Overfishing over historical times is largely responsible for the recent collapse of coastal ecosystems around North America and Australia, concludes a team of 18 scientists that has sifted through mounds of geological, archaeological, historical and modern ecological data.
Global climate change, pollution and the invasion of non-native species into new habitats have all been blamed for marine ecosystem problems in recent decades. But overfishing appears to be the biggest factor, according to the scientists, who include Susan Kidwell, a Professor in Geophysical Sciences at the University of Chicago.
“This study examines the question of ecosystem change on a much longer timescale than we typically consider when thinking about the drivers for present-day changes,” Kidwell said. “This group concludes that the depletion or near-complete removal of top predators and of habitat-modifying organisms like oysters by human fishing is the fundamental culprit, and this has been going on for centuries in some areas. A lot of the deterioration that we see today is the final outcome of a collapse that has been occurring over much longer periods of time.”
The team will publish its conclusions in the July 27 issue of the journal Science. The study was led by Professor Jeremy Jackson of the Scripps Institution of Oceanography in La Jolla, Calif. He and other ecologists have noted that the ecosystems they have been studying in recent decades have been so significantly altered by human impact that the environments no longer could be viewed as natural. So Jackson put together a team of scientists that included geologists, paleontologists and archaeologists as well as marine ecologists in order to assess more precisely the condition of selected marine environments for thousands of years. Specifically, the team searched for key biological information on the changing population and body sizes of fish, marine mammals, seagrasses, kelp forests and reefs. The team also compiled information on other marine health indicators such as the incidence of marine diseases, invasive species, low-oxygen events (red tides), and the relative productivities of microbes versus animals on the sea floor and in the water.
The timing of changes in these conditions were then compared to the local history of human impacts, such as fishing of fin and shellfish, sediment and nutrient run-off from land clearance and agriculture, and pollution levels from industry and other urbanization.
Jackson assembled a team that could draw together this variety of information ranging from modern ecological studies and government fishery surveys to historical reports written by sea captains hundreds of years ago. Moving farther back in time, archaeological studies of native settlements provided clues to the condition of selected ecosystems before the arrival of colonists in North America and Australia.
Sedimentary cores drilled from estuaries, where river meets ocean, provided data extending from the modern era back into even older times. A variety of chemical and fossil information recorded in the sediments of these estuaries has enabled scientists to reconstruct an environmental and biological record stretching back as far as 20,000 years.
The data revealed a pattern of progressive “fishing down” that began with the removal of the largest food fish, which tend to be top predators of their environments. Cod, swordfish and halibut are familiar examples. Once fishermen had depleted those species locally, they moved farther afield, but they also shifted down to the next-largest edible fish. Eventually, commercial fisheries ended up depleting top predators in even the most distance parts of the globe, and providing what formerly had been regarded as trash fish or bait fish to restaurants for human consumption.
“This same progression has occurred for marine mammal communities owing to the fur and oil trades, and the removal of so much of the higher portions of the food chain fundamentally transforms the residual ecosystem,” Kidwell said.
The authors of the Science paper were able to isolate overfishing as a major problem because the timing and nature of human impact on coastal ecosystems has been highly variable on the east and west coasts of North America and in Australia.
Initially, native peoples living along the sea coast depended upon shell fish and finned fish as an important food source. Then came European colonists who did likewise. But the colonists also cleared the land for agriculture, which increased sediment runoff, and exploited fisheries for distant commercial markets. Next, the widespread application of chemical fertilizers during the World War II era led to an increase in nutrient runoff, as has coastal urbanization.
Most recently, factory ships and mechanical dredging methods have vastly increased the rate of take that is possible. Fast-traveling tankers discharging large volumes of foreign seawaters into local ports have applied additional human stresses on coastal ecosystems, Kidwell said.
The scientists did find links between increased nutrient runoffs and modern problems such as red tides, low-oxygen waters discolored by population blooms of often-deadly micro-organisms. “Sediment cores indicate that low-oxygen events have actually been going on for some time and are naturally common in many estuaries,” Kidwell said. “In recent decades, however, such events have become more frequent and severe. The ecosystem, because it has been degraded through overfishing, is unable to handle those nutrient inputs, whereas in the past it was able to.”
Such insights suggest new and more effective ways in which collapsed ecosystems may be restored and managed. Until now, proposals for controlling red tides have focused primarily on legislating nutrient runoff. But a more successful strategy might be to restore key organisms to shades of their former glory as well, Kidwell said. This lets the more natural food-web do some of the work.
The Science study shows how important oysters are to maintaining water quality in the Chesapeake Bay, for example. Early historical accounts talk about the incredible clarity of the now-cloudy water in Chesapeake Bay. Early accounts also describe the abundance of oyster reefs in the bay. “There were so many oyster reefs, and they came up so close to the surface of the bay, that they were a genuine hazard to navigation,” Kidwell said.
Native peoples and oyster fishermen had exploited these reefs for centuries. But in the 1870s, the fishermen began using mechanical dredging machines that could reach reefs throughout the bay and that were much more destructive and complete in removing reef structure, Kidwell said. Oyster catch dropped to just a few percent of former peak values by the early 20th century. Only after this collapse of the oyster fishery did low-oxygen events and other symptoms of nutrient-overload begin to occur, in the 1930s, she said.
“Oysters filter huge amounts of water every day. Calculations are that in the time when the oyster fishery was healthy in the Chesapeake Bay, the oyster population would pump the entire volume of the bay clean within three days,” Kidwell said. “These populations were able to clear nutrient and sediment inputs into the bay, which had been elevated from the start of agriculture during colonial times.”
Experiments conducted in Pamlico Sound off the North Carolina coast by study co-authors Hunter Lenihan and Charles Peterson at the University of North Carolina, Chapel Hill, show the benefits of rebuilding reefs to their former heights and sizes. The rebuilt reefs enable oysters to maintain healthier waters even in the face of modern run-off conditions, suppressing the oxygen-poor waters that rise naturally from the bottom of the sound each summer.
Kidwell said it is time for more such experiments to be conducted on a larger scale. “This is absolutely not to say that nutrient and pollution inputs don’t matter‹these are definitely exacerbating the situation,” she said. But overfishing will prove to be the key, she predicted.
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