As humans expand into natural environments, populations of wild organisms may become relegated to marginal habitats at the boundaries of their distributions. In the ocean, mesophotic coral ecosystems (30–150 m) at the depth limit of photosynthetic reefs are hypothesized to act as refuges that are buffered from anthropogenic and natural disturbances, yet the viability and persistence of subpopulations in these peripheral habitats remains poorly understood. To assess the potential for mesophotic reefs to support robust coral reef fish populations, we compared population density and structure, growth, size, and reproductive output of the bicolor damselfish (Stegastes partitus) from shallow (<10 m), deep shelf (20–30 m), and mesophotic reefs (60–70 m) across the Florida Platform. Population densities decreased and size and age distributions shifted toward older and larger individuals in deeper habitats. Otolith-derived ages indicated that S. partitus found on mesophotic reefs reach larger asymptotic sizes and have longer lifespans than fish in shallower habitats. Based on measurements of oocyte area and batch fecundity, mesophotic fish also have higher reproductive investment. These demographic patterns indicate that mesophotic fish populations composed of large, fecund individuals produce high condition larvae and rely on longevity of individuals for population persistence and viability.

Environmental clines such as latitude and depth that limit species' distributions may be associated with gradients in habitat suitability that can affect the fitness of an organism. With the global loss of shallow-water photosynthetic coral reefs, mesophotic coral ecosystems (*30-150 m) may be buffered from some environmental stressors, thereby serving as refuges for a range of organisms including mobile obligate reef dwellers. Yet habitat suitability may be diminished at the depth boundary of photosynthetic coral reefs. We assessed the suitability of coral-reef habitats across the majority of the depth distribution of a common demersal reef fish (Stegastes partitus) ranging from shallow shelf (SS, \10 m) and deep shelf (DS, 20-30 m) habitats in the Florida Keys to mesophotic depths (MP, 60-70 m) at Pulley Ridge on the west Florida Shelf. Diet, behavior, and potential energetic trade-offs differed across study sites, but did not always have a monotonic relationship with depth, suggesting that some drivers of habitat suitability are decoupled from depth and may be linked with geographic location or the local environment. Feeding and diet composition differed among depths with the highest consumption of annelids, lowest ingestion of appendicularians, and the lowest gut fullness in DS habitats where predator densities were highest and fish exhibited riskaverse behavior that may restrict foraging. Fish in MP environments had a broader diet niche, higher trophic position, and higher muscle C:N ratios compared to shallower environments. High C:N ratios suggest increased tissue lipid content in fish in MP habitats that coincided with higher investment in reproduction based on gonado-somatic index. These results suggest that peripheral MP reefs are suitable habitats for demersal reef fish and may be important refuges for organisms common on declining shallow coral reefs.

, J. Reed, and S. Sponaugle. 2016. Habitat availability and depth-driven population demographics regulate reproductive output of a coral reef fish. Ecosphere 7(11):e01542. 10. 1002/ecs2.1542 Abstract. Global habitat decline may displace organisms from optimal environments, increasing reliance on ecosystems with lower habitat suitability and availability. For coral reef fishes, potentially marginal mesophotic coral ecosystems (~30-150 m) may be buffered from anthropogenic stressors; however, variation in habitat quality across depths can alter population demographics, reproductive output, and subpopulation size, potentially restricting the ability for peripheral habitats to support declining populations through larval supply. This study incorporated population density, benthic habitat, and depth-stratified population demographics to assess bicolor damselfish (Stegastes partitus) subpopulation reproductive output from a broad geographic region encompassing the known depth distribution of the species, including coral reefs in the Florida Keys (0-35 m depths) and mesophotic reefs (~60-90 m) at Pulley Ridge (PR) on the west Florida Shelf. Results indicated that densities of bicolor damselfish peaked in mid-shelf (10-20 m) and deep-shelf (20-30 m) habitats, and subpopulation sizes and reproductive output peaked at mid-depths (10-20 m) in the Florida Keys and declined as depth increased. Subpopulation egg production was affected by differences in demographics across depths, including fish size, sex ratios, and a lower probability and frequency of spawning in deeper habitats. Despite low population densities on mesophotic reefs, the expansive reef area at PR resulted in an estimated subpopulation size that comprised ~14% of the population in the study region, and ~9% of the total reproductive output, indicating that peripheral mesophotic reefs may be sources of larvae that can subsidize declining populations. Larval dispersal and population connectivity models used to inform ecosystem management should incorporate spatially explicit demographics across depth distributions and habitat availability that have substantial effects on egg production and larval supply.

Pacific capelin Mallotus catervarius are planktivorous small pelagic fish that serve an intermediate trophic role in marine food webs. Due to the lack of a directed fishery or monitoring of capelin in the Northeast Pacific, limited information is available on their distribution and abundance, and how spatio-temporal fluctuations in capelin density affect their availability as prey. To provide information on life history, spatial patterns, and population dynamics of capelin in the Gulf of Alaska (GOA), we modeled distributions of spawning habitat and larval dispersal, and synthesized spatially indexed data from multiple independent sources from 1996 to 2016. Potential capelin spawning areas were broadly distributed across the GOA. Models of larval drift show the GOA’s advective circulation patterns disperse capelin larvae over the continental shelf and upper slope, indicating potential connections between spawning areas and observed offshore distributions that are influenced by the location and timing of spawning. Spatial overlap in composite distributions of larval and age-1+ fish was used to identify core areas where capelin consistently occur and concentrate. Capelin primarily occupy shelf waters near the Kodiak Archipelago, and are patchily distributed across the GOA shelf and inshore waters. Interannual variations in abundance along with spatio-temporal differences in density indicate that the availability of capelin to predators and monitoring surveys is highly variable in the GOA. We demonstrate that the limitations of individual data series can be compensated for by integrating multiple data sources to monitor fluctuations in distributions and abundance trends of an ecologically important species across a large marine ecosystem.