The whale carcass debate
It’s an incredible sight to see: sharks feasting on a whale carcass, but could it be that this is encouraging more human-shark interactions?
Dr Blake Chapman
Dr Blake Chapman is Australian Geographic’s shark Editor-at-large.
SUCCESS STORIES are great, right? There are few recovery stories that are as impressive as that of Australian humpback whales. These whales were hunted so heavily in the early 20th century that populations were decimated to near extinction. They became so rare that the whaling industry became unsustainable. In a way, this was good news, as the hunting pressure quickly subsided.
The whaling industry ended in Australia in 1978, and since this time, many conservation measures have been employed. All whales are protected in Australian waters under the Environment Protection and Biodiversity Conservation Act 1999 and five species, including the humpback, are listed under the Act as nationally threatened, with recovery plans in place.
The cessation of hunting and subsequent conservation and recovery measures have worked better than expected for humpbacks, and some populations have been booming. In fact, the eastern Australian population has been increasing at a long-term average rate of nearly 11 per cent a year. It’s likely that the Western Australian population has also increased rapidly, but the evidence is not quite as definitive there. Amazingly, according to recent estimates, it is thought that the eastern population has essentially recovered to pre-whaling numbers. Yet, the exponential growth continues.
The recovery of (certain) whale populations has not only been great for the species in question, but also for their natural predators. However, such alarming recovery presents somewhat unprecedented management challenges. In the case of humpback whales, natural predators include white sharks, and other large, potentially dangerous species.
The overwhelming recovery of humpback whales has been considered by some to be a factor in increasing human-shark interaction. This is on the premise that, as some sharks are known to follow whale migrations, more whales mean more food, more motivated sharks and the capacity to support larger shark populations. However, as with all theories on human-shark interaction, this one, too, is met with opposition.
The counter-argument is that whale migration patterns would rarely bring predators into environments that commonly overlap with human use. Interestingly, a very similar situation is evolving in the Cape Cod region of the United States. However, there, the situation is even more challenging, as the recovering grey seal population attracts its natural predators right to the shoreline.
The ‘closer to home’ problem here in Australia arises when whales die, and carcasses are transported by wind or currents to the coast or they strand on beaches. According to an article published by The West Australian, last year ‘44 dead whales have been reported on or close to the WA coast — not including 105 killed in a mass stranding in March — which is more than half the 85 carcasses the Department of Biodiversity and Conservation has been made aware of in the past eight years.’
Current Australian management practices for stranded whale carcasses revolve around one of four responses. These include the carcass being left alone (on land or at sea), buried on the beach, towed out to sea or transported to a waste management plant. The management of whale carcasses can be quite complex, and each stranding presents significant social, logistic, economic and environmental challenges.
Towing a whale carcass out to sea provides the most natural solution, allowing the animal to be cycled through the marine food chain. Being so high in fat, whale carcasses provide an extremely valuable food source for a whole range of animals that feed from the surface all the way down the water column to the ocean floor. In fact, my all-time favourite quote from a scientific publication describes two white sharks feeding at a whale carcass,
‘On July 5 at 1711 hrs, 7 sharks were simultaneously feeding on the whale. In this instance, two individuals were feeding at 1.5 m apart from one another. As one of the sharks continued to remove blubber, it worked its way along the flank of the whale, subsequently biting the head of the neighboring shark, leaving 2 teeth embedded in the shark’s head. However, neither of the sharks appeared to be affected by this interaction, as both continued to feed along the whale’s flank without any responsive behavior observed’ (Fallows et al., 2013).
If this doesn’t paint a different picture of white sharks – and one of clear food bliss – I don’t know what does!
The decomposition process of whale carcasses also occurs far more rapidly in the water than in the sand, and this process is less expensive than beach burial or transport to a waste facility. However, towing carcasses to sea does not often occur, amazingly, as this action is seen to present a potential navigation hazard for watercrafts, and the managers responsible for towing could be held liable.
As a result, beach burial is being investigated as the most promising option. Anecdotal reports have suggested that shark prevalence in an area can increase following whale carcass burial, and this extends beyond white sharks. In particular, bull sharks are often noted in these environments. However, a recent study based on lab tests and a field trial with 360 kg of humpback whale blubber and muscle concluded that, if done properly, burying whales on the beach should not be considered to be a localised attractant for sharks.
The world's largest great white shark was spotted eating a dead whale. 😱 pic.twitter.com/1Olo8BoSyk
— Seeker (@Seeker) February 17, 2019
The study’s lead author, Southern Cross University PhD student James Tucker, said that he expected to see traces of contamination entering the water, but was surprised with how quickly the chemical traces disappeared, and the short distance that they migrated through the sand. While the study was careful to acknowledge that it did not cover all possible situations and scenarios of burial, the team believed that they had enough evidence to assist managers in developing beach burial as a safe, viable option into the future.
While developing management plans, it is important to consider that sharks have very strong sensory capability, which, in many cases, outperform ours. It is also important to remember that our knowledge of their sensory capabilities is still quite rudimentary. With having such limited understanding in this space, it is difficult for us to know exactly what and how they are sensing, discriminating and honing in on cues that are important to them in their environment. It would also be good to know how long sharks tend to stick around an area after a whale carcass has stranded, so that beaches can be managed as needed during this period. As such a valuable food item, it is assumed that sharks would be very motivated to get to the source and may spend a considerable amount of time trying to do so.
Presently, it seems that there is little communication to the public around stranded and buried whale carcasses. Carcass disposal is left to regional councils/managers, and as a result, bespoke processes are seen. Unfortunately, responses do not always seem to consider scientifically developed recommendations or best practice, and inappropriately buried whales have had to be exhumed and otherwise disposed of following public backlash and potential environmental and even human health concerns.
In line with many shark risk management surveys, the general public most often just wants more information in order to make informed decisions about how they use the water. Therefore, it seems that public notification and appropriate signage at locations of stranded and buried whale carcasses is the most obvious and actionable immediate management strategy. Given the recently published projection that the eastern Australian humpback whale population may peak between the years 2021 and 2026, and then potentially experience significant subsequent declines, further optimising management strategies that consider the possibility of a large number of carcasses is warranted. It is great to see that research in this area has already begun!