When I sat down to read about shark declines around the world, I had thought I was prepared for the numbers I was about to encounter: it’s very bad, we know this now, time to reserve the trend. But as I became more and more depressed about the current situation of shark species, I realized I hadn’t before recognized either the magnitude of the problem or how long we’ve had an inkling it was occurring. And that makes me feel like we’ve dropped the ball on this one, although it’s probably par for the course when we consider our general treatment of the world’s oceans.
Great Hammerhead Shark
What types of declines are we talking about? Well, it feels a little crazy to realize that I am not throwing out these numbers in an attempt at exaggeration, but it looks very, very bad. Let’s start with some of the rosier views: a 2004 assessment of shark species in the Mediterranean suggested that about 46% of sharks and their relatives were threatened, although not enough data were available on another 30% (Abdulla 2004); Baum et al. (2003) suggested that tiger sharks in the northwestern Atlantic had declined by 65% between 1986 and 2000 and that the average decline in shark species they had investigated was over 75% during that time. The numbers only get scarier from here: hammerheads in the northwestern Atlantic declined by 89% between 1986 and 2000 (Baum et al. 2003); semi-pelagic hammerheads around the world have declined 64-99% overall (Camhi et al. 2009). Of the 20 species of large predatory sharks in the Mediterranean, Ferretti et al. (2008) could only find enough abundance data to analyze trends for 5 species- over the last 200 years, all 5 saw declines of over 96%. 96-99% over 200 years sounds horrible- but it actually pales in comparison to the plight of oceanic whitetip sharks in the Gulf of Mexico- that population declined by more than 99% between the 1950s and the 1990s (Baum & Myers 2004). When we consider that Gulf of Mexico whitetips were probably down from their historical numbers by the 1950s, we’re talking about less than 1% of the population left. And I have to admit that I’m a little annoyed that we let things get this bad before taking broadscale action- in 1991, South Africa had already recognized population declines in great whites and implemented protection of the species, and California followed suit in 1993 (Heneman & Glazer 1996), but the US didn’t act to ban finning of shark species until 2000 (Cunningham-Day 2002). Governments such as the US and Australia are still working to get global consensus and cooperation for shark conservation (more on that in a future post). It seems like we could have done more and done it earlier.
Why are shark populations declining so much? I think we can look at this one on two levels: the actions removing sharks directly from the oceans, and then the circumstances keeping populations from recovering. Sharks are killed on a massive scale. Some of that is unintentional, for example when they get caught in fishing nets or on long-lines set out for other species- according to Camhi et al. (2009), 28% of the fish caught on long-lines set out for migratory species are discarded, and blue sharks make up the largest portion of that discard. Some sharks are intentionally targeted to supply several industries: sharks are now caught for their fins (for soup), their livers (medicinal practices), their meat (food), their cartilage (medicinal uses), and for the sport of catching a shark. They also get caught in nets set up at beaches to protect bathers. Taken all together, the numbers are a bit staggering- in 2002, it was estimated that about 3000 sharks were killed in drift gill nets per day (Cunningham-Day 2002); Clarke et al. (2006) estimated that 10.7 million blue sharks were captured each year for the fin trade alone! Why can’t populations sustain this level of harvest? There are a few reasons for this. Habitat loss, that perennial conservation issue, contributes to shark issues, for example in areas where juvenile sharks congregate (sometimes called “nurseries”). Many species of shark are slow to mature and have low reproductive rates- the dusky shark, for example, requires roughly 2 decades to reach maturity. If a species is experiencing heavy fishing pressure, juveniles may not live long enough to reproduce. Some species segregate by gender except for breeding, for example females staying near the coast, and so shark fishing which concentrates in specific locations may create gender biases in the population (Hueter 1998)- there may be females who can’t find males or vice versa. When you put these issues together, it can take a very, very long time for shark populations to recover from being decimated- basking sharks off the coast of Ireland have yet to fully recover after more than 50 years of protection (Cahmi et al. 2009).
Why should we care about losing these fearsome animals? I could certainly make the argument that biodiversity in and of itself is a good thing, because it makes the world a more fascinating place. Or I could suggest that complex ecological systems are better able to withstand change, which will probably be helpful given the current and future impact of global climate change. I think that those are both very important reasons, but sometimes people need a more immediate reason to protect species, so think about what happens when the top predator is removed from a system- things can get a little crazy in what is known as a ‘trophic cascade’- smaller predators might suddenly become overly abundant because they are now at the top of the chain, prey populations can expand until they exhaust their food base, etc. (If you want to get a sense of the implications of trophic cascades in a variety of habitats, I suggest looking into sea otter-sea urchin dynamics along the Pacific coast, and mule deer-cougar/elk-wolf dynamics in national parks.) To give you a sense of how sharks allow us to continue harvesting certain resources, the decline of large sharks in the northwestern Atlantic has been linked to the collapse of the bay scallop fishery (Baum et al. 2003).
So sharks are clearly in trouble for a number of reasons, although overharvesting does stand out as a glaring issue. Over the past decade or so, researchers have really concentrated on exploring options for shark conservation- for my next post, I’ll dive into the strategies than have been proposed and implemented to support shark populations. We’ve done a pretty good job of destroying shark populations- hopefully we will be just as good at helping them recover.
Works cited:
Abdulla, A. 2004. Status and Conservation of Sharks in the Mediterranean Sea. IUCN Technical Paper. Pgs. 7.
Baum, JK and RA Myers. 2004. Shifting baselines and the decline of pelagic sharks in the Gulf of Mexico. Ecology Letters 7: 135-145.
Baum, JK, Myers, RA, Kehler, DG, Worm, B, Harley, SJ, and PA Doherty. 2003. Collapse and conservation of shark populations in the northwest Atlantic. Science 299 (5605): 389.
Camhi, MD, Valenti, SV, Fordham, SV, Fowler, SL, and C Gibson. 2009. The Conservation Status of Pelagic Sharks and Rays: Report of the IUCN Shark Specialist Group Pelagic Shark Red List Workshop. IUCN Species Survival Commission Shark Specialist Group. Newbury, UK.
Clarke, SC, McAllister, MK, Milner-Gulland, EJ, Kirkwood, GP, Michielsens, CGJ, Agnew, DJ, Pikitch, EK, Nakano, H, and MS Shivji. 2006. Global estimates of shark catches using trade records from commercial markets. Ecology Letters 9: 1115-1126.
Cunningham-Day, R. 2002. Sharks in danger. Biologist 49: 118-122.
Ferretti, F, Myers, RA, Serena, F, and HK Lotze. 2008. Loss of large predatory sharks from the Mediterranean Sea. Conservation Biology 22: 952-964.
Heneman, B and M Glazer. 1996. More Rare Than Dangerous: A Case Study of White Shark Conservation in California. Pp. 481-491. In: AP Klimley & DG Ainley (ed.) Great White Sharks: The Biology of Carcharodon carcharias, Academic Press, San Diego.
Hueter, RE. 1998. Philopatry, natal homing and localized stock depletion in sharks. Shark News 12: 1-2.