US Navy

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Marine Species Monitoring

Acoustic Monitoring and Evaluation of Tursiops Response to MINEX Training activities

Introduction & Objectives

To better understand the potential effects of mine warfare training activities on marine mammals, an effort was initiated in August 2012 (and is currently still ongoing) to monitor odontocete activity at the W-50 Mine Neutralization Exercise (MINEX) training area in the VACAPES Range Complex using passive acoustic methods. This work is primarily being conducted by Oceanwide Science Institute

The initial objectives of the project were:

  1. Establish the daily and seasonal patterns of occurrence of dolphins in the W-50 MINEX training range.
  2. Detect underwater detonations (UNDETs) related to MINEX activities.
  3. Determine whether dolphins in the area show evidence of a behavioral response to UNDETS, and if so at what ranges.

During the second year of the study (August 2013–July 2014), these objectives were expanded to also address the following questions:

  1. At what distance from the explosion site is an acoustic response observable?
  2. Do dolphins show evidence of re-distribution as a result of MINEX activities?
  3. At what distance from MINEX explosions do dolphins occur?

Technical Approach

Four Ecological Acoustic Recorders (EARs; autonomous recorders) were deployed in August 2012 and refurbished approximately every 2 months. The units were initially deployed in pairs and programmed to achieve continuous monitoring by operating on alternating 50% duty cycles.  The EARs sampled at a rate of 50 kHz for 180 seconds (3 min) every 360 seconds (6 min), providing approximately 25 kHz of Nyquist bandwidth recording at a 50 percent duty cycle. This bandwidth is sufficient to detect signals (whistles and the low frequency end of clicks) from bottlenose dolphins (Tursiops truncatus) and other delphinid species potentially occurring in the area that produce signals at frequencies less than 25 kHz; however, harbor porpoise (Phocoena phocoena) clicks, with center and peak frequencies of 130 to 140 kHz (Goodson and Sturtivant 1996), are above the recording range of these EARs.

Individual units within each pair were located approximately 1 km apart.  Units A and B were placed in 13 m and 14 m water depths respectively and approximately 1 km from a location that is considered to be the ‘epicenter’ of MINEX training activity. This is a search field location where the majority (approximately 95 percent) of MINEX detonations is expected to occur each year. Units C and D were deployed in 15 m and 16 m water depth respectively, approximately 5 km to the south-southeast of EARS A and B, near another mine search field area.

Beginning in September of 2013, the EAR configuration was modified to a ‘linear coastal array’ with the units spaced at distances of 1 km, 3 km, 6 km and 12 km from the primary MINEX epicenter. The data obtained from linear coastal array deployments are used to examine the acoustic activity of dolphins at the four locations during the days before, during, and after MINEX training events to determine the range at which an acoustic response by dolphins is observed. Data from the four coastal locations were also used to assess whether or not there is a re-distribution of animals following MINEX training activities. The linear coastal array was shifted to the south, east and north during alternating EAR redeployments.

In 2014 alternate deployments placed the EARs in a ‘localization array’ configuration, with the units separated by approximately 150m. This array configuration is being used to localize dolphins during periods of MINEX training using time-of-arrival differences of dolphin signals recorded on the four EAR units. A Trimble high-accuracy global positioning system is used to precisely record EAR deployment locations. The four EAR units are programmed to record simultaneously at a 50 percent duty cycle, allowing them to record the same dolphin signals and explosions. In order to accurately localize signals during post-processing, the recordings must be precisely time-aligned. This time-alignment of the EAR clocks is accomplished by deploying an ARS-100 pinger with one of the moorings in the localization array deployment. The known location of the pinger is used to calculate the time-delay between EARs in order to align the recordings. Analysis of data from these deployments is currently in progress. The information obtained will be used to determine the approximate distance animals occurred from the explosion site and compare the spatial distribution of dolphins immediately prior to and following an UNDET.

 

Progress & Results

As of February 2015, there have been twelve deployments since the beginning of the project in August 2012 with 19 out of units successfully collecting high-quality data. In total, 344,510 3-minute recordings have been made, totaling 17,225 hours of data.

The data from the second year of work have generally confirmed the findings previously reported (Lammers et al, 2014). Seasonally, there appears to be a consistent period of 1–3 months of low occurrence or reduced acoustic activity centered on February. Dolphin occurrence within some other months of the year also varied from year to year, demonstrating some natural inter-annual variability in the occurrence of dolphins in the area around the ‘epicenter’ of MINEX training. The sample sizes analyzed from the linear coastal EAR arrays are still too small to draw any firm conclusions, but the data examined to date do not suggest that dolphins follow a consistent pattern of re-distribution away from the epicenter after a MINEX training event. There is some evidence that dolphins may be more acoustically active or abundant 3 km from the epicenter during the early morning hours of the day after an exercise, but this trend may or may not hold as data from additional deployments are collected and/or analyzed.

The last deployments have been retrieved and final analysis and results will be presented in a final project report available in early 2017.