Effects of freshwater release on oyster reef density, reproduction, and disease in a highly modified estuary

Few estuaries remain unaffected by water management and altered freshwater deliveries. The Caloosahatchee River Estuary is a perfect case study for assessing the impact of altered hydrology on natural oyster reef (Crassostrea virginica) populations. The watershed has been highly modified and greatly enlarged by an artificial connection to Lake Okeechobee. Accordingly, to generate data to support water management recommendations, this study monitored various oyster biometrics over 15 years along the primary salinity gradient. Oyster reef densities were significantly affected by both prolonged high volume freshwater releases creating hyposaline conditions at upstream sites and by a lack of freshwater input creating hypersaline conditions at downstream sites. Low freshwater input led to an increase in disease caused by Perkinsus marinus and predation. Moderate (< 2000 cfs) and properly timed (winter/spring) freshets benefited oysters with increased gametogenesis, good larval mixing, and a reprieve from disease. If high volume freshets occurred in the late summer, extensive mortality occurred at the upstream site due to low salinity. These findings suggest freshwater releases in the late summer, when reproductive stress is at its peak and pelagic larvae are most vulnerable, should be limited to < 2000 cfs, but that longer freshets (1-3 weeks) in the winter and early spring (e.g., December-April) benefit oysters by reducing salinity and lessening disease intensity. Similar strategies can be employed in other managed systems, and patterns regarding the timing of high volume flows are applicable to all estuaries where the management of healthy oyster reefs is a priority.

Mercury accumulation in sharks from the coastal waters of southwest Florida

As large long-lived predators, sharks are particularly vulnerable to exposure to methylmercury biomagnified through the marine food web. Accordingly, nonlethal means were used to collect tissues for determining mercury (Hg) concentrations and stable isotopes of carbon (δ(13)C) and nitrogen (δ(15)N) from a total of 69 sharks, comprising 7 species, caught off Southwest Florida from May 2010 through June 2013. Species included blacknose (Carcharhinus acronotus), blacktip (C. limbatus), bull (C. leucas), great hammerhead (Sphyrna mokarran), lemon (Negaprion brevirostris), sharpnose (Rhizoprionodon terraenovae), and tiger sharks (Galeocerdo cuvier). The sharks contained Hg concentrations in their muscle tissues ranging from 0.19 mg/kg (wet-weight basis) in a tiger shark to 4.52 mg/kg in a blacktip shark. Individual differences in total length and δ(13)C explained much of the intraspecific variation in Hg concentrations in blacknose, blacktip, and sharpnose sharks, but similar patterns were not evident for Hg and δ(15)N. Interspecific differences in Hg concentration were evident with greater concentrations in slower-growing, mature blacktip sharks and lower concentrations in faster-growing, young tiger sharks than other species. These results are consistent with previous studies reporting age-dependent growth rate can be an important determinant of intraspecific and interspecific patterns in Hg accumulation. The Hg concentrations observed in these sharks, in particular the blacktip shark, also suggested that Hg may pose a threat to shark health and fitness.

Mercury in the Gulf of Mexico: sources to receptors

Gulf of Mexico (Gulf) fisheries account for 41% of the U.S. marine recreational fish catch and 16% of the nation's marine commercial fish landings. Mercury (Hg) concentrations are elevated in some fish species in the Gulf, including king mackerel, sharks, and tilefish. All five Gulf states have fish consumption advisories based on Hg. Per-capita fish consumption in the Gulf region is elevated compared to the U.S. national average, and recreational fishers in the region have a potential for greater MeHg exposure due to higher levels of fish consumption. Atmospheric wet Hg deposition is estimated to be higher in the Gulf region compared to most other areas in the U.S., but the largest source of Hg to the Gulf as a whole is the Atlantic Ocean (>90%) via large flows associated with the Loop Current. Redistribution of atmospheric, Atlantic and terrestrial Hg inputs to the Gulf occurs via large scale water circulation patterns, and further work is needed to refine estimates of the relative importance of these Hg sources in terms of contributing to fish Hg levels in different regions of the Gulf. Measurements are needed to better quantify external loads, in-situ concentrations, and fluxes of total Hg and methylmercury in the water column, sediments, and food web.