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FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013 given that they have been exposed and respond to that exposure” versus “number of times marine mammals might be exposed”). 1. THE U.S. NAVY’S “TAKE” ESTIMATES FOR THE 2006 RIM OF THE PACIFIC EXERCISE. To estimate the number of marine mammals that were likely to be “taken” as a result of their exposure to mid-frequency active sonar during the 2006 Rim of the Pacific exercise, the U.S. Navy developed and ran computer simulations that made several assumptions about marine mammals in the Hawai’i Range Complex (U.S. Navy 2006; see Appendix C of that document for a more detailed presentation of the Navy’s modeling procedures): 1. acoustic energy would be constant throughout the vertical water column at a given horizontal range from the source; 2. marine mammal hearing is omni-directional; 3. marine mammals were static (not moving) at the maximum acoustic energy depth at any range. The Navy’s models assumed ship speeds of 10 knots for all exercises except those taking place in Area 4 where speeds were modeled at 20 knots. All active sonar was modeled using the operational characteristics of the AN/SQS- 53 and its variants. The model the Navy used in 2006 included several other considerations or assumptions about marine mammals. First, the model considered the density and hearing of marine mammals, but did not attempt to predict an animal’s location relative to the point where active sonar transmissions began or the animal’s behavioral response to sound in the water. That is, the model assumed that marine mammals would not leave an area once they were exposed to active sonar transmissions. Second, the model assumed that mammals would be exposed to the maximum received levels calculated for the horizontal distance to the source at any water depth for that distance although direct path sound transmission was not always likely. Both of these assumptions — assuming that marine mammals are not likely to try to avoid exposure or continued exposure to active sonar and that they would be exposed to the maximum received levels calculated for the horizontal distance to a sound source — would tend to overestimate the number of marine mammals that might be exposed, because marine mammals are highly mobile and are likely to use their mobility to avoid stimuli like active sonar, just as they avoid vessel traffic. Consequently, the results of this approach would be conservative, in the sense that they would tend to overestimate the number of animals that were likely to have been “taken” during the 2006 Rim of the Pacific Exercise. 2. U.S. NAVY EXPOSURE ESTIMATES FOR PROPOSED ACTIONS IN THE HAWAI'I RANGE COMPLEX. Over the past year, the U.S. Navy updated its approach to estimating the number of marine mammals that might be exposed to the activities the U.S. Navy plans to conduct in the Hawai’i Range Complex over the five-year period beginning in January 2009. What follows is a brief summary of the Navy’s current approach (for more details, refer to Appendix K of the U.S. Navy’s Environmental Impact Statement — draft or final — on the Hawai'i Range Complex; U.S. Navy 2007 and 2008a). The U.S. Navy’s updated approach focuses on a suite of representative provinces based on sound velocity profiles, bathymetries, and bottom types. Within each of these provinces, the U.S. Navy modeled transmission losses in 5 meter increments and used the results to build sound fields (based on maximum sound pressure levels). The U.S. 68
FINAL PROGRAMMATIC BIOLOGICAL OPINION ON U.S. NAVY ACTIVITIES IN THE HAWAII RANGE COMPLEX 2008-2013 Navy then calculates an “impact volume,” which is the volume of water in which an acoustic metric exceeds a specified threshold; in this case, the Navy used one of three acoustic metrics: energy flux density (in a limited band or across a full band), peak pressure, or positive impulse. By multiplying these “impact volumes” by estimates of animal densities in three dimensions (densities distributed by area and depth), the U.S. Navy estimated the expected number of animals that might be exposed to an acoustic metric (energy flux density, peak pressure, or positive impulse) at levels that exceed thresholds that had been specified in advance. Specifically, the U.S. Navy calculated impact volumes for sonar operations (using energy flux density to estimate the probability of injury), peak pressure, and a Goertner modified positive impulse (for onset of slight lung injury associated with explosions). To calculate “impact volumes,” the U.S. Navy used a “risk continuum” or a curve that the U.S. Navy and <strong>NMFS</strong> developed that relates the probability of a behavioral response given exposure to a received level that is generally represented by sound pressure level, but included sound exposure level to deal with threshold shifts. The risk continuum, which the U.S. Navy and <strong>NMFS</strong>’ Permits Division adapted from a mathematical model presented in Feller (1968), was estimated using three data sources: (1) data from controlled experiments conducted at the U.S. Navy’s Space and Naval Warfare Systems Center in San Diego, California (Finneran et al. 2001, 2003, 2005; Finneran and Schlundt 2004; Schlundt et al. 2000), (2) data from a reconstruction of an incident in which killer whales were probably exposed to mid-frequency active sonar (Fromm 2004, Department of the Navy 2003), and (3) a suite of studies of the response of baleen whales to low-frequency sound sources (Nowacek et al. 2004). The U.S. Navy and <strong>NMFS</strong>’ Permits Division estimated the proportion of a population that is expected to exhibit behavioral responses that <strong>NMFS</strong>’ would classify as “take” (as that term is defined by the MMPA) by multiplying the different “impact volumes” at particular received levels by the “risk continuum.” Like the approach the Navy used to estimate the number of marine mammals that might be “taken” during the 2006 Rim of the Pacific Exercise, this approach would also tend to overestimate the number of marine mammals that might be exposed, because marine mammals are highly mobile and are likely to use their mobility to avoid stimuli like active sonar, just as they avoid vessel traffic. Consequently, the results of this approach would be conservative, in the sense that they would tend to overestimate the number of animals that were likely to have been “taken” during the 2006 Rim of the Pacific Exercise. 3. <strong>NMFS</strong>’ EXPOSURE ESTIMATES USING COMPONENTS OF AN ECOLOGICAL PREDATOR-PREY MODEL. The models the U.S. Navy used provide estimates of the number of marine mammals that might be “taken,” as that term is defined by the MMPA, by active sonar and underwater detonations, particularly as a result of either noise-induced hearing loss (temporary or permanent threshold shifts) or behavioral responses. However, our jeopardy analyses must consider all potential effects of proposed actions, including direct or indirect beneficial and adverse effects that do not necessarily rise to the level of “take.” For example, jeopardy analyses must consider the direct beneficial or adverse effects of actions on endangered or threatened individuals as well as indirect effects that results from how competitors, prey, symbionts, or the habitat of those listed individuals respond to an action. We cannot begin those analyses with estimates of the number of individuals that might be “taken” (as that term is defined by the MMPA) because our analyses must consider direct and indirect effects that do not necessarily represent one or more form of “take.” 69
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