Abstract
Acoustic deterrents can reduce marine mammal interactions with fisheries and aquacultures, but they contribute to an increasing level of underwater noise. In Southern California, commercially produced explosive deterrents, commonly known as “seal bombs,” are used to protect fishing gear and catch from pinniped predation, which can cause extensive economic losses for the fishing community. Passive acoustic monitoring data collected between 2005 and 2016 at multiple sites within the Southern California Bight and near Monterey Bay revealed high numbers of these small-charge underwater explosions, long-term, spatio-temporal patterns in their occurrence, and their relation to different commercial purse-seine fishing sectors. The vast majority of explosions occurred at nighttime and at many nearshore sites high explosion counts were detected, up to 2,800/day. Received sound exposure levels of up to 189 dB re 1 μPa2-s indicate the potential for negative effects on marine life, especially in combination with the persistence of recurring explosions during periods of peak occurrence. Due to the highly significant correlation and similar spatio-temporal patterns of market squid landings and explosion occurrence at many sites, we conclude that the majority of the recorded explosions come from seal bombs being used by the California market squid purse-seine fishery. Additionally, seal bomb use declined over the years of the study, potentially due to a combination of reduced availability of market squid driven by warm water events in California and regulation enforcement. This study is the first to provide results on the distribution and origin of underwater explosions off Southern California, but there is a substantial need for further research on seal bomb use in more recent years and their effects on marine life, as well as for establishing environmental regulations on their use as a deterrent.
Highlights
The increased occurrence, power, and pervasiveness of anthropogenic noise sources in the ocean has increased ambient sound levels (Richardson et al, 1995; Hildebrand, 2009)
We did not correct for temporal autocorrelation of the predictor variable in the generalized additive modeling (GAM), as we mainly examined the effects of calendar week, as proxy for seasonal pattern, which itself is depicted by the autocorrelation within the data, and we did not evaluate the effect of any environmental parameters
This study aims to improve understanding of where and when seal bombs are being utilized and of their likely impact on marine life, so that their future use can be better regulated
Summary
The increased occurrence, power, and pervasiveness of anthropogenic noise sources in the ocean has increased ambient sound levels (Richardson et al, 1995; Hildebrand, 2009). Low-frequency ship noise has increased average ambient noise levels recorded in the Southern California Bight by about 2–3 dB per decade since the 1960s (Andrew et al, 2002; McDonald et al, 2006; Haren, 2007). While low-frequency ship noise affects both basin-wide and regional areas, other underwater noise sources, such as sonar, acoustic deterrents and explosives, can have strong local impacts on marine life (Hildebrand, 2009). Commercial fisheries are an important sector of Southern California’s economy and there is potential for adverse effects from interactions with the area’s two most abundant pinniped species: California sea lions (Zalophus californianus) and Pacific harbor seals (Phoca vitulina richardsii; Beeson and Hanan, 1996; Scordino, 2010). A variety of deterrents have been used to keep these species away from fishing operations and areas
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