Underwater noise and zones of masking with respect to hopper dredging and manatees in the St. Johns River in Jacksonville, FL

Abstract

Underwater noise radiating from dredging can effectively obscure or mask biological and other important sounds. This study recorded underwater acoustic characteristics of hopper dredging in the St. Johns River, Jacksonville, FL, to evaluate noise impacts in the waterway with respect to the endangered West Indian manatee. Of particular interest was the extent and range that dredging noise may mask the sounds of approaching commercial and recreational vessels. Vertical hydrophone arrays and a multi-channel PC-based recording system were used to measure dredging noise at various distances. Ambient noise surveys, active propagation of calibrated sources, and controlled boat noise measurements were conducted along the waterway. These data were integrated with behavioral hearing data to estimate zones of masking surrounding dredging. Three discernable noise sources that masked boat noise were (1) cavitation from dredge propellers, (2) draghead vacuuming, and (3) noise from submerged slurry pipelines. Sustained high ambient noise levels from dredging can significantly increase the risk of manatee-boat collisions by masking the sounds of approaching vessels over large radii (up to 2.5 miles). Mitigations suggested include ship quieting, reducing propeller cavitation, insulating or elevating slurry pipelines, and minimizing transects to pump out stations. [Work funded by the City of Jacksonville Waterways Commission.]

Evaluating effects of dredging-induced underwater sound on aquatic species : a literature review

“…Many of the controlled experiments conducted on marine mammals were tonal or playback sounds. Only a few studies were found that estimated effects of dredgingrelated sounds on marine mammals (Richardson et al 1990;Gilmartin 2003;Gerstein et al 2006;Hoffman 2010;Heinis et al 2013).…”

“…A few studies were identified that estimated the influence of low-frequency continuous sounds from shipping and dredging operations to communication or auditory masking of marine mammals (manatee and whales). Gerstein et al (2006) estimated zones of auditory masking to manatee during TSHD operations in the St. Johns River in Jacksonville, FL. The primary goal was to discern whether manatees had an increased risk of vessel strikes due to the underwater sound of dredging activities.…”

“…The major sources of underwater sound that were relatively continuous included cavitation from dredge propellers, draghead vacuuming, and the submerged slurry pump out pipeline. Sound pressures levels measured during operations ranged from 169 to 177 dB re 1 µPa at 1 m. Gerstein et al (2006) estimated that TSHD acoustical properties created zones of auditory masking in the vicinity of the operations; the zones may increase the probability of vessel strikes. Zones of masking for manatee were estimated to be 512 m and 4,096 m from the dredge, for boats approaching at speeds of 5 and 24 miles per hour (mph), respectively.…”

“…Based on observational studies, pinnipeds (seals) did not exhibit avoidance or altered behavior near dredging activities (Gilmartin 2003). There is some evidence that sirenians (manatee) may be susceptible to low-frequency sounds masking boating sounds (Gerstein et al 2006). Significant data gaps exist in terms of sea turtle responses to underwater sound.…”

“…There is some evidence that low-frequency sounds from dredging activities may mask approaching boating sounds to sirenians (manatee, etc.) (Gerstein et al 2006). It should be noted that there is uncertainty as to the appropriateness of using sound metrics for predicting biological responses.…”