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How severe are shark population decreases, and how do we know?

John Shepherd once said that counting fish is like counting trees, except that fish move and you can’t see them because they’re underwater. This is true with sharks as well. It’s basically impossible to know how many sharks there are. Fortunately, a variety of methods exist that can be used to determine population trends. In other words, even if we can’t know how many sharks there are, we can tell if there are more or less than there used to be. Presented here are brief descriptions of some of these methods and the conclusions of major shark conservation studies that used them. Though no one method is perfect, the fact that so many different methods have such similar conclusions is quite telling.

Fisheries-dependent population surveys
Many commercial fisheries are legally required to keep extremely detailed records of their catch (and bycatch), and some of these records are publicly available. Population studies that utilize the records of commercial fisheries are referred to as “fisheries dependent”. There are many, many more fishing boats than scientific research vessels, which results in much more data.  All other variables being equal, if fishing boats catch fewer sharks per trip, it means that fewer sharks are there.

The most famous example of a fisheries-dependent population survey of sharks came from Dr. Julia Baum. She and her team analyzed fisheries data from the United States open-ocean longline fleet, which focuses on swordfish and tuna but often catches sharks. In between 1986 and 2000, over 200,000 longlines were set in the Northwest Atlantic Ocean. Fishing vessels recorded the number of sharks of various species caught during each longline deployment, and Dr. Baum calculated the trends in shark catch over time from these data.

Her results were startling, and the resulting research paper (Collapse and Conservation of Shark Populations in the Northwest Atlantic) has been cited over 500 times. Since 1986, Dr. Baum’s analysis of logbook data suggests a population decline of 89% in hammerhead sharks, 79% in great white sharks, 65% in tiger sharks, 80% in thresher sharks, 60% in blue sharks, and 70% in mako sharks.

From Baum 2003, showing population declines over time in many shark species

 

In a similar paper focusing on longline fishing in the Gulf of Mexico, Dr. Baum found other alarming trends. In the 1950’s, sharks were about 17% of the total catch on longlines (approximately 7 sharks caught per 1,000 hooks set). In the 1990’s, sharks were only 2% of the total longline catch (approximately 1 shark per 1,000 hooks). Oceanic whitetip sharks, which were once considered the most abundant large predator on Earth, were caught in 64% of longline sets in the 1950’s and were almost never caught in the 1990’s. This indicates that the population of oceanic whitetip sharks declined by more than 99% since the 1950’s.

 

From Baum and Myers 2004, showing catch rates of different species per 1000 hooks in the 1950's (left) vs the 1990's (right)

Many fisheries scientists have concerns about using fishery-dependent data. Though fishery-dependent datasets are generally much larger, the data that is collected is not the focus of the voyage- it is incidental to the fishing. Numerous questions have been raised about how accurate this kind of data is, since some fishermen may inaccurately report their catch intentionally to avoid fines for catching too much of the wrong species, while others may simply be unable to accurately identify the difference between closely related species. In fact, a great deal of fishery-dependent data simple says “shark” without even attempting to identify the species in question.

Additionally, it is important to note that fisheries-dependent studies like Dr. Baum’s do not necessarily show that there are 65% fewer tiger sharks in the Northwest Atlantic- the data show that fishermen reported catching 65% fewer tiger sharks in the Northwest Atlantic. Some of the decline might be due to changes in how catches are reported. Some of the decline might be due to the fishing fleet changing gear that results in catching less bycatch (including sharks). With reported declines this severe, however, it seems likely that actual population declines are a major piece of the puzzle.

Fishery independent surveys
An alternative method to tracking the population of marine animals is known as fisheries-independent surveys. With these surveys, a research vessel attempts to catch fish specifically to assess their populations. Trained experts record the catch, eliminating observer bias. These surveys typically use the same gear and visit the same sites year after year specifically to avoid the biases described in the last paragraph. These surveys are typically run by universities or governments (state or Federal).

Many fisheries independent surveys found similar results as the studies described above. A 2007 paper called “Cascading effects of the loss of apex predatory sharks from a coastal ocean” used data from a fishery-independent survey in North Carolina state waters that has been ongoing since 1972. All of the large sharks in the survey have decreased in population in the last 35 years, some by more than 90%.

 

From Myers et al 2007, showing population declines of several shark species since 1972

Beach nets
Several resort communities in Australia and South Africa place “shark nets” along their beaches. While these are environmentally damaging and do very little to protect people from shark attacks, they do provide a useful method for indirectly tracking shark populations over time.  (Feretti 2010, Holden 1977). The nets are set up in approximately the same area year after year, and net technology hasn’t really changed very much lately.

Francesco Feretti analyzed catch data from beach netting programs, and found that during the brief period from 1952 to 1972, catch rates for shark species in South Africa declined between 27% and 99%. Additionally, since the 1960’s, Australian shark nets have decreased their total catch by about 85%. The net programs themselves aren’t the cause of this decline (there aren’t enough nets to do this kind of ecological damage), but they show that declines have occurred pretty convincingly.

From Feretti et al. 2010. The Y axis is "sharks per km of beach net".

 

Diver surveys
One of the groups most likely to encounter sharks is the recreational SCUBA diving community. Many SCUBA divers take part in “visual census” surveys, reporting what they see on a dive to scientists. These databases are treasure troves for scientists, particularly those studying fish populations in the Caribbean.

Shark researcher Christine Ward-Paige analyzed over 75,000 dives worth of survey data from throughout the Caribbean Sea, and found that divers were much more likely to see sharks in isolated areas away from large groups of humans, or in areas which have enacted shark conservation policies. Sharks were rarely, if ever, seen on dives near large concentrations of humans and the associated fishing fleets.

From Ward-Paige et al 2010. Grey, green, and light blue indicates few sharks seen by divers. Red and dark blue indicate more sharks.


Observing pristine systems
Another way to measure how severe shark population declines are is to observe the natural abundance of sharks in pristine, unfished ecosystems. Unfortunately, as fishing boats get more and more high-tech, there are precious few of these pristine ecosystems left.

However, a recent expedition to the relatively-pristine Northern Line Islands made a shocking discovery. In unexploited reef systems, there are a LOT of sharks, which the authors refer to as an “inverted trophic pyramid” ! Additionally, the relative number of sharks changes proportionally with fishing pressure.

Figure from Sandin et al. 2010. Black bars indicate apex predators like sharks. The scale moves from unexploited (left) to exploited (right)

Conclusions
We may never know exactly how many sharks are out there, or exactly how many are killed each year. What we do know, from a variety of different types of analysis, is that many species of sharks are decreasing in population at alarming rates. Perhaps more importantly, we know why so many sharks are being killed, and we know how to stop it.


ResearchBlogging.org

Baum JK, Myers RA, Kehler DG, Worm B, Harley SJ, & Doherty PA (2003). Collapse and conservation of shark populations in the Northwest Atlantic. Science (New York, N.Y.), 299 (5605), 389-92 PMID: 12532016

Baum, J., & Myers, R. (2004). Shifting baselines and the decline of pelagic sharks in the Gulf of Mexico Ecology Letters, 7 (2), 135-145 DOI: 10.1111/j.1461-0248.2003.00564.x

Burgess, G., Beerkircher, L., Cailliet, G., Carlson, J., Cortés, E., Goldman, K., Grubbs, R., Musick, J., Musyl, M., & Simpfendorfer, C. (2005). Is the collapse of shark populations in the Northwest Atlantic Ocean and Gulf of Mexico real? Fisheries, 30 (10), 19-26 DOI: 10.1577/1548-8446(2005)30[19:ITCOSP]2.0.CO;2

Ferretti, F., Worm, B., Britten, G., Heithaus, M., & Lotze, H. (2010). Patterns and ecosystem consequences of shark declines in the ocean Ecology Letters DOI: 10.1111/j.1461-0248.2010.01489.x

Myers, R., Baum, J., Shepherd, T., Powers, S., & Peterson, C. (2007). Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean Science, 315 (5820), 1846-1850 DOI: 10.1126/science.1138657

Sandin SA, Smith JE, Demartini EE, Dinsdale EA, Donner SD, Friedlander AM, Konotchick T, Malay M, Maragos JE, Obura D, Pantos O, Paulay G, Richie M, Rohwer F, Schroeder RE, Walsh S, Jackson JB, Knowlton N, & Sala E (2008). Baselines and degradation of coral reefs in the Northern Line Islands. PloS one, 3 (2) PMID: 18301734

Ward-Paige CA, Mora C, Lotze HK, Pattengill-Semmens C, McClenachan L, Arias-Castro E, & Myers RA (2010). Large-scale absence of sharks on reefs in the greater-Caribbean: a footprint of human pressures. PloS one, 5 (8) PMID: 20700530