Anthropogenic nutrient enrichment and physical characteristics result in low dissolved oxygen concentrations (hypoxia) in estuaries and semienclosed seas throughout the world. Published research indicates that within and near oxygen-depleted waters, finfish and mobile macroinvertebrates experience negative effects that range from mortality to altered trophic interactions. Chronic exposure to hypoxia and fluctuating oxygen concentrations impair reproduction, immune responses, and growth. We present an analysis of hypoxia, nitrogen loadings, and fisheries landings in 30 estuaries and semien-closed seas worldwide. Our results suggest that hypoxia does not typically reduce systemwide fisheries landings below what would be predicted from nitrogen loadings, except where raw sewage is released or particularly sensitive species lose critical habitat. A number of compensatory mechanisms limit the translation of local-scale effects of hypoxia to the scale of the whole system. Hypoxia is, however, a serious environmental challenge that should be considered in fisheries management strategies and be a direct target of environmental restoration.

Human alteration of land cover (e.g., urban and agricultural land use) and shoreline hardening (e.g., bulkheading and rip rap revetment) are intensifying due to increasing human populations and sea level rise. Fishes and crustaceans that are ecologically and economically valuable to coastal systems may be affected by these changes, but direct links between these stressors and faunal populations have been elusive at large spatial scales. We examined nearshore abundance patterns of 15 common taxa across gradients of urban and agricultural land cover as well as wetland and hardened shoreline in tributary subestuaries of the Chesapeake Bay and Delaware Coastal Bays. We used a comprehensive landscape-scale study design that included 587 sites in 39 subestuaries. Our analyses indicate shoreline hardening has predominantly negative effects on estuarine fauna in water directly adjacent to the hardened shoreline and at the larger system-scale as cumulative hardened shoreline increased in the subestuary. In contrast, abundances of 12 of 15 species increased with the proportion of shoreline comprised of wetlands. Abundances of several species were also significantly related to watershed cropland cover, submerged aquatic vegetation, and total nitrogen, suggesting land-use-mediated effects on prey and refuge habitat. Specifically, abundances of four bottom-oriented species were negatively related to cropland cover, which is correlated with elevated nitrogen and reduced submerged and wetland vegetation in the receiving subestuary. These empirical relationships raise important considerations for conservation and management strategies in coastal environments.

Building on previous analyses suggesting that the composition of fishery landings reflects the effects of eutrophication on mobile fish and benthos, we examined landings composition in relation to nitrogen loading and the spatial extent of hypoxia in a cross-system comparison of 22 ecosystems. We hypothesized that explicit consideration of both N and hypoxia is important because nutrient enrichment has been shown to have contrasting direct and indirect effects on fisheries. Consistent with this premise, patterns in landings composition differed with respect to N load and the spatial extent of hypoxia. For example, the ratios of pelagic to benthic and demersal biomass in fishery landings (P/D) exhibited a decreasing trend across ecosystems with progressively higher N but were significantly and positively correlated with the spatial extent of hypoxia. The P/D ratios were particularly high in systems with extensive and persistent hypoxia and particularly low in several estuaries where purse seining is prohibited or not used. In analyses that considered all systems, benthic and demersal landings did not decrease at high N as predicted by previous conceptual models, and the negative association with the spatial extent of hypoxia was statistically significant only when the Black Sea was included in the analysis. Landings of pelagic planktivores did not vary with the spatial extent of hypoxia but were positively related to N for all systems combined and for semi-enclosed seas. The trophic and size composition of fishery landings were not related to N or hypoxia, perhaps because landings of large, high-trophic-level species are more influenced by fishery exploitation or practices that mask the effects of water quality. Our results suggest that the response of fisheries to eutrophication differs from prevailing paradigms, which do not clearly distinguish between nutrient and hypoxia effects on fishery landings and do not consider the important influence of fishing practices and regulations on patterns in landings data.

Stable isotope analysis has become a common tool for mapping trophic relationships, describing foodweb changes, and assessing ecosystem health. Clear interpretation of stable isotopes is facilitated by understanding how environmental factors can affect isotopic values; in estuarine systems, these factors may include salinity, land use, and shoreline habitat. To evaluate these factors, fish were collected from shallow-water habitats next to hardened (bulkhead and riprap) and unhardened (beach and marsh) shorelines within five subestuaries of the Chesapeake Bay that differed in predominant land use and salinity. This study focused on three common mid-Atlantic fish species: mummichog, Fundulus heteroclitus, Atlantic silverside, Menidia menidia, and white perch, Morone americana. Multiple regression analyses pointed to standard length, salinity, % of watershed as developed or crop land, and shoreline habitat type as important predictors for δ15N in all three species and for δ13C in mummichog and white perch. Further analysis controlling for the effects of salinity, land use, and fish size demonstrated that δ13C and δ15N were lower in tissues of fish collected next to marsh compared with hardened or beach habitat. Habitat effects were strongest for mummichog. This study focused on overarching patterns driving stable isotope signatures in fish; however, it also indicated potentially important interactions between nearshore habitat type and land use or salinity that deserve further analysis. Results have implications for the scale of isotope inquiry and give justification for more detailed follow-up studies of foodweb structure along modified and natural shorelines.

Coastal shoreline hardening is intensifying due to human population growth and sea level rise. Prior studies have emphasized shoreline-hardening effects on faunal abundance and diversity; few have examined effects on faunal biomass and size structure or described effects specific to different functional groups. We evaluated the biomass and size structure of mobile fish and crustacean assemblages within two nearshore zones (waters extending 3 and 16 m from shore) adjacent to natural (native wetland; beach) and hardened (bulkhead; riprap) shorelines. Within 3 m from shore, the total fish/crustacean biomass was greatest at hardened shorelines, driven by greater water depth that facilitated access to planktivore (e.g., bay anchovy) and benthivore-piscivore (e.g., white perch) species. Small-bodied littoral-demersal species (e.g., Fundulus spp.) had greatest biomass at wetlands. By contrast, total biomass was comparable among shoreline types within 16 m from shore, suggesting the effect of shoreline hardening on fish biomass is largely within extreme nearshore areas immediately at the land/water interface. Shoreline type utilization was mediated by body size across all functional groups: small individuals (≤60 mm) were most abundant at wetlands and beaches, while large individuals (>100 mm) were most abundant at hardened shorelines. Taxonomic diversity analysis indicated natural shoreline types had more diverse assemblages, especially within 3 m from shore, although relationships with shoreline type were weak and sensitive to the inclusion/exclusion of crustaceans. Our study illustrates how shoreline hardening effects on fish/crustacean assemblages are mediated by functional group, body size, and distance from shore, with important applications for management.

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