From 14 to 28 June 2002, the U.S. Geological Survey conducted seismic-reflection surveys in the Santa Barbara Channel area off of southern California. As a part of this project, Cascadia Research was contracted by the USGS to monitor marine mammals from the survey platform and provide mitigation on impacts on marine mammals by requesting shutdown of the sound sources when marine mammals were close to the operations. This report summarizes the results of the marine mammal mitigation and monitoring program conducted in conjunction with this USGS surveys. In addition to mitigating from the survey ship there was an effort to tag large whales ahead of the research vessel and monitor both their behavior and the levels of sound received by the animal from the survey vessel.
A small two-chamber generator-injector (GI) airgun was used during daylight hours only. The GI gun of the size we used has a sound-pressure level (SPL) of about 220 dB re 1 µPa-m RMS with a sound pulse duration of 10 ms. Problems with the airgun on 22 June required that it primarily be used with only a sleeved single chamber. This reduced capacity from 70 in3 down to 24 in3 and reduced pressure (3000psi to 2000psi). Other lower-power sound sources were also used including a high-resolution Huntec™ boomer system, an Edgetech 512i Chirp sub-bottom profiler, and a minisparker. Two sets of safety zones were used, one for the airgun and a smaller one when only the lower power sound sources were in use.
The primary objectives of the marine mammal study were to: 1) help mitigate impacts on marine mammals by providing immediate information on the presence of any marine mammals close enough to the sound source to risk injury so that the sound source can be turned off, 2) document the presence and number of marine mammals present in the vicinity of USGS survey operations, and 3) document reactions of marine mammals to the survey ship and sound sources. We also had secondary objectives to attach tags to blue and humpback whales in the vicinity of the seismic-reflection survey as well as examine changes in distribution of whales in reaction to the passage of the survey vessel.
The research effort was primarily conducted directly from the seismic-reflection survey vessel (Auriga). Observers conducted 24-hour-a-day observations from the survey ship during all seismic-reflection operations. There was a total of 289.3 hours of observation during day and night in the study area including 85.7 hours of observation while the airgun was firing.
The mammal observers requested shut-down of sound source operations for marine mammals 83 times, 64 during the day and 19 at night. A total of 38 shutdowns called while the airgun was in operation (termed high power) and 45 shutdowns occurred while the airgun was not in use but one of the other low power sources were in use. The principal species triggering shut-downs (45%) were common dolphins. Observers made 504 sightings of 6,537 marine mammals representing 11 species over the course of the survey. California sea lions were the most common followed by common dolphins and humpback whales. Marine mammals were observed exhibiting a variety of behaviors during the period of observation with no clear indication of distress or problems related to sound source operation. Animals tended to be oriented away from the ship more often than toward the ship in all types of operation modes.
We deployed suction-cup attached tags with acoustic recorders to blue and humpback whales in the general vicinity as the seismic-survey operations. Unfortunately it proved difficult to opportunistically get these tags on animals directly ahead of the path of the survey ship. Despite these problems we did place tags on several animals within a few km of the ship while the ship was operating the single-chamber airgun. While these tag deployments did not allow an evaluation of changes in whale behavior in response to specific received sound levels from the Auriga, we did obtain useful data on whale behavior and the tags on two occasions obtained recordings of the airgun in the distance.
We were able to evaluate any changes in blue whale distribution in response to the single-chamber airgun on one day where we conducted repeated transects with a 2nd vessel through an area of blue whale concentration before, during, and after passage of the survey vessel. These did not indicate any dramatic shift in blue whales away from the area where the ship operated.
There has been heightened concern in recent years about the potential impacts of underwater sounds on marine mammals. This concern has been heightened by recent evidence of strandings of marine mammals in relation to operation of mid-frequency sound sources by the military. In 2002, the stranding of several beaked whales was documented in the Sea of Cortez in close proximity to operation of a large air-gun array. The sound sources involved in the current study were dramatically smaller (less than 100 in3 compared to several thousand in3). While animals seemed to orient away from the survey vessel and in general were sighted farther away when the airgun was firing, we did not see any signs of distress or shifts in overall distribution in response to this survey.
Citation:
Calambokidis, J., T. Chandler, and A. Douglas. 2002. Marine mammal observations and mitigation associated with USGS seismic-reflection surveys in the Santa Barbara Channel 2002. Report to U.S. Geological Survey, Menlo Park, CA. Cascadia Research, 218½ W Fourth Ave., Olympia, WA 98501. 30pp
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