The Far-Reaching Hand of Anthropogenic Change

More shark history this post.  Not fossilized extinct sharks this time, but the use of historical records to trace the extinction - more correctly extirpation, more on that later - of a shark population in recent history.

The Saint Peter and Saint Paul Archipelago is often called Saint Paul’s Rocks, and it is just that, a largely barren outcropping of small rock islands.  The archipelago sits atop the Mid-Atlantic Ridge just a scooch north of the equator, putting it midway between Brazil and the west coast of Africa.  It’s hard to imagine a more remote oceanic locale, and yet a new paper by Luiz & Edwards shows that even this place is not untouched by human effects.

Despite the remoteness of the Rocks, they have been visited periodically by ships.  Seamounts draw fish, and were historically a popular stop for replenishing food stores on long ocean voyages.  Initially the visitors were ocean explorers and naturalists - Charles Darwin stopped there aboard the Beagle in 1832 - and more recently there have been dedicated scientific missions and an increasing number of fishing vessels.  These occasional visitors recorded their observations at the Rocks, and one of the most remarkable aspects of this paper is the authors’ collection of historic reports about the islands.  Some of the comments have to do with the lack of land animals or even vegetation, but the most compelling records concern the vast numbers of sharks that were observed through the 18th, 19th and early-mid 20th centuries.

A few examples are below, and there are many others in the paper, which is worth reading for these excerpts alone.  (The quotes are taken directly from Luiz & Edwards, and original references can be found in the manuscript):

HMS Beagle: February 1832
‘‘While our party were scrambling over the rock, a determined struggle was going on in the water, between the boats’ crews and sharks. Numbers of fine fish, like the groupars [sic] (or garoupas) of the Bermuda Islands, bit eagerly at baited hooks put overboard by the men; but as soon as a fish was caught, a rush of voracious sharks was made at him, and notwithstanding blows of oars and boat hooks, the ravenous monsters could not be deterred from seizing and taking away more than half fish that were hooked.”

SY Scotia: December 1902
‘‘Dec. 10th, St. Paul’s Rocks. Sharks innumerable. Secured eight specimens, and took dimensions and weight of each . . .  Several fish seen but none caught, as the sharks took every bait.’’

USS Atka: March 1955
‘‘The numerous sharks, which swarm in the waters of the cove and around the Rocks, speedily attack hooked fish and either snatch the whole fish off the line or leave only a half fish or head on the hook for the fisherman. On the ATKA, the chief medical corpsman hooked a beautiful tuna-like fish from the fantail several hundred yards off the Rocks, but when he hauled in his catch all that remained was an enormous head fully a foot high . . .’’

RRS Bransfield: May 1971
‘‘. . . the ship’s launch was used to catch fish just off-shore from the Rocks. . . . Difficulty was experienced in obtaining these specimens as fish once hooked were frequently taken by marauding sharks before they could be brought on board.’’

The sharks that were so abundant at St. Paul's Rocks were likely of two species, the Galapagos shark (Carcharhinus galapagensis), and the silky shark (Carcharhinus falciformis).  Galapagos sharks tend to have restricted ranges, keeping close to reefs and island shores and seldom venturing into open water.  Silky sharks in contrast are more pelagic in their behavior, but are often found at the edges of reefs and are known to feed at night.

Beginning in the late 1900s, observer's reports from St. Paul's Rocks carry a different tone.

Cambridge Expedition: September 1979
‘‘It was notable that during the day sharks hardly interfered with line fishing activities and were only rarely seen at the surface. . . . suggest that the shark population may have declined somewhat in recent years; our observations are in agreement with such a conclusion. In this respect it is perhaps worth noting that the Rocks have recently been subject to occasional visits by Brazilian fishing boats; one of these recorded capturing two tons of sharks by accident in one evening while fishing for commercial species.’’

Segredos Submersos Expedition: November 1993
‘‘All dives we made were magnificent, but the lack of sharks was noticeable. We carried luparas (sticks with explosive tips) and electric end sticks in order to repel the sharks we expected to find, . . . truly, we never had to use these.’’

Scientific surveys beginning in 1998 that were specifically directed at assessing the fish populations at the Rocks failed to find any sharks.  What happened between the 1970s and the 1990s?  The Cambridge report from 1979 gives a clue to one change that took place, in the mid-1950s the Brazilian government opened the region to commercial fishing, and since 1988 these waters been fished on a daily basis.  Sharks are not the target of this fishery, the region is rich in tuna and other desirable pelagic fish.  Sharks are caught as bycatch at high levels in longline fisheries however, and the article documents significant rates of shark bycatch through the 1970s.  By the 1980s there were no longer any sharks to catch.

Species declines due to fishing are difficult to gauge in remote areas, particularly when there is little information about baseline numbers prior to human activities.  This paper does a great job of using historical reports to demonstrate the pre-fishery abundance of sharks at St. Paul's Rocks, allowing the full magnitude of these species' decline to be understood.  Although reliance on such accounts can be fraught with error, the authors applied rigorous criteria in selecting reports to use, and in gauging their reliability.  And the fact remains, since 1993 no sharks have been seen at St. Paul’s Rocks.  C. galapagensis and C. falciformis have been extirpated (the word means to become extinct within a portion of a species’ range) in this region.

The paper is: Osmar J. Luiz and Alasdair J. Edwards. (2011) Extinction of a shark population in the Archipelago of Saint Paul’s Rocks (equatorial Atlantic) inferred from the historical record. Biological Conservation, In press.