Fairy Penguin: Richard.Fisher CC BY 2.0

The thrum of engines 24/7. Ships, tenders, feeders, bathers, water jets 24/7. Tens of thousands of underwater 'cracker bombs' to deter seals. Richard Flanagan detailed some of these annoyances in his book, Toxic, but too many more are going untold. Too many people are suffering and their struggles to be heard are drowned out by the pursuit of profit. And too many animals are experiencing even worse. Meanwhile, the regulator turns a deaf ear. 



We focus here on the hidden impacts of seal bombs. For background on why these are used, go to our page on seals. Seal bombs, or seal crackers, are underwater bombs used to deter seals. More than 77,000 cracker bombs were deployed by the salmon industry between January 2018 and May 2021 [1]. Their effect on Tasmanian wildlife is completely unstudied, and we have some concerns about that. 


Noise is worse for aquatic animals than for humans... much worse

Try to imagine being a whale or dolphin, who uses subtle variations in sound frequencies for communication, navigation, finding food, avoiding predators, and social cohesion. We humans can't perceive these subtleties. Now imagine explosions sounding like they are just centimetres from your head. 

Underwater, noise travels much farther than it does on land or through air. And it can be very difficult to perceive the direction because, as every scuba diver knows all too well, underwater noise sounds like it's coming from everywhere. 

Shipping noise is the primary component of overall ocean noise pollution. A close second is what are known as 'seismic airgun surveys'. This is how offshore fossil fuel deposits are found. Arrays of airguns are trawled behind a ship, exploding at regular intervals, e.g., every 10-80 seconds. The shock wave hits the seafloor and reflects back, allowing the scientists to 'see' the seafloor acoustically. The acoustic energy from these airguns doesn't only go down, it goes everywhere, and it can be detected up to 50-75 km in shallow waters and up to 4000 km in deep waters [2]. For reference, the salmon farming area of Tasmania's Storm Bay with its associated estuaries and channels, is about 26 km long by 40 km wide: it seems possible that seal crackers detonated anywhere in this system would be felt everywhere in this system. 

To put these blasts into perspective, each is a sharp, loud sound in the range of 230-255 dB [2]. Chainsaws are just a wussy 110. Jackhammers are 130. The upper threshhold of pain is at 140. Gunshots are 150. Ear drums burst at 160. Ok, to be fair, sound is measured differently underwater, so underwater 230 might be more like 200 or so on land. But it's a lot. It is, after all, an explosion. 

There's a fair bit of research on the effect of seismic airgun surveys on marine mammals, seabirds, fish, even plankton. All of it shows the same result: bomb noise harms living things. This isn't entirely surprising, given that when a bomb rips apart a building, the part doing the damage is the sound waves, turning masonry to dust and bending steel as if it were gummy snakes. 

The surprising part about seismic survey bombs, however, is that seal deterrent 'cracker bombs' – the ones used by the Tasmanian salmon industry – are so similar, clocking in at about 234 dB [3]. They are, after all, explosions. At close range, they shatter bones and herniate brain tissue. But just because they don't dismember animals at long range, doesn't mean they don't cause harm. Indeed they do. 


Marine mammals 

Underwater explosions cause tissue damage and can be lethal, and we already know from the salmon industry's and Government's documents that close-range weapons are routinely used against seals in Tasmania [4]. We can hope that salmon farmers are not deploying explosives near dolphins and whales. But even at distance, the more likely risk for these animals is barotrauma, or tissue damage resulting from rapid changes in pressure, often resulting in hearing impairment. Other impacts on mammals may include startling or fright, avoidance of areas with loud noises, and changes in behaviour or vocalisation patterns, and these impacts can occur at tens to hundreds of kilometres [5].


Penguins and other seabirds

Few studies are published examining the impact of underwater bombs on seabirds. However, one study on African penguins is intriguing [2]. Penguins within 100 km of seismic surveys showed a clear pattern of foraging farther away from areas where acoustic disturbance took place, but the researchers could not disentangle whether the behaviour was due to direct avoidance of noise or change in prey fish distribution, which was possibly as a result of the noise. Either way, the birds had to expend more energy foraging and were away from their nests longer, leaving the young more vulnerable. 

Also in that study, penguins resumed their normal foraging behaviour soon after the cessation of noise. The extent to which this may apply to Tasmanian penguins is unknown, because seal bombs are deployed in thinner clusters but all year, every year. There is no cessation. The cumulative impact of ongoing seal bombs and noise from ships, engines, and general industrial activities in Tasmania should be a high priority for research. 



An obvious question may be what effect seal bombs might have on salmon, given that they are typically the closest organism other than seals to the blasts. A study led by a researcher at Curtin University in Western Australia investigated this very question by testing effects of air-gun noise on the hearing system of fish [6]. Fish ears are similar to ours and those of other vertebrates, employing sensory hair cells in an accelerometer. Trials on pink snapper compared the effects on fish examined 18 hours after air-gun stimulation versus 58 days later. At 18 hours, minimal damage was visible. The fish analysed 58 days after exposure continued to grow and appear normal, but in fact, their ear damage was profound. Blistering and holes suggested that hair cells had been ripped from the epithelium or had exploded after exposure due to cell death. Importantly, the fish in this study were in cages and could not swim away from the blasts. Farmed salmon are also confined in this way.

Another interesting paper reviewed 28 studies on the effect of air-guns on fish [7]. Of these, 19 found a range of physical, behavioural, and physiological impacts to adult and/or juvenile fish at realistic exposure levels. As well, fishes with hearing impairment may be more vulnerable to predators and find it more difficult to locate prey. 

These studies raise concerns about effects on the iconic weedy sea dragon, a relative of the seahorses native to southeastern Australia, and the critically endangered handfish in southern Tasmania, found nowhere else in the world. Weedy sea dragons live in kelp-covered rocky reefs and seagrass meadows, which are patchy and unlikely to be abandoned. Handfish are sedentary and localised to small reefs, and are unlikely to attempt to find – or be successful at finding – alternative suitable habitat. 



At least 33 studies have examined the question of impacts of seismic air-guns on marine invertebrates, and were reviewed in 2016 [7]. Of these, at least seven found measurable responses at realistic exposure levels, while another 11 found possible response / conflicting or anecdotal results. The most frequently studied invertebrates are shellfish of commercial value, e.g. scallops, crabs. However, two studies have focused specifically on soft-bodied cephalopods, providing valuable insight. One reviewed 17 records of stranded and surface-floating giant squids, and found "acute tissue damage... that probably caused the deaths of these animals" [8]. The other study compared the effects of seismic survey sounds in four cephalopod species, i.e., two types of squids, one type of cuttlefish and one type of octopus [9]. This study concluded that exposure to low-frequency sounds caused "massive acoustic trauma, not compatible with life" in these species. 



Damage from explosives isn't limited to big species like mammals, seabirds, fish, and squid. Even tiny planktonic organisms are impacted by underwater bombs. Severely so. There are two main groups of plankton: plant plankton, called phytoplankton, and animal plankton, called zooplankton. Zooplankton graze on phytoplankton, and zooplankton support nearly the entire food chain. In a 2017 study involving five UTAS scientists, experimental seismic air-gun exposure was found to have caused a two- to three-fold increase in dead adult and larval zooplankton [10]. Previously, an impact range of 10 m was assumed, but this study sampled up to 1.2 km and found impact throughout. In fact, they found that all larval krill in the study zone were killed. 

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The extent to which the above impacts can be extrapolated to Tasmanian species is unknown, but certainly we must allow for the possibility that their responses are similar.

Also unknown, is the way that blast sounds and shockwaves travel through Tasmanian waterways. Storm Bay, the D'Etrecasteaux Channel, the Huon, the Derwent Estuary, and the Tasman Peninsula all have unique structural and sedimentary features that may worsen or mitigate the propagation of injury-causing sound waves.

Finally, the threshhold of annoyance for Tasmanian species is unclear. Seismic surveys deploy many more explosions than salmon farmers do, but salmon farmers detonate these bombs in a smaller, shallower area than the open ocean where the seismic surveys take place. It could be that coastal species are more adapted to noise, or less.

What is clear, however, is that seal bombs are capable of causing catastrophic injury to a broad array of native species. And without studying this question here, the salmon industry's current practices may be decimating native species in ways that have been previously unimaginable. 



Click here to go to Noise Pollution References

"Seriously, Who Hurts a Penguin?"