This Google Earth image shows a fishing weir along the Persian Gulf. Fish swimming close to shore get trapped by the weirs at low tide.
Fishing traps known as weirs that jut from coastlines may be snaring six times more fish in the Persian Gulf than what is officially reported, according to a new estimate based, in part, on satellite imagery available through Google Earth.
Scientists turned to the Internet search giant's mapping tool as a way to cross-check catch data reported to the United Nations Food and Agriculture Organization by six countries in the gulf, a region of the world where marine ecosystems are understudied.
"The first thing we did was survey the coast of the Persian Gulf and just started counting the number of weirs that we saw," Dalal Al-Abdulrazzak, a graduate student in fisheries science at the University of British Columbia in Vancouver, Canada, told NBC News.
She and colleague Daniel Pauly counted 1,656 weirs along the coast in images from 2005, the most recent year with the greatest coverage of the coast at low tide. Accounting for gaps in coverage and areas with low resolution, they revised the count upward to about 1,900.
Al-Abdulrazzak then spoke with fisheries scientists from the region to estimate daily catch rates for weirs and used that to calculate a total annual haul of more than 34,000 tons. The official reported number was 5,908 tons total for the six countries.
Weirs have been used to catch fish for at least 3,000 years and are still commonly used throughout the Persian Gulf. "They basically work with differences in the tide," Al-Abdulrazzak explained.
At high tide, fish come in closer to shore and swim parallel to the coastline. When they hit the long wall of the weir they instinctively swim for deeper waters. When they do, they swim into an enclosure, an entrapment area.
"Once they are in there and the tide starts receding, they are stuck. … At low tide, the fishermen can just walk out and start collecting the fish," she said.
According to official estimates, weirs account for about 1 percent of the total catch in the Persian Gulf. "My estimate says it is closer to 10 percent," Al-Abdulrazzak said. "So, it is a lot more significant than they are giving these traps credit for."
Informing fisheries management
Fisheries, added Al-Abdulrazzak, are the second most important resource in the Persian Gulf after oil and are a primary source of protein for the region's population. The amount of fish removed by weirs "could have dire consequences for fisheries stock in the long term."
In particular, she noted, weirs are constructed in shallow waters that are preferred by juvenile fish and thus trap young fish that have yet to spawn.
She hopes that this new data will be used to improve the quality of catch statistics and inform management strategies in the Persian Gulf, such as the establishment of marine protected areas in known nursery zones.
The new research is the "first example of fisheries catch estimates from space," Al-Abdulrazzak and Pauly write in a paper published Monday in the ICES Journal of Marine Science.
In the future, they add, such imagery could be used to verify country-reported data and monitor the oceans for illegal fishing activity.
John Roach is a contributing writer for NBC News. To learn more about him, visit his website.
First published November 26 2013, 12:45 PM