Nagelkerken, I.

Differences in fish community structure between different estuaries, lagoons and bays can be very large, and generalisations are complicated by the use of a wide variety of sampling methods. In the present study, fish communities of subtidal seagrass beds and sandy seabeds in 13 marine embayments of a single Caribbean island were therefore sampled using a uniform method. The objective of the study was to determine whether the seagrass and sandy seabed habitats of various embayments are characterised by typical fish assemblages which differ in terms of taxa (species, families), size classes (life stages) and functional groups (ecological species groups, feeding time and diet). This was linked to the hypothesis that differences in fish assemblages between habitats in different embayments are larger at taxonomic levels than at the level of functional groups. A second objective was to determine the most useful discriminating features between the two habitat types. The above hypothesis was rejected, since differences in fish assemblages from different seagrass and sandy seabed sites did not increase from functional to taxonomic level, but from size class to diet/species to family/feeding time to ecological species group. However, the seagrass and sandy seabed habitats could each be characterised by typical fish assemblages which differed in taxonomical and functional group composition, irrespective of differences in environmental and biotic variables between the embayments in which these habitats were situated. The two habitat types could be best characterised on the basis of fish family, ecological species group, feeding time and size distribution. Seagrass beds mainly harboured nocturnally active nursery species (Haemulidae, Lutjanidae, etc.), whose relative abundance was related to vegetation (mainly seagrass) cover. Sandy seabeds mainly harboured diurnally active bay species (Gerreidae, etc.) whose relative abundance was related to cover of bare sand. Similarities in taxonomical and functional traits of fish species predicted whether they occurred more abundantly in seagrass beds or in sandy seabeds.

The objective of this study was to quantify the extent and nature of submerged debris at beaches in Curacao.

Juvenile French grunts (Haemulon flavolineatum) were caught, measured in length and weight and introduced in cages for 6 to 8 weeks to grow in their natural environments, the reef, the seagrass beds and the mangroves. After this period, weighing and measuring length was repeated and the growth could be calculated. The mangroves and seagrass beds are identified as nursery habitats for juvenile reef fish and are supposed to increase their survival chances. The nursery hypothesis claims nurseries are beneficial to the juveniles in several ways, including higher growth rates. This research, conducted on the Caribbean islands of Curaçao and Aruba proved the opposite for juveniles between 3.5 and 4 cm. The growth in length and weight of the fishes were higher, although only significantly in Aruba, in the reef habitat compared to the nurseries. Food samples collected on Aruba at each site were comparable with the growth results, because the highest food availability was found in th reef habitat. When comparing the growth of the fishes between two different locations (each location containing all three habitats) on the island of Aruba, significant differences in growth in length were found. Apart from the fact that the highest growth was found on the reef on both islands, there were little similarities found in growth between the islands. The correlation between environmental factors and the growth of the fishes were small, with only a correlation detected between water clarity and growth in weight and weight/length ratio on Aruba. Differences between the two main areas on Aruba and between the islands could, however, not be explained by this. In conclusion, mangrove and seagrass nurseries do not contribute to a higher growth rate of small juvenile French grunts.

The importance of seagrass beds and mangroves as a juvenile habitat as opposed to other shallow water habitat types is investigated using a single sampling method on four islands in the western Indian Ocean for Cheilinus undulatus, and on one island in the southern Caribbean Sea (Aruba) for Scarus guacamaia. Both species occur on the Red list of threatened species. Juveniles of Cheilinus undulatus were predominantly found on seagrass beds while adults were limited to the coral reef. The presence of seagrass beds resulted in significantly higher densities of the species on coral reefs in front of these habitats, indicating the importance of seagrass beds as a juvenile habitat. For Scarus guacamaia, juveniles were exclusively observed in mangroves while adults only occurred on the coral reef. Adult S. guacamaia occurred on all coral reefs along the sheltered coast of the island containing mangroves, but no relationship with distance to mangroves was observed. This could indicate the importance of mangroves for the occurrence of adults of this species on the scale of an entire island.

Alterations in predation pressure can have large e ects on trophically-structured systems. Modi cation of predator behaviour via ocean warming has been assessed by laboratory experimentation and metabolic theory. However, the in uence of ocean acidi cation with ocean warming remains largely unexplored for mesopredators, including experimental assessments that incorporate key components of the assemblages in which animals naturally live. We employ a combination of long-term laboratory and mesocosm experiments containing natural prey and habitat to assess how warming and acidi cation a ect the development, growth, and hunting behaviour in sharks. Although embryonic development was faster due to temperature, elevated temperature and CO2 had detrimental e ects on sharks by not only increasing energetic demands, but also by decreasing metabolic e ciency and reducing their ability to locate food through olfaction. The combination of these e ects led to considerable reductions in growth rates of sharks held in natural mesocosms with elevated CO2, either alone or in combination with higher temperature. Our results suggest a more complex reality for predators, where ocean acidi cation reduces their ability to e ectively hunt and exert strong top-down control over food webs. 

Various species of aquatic animals have complex life cycles and utilize different habitats during consecutive phases of their life cycles. For example, many marine fish species occupy different habitat types during juvenile and adult life stages. Juveniles of some species recruit to inshore nursery habitats such as mangroves and seagrass beds, whereas large adults tend to dominate coral reefs. The mechanisms underlying apparent cross-habitat distribution patterns by life stage remain uncertain for many species. Here, we investigated potential mechanisms that produce a 5-phase, and possibly even a 6-phase life cycle pattern in a common Caribbean coral reef fish species Haemulon flavolineatum (French grunt) across multiple coastal habitats. At each discrete life stage, individuals were faced with important and stage-specific ecological trade-offs that could significantly augment fitness. Pelagic larvae settled on rubble habitats near bay entrances where they reached an optimum between predation risk (survival) and food abundance (growth). Individuals subsequently shifted to seagrass beds, likely as a result of increased food resources, followed by a shift to mangroves as predation refugia. Before the uni-directional movement between bays and coral reefs, some fishes shifted from mangroves to boulder/notch habitats. Likely, this habitat serves as an intermediate stop before their final shift to the coral reef, where they reach maturity and reproduce. This study reveals ecological linkages and flows among habitat types that are of direct conservation importance to these ecosystems. Furthermore, the identification of mechanisms that give rise to cross-habitat distribution patterns of marine fishes in general might lead to enhanced conservation management solutions to declines in fisheries at larger scales. 

Abstract

The importance of mangrove and seagrass lagoonal habitats as nursery areas for many reef-associated fish species is well established in the scientific literature. However, few studies have examined the relative use by nursery species of different sub-habitats within such systems. Here, we investigated fish community structure of a variety of interconnected sub-habitats of the tropical lagoon of Lac Bay in Bonaire, Dutch Caribbean. Visual census was used to test the degree to which these sub-habitats may differ in their use by fishes of different species and life stages. We quantitatively sampled the fish species abundance, composition, and size structures at a total of 162 sites distributed among nine different sub-habitats that are common to mangrove and seagrass ecosystems. Fish community variables differed consistently among sub-habitats and were mainly influenced by the presence of mangrove root structure or seagrass cover. Mangrove fringe sub-habitats were a premier habitat since multiple life stages of a variety of species showed highest densities and biomass there. Several reef fish species had a distribution pattern suggesting a unique stepwise post-settlement life cycle migration in which larger juveniles and/or subadults appear to move from the open bay environment (seagrass beds or bay mangrove fringe) to the interior mangrove fringes along mangrove pools before later departing to the adult habitat of the coral reef. In the case of the well-lit and well-circulated central bay sub-habitat, the limiting factor to fish abundance and diversity appeared to be the paucity of three-dimensional shelter due to the lack of Thalassia seagrass beds. In the warm and hypersaline backwaters, physiological tolerance limits were likely a key limiting factor. Long-term changes driven by mangrove expansion into this non-estuarine lagoon have been steadily reducing the net coverage of clear bay waters, while the surface of shallow, muddy, and hypersaline backwaters, unusable by key nursery reef fish species, has been increasing by an almost equal amount. Our study shows how fish density varies along the full gradient of sub-habitats found across a tropical bay to provide insight into the potential consequences for nursery habitat function when the availability and quality of these sub-habitats change in response to the long-term dynamic processes of mangrove land reclamation and climate change.

Abstract:

Marine spatial population dynamics are often addressed with a focus on larval dispersal, without taking into account movement behavior of individuals in later life stages. Processes occurring during demersal life stages may also drive spatial population dynamics if habitat quality is perceived differently by animals belonging to different life stages. In this study, we used a dual approach to understand how stage-structured habitat use and dispersal ability of adults shape the population of a marine fish species. Our study area and focal species provided us with the unique opportunity to study a closed island population. A spatial simulation model was used to estimate dispersal distances along a coral reef that surrounds the island, while contributions of different nursery bays were determined based on otolith stable isotope signatures of adult reef fish. The model showed that adult dispersal away from reef areas near nursery bays is limited. The results further show that different bays contributed unequally to the adult population on the coral reef, with productivity of juveniles in bay nursery habitat determining the degree of mixing among local populations on the reef and with one highly productive area contributing most to the island’s reef fish population. The contribution of the coral reef as a nursery habitat was minimal, even though it had a much larger surface area. These findings indicate that the geographic distribution of nursery areas and their productivity are important drivers for the spatial distribution patterns of adults on coral reefs. We suggest that limited dispersal of adults on reefs can lead to a source–sink structure in the adult stage, where reefs close to nurseries replenish more isolated reef areas. Understanding these spatial population dynamics of the demersal phase of marine animals is of major importance for the design and placement of marine reserves, as nursery areas contribute differently to maintain adult populations.

Free-swimming larvae of tropical corals go through a critical life-phase when they return from the open ocean to select a suitable settlement substrate. During the planktonic phase of their life cycle, the behaviours of small coral larvae (<1 mm) that influence settlement success are difficult to observe in situ and are therefore largely unknown. Here, we show that coral larvae respond to acoustic cues that may facilitate detection of habitat from large distances and from upcurrent of preferred settlement locations. Using in situ choice chambers, we found that settling coral larvae were attracted to reef sounds, produced mainly by fish and crustaceans, which we broadcast underwater using loudspeakers. Our discovery that coral larvae can detect and respond to sound is the first description of an auditory response in the invertebrate phylum Cnidaria, which includes jellyfish, anemones, and hydroids as well as corals. If, like settlement-stage reef fish and crustaceans, coral larvae use reef noise as a cue for orientation, the alleviation of noise pollution in the marine environment may gain further urgency. 

Findings:

The main conclusion of this study is that the shallow, warm and saline back-water habitat which is continuing to increase in importance within Lac Bay is unable to support meaningful mangroves, seagrass or algal meadows, nor the key nursery species. As the natural process of land reclamation by mangroves carries on, the bay’s important nursery habitats will come under additional salinity stress and likely continue to decrease in coverage and quality at an accelerated rate.

Distribution of sea grass and algal beds in Lac Bay

• The valuable seagrass and mangrove habitats of Lac are currently trapped in an enclosed bay.
• High light-intensity and well-circulated shallow habitats that fringed the mangroves of the central bay have the richest assemblages with the highest biotic coverage.
• Isolated mangrove pools have the lowest total cover, species richness and biodiversity of all habitats.
• Biotic diversity and cover decrease towards the deeper parts of the bay.
• There is an alarmingly rapid invasion of the bay by the invasive seagrass H. stipulacea.

Fish species utilization of contrasting habitats in Lac Bay

• Fish community variables differ consistently among habitats and are influenced by the percent cover of seagrass vegetation or presence of mangrove-root structure.
• Mangrove fringe habitats are a premier habitat since multiple life stages of a variety of species showed highest densities there. Mangrove fringing open waters had highest overall fish densities and species diversity.
• The various vegetated sub-habitats all play a unique role for different size-classes of different fish species. 

Management Recommendations:

• Management action is needed to stem further erosion of nursery habitat quality and ensure that a tipping-point is not reached beyond which recovery may be difficult or impossible.
• Measures should be taken to help restore water depth and circulation to relieve the bay’s ecosystem of thermal and salinity stress caused by the shallow backwaters. This includes excavating accumulated erosional and biogenic sediments as well as dredging to restore former feeder channels by removal of mangrove overgrowth (as already started by Stinapa).
• Further studies to assess the impacts of the invasive seagrass H. stipulacea on the bay’s flora and fauna.