Surfactants as chemical shark repellents: Past, present, and future

The development of Shark Chaser by the U.S. Navy during World War II was th e first serious effort to develop a chemical shark repellent. In the decade following the war reports of Shark Chaser ineffectiveness led the Office o f Naval Research to search for a more efficacious shark repellent. After ye ars without success, ONR eventually canceled the use of Shark Chaser and ab andoned the search for a chemical shark repellent. In the early 1970s, inte rest in chemical shark repellents was renewed by the discovery of pardaxin, a natural shark repellent secreted by the Red Sea Moses sole, Pardachirus marmoratus. The surfactant-like nature of pardaxin led investigators to tes t the potential of various surfactants as repellents. Subsequent studies in dicated that the shark repellent efficacy of the effective alkyl sulfate su rfactants was due to their hydrophobic nature. Here we report tests conduct ed on juvenile swell sharks, Cephaloscyllium ventriosum, to determine if th e noxious quality of alkyl sulfates is affected by surfactant hydrophobicit y [carbon chain length and ethylene oxide (EO) groups] and counterions. Our results indicate that the aversive response of sharks to alkyl sulfate sur factants increases with carbon chain length from octyl to dodecyl, decrease s with the addition of EO groups and is not affected by counterions. This s tudy confirms that dodecyl sulfate is the most effective surfactant shark r epellent, but it does not meet the Navy's potency requirement for a nondire ctional surrounding-cloud type repellent of 100 parts per billion (0.1 mug ml(-1)). Thus, dodecyl sulfate is only practical as a directional repellent such as in a squirt application. Future research should test the action of alkyl sulfates on cell membranes, the potential of other biotoxic agents, and semiochemicals in the search for an effective chemical shark repellent.