Simple ecological trade-offs give rise to emergent cross-ecosystem distributions of a coral reef fish

Ecosystems are intricately linked by the flow of organisms across their boundaries, and such connectivity can be essential to the structure and function of the linked ecosystems. For example, many coral reef fish populations are maintained by the movement of individuals from spatially segregated juvenile habitats (i.e., nurseries, such as mangroves and seagrass beds) to areas preferred by adults. It is presumed that nursery habitats provide for faster growth (higher food availability) and/or low predation risk for juveniles, but empirical data supporting this hypothesis is surprisingly lacking for coral reef fishes. Here, we investigate potential mechanisms (growth, predation risk, and reproductive investment) that give rise to the distribution patterns of a common Caribbean reef fish species, Haemulon flavolineatum (French grunt). Adults were primarily found on coral reefs, whereas juvenile fish only occurred in non-reef habitats. Contrary to our initial expectations, analysis of length-at-age revealed that growth rates were highest on coral reefs and not within nursery habitats. Survival rates in tethering trials were 0% for small juvenile fish transplanted to coral reefs and 24–47% in the nurseries. As fish grew, survival rates on coral reefs approached those in non-reef habitats (56 vs. 77–100%, respectively). As such, predation seems to be the primary factor driving across-ecosystem distributions of this fish, and thus the primary reason why mangrove and seagrass habitats function as nursery habitat. Identifying the mechanisms that lead to such distributions is critical to develop appropriate conservation initiatives, identify essential fish habitat, and predict impacts associated with environmental change

Effects of Marine Reserves versus Nursery Habitat Availability on Structure of Reef Fish Communities

No-take marine fishery reserves sustain commercial stocks by acting as buffers against overexploitation and enhancing fishery catches in adjacent areas through spillover. Likewise, nursery habitats such as mangroves enhance populations of some species in adjacent habitats. However, there is lack of understanding of the magnitude of stock enhancement and the effects on community structure when both protection from fishing and access to nurseries concurrently act as drivers of fish population dynamics. In this study we test the separate as well as interactive effects of marine reserves and nursery habitat proximity on structure and abundance of coral reef fish communities. Reserves had no effect on fish community composition, while proximity to nursery habitat only had a significant effect on community structure of species that use mangroves or seagrass beds as nurseries. In terms of reef fish biomass, proximity to nursery habitat by far outweighed (biomass 249% higher than that in areas with no nursery access) the effects of protection from fishing in reserves (biomass 21% lower than non-reserve areas) for small nursery fish (≤25 cm total length). For large-bodied individuals of nursery species (>25 cm total length), an additive effect was present for these two factors, although fish benefited more from fishing protection (203% higher biomass) than from proximity to nurseries (139% higher). The magnitude of elevated biomass for small fish on coral reefs due to proximity to nurseries was such that nursery habitats seem able to overrule the usually positive effects on fish biomass by reef reserves. As a result, conservation of nursery habitats gains importance and more consideration should be given to the ecological processes that occur along nursery-reef boundaries that connect neighboring ecosystems

Results of SIMPER analysis for nursery species, showing which species best explained differences in fish community between sites close to vs. isolated from nurseries (average dissimilarity = 53.2).

<p>Results of SIMPER analysis for nursery species, showing which species best explained differences in fish community between sites close to vs. isolated from nurseries (average dissimilarity = 53.2).</p

Results of linear mixed-effects models for fish biomass of nursery as well as all species with reserve presence and isolation from nurseries as crossed fixed effects and site as a random factor.

<p>No. of observations = 108,</p>*<p>P<0.05, NS = not significant.</p><p>Size indicates total fish length.</p

Map of the study area.

<p>Numbers 1–9 show the sampled reef sites (O) in fishery reserves (marine park zone) and in fished areas (non-reserves). Reef sites close (<1 km distance) to nursery habitats are site # 1, 2, 3, and 7, while those isolated (>3.5 km distance) from nurseries are site # 4, 5, 6, 8, and 9; site # 3 falls completely within the small northern marine park zone. The position of the replenishment zones (line fishing allowed on fish>20 cm in body length) and the environmental zone (no fishing of any kind allowed) is also indicated. Location of the various zones was obtained from the Cayman Islands marine park brochure. Grey represents land mass.</p

Fish biomass in marine reserves vs. fished areas with different proximity to nurseries (close vs. isolated).

<p>Mean total biomass per 100 m² (±standard error) across reef sites is shown for the entire size range (A) of nursery species and all species, and split (B, C) for small (≤25 cm total length) and large (>25 cm total length) fish. The black arrow indicates the reserve effect in absence of nurseries, whereas the dashed arrow indicates the nursery habitat effect in fished areas on small individuals of nursery species.</p

Non-metric multi-dimensional scaling plot for biomass of nursery species.

<p>The plot shows the ordination of the fish community at reef sites (numbered 1–9, see Fig. 1) that differ in fishery protection (reserve vs. fished) and nursery proximity (close vs. isolated).</p

Data from: Direct and indirect effects of nursery habitats on coral-reef fish assemblages, grazing pressure, and benthic dynamics

Migrating species are common within seascapes, but the potential for these movements to alter the populations and functional roles of non-migrating species (e.g. by increasing predation) is rarely investigated. This study considers whether the presence of nursery habitats (mangroves and seagrass) simply enhances the abundance of nursery-using parrotfishes and piscivores on nearby coral reefs, or also affects other parrotfishes. Data from 131 reef sites and multiple seascape configurations across 13 degrees of latitude were used to model correlations between biophysical variables, including nursery habitat connectivity, and the abundance and grazing pressure of both nursery-using species and other parrotfishes and piscivore biomass. Connectivity to mangroves and dense seagrass was positively correlated with the biomass of nursery-using species, but was also negatively correlated with non-nursery parrotfish populations. This reduction may be caused indirectly by nursery habitats increasing confamilial competition and predation by nursery-using piscivores, particularly affecting small parrotfishes settling directly onto reefs. As key reef grazers, parrotfishes affect coral demographics. Consequently, a spatial simulation model predicted the impacts after five years of changes in grazing pressure because of nursery habitat connectivity. The model demonstrated that high nursery connectivity was correlated to changes in grazing pressure on nearby reefs that could potentially lead to differences in coral cover of ∼3–4% when compared to low connectivity reefs. However, the direction of this change depended on the seascapes’ characteristics. Historically, large-bodied, nursery-using parrotfish would have increased grazing in all nursery-rich seascapes. Overfishing means that nursery availability may have spatially variable impacts on coral cover, influencing reserve design. This study suggests that nursery availability may directly and indirectly modify an ecological process, and alter an ecological cascade (migrating species increase predator and competitor abundances, affecting other grazers and consequently corals). Therefore, elucidating the multi-species impacts of animal movements is required to better understand ecosystem functioning