Shark Science Blog

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.

http://dx.doi.org/10.1016/j.biocon.2011.08.004

400 Million Years and Counting

There’s an often-reported statistic that sharks have remained unchanged for 400 million years.  What does this really mean?  Our familiar sharks weren’t around 400 million years ago, and what do we really know about the extinct shark species that have existed between then and now?

Fischer et al now report on new shark fossils from the middle to late Triassic period (250-200 million years ago).  These fossils were found at a site called Madygen, in Kyrgyzstan in central Asia.  During the Triassic period, Madygen is believed to have been a large inland freshwater lake.

Unlike bony animals, the cartilaginous skeletons of sharks fossilize poorly, so most extinct shark species are identified from little more than their teeth.  The shapes and sizes of the animals belonging to these teeth must be inferred from similar species alive today, and hoping to understand anything about the lives and behaviors of these sharks is usually impossible.

Fischer et al found fossilized shark teeth at Madygen that were different than those that had previously been described, and eventually determined that they belonged to three new species of freshwater shark.  The term ‘freshwater shark’ sounds odd today; all but a few of the 400+ species of selachians (sharks, exclusive of rays and skates) currently alive are marine animals.  The bull shark can live in both salt and fresh water, allowing it to move far up into river systems, and there is a genus of ‘river sharks’ in Asia, but most sharks live in the ocean.  Freshwater elasmobranchs appear to have been abundant in shark history, however, based on large numbers of fossil shark teeth found in the deposits of inland lakes.

In addition to teeth, the embryonic egg cases of sharks can also fossilize, and while the embryonic sharks within them remain only as tiny teeth, studying them can sometimes tell us about the lives of these animals.  Madygen is a particularly rich site for fossil shark egg cases and the teeth of the developing young they once held.  The researchers found many egg cases, from two different species, in an area that at one time encompassed the shallow near-shore waters of this ancient lake.  Few adult teeth were found nearby, however, suggesting only young sharks inhabited the area -- a shark nursery.

Many shark species today use shallow near-shore ocean habitats as juvenile nurseries, where young sharks can grow, with abundant food and reduced risk of predation, until they are large enough to join the adults of their species.  It seems things weren’t all that different for sharks more than 200 million years ago.

The paper is: A selachian freshwater fauna from the Triassic of Kyrgyzstan and its implication for Mesozoic shark nurseries. (2011) Fischer, J, Voigt, S, Schneider, JW, Buchwitz, M and Voigt, S. Journal of Vertebrate Paleontology, 31:937-953.

It can be found at: http://dx.doi.org/10.1080/02724634.2011.601729

Sharks in Decline

We all know sharks are in trouble.  We talk about the dramatic declines in shark populations, a result of high catch rates for shark fin, and as bycatch in fisheries targeted at other species.  But what is the data these statements are based on?  How are such declines calculated?

Like many researchers who study sharks, I frequently give seminars where I talk about declining shark numbers.  Recently I went back to review much of the research literature supporting the decline in shark numbers, and I'll highlight two of the most compelling articles here.

In the first report, Baum et al took a historical approach to estimate the change in numbers of large pelagic sharks by compiling data from fisheries logbooks.  Specifically, they looked at the logs of the US tuna and swordfish longline fishing fleet in the Northwest Atlantic, from 1986-2000.  While sharks are not targeted by this fishery, longline hooks catch many sharks who attempt to eat either the bait on the hook, or the captured prey.  Each fishing boat records the number and species of the sharks they catch, and these numbers give an ongoing estimate of the abundance of each shark species in the region.  The calculations of Baum et al found that 8 species of shark studied have declined by more than 50% in the past 15 years, with scalloped hammerheads (Sphyrna lewini) decreasing by 89%, thresher sharks (Alopias vulpinus and Alopias superciliosus) by 80% and white sharks (Carcharodon carcharias) by 79%.

What about coastal reef shark species, are they doing any better?  In a separate article Robbins et al analyzed numbers of two common coastal sharks off the Australian Great Barrier Reef, whitetip reef sharks (Triaenodon obesus) and grey reef sharks (Carcharhinus amblyrhynchos).  They counted sharks in areas with active fishing, in “no take” marine reserves, where boats are permitted but fishing is not allowed, and in “no go” marine protected areas, where boats are not allowed to enter.  As a control for an undisturbed shark population - difficult to find these days - they counted sharks at the remote Cocos Islands (the Indian Ocean Cocos, not the Pacific Cocos), where there is little or no fishing.

Robbins et al found that “no go” areas carried far higher numbers of sharks than areas without this protection - close to the numbers of sharks seen at Cocos Island.  Regions that were not entirely closed to boat traffic, however, the “no take” zones, showed 80% fewer whitetip reef sharks and 97% fewer grey reef sharks, even with fishing restrictions in place.  Restricted fishing reserves thus appear to offer little protection for sharks, at least in this area, probably because enforcement is difficult, and much illegal fishing occurs.  The authors estimated annual changes in populations of these two species, showing declines of 7% per year for whitetip reef sharks and 17% per year for grey reef sharks - rates that indicate likely extirpation (regional extinction) of these species within 20 years.

Think these statistics are depressing?  Keep in mind that these studies are 5-8 years old, and there is little evidence that the status of sharks has improved since then.  Recent victories in restricting shark fishing and regulating the fin trade are essential to prevent extinction of many shark species, but it will take a long time for these actions to impact such depleted populations.

The articles are:
Baum, JK, Myers, RA, Kehler, DG, Worm, B, Harley, SJ and Doherty, PA. (2003) Collapse and conservation of shark populations in the Northwest Atlantic. Science, 299: 389-392.

It can be found at: http://www.sciencemag.org/content/299/5605/389.abstract

Robbins, WD,  Hisano, M, Connolly, SR, Choat, JH. (2006) Ongoing collapse of coral-reef shark populations. Current Biology, 16:2314-2319.

It can be found at: http://www.cell.com/current-biology/abstract/S0960-9822%2806%2902276-7

Where the Boys Are

Whale sharks (Rhincodon typus) are typically studied in feeding aggregations that occur annually in locations around the world.  Travel to Djibouti in January, the Philippines in March, Western Australia in May, or Mexico’s Yucatan Peninsula in July, and you will find large numbers of whale sharks massed to feed on a transient food source.  Usually this is a fish or coral spawning event, and the sharks arrive yearly like clockwork.  As a study population these aggregations are imperfect; most are composed largely of juvenile male sharks less than 8m in length.  Adult males, and females of all ages, only rarely join these planktonic feeding frenzies.  While these aggregations can tell us only about the life history of young male whale sharks, in their exclusivity they actually highlight one of the most tantalizing questions about these mysterious creatures.

We know little about whale shark reproduction, and the data we do have comes almost entirely from a single pregnant female caught in a Taiwan fishery in 1995.  This animal carried 300 developing pups in her uteri; if this number is typical for whale sharks it means a huge number of pups are born each year.  These tiny sharks then immediately vanish - only 15 newborn whale sharks have been scientifically documented!  Where do all the pups go?  Many are probably eaten by large fish and other sharks, but there is still a big disconnect between whale shark fecundity (how many offspring a female has) and the number of small whale sharks seen.  Many researchers suspect that newborn whale sharks utilize a unique habitat we have not yet discovered, where they spend the first year or two of their lives growing large enough to survive in the open ocean.

When we first find young whale sharks in significant numbers they are about 2.5m in length, and to an amazing degree they are off the coast of Djibouti.  Djibouti has the smallest average size of any whale shark aggregation, as reported in a paper by David Rowat and crew.  (David is the director of the Marine Conservation Society Seychelles, and an SRI International Director.)  As reported by David and his team, the mean size for Djibouti sharks is only 3.7m, with animals as small as 2m.  How old is a 2m whale shark?  A newborn whale shark raised in captivity in Taiwan, about 60cm at birth, reached 1.4m before he died at the age of 4 months.  Beyond that age there is no information until ~4m in length, where a few aquariums have documented whale shark growth rates.  The rapid growth of newborn animals is not sustained - the growth rate of the 4m animal was much slower - so perhaps those 2m sharks are just reaching their first birthdays?

Rowat et al used spot pattern identification to study the Djibouti aggregation, and a separate population in the Seychelles, over a number of years.  The Seychelles hosts a different population of animals, with a mean length of 5.8m.  They found differences in the number of years that animals visited the two aggregations, with Djibouti animals staying for a shorter period of time than Seychelles.  The small size and shorter return period of the Djibouti sharks may indicate that this region is a sort of entry point for young whale sharks, and they stay only long enough to grow to a size where they can compete with larger animals at other aggregation sites.  I sometimes explain whale shark feeding aggregations as high schools, with their large populations of adolescent males.  By that analogy Djibouti is a whale shark primary school, a place where young animals first enter the whale shark social scene.  Where do they come from?  Where do they go when they leave?  So many questions…..

The paper is: Rowat, D, Brooks, K, March, A, McCarten, C, Jouannet, D, Riley, L, Jeffreys, G, Perri, M, Vely, M and Pardigon, B. (2011) Long-term membership of whale sharks (Rhincodon typus) in coastal aggregations in Seychelles and Djibouti. Marine and Freshwater Research, 62: 621-627 .

It can be found here: http://www.publish.csiro.au/?paper=MF10135