Overfishing Goes Back Centuries, Log Books Reveal

Image showing the division of catch among whaling ship owners, captains, and crew. (Image credit: HMAP)

Overfishing led to shrinking sizes of freshwater fish caught by Europeans all the way back in medieval times. And the real revolution in deep-sea fishing came not with modern day trawlers, but back in the 1600s when pairs of boats began dragging a net between them.

Those are just a few of the facts unearthed by marine historians who want to find out when ocean life populations and natural sizes began to shrink.

The evidence shows that much of the decline took place even before the modern fishing industry really got going.

"The important point is not when modern technology was being used, it was mass commercialization," said Poul Holm, an environmental historian at the University of Dublin-Trinity College in Ireland who heads the global history project for the Census of Marine Life.

People who had once fished to feed themselves increased their catches by just using traditional technologies, Holm noted. The ramped-up fishing came in response to the demand of an increasingly global market.

Researchers are scheduled to present such findings at the Census of Marine Life conference in Vancouver from May 26-28.

"We have a lot of evidence now that major reductions were made in the latter half of the 19th century," Holm told LiveScience. "People were still using sail power, but the impact was just immense."

{{ video="LS_090526_Overview" title="Discovering the History of Marine Animal Populations" caption="The Census of Marine Life is tracking changes in marine life populations and natural sizes through science and historical records. Credit: Census of Marine Life" }} 

The investigatory tools

Modern fisheries have only begun keeping serious track of some marine populations for the past 20 or 30 years. That leaves a huge knowledge gap that researchers try to bridge with a variety of methods.

Scientists can use sediment core sampling from the seabed to assess past ocean populations indirectly. Another trick involves looking at archaeological evidence from land to see how much food humans harvested and ate from the ocean.

Yet perhaps the richest treasure trove of knowledge comes from historical documents such as ship logs, literary texts, tax accounts and legal documents. Old restaurant menus and artifacts such as whalebone buttons or mounted trophies can tell their own tales, too.

One text written in Sicily in 1153 described North Atlantic marine life as being so big that islanders built their homes and tools out of their bones.

Such historical evidence has often proved more reliable than modern data, Holm noted. Back in the day, ship skippers would keep logbooks for their own information that no one else would read, and so had no reason to alter the facts of their day's catch. But modern fishing fleets face quotas and limits on their catches that can encourage some truth-bending.

"Much of the fishery data today relies on logbooks that have been falsified or skewed, so that under-recording happens all the time," Holm said.

{{ video="LS_090526_White-Barents-Sea" title="Tracking Salmon in the Barents Sea" caption="Researchers uncover past salmon populations in the Barents Sea by using the historical records of a Russian monastery. Credit: Census of Marine Life" }} 

A bountiful ocean much reduced

Humans began turning to the shellfish, finfish and marine mammals for food as early as the Middle Stone Age from 300,000 to 30,000 years ago, Census researchers said. That's 10 times earlier than scientists once believed.

Latin and Greek passages from the second century A.D. note that Romans began trawling the seas with nets. And since then, evidence of the human impact on fisheries has flowed in from around the world.

New Zealand waters once hosted anywhere from 22,000 to 32,000 southern right whales in the early 1800s, based on estimates derived from whaling logbooks, according to New Zealand and U.S. researchers. Just 25 reproductive female whales still survived by 1925, although the population has started to recover since then to reach 1,000 animals today.

Elsewhere, the European North Sea saw a jellyfish boom that seriously altered the food chain. That occurred after North Sea nations devastated the herring population which usually preyed on the jellyfish.

"By 1870, they were taking out an amount of herring equivalent to 300,000 tons of herring per year," Holm explained — the equivalent of the allowable yearly catch for today's fishing industry.

{{ video="LS_90526_Australia" title="Australian Fisheries Underwent Steep Decline" caption="Researchers find that certain fish species have almost vanished since heavy fishing started in the waters of southeast Australia the 20th century. Credit: Census of Marine Life" }} 

Fishery pirates and future challenges

The historical findings have allowed scientists to better improve projections of how ocean life is impacted by modern-day fishing, and hopefully get more species back on a road to recovery.

However, a largely lawless ocean still presents challenges and even dangers for researchers, who want to understand more recent impacts from fishing fleets operating out of ports such as Hong Kong.

"They are really behaving like pirates in the open sea by going into fisheries and wreaking havoc," Holm said. He added that the lack of national or international jurisdiction allowed the fleets to use a "gold-mining strategy" that left marine life populations in shambles.

One new source of data may come from the oral histories of people who live in exploited regions. Marine historians and scientists have spent much of the past several years figuring out the right methods to verify and use such evidence, despite the lack of written records.

"They can tell us about sightings of animals which are not seen anymore and are not recorded by any means," Holm said. He and other researchers want to begin training local peoples to observe and record the massive changes in the ocean around them — and to tie that into their own human stories.

A final report from the Census of Marine Life is slated for release in October 2010.

Jeremy Hsu
Jeremy has written for publications such as Popular Science, Scientific American Mind and Reader's Digest Asia. He obtained his masters degree in science journalism from New York University, and completed his undergraduate education in the history and sociology of science at the University of Pennsylvania.