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Assessment of dolphin whistles and bridge construction noise levels

by Kathleen M. Dudzinski, Ph.D., Senior Marine Scientist

PLANO, TEXAS (February 16, 2009)

In June 2008, more than two-dozen bottlenose dolphins (Tursiops truncatus) entered the estuary system of the Navesink and Shrewsbury Rivers in northern New Jersey. The news media and members of the general public observed the antics of these dolphins throughout the summer months.

When the weather turned cold, concern grew about whether the dolphins would return to the ocean, or even if they ‘could’. That is, concern was expressed about the possibility that construction activity to repair and refurbish the Highlands Bridge might be creating a ‘noise barrier’ through which the dolphins would not pass on a return trek to the ocean.

Into the middle of November, 10 to 12 dolphins still called these rivers home. NOAA and members of the New Jersey Government received numerous contacts (letters, emails, phone calls) from members of animal rights organizations and of the general public regarding these dolphins. Construction of the Highlands Bridge was ongoing, though no dolphins were observed within 200 m of the bridge during construction activity.

In late November 2008, NOAA contracted GMI and scientists from the Rutgers University Marine Field Station to record sounds from the river in two locations in an attempt to determine if the dolphins seemed to be vocalizing normally. One recording location was adjacent to the Highlands Bridge and the second spot was about 1 km distant at a private dock near a hotel.

Five deployments of two passive acoustic monitoring systems let us record more than 237 hrs of data that were collected over four weeks. These data have allowed us to assess sound pressure levels (SPLs) from pile driving activities during bridge construction and to investigate these waters for dolphin vocal activity. (SPLs relate to how loud the sounds might be perceived at set and identified distances from the actual construction. Think about when you turn your stereo up loud when drums are playing: the ‘loudness’ is different if you are right next to the speakers as opposed to in the other room, or even outside.)

All data were examined and SPLs were indicated at well below accepted safety levels (140 dB re 1mPa at 1 m) where dolphins might be observed. No whistles or clicks from dolphins were recorded or found within the data recordings; however, data were logged at a sampling rate (4.4 kHz) that would represent only the lowest end of the frequency range for bottlenose dolphin whistles. Data collected provided evidence to conclude that bridge construction did not adversely affect the bottlenose dolphins in the Navesink and Shrewsbury Rivers. As of late January 2009, no bottlenose dolphins have been sighted in these rivers. Thus, it is likely that the remaining dolphins returned to the ocean some time after the New Year began.

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