of the invader allows a more complete understanding of the population dynamics of marine species invasions.     

Effects of coral transplantation and giant clam restocking on the structure of fish communities on degraded patch reefs

Active restoration is being practiced to supplement conservation activities for the purpose of reversing the trend of reef degradation. In the last decade, the feasibility of different restoration approaches such as coral transplantation and restocking of other marine biota has been the focus of research and relatively few have examined experimentally its effects on the resultant communities. In this study, coral transplantation and giant clam restocking were applied on 25 degraded patch reefs (~ 25 m²) inside a marine sanctuary in Pangasinan, northwestern Philippines to examine their effects on the community structure of reef fishes. Five interventions or treatments were employed: 1) “coral” consisted of transplantation of a combination of Acropora spp. and Pocillopora spp. on concrete blocks; 2) “clam” consisted of restocking of Tridacna gigas; 3) “clam+coral” consisted of restocking of T. gigas with Acropora spp. transplanted on their shells; 4) “shell” consisted of deployment of T. gigas shells; and 5) “control” consisted of no intervention. Fish communities on the patch reefs were monitored monthly for 3 months before the intervention and were monitored further for 11 months after the intervention, including 1 recruitment season. After the intervention, the coral cover and the “other biota” category increased in the coral and clam+coral treatments, due to the transplanted corals and deployed giant clams. Consequently, the complexity of the substrate was enhanced. A month after the intervention, a rapid increase in the abundance and species richness of reef fishes on the coral, clam+coral and clam treatments was observed compared to the shell and control treatments. A change in species composition of reef fish assemblage was also apparent in the coral and clam+coral treatments relative to the clam, shell and control, especially 4 months after the intervention. The present experiment demonstrates the feasibility of improving the condition of degraded patch reefs, which can subsequently enhance the fish community. Results also show the importance of the underlying substratum and the abundance of live corals and clams to reef fishes.     

Forgotten underwater forests: The key role of fucoids on Australian temperate reefs

Kelp forests dominated by species of Laminariales are globally recognized as key habitats on subtidal temperate rocky reefs. Forests characterized by fucalean seaweed, in contrast, receive relatively less attention despite being abundant, ubiquitous, and ecologically important. Here, we review information on subtidal fucalean taxa of Australia's Great Southern Reef, with a focus on the three most abundant and widely distributed genera (Phyllospora, Scytothalia, and Sargassum) to reveal the functionally unique role of fucoids in temperate reef ecology. Fucalean species span the entire temperate coastline of Australia (~71,000 km²) and play an important role in supporting subtidal temperate biodiversity and economic values on rocky reefs as well as in adjacent habitats. Climatic and anthropogenic stressors have precipitated significant range retractions and declines in many fucoids, with critical implications for associated assemblages. Such losses are persistent and unlikely to be reversed naturally due to the life history of these species and colonization of competitors and grazers following loss. Active restoration is proving successful in bringing back some fucoid species (Phyllospora comosa) lost from urban shores and will complement other passive and active forms of conservation. Fucalean forests play a unique role on subtidal temperate reefs globally, especially in Australia, but are comparatively understudied. Addressing this knowledge gap will be critical for understanding, predicting, and mitigating extant and future loss of these underwater forests and the valuable ecosystem services they support.     

Reef fish assemblage structure affected by small-scale spacing and size variations of artificial patch reefs

To examine how varying the distance between patch reefs affects reef fish assemblage structure, replicate concrete reef modules (∼ 1 m³ each) were deployed on sand bottom at 8 m depth off Ft. Lauderdale, Florida, USA (26°07N, 80°05W). Modules were positioned at the apices of one of four differently sized equilateral triangles. Triangular configurations had side lengths of: 25 m, 15 m, 5 m, and 0.33 m; each treatment with two replicates. Two additional configurations: (1) a solitary module (Single) and (2) two modules side by side (Double), also with two replicates, were deployed in order to examine the interaction of reef size with fish assemblages. SCUBA divers censused fishes monthly, for 2 years, recording the species present, their abundance and sizes (TL). Fishes were assigned to one of five length categories: < 2 cm, > 2-5 cm, > 5-10 cm, > 10-20 cm, and > 20 cm. In general and excluding the smallest three-module spacing treatment (0.33 m treatment), which may have provided unique treatment-specific refuge, total fish abundance and richness were shown to increase when isolation distance increased. However, there were also species-specific and size class differences in response to isolation distance. The second part of this study indicated varying reef size, by doubling and tripling the number of reef modules, increased total fish abundance and species richness. Nevertheless, fish abundance and species richness did not change by an identical multiplier (e.g., doubling modules ≠ double abundance). These results suggest that scientists and marine managers alike should consider reef size and isolation as habitat attributes capable of altering the structure and dynamics of reef fish assemblages.     

Movements of hatchery-reared lingcod released on rocky reefs in Puget Sound

Fourteen sub-adult hatchery-reared lingcod (Ophiodon elongatus) were released onto reefs in South Puget Sound, Washington, USA to evaluate their movement behavior. Acoustic telemetry revealed variation in movement among individuals that was related to body size. Larger lingcod tended to leave the release reef sooner than smaller lingcod. Four lingcod left the reefs less than 10 days after release, while three lingcod left between one and 4 months after release. Seven lingcod remained at the release reefs for the entire 5-month study, though they did make apparent short-term (< 24 h duration) excursions away from the reefs. Data suggest that the frequency and duration of excursions increase with age and size in both wild and hatchery lingcod. Movement data from these hatchery lingcod and previously published studies on wild lingcod are compared.