The number of oxygen-starved "dead zones" in coastal waters around the world continues to increase, but recent studies question whether worsening water quality in places like the Chesapeake Bay is the primary factor that limits fish populations.
A study published in the Aug. 15 issue of Science identified 405 areas of hypoxia-low oxygen conditions-in coastal areas worldwide. Since publication, the tally has risen to "close to 450" said Robert Diaz, a professor at the Virginia Institute of Marine Science, who co-authored the paper with Rutger Rosenberg in Sweden.
"The information keeps rolling in," Diaz said. "It's unfortunate that they're still going up."
Hypoxia is one of the most notable impacts of eutrophication, the process in which excessive amounts of nitrogen and phosphorus flow into water bodies, where they also cause algae blooms, loss of underwater grass beds and other impacts.
Although the number of coastal waters that suffer hypoxia is growing, there is little convincing evidence that it is limiting fish production in most of those areas when the issue is considered at the scale of the whole estuary, according to a recent paper by Denise Breitburg, a senior scientist at the Smithsonian Environmental Research Center in Edgewater, MD, on which Diaz was a co-author.
Their paper, which appears in the 2009 Annual Review of Marine Science, concluded, "our analysis indicates that in many, perhaps most, temperate and subtropical estuaries and semi-enclosed seas, hypoxia is not likely to be the most important factor limiting landings of finfish and mobile macroinvertebrates."
Breitburg said fish populations are affected by multiple factors, and in many systems, fishing pressure is more important than water quality. While reducing nutrient pollution is important, that alone likely would not rebuild low fish populations, she said, and could create unrealistic expectations for the public.
"It's a matter of doing the right thing for the right reason," Breitburg said. "If we don't understand which problems are caused by nutrient pressures, and which are caused by fishing pressures, then we don't have any hope of reaching the conditions we would like to see."
For example, the loss of underwater grass habitat, which offer refuge for juvenile crabs, is often cited as an important factor for today's low blue crab abundance. But the population was higher in the early 1980s, when grass acreage in the Chesapeake was half of what it is now-a strong suggestion that habitat impacted by water quality is not the main factor limiting crab numbers.
"We know there are habitats that the crab likes, but are there habitats that are truly essential?" Diaz said. "That is the issue."
An alternative hypothesis is that the algae-filled water that blocks sunlight and kills grasses may also have the impact of protecting blue crab juveniles from visual predators, potentially increasing production, Breitburg said.
Overall, the analysis of 30 coastal waterways around the world, including the Chesapeake, found "little evidence" that increased nitrogen loadings or increased hypoxia reduce fish landings except in localized areas where raw sewage was discharged on an ongoing basis or where habitat for a particularly sensitive species was impacted. For instance, fluctuations in the amount of hypoxia in areas of the Baltic Sea used for spawning by Baltic cod affects egg mortality, and therefore the cod population.
Scientists found that total fish landings tended to increase with nitrogen inputs. The Chesapeake Bay, had annual nitrogen loadings of 14 metric tons per square kilometer, but had the highest landings of fish and mobile invertebrates of any coastal area examined in the study.
Yet fisheries data show that from the 1950 through the 1990s, total fisheries landings in the Bay have increased, except for bivalves, even though the amount of hypoxia has increased sharply. What has changed is the makeup of those landings-the menhaden catch has grown, for instance.
The difficulty in assessing the water quality impact on fish stems from a trade-off associated with nitrogen. By fertilizing the water, it spurs the growth of algae, which form the base of much of the aquatic food chain. Previous studies have shown that fish production typically increases with nitrogen inputs, and in some areas declined when nitrogen levels were reduced.
But when nitrogen levels become excessive and produce more nitrogen than can be consumed, it depletes oxygen from the water, causing large areas of hypoxia that are off-limits for most species. The algae blooms also cloud the water, causing the die-off of underwater grass beds, an important habitat for many species.
In some cases, the reduced habitat may squeeze fish into areas where they are more easily caught, which could mask a population decline, the paper noted.
But other factors could explain the lack of impact. Given the chance, fish likely avoid areas of low dissolved oxygen by swimming away. Some fish may adapt, perhaps by switching to a food source found away from hypoxic areas.
Nitrogen reductions are still warranted, the scientists said, because eutrophication causes a host of potential problems for fish. Exposure to moderate hypoxia can lead to reproductive impairment and lab experiments subjecting crabs and shrimp to hypoxia and pathogens found they were more susceptible to disease and mortality.
When predators and prey have different tolerances to hypoxia, it can disrupt aquatic food webs. And in the Chesapeake, scientists have suggested that hypoxic conditions make striped bass in the Bay more susceptible to mycobacteriosis, a disease that affects roughly 75 percent of the Bay's rockfish, by forcing them out of the deep, cooler waters they prefer in the summer and into warmer waters near the surface.
For individual species, the habitat impacts can be significant-bay scallops, for example, clearly need seagrass beds. "It would be kind of foolish to be seeding scallops into an area with no grass," Diaz said.
Further, the full impact of overfertilizing coastal waters may not yet be realized. "We've turned these systems into poorly managed agricultural systems that still seem to be good at producing protein, without knowing the long-term consequences," Breitburg said.
Another reason that habitat and fish landings don't show a strong link is that populations in many areas, including the Chesapeake, are far below historic levels, she said. In an unfished system, the loss of habitat from water quality may well limit fish populations. "We are overharvesting the populations and keeping them below the habitat-determined carrying capacity," Breitburg said.
Diaz said the studies, taken together, show the need to move toward ecosystem-based fisheries management, which seeks to manage fish-and their habitats.
"The only sensible thing to do would be to look at a fish population that you want to restore from a holistic view, which takes into account the pressures of fishing and the pressures of poor water quality," he said.
Indeed, in the past, it is likely fish populations were much higher, even though nitrogen inputs were lower, Diaz said, at least in part because organisms used nitrogen more efficiently, recycling it many times-something unneeded when the system is awash with the nutrient.
"I think everyone would agree that there was a lot less nitrogen in the old days than there is now," Diaz said. "I don't think anyone would argue that."