When you think about threats to sharks, I imagine the first things that come to mind are finning, climate change and overfishing. And whilst it's true that these are all enormous problems, do you know that shipping is also a serious threat to sharks? International shipping is an absolutely ENORMOUS industry, with thousands of boats traversing hundreds of shipping lanes all around the world to deliver millions of tonnes of products every year. You probably don't give it much thought, but your food, fuel, toiletries, clothes, toys, internet orders... etc. etc. all rely on international transit, often via boat. So what effect does all this maritime traffic have on sharks? Does shipping affect the marine environment? And is it sustainable?
I'm On a Boat
International shipping is a huge industry. Shipping accounts for 80% of the world's international trade of merchandise and the demand for the movement of products across our seas is growing exponentially. In 2015 alone seaborne trade exceeded 10 billion metric tons of merchandise. In order to keep up with the rising demands more and more ships are taking to our oceans and they are growing ever-larger in size to be able to carry all the goods we require. Over the last 25 years the world's commercial shipping fleet has expanded from 1771 to over 94,000 vessels (Pirotta et al, 2019; Womersley et al, 2022).
All these ships don't just sail around the world willy-nilly. Instead they follow shipping lanes. These routes are like marine roads that are well-navigated and can be followed to get from point A to point B. As our demand for trade continues to rise these shipping lanes are becoming ever-more densely populated by traffic, especially around Europe and east Asia (Pirotta et al, 2019).
New shipping lanes are also popping up to make international trade more economical. For example, new routes have been scouted through the Arctic, where melting sea ice as a result of climate change, has opened up regions of the ocean which were previously impassable (Pirotta et al, 2019).
International shipping has obvious implications for the environment, just like all transportation services do. Their engines emit greenhouse gases that are contributing to climate change and associated ocean deoxygenation and acidification (to learn more about how these issues affect sharks you can check out How Low Can You O?). Rising sea temperatures thanks to anthropogenic climate change could also shift the distribution of prey species, affecting where specialised feeders like basking sharks (Cetorhinus maximus) are able to find food (Pirotta et al, 2019; Womersley et al, 2022)
"Shipping now produces close to 1 billion metric tons of CO2 annually (comprising 2.5% of global greenhouse gas emissions), with emissions projected to increase by 50–250% by 2050"
- Pirotta et al, 2019
Shipping accidents have also been responsible for enormous oil spills. Tankers running aground can spew millions of barrels of oil into the oceans, which can have catastrophic short- and long-term impacts on marine wildlife, including sharks (to learn more about this you can head over to Not So Slick) (Pirotta et al, 2019; Womersley et al, 2022).
Going Down with the Ship
Recently scientists have become increasingly concerned about the impacts of noise pollution in the marine environment. Famously this has caused mass stranding in marine mammals like whales, but it is now known that noise pollution also has a large impact on the behaviour of a wide range of sea creatures, including sharks (to learn more you can check out Keep the Noise Down) (Pirotta et al, 2019).
"Global shipping networks have added an estimated 12 decibels to ocean ambient noise levels over the past several decades"
- Pirotta et al, 2019
As shipping is so common now, it has contributed an enormous amount of noise pollution to our oceans by their sheer volume. Whatsmore, as ships create low frequency sounds (ranging between 5 to 500 Hertz), their noise travels huge distances and can cause disturbance tens of kilometers away from the shipping lane itself (Pirotta et al, 2019).
Whilst research to understand exactly how noise pollution affects sharks is in its infancy and their is a lot we simply don't know yet, we do know that noise pollution causes behavioural changes in other aquatic animals, including reduced foraging, avoidance behaviours, changes in movement patterns and disrupted communication. As sharks are especially sensitive to low frequency sounds (between 20 Hz and 1.5 kHz, depending on the species), there is concern that shipping noise in their environment will almost certainly be affecting them in some way (Casper et al, 2012; Pirotta et al, 2019).
Ships Passing in the Night
An effect of shipping that is already entirely apparent is the physical disruption that shipping lanes cause to sharks around the world. Like a road in the terrestrial environment, shipping channels create a barrier that must be crossed by migratory animals that spend time near to the surface, like whale sharks (Rhincodon typus) (Womersley et al, 2022).
Scientists studying the movement patterns of whale sharks have found that their oceanic migratory routes and their coastal aggregations sites regularly intersect with extremely busy shipping lanes. Researchers have discovered that there are hostspots of collision risk - where whale sharks and boats are sharing the ocean in high densities - in all major oceans around the world. Especially high risk areas were found in the Red Sea and the Arabian Sea where GPS tracking of sharks showed their movements almost constantly overlapped with shipping routes (Womersley et al, 2022).
Ships strikes are frequently reported in other large animals like whales and as we know they live in such heavy-traffic areas, scientists now suspect that fatal strikes on whale sharks may be common, but rarely reported. Whale sharks are flagged as Endangered on the IUCN Red List of Threatened Species as they have suffered large population declines. They have therefore been offered protection in many places around the world and are very rarely fished, but their populations are yet to begin to bounce back. Scientists now wonder if lethal collisions with boats, that are not reported or monitored (known as "cryptic strikes"), are the reason that whale shark popuations have yet to recover (Pirotta et al, 2019; Womersley et al, 2022).
"Lethal ship strikes... could explain why whale shark population declines continue despite international protection and low ﬁshing-induced mortality"
- Womersley et al, 2022
Has the Ship Sailed?
So with shipping being such an enormous and vital industry, is there anything we can do to reduce the threat it poses to sharks? The problem is undeniably large, but that does not mean there is no hope! Conservationists are working on it!
Recognition of the damage caused by noise pollution has lead to the International Maritime Organization (IMO) introducing guidelines to reduce the underwater noise produced by ships. This includes updating ship designs to reduce their noise output and implementing best practices that decrease the amount of noise generated by ships already in use. Hopefully in the future we may see some positive changes from these new measures (Pirotta et al, 2019)
Conservationists have also suggested that broadening exclusion zones (areas where cargo ships and tankers may not enter) may reduce the impacts of shipping lanes in marine animals like sharks. As scientists are increasingly able to study shark movement ecology in more depth (thanks to the advancement of tracking technologies), we are now in a position to be able to advise the shipping industry how to avoid interactions with endangered species and how to divert their routes around critical habitats. The IMO has already relocated certain shipping lanes and implemented traffic separation schemes specifically to help to reduce boat strikes with whales, so there is no reason to expect they would not also be open to similar protective measures for sharks (Silbera et al, 2012; Pirotta et al, 2019).
It is a big job, but if scientists, conservationists and the shipping industry can all work together to design effective protective measures, then maybe in the future people and sharks will be able to traverse the high seas more harmoniously. If human beings have the ingenuity to be able to direct and manage an unimaginable amount of ships around the world as we do today, there is no excuse for us not to turn that creativity towards doing so with more consideration for the creatures that live in those waters!
Casper BM, Halvorsen MB & Popper AN (2012). Are sharks even bothered by a noisy environment? Advances in Experimental Medicine and Biology, 730. Access online.
Pirotta V, Grech A, Jonsen ID, Laurance WF & Harcourt RG (2019). Consequences of global shipping traffic for marine giants. Frontiers in Ecology and the Environment. Access online.
Rider MJ, Kirsebom OS, Gallagher AJ, Staaterman E, Ault JS, Sasso CR, Jackson T, Browder JA & Hammerschlag N (2021). Space use patterns of sharks in relation to boat activity in an urbanized coastal waterway. Marine Environmental Research, 172: 105489. Access online.
Silbera GK, Vanderlaan ASM, Arceredillo AT, Johnson L, Taggart CT, Brown MW, Bettridge S & Sagarminaga R (2012). The role of the International Maritime Organization in reducing vessel threat to whales: Process, options, action and effectiveness. Marine Policy, 36:6. Access online.
Womersley F, Humphries NE, Queiroz N & Vedor M (2022). Global collision-risk hotspots of marine traffic and the world’s largest fish, the whale shark. Proceedings of the National Academy of Sciences, 119:20. Access online.