Explosive Deterrents "Seal Bombs" in Fisheries and their Effects on Small Cetaceans in Southern California

Abstract

In Southern California, commercially produced explosive deterrents, commonly known as “seal bombs”, are used to protect fishing gear and catch from pinniped predation. Common U.S. made seal bombs usually contain about 2.3 g of an explosive flash powder mixture, with a waterproof fuse at one end and weighted with sand or silica at the other end to sink and explode approximately up to 4 m below the water surface. In 1990 their use was banned for the tuna purse-seine fishery where they have been used to catch fish while their general use as a pinniped deterrent is still legal and unregulated. Using passive acoustic monitoring data collected between 2005 and 2016 at 21 sites within the Southern California Bight and near Monterey Bay, it was shown that about 94% of explosions occurred at nighttime and at many nearshore sites high explosion counts were detected, up to 2,800/day. Due to similar spatio-temporal patterns and a strong correlation with market squid landings (Doryteuthis opalescens) at many sites, most recorded explosions likely come from seal bombs being used by the California market squid purse-seine fishery. To determine source levels of seal bombs, an experiment offshore off San Diego was conducted in which > 500 seal bombs were deployed and exploded underwater in different distances to a floating hydrophone system resulting in a peak source pressure level of 234 dB re 1 µPa and a sound exposure source level of 203 dB re 1 µPa2s. Taken those values as a basis, a local transmission loss model for seal bombs in Monterey Bay revealed that harbor porpoises (Phocoena phocoena) would experience permanent and temporary threshold shifts at ranges out to 150 and 650 m from a seal bomb explosion, respectively. A temporary threshold shift from cumulative exposure of 6 seal bomb explosions was estimated to occur within 2 km range. The passive acoustic monitoring data also revealed that Risso’s dolphins (Grampus griseus) were exposed to seal bomb noise for > 30 % of the hours they spent around certain sites, with mean received cumulative sound exposure levels of 160-170 dB 1 µPa2s and thus great potential for hearing damage and other physiological effects. Whereas Pacific white-sided dolphins (Lagenorhynchus obliquidens) showed much less overlap and seemed to avoid the noise, at least during times of high noise. Risso’s dolphins prey heavily on squids, sharing the same main target with the fishery using seal bombs, while Pacific white-sided dolphins are more opportunistic feeders, which can explain the different overlap and effects. The results of this dissertation aim to support the implementation of regulations to protect cetaceans and other taxa from being harmed by seal bomb noise.