Scientists Surveyed the Sea off South Africa—1890s Style - Hakai Magazine

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From 1897 to 1906, J. D. F. Gilchrist, a marine biologist for the former British Cape Colony in what is now South Africa, repeatedly set out aboard the SS Pieter Faure to document the sea life of the Agulhas Bank, off Africa’s southern tip. In a series of surveys, he used trawl nets to snag fish from the bank with the aim of determining whether it could support industrial fishing.

Gilchrist’s research was meticulous. He made thorough notes of currents, the seafloor, and where each kind of fish was found. His records show a sea teeming with kabeljou, with several hauls bringing in thousands of these fish from the deep. The surveys ushered in a booming fishing industry. Soon, trawl nets overflowed with cob, panga, and east coast sole, scraped from the brimming belly of the Agulhas Bank.

One hundred and eleven years later, Gilchrist’s data is an oddity—and an opportunity. It offers a detailed glimpse into the state of long-exploited fishing grounds before industrial fishers began picking them clean.

On the lookout for a topic for his doctoral thesis, marine scientist Jock Currie, who is now a postdoctoral researcher at the South African National Biodiversity Institute, wanted to replicate Gilchrist’s detailed surveys to see how much things had changed. But to make the comparison as accurate as possible, he needed to conduct the repeat surveys with the same fishing gear that was used from 1897 to 1906.

The key to doing that lay in the trawl net. A near-exact replica was integral. Even if Currie could repeat the surveys under similar conditions, using different gear would mean that he wouldn’t be able to tease apart which changes were due to the equipment and which reflected actual changes in fish populations.

Currie hit a stumbling block straight away. Trawl nets have changed a lot over time, and Currie and his colleagues assumed Gilchrist would have recorded the specifics of his. Yet none of Gilchrist’s meticulous reports included such details.

The search took Currie to England, where he located detailed plans from 1903 of a net similar to one that Gilchrist would have used. Piecing together these details with hints captured in pictures from the SS Pieter Faure, Currie settled on an early Granton otter trawl net made of tarred Manila hemp, materials that have been phased out in the production of fishing nets in favor of synthetic rope. To keep the mouth of the net open, he also had to construct otter doors: two flat wooden boards in a steel frame that connect to the net headline and groundrope.

With the help of a local trawl gear expert, the net was handwoven at a Cape Town, South Africa–based netting manufacturer, and then slathered with tar. For the otter boards, an engineering firm was called in. Currie sourced a suitably strong pine as an alternative to hardwood, though the latter would have been used in Gilchrist’s time.

More than 100 years after J. D. F. Gilchrist and his team surveyed the Agulhas Bank off South Africa, Jock Currie and his colleagues returned to do the same. Photos courtesy of Jock Currie

The ship Currie attached his net to was as close as could be, too. While the steam-powered SS Pieter Faure was a different shape and had different proportions than the diesel engine-powered vessel Currie used, both were side trawlers, on which the net swings around to the side of the boat. Side trawlers operate more slowly than the stern trawlers commonly in operation today, he says. “I was glad we had a vessel that used a similar, less-efficient approach to that employed historically.”

“At times it seemed like I bit off more than I could chew,” Currie says. But almost a year after he started, with ample support from colleagues and industry experts, Currie pulled it off. In 2015, more than a century after Gilchrist trawled the Agulhas Bank, one of the last remaining side trawlers in the local industry swept through to see what fish the bank would offer.

The key results, published in a recent study, are surprising. Though they were pulled from the same water with almost the same gear, the fish caught by Currie and those caught by Gilchrist scarcely align. While Gilchrist’s catch was full of kabeljou, not one was found in the 2015 survey. Instead, Currie’s nets caught mostly gurnards, Cape horse mackerel, spiny dogfish, and shallow-water hake. White sea catfish made up 85 percent of Currie’s catch, compared with a historical three percent. Many of the findings do not line up with common theories on which species should do better or worse under intense commercial fishing.

To explain the difference, Currie says, you need to consider how the Agulhas Bank itself has been changed. The main species of the historical catches are associated with reef habitats, whereas a far greater proportion of the modern catches prefer sand or mud habitats. This indicates that trawling probably changed the seafloor, which in turn led to changes in fish communities. “It seems obvious in retrospect,” says Currie.

If not for the historical data and meticulous repeat survey, this insight would be obscured forever. “We know so little of how our oceans were a couple of hundred years before,” says Currie. “But to know where we want to go in the future, we need to understand our history.”

Ruth Thurstan, cochair for the International Council for the Exploration of the Sea’s working group on the history of fish and fisheries, who was not involved in the research, agrees that historical perspectives are crucial. “Without this long-term perspective we tend to underestimate the scale of changes that have occurred,” says Thurstan, something that is especially true for the marine environment. “Because we cannot see underneath its surface, we underestimate our impact on this vast space.”

The Link Lonk


December 18, 2020 at 03:06PM
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Scientists Surveyed the Sea off South Africa—1890s Style - Hakai Magazine

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