Compound Interest: Shark Week Special: A Brief History of Chemical Shark Repellents

The human fear of sharks is an age old one. Even though your odds of being killed by a shark are estimated to be approximately 1 in 3.7 million, versus a 1 in 218 chance of simply dying from a fall, the fear persists, and over the years many chemists have turned their attentions to trying to find an effective chemical shark repellent – with somewhat mixed results.

The search for chemical shark repellents began in earnest back in the 1940s. A large number of compounds were tested for their effectiveness, including chlorine gas, but whilst most of these compounds succeeded in killing the sharks due to their toxicity, no real repellent effect was seen for the majority. Decomposing shark meat, however, was shown in tests to have some efficacy, so attempts were made to isolate the particular chemicals responsible. The acidic nature of the decomposing flesh was attributed to acetic acid, and this was combined with copper ions, which had illustrated that they could slow the rate of shark feeding in other tests, to produce copper acetate. Although field results for this combination were mixed, researchers pursued it anyway, and it was combined with black dyes in further tests.

After the sinking of the USS Indianapolis during the Second World War in 1945, the number of sailors who were killed by shark attacks whilst awaiting rescue prompted the US Navy to issue the combination of copper acetate and black dye in caked form to all US Military personnel. Branded as ‘Shark Chaser’, other countries swiftly followed suit, and the supposed repellent was to be used for several decades subsequently.

However, doubts about the efficacy of Shark Chaser persisted. A number of further tests failed to show that it was any more effective than a control in repelling sharks, and ultimately, in 1974, the US Navy withdraw it from use. It’s thought that any efficacy it seemed to have was most likely largely due to the black dye causing reduced visibility for the sharks, rather than there being any actual repellent effect.

Thrown back to square one, chemists again resorted to testing a wide range of chemicals for their shark repellent effects, with mixed results. Lachrymatory agents, human sweat, sea cucumber secretions, and flatfish secretions were all tested, with mainly ineffective results. Another type of compound tested were surfactants – these are molecules such as sodium lauryl sulfate, commonly found in many shampoos and toothpastes. Researchers found that sodium lauryl sulfate did have some effect on sharks – but relatively high concentrations were required. To be precise, a concentration of 35 micrograms per millilitre of water was required to see repellent effects; the US Navy had previously stated that, in order to be considered useful as a non-directional repellent, repellent effects needed to be seen at concentrations of no more than 0.1 micrograms per litre.

Further success was achieved in the 1990s as chemists started to examine semiochemicals as possible repellents. ‘Semiochemical’ is a general term, used to refer to any substance that is used for communication or messaging in the natural world. They theorised that sharks may possess selective electrical signals, triggered by the detection of chemicals associated with their natural predators, which could also be used to instigate a repellent effect. They performed several tests with extracts from American Crocodiles, and observed successful results with a number of juvenile sharks. Strangely enough, no further work seems to have been carried out on the exact chemicals isolated in these studies, so their further potential is unknown.

However, elsewhere, further advances with semiochemicals were observed. One study using extracts from putrified shark remains found that effective and consistent repellent action could be observed in a number of species. Since 2008, a company has been producing aerosol canisters capable of dispensing the chemical mixture. Work still continues, however, on improving chemical shark repellents still further. Additionally, electrical devices are also being developed which have shown some degree of effectiveness.