Human impacts on fisheries across the land-sea interface.

A long-held paradigm is that estuaries are important to the welfare of estuarine-dependent coastal fisheries, providing spawning areas, suitable salinity gradients for development, protection from predators, and food. Many publications link the beneficial nature of estuaries with fisheries production and decry human activities that modify habitat, and therefore potentially decrease fisheries production. However, what if the water quality of the estuary, often human-driven, results in less than suitable habitat for coastal fisheries productivity, both in the estuary and in the adjacent offshore waters? Finding the data to support relationships (positive and negative) among estuaries and coastal fisheries production is difficult because of inadequate data, highly variable data, or data with no clear linkages to processes. Hughes et al. (1), however, amassed a dense and multifaceted database of declining estuarine water quality in the form of worsening hypoxia to declines of fish diversity, reduced estuarine habitat suitability, and declines in offshore fisheries. The authors defend a plausible mechanism across the land– sea interface. Relationships among hypoxic waters and fisheries productivity (either catch per unit effort or landings) remain as elusive as relationships of nitrogen loading and fisheries productivity. Efforts to understand these ocean trophic models shifted over the 20th century from agricultural models of food stimulated by nutrients that feed higher primary producer standing stocks and secondary production to concerns of higher N and P loads into water bodies, excess phytoplankton biomass, and the deleterious effects of eutrophication, including harmful algal blooms and hypoxia (Fig. 1) (2–6). A meta-analysis by Micheli (7) of 47 food webs (natural and experimental) verified a relationship between nitrogen loading and primary production but not necessarily standing stocks of secondary consumers in the form of fisheries. Breitburg et al. (8) followed with a broad global analysis of 30 estuaries and semienclosed seas and found few strong relationships between N loading and fisheries production and hypoxia and fisheries production. Their divisions for fisheries were pelagic planktivores, benthopelagics, and benthics. Fisheries landings were positively related to N loading only for the benthopelagics and pelagics. None were related to the percent of the bottom water that was hypoxic. On more local or regional scales, however, there are documented decreases in landings or catch per unit effort with increase in area of hypoxia. Cod landings in the Baltic decline with an increase in stratification that affects the buoyancy of cod eggs so that they do not reach suitable habitat for development and with an increase in volume of hypoxic bottom-water that negatively affects survivability (9, 10). A similar decrease in cod landings in the western Atlantic is potentially related to increasing nutrient loads in the St. Lawrence River estuary and subsequent hypoxia (11, 12). The bottom-waters of the northern Gulf of Mexico west of the Mississippi River delta are severely low in dissolved oxygen on an annual basis through much of the spring and summer, and are related to increased N and P loading and shifting nutrient ratios (13–15). Two analyses (16, 17) found significantly negative relationships (P < 0.05) over the period from 1985–1997 and 1985–2004, respectively, when the brown shrimp catches from both Louisiana and Texas fisheries were combined and compared with the area of bottomwater hypoxia in midsummer. No statistically significant relationships were found between hypoxic zone area and white shrimp annual catch. White shrimp spawn at the time of year and location of the severest hypoxia. Brown shrimp are emigrating from the estuaries into severely hypoxic water at a different time of the year, and offshore migration is halted by the physical geographic location of the “dead zone.” These life history characteristics may explain part of the differences between the white and brown shrimp, but many other factors are equally important, such as fishing effort, timing of fishing effort, distance from port × price of fuel × small/large boat fishery, salinity of the nursery areas, economic impact of imported shrimp on the market, human-induced loss of habitat because of increased nutrient loads, and acreage of nursery area, which is proportional to the shrimp landings (18). Chesney and Baltz (19) further argue that the penaeid shrimp fishery is highly resilient to environmental stress and speculate that it is likely that other quantifiable impacts of greater magnitude may currently have more significant effects than hypoxia on the community structure and secondary production of nekton populations in the northern Gulf of Mexico.