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BOEMRE ENVIRONMENTAL STUDIES PROGRAM: ONGOING STUDIES | ||
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BOEMRE OCS Region: |
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Planning Area: |
Gulfwide | |
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Title: |
Sperm Whale Acoustic Prey Study (SWAPS) (GM-09-05) | |
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Total Cost: $550,000 |
Period of Performance: FY 2009-2012 | |
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Conducting Organization: |
NOAA, National Marine Fisheries Service | |
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BOEMRE Contact: |
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Description: Background: Sperm whales (Physeter macrocephalus) in the northern Gulf of Mexico (GOM) are protected by both the Endangered Species Act (ESA) and the Marine Mammal Protection Act (MMPA). Sperm whales are present throughout the GOM year-round. Preliminary results from several studies of genetics, size distribution, and acoustic behavior suggest that this population is distinct from others in the North Atlantic Ocean (Waring et al. 2008). The most recent abundance estimate for the portion of the stock residing in the northern GOM (within the U.S. Economic Exclusivity Zone (EEZ)) is 1,665 animals (coefficient of variation = 0.20, Mullin 2007). Based upon this abundance estimate the annual number of human caused mortalities that can be sustained by the stock without threatening its recovery (i.e. Potential Biological Removal or PBR) is 2.8 animals (Waring et al. 2008).
The northern GOM is one of the most heavily industrialized bodies of water in the world and is the site of intensive energy exploration and exploitation. There are approximately 4,000 offshore oil platforms and 25,000 miles of active oil and gas pipeline on the GOM sea floor with the majority of these resources concentrated on the continental shelf and continental shelf break west of the Mississippi River. In addition, there is extensive and ongoing exploration of the Gulf for additional energy resources over continental slope waters. These exploration activities typically employ air-gun arrays that radiate high intensity, broad-band frequency sounds. These noise sources may result in behavioral or physiological impacts to protected marine mammals, including sperm whales. As a result, BOEMRE has sponsored numerous studies of sperm whale abundance, spatial distribution, habitat, and response to sound sources since the 1990’s. The most recent of these studies was the Sperm Whale Seismic Study (SWSS) which conducted fieldwork between 2002 and 2005 to develop baseline information on the biology and behavior of sperm whales, characterize habitat use, and assess changes in behavior associated with exposure to sounds from seismic air-guns (Jochens et al. 2008).
Analyses of sperm whale spatial distribution during the first two years of the SWSS cruises (2002 and 2003) and from prior studies in the summers of 2000 and 2001 demonstrated a strong relationship between sperm whale occurrence and mesoscale physical features. Most notably, Loop Current eddies and cyclonic circulation along the 1,000 m isobath in the central northern GOM resulted in off-margin flow and locally elevated surface chlorophyll concentrations. Under these conditions of elevated surface layer primary productivity, there were localized high densities of sperm whales (Biggs et al. 2005). Localized acoustic studies of sperm whale distribution during the SWSS studies demonstrated that areas of high sperm whale concentration were also related to elevated levels of back-scatter from scientific echosounders as an indicator of the biomass of prey resources at depth.
Diving studies conducted during SWSS demonstrated that the depth range between 400-600m was most consistently associated with sperm whale dives and acoustic behavior consistent with feeding. Data from scientific echosounders (70 kHz and 38 kHZ Simrad EK60) indicate two primary scattering layers in regions where feeding sperm whales were observed. The first consisted of vertically migrating organisms that are at depths between 350-550 m during the day that rose rapidly to the surface layer at night. The second layer is more horizontally patchy, but had high backscatter levels during both day and night hours at depths greater than 500 m (Jochens et al. 2008). Based on foraging dive depths, sperm whales apparently feed near the bottom of the primary scattering layer during the daylight hours and near the top of the secondary deep scattering layer at night. The actual species composition of sperm whale prey in both scattering layers is not known, and there has been little systematic study of the deep and mid-water pelagic community of the northern GOM. The diets of sperm whales globally are presumed to be dominated by mid-water squid species (Clarke et al. 1993). A study of sperm whale stomach contents from the Azores found that the modal mass of individual squid was between 400-500 g with most taxa typically having mantle lengths between 100 – 300 mm (Clarke et al. 1993). A study of sperm whale diets inferred from both stranded animals and fecal collections in the northern GOM found a similar size range and species composition to that of the Azores study. Sperm whale diets included squid prey from 13 species within 10 families, and squids were the only prey type observed. The diets were dominated by Histoteuthid squids that were inferred to have a mean mantle length of 80 mm (range: 60-90 mm) and an average mass of 194 g (range: 99-303 g, Barros 2003). There are two species of Histoteuthid squids documented in the northern GOM. Both species occur throughout the water column from the surface to 2,000 m and have varying degrees of vertical migration (Voss et al. 1998). However, it should be noted that these data are from a very small sample size collected in a localized area, and may not fully reflect the diversity of sperm whale prey in the Gulf. | ||
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Objectives: The goal of this study is to characterize the species composition and biomass of mid-water squid and small pelagic fish in the GOM that represent the apparent forage base for sperm whales. Acoustic backscatter provides a high resolution picture of the vertical and horizontal distribution of secondary productivity. However, the actual composition of the prey may vary significantly and may not be reflected in the backscatter. Similarly, the return from the 38 kHz echosounder may represent plankton and micro-nekton that are the prey of mid-water squids, rather than the squids themselves. To better characterize the forage base for sperm whales in the Gulf this study will:
The quantitative sampling of the mid-water squid and fish community is a significant challenge. There are relatively few studies of this community in the global oceans, reflecting the difficulty in efficiently catching these relatively large and fast-moving prey. A pilot study was conducted during the summer of 2009 in association with a marine mammal assessment survey aboard the NOAA ship Gordon Gunter. The primary goal of the pilot study was to develop fishing methods using a large mid-water trawl to target specific layers of prey apparent in the acoustic backscatter data. A dedicated cruise was completed on the NOAA ship Pisces in January - March of 2010 immediately prior to the Deepwater Horizon (DWH) event. This cruise was a larger scale effort to sample prey both acoustically and with nets while simultaneously quantifying sperm whale distribution and abundance throughout the GOM. | ||
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Methods: The pilot project was conducted during a Marine Mammal Assessment survey between June – August 2009 aboard the NOAA Ship Gordon Gunter. The survey covered waters between the 200 m isobath and the U.S. EEZ encompassing sperm whale habitats of the continental shelf break and upper continental slope throughout the Gulf.
The survey data collection included information on sperm whale (and other marine mammal species) spatial distribution and abundance simultaneous with data collected on prey biomass, composition, and spatial distribution. Data on prey composition and biomass was collected with two primary tools: scientific echosounders and a mid-water trawl.
The scientific echosounder aboard the Gordon Gunter is similar to that used during the SWSS studies (SimRad EK60). This is a split-beam echosounder with both 38 kHz and 120 kHz frequencies. The echosounder was calibrated both at the beginning and end of the study using targets of known return strength to allow reliable quantification of backscatter. Throughout the survey, echosounder data was collected continually and recorded to hard-disk to provide information on the vertical and horizontal distribution of backscatter on both frequencies throughout the cruise. Finally, the echosounders were used to provide detailed data on the vertical distribution of scattering layers in the presence of foraging sperm whales. Initial deployment and testing of the gear was conducted during a dedicated 7-day period at the beginning of the survey. The focus of the work was on targeting specific depth layers (using the scientific echosounder), fishing in those layers, and verifying the effectiveness of the gear at collecting target species. During the remaining 50 days-at-sea of the survey, the primary objective of the cruise was to conduct visual line transect surveys for marine mammals. Additional mid-water trawling was conducted throughout the survey, generally in areas with aggregations of feeding sperm whales. Stations were selected on an adaptive basis and included both day and night sampling across a range of habitats throughout the Gulf such as within or near Loop Current eddies and along the continental shelf break of the eastern, central, and western Gulf. Twenty-five were sampled with the mid-water trawl throughout the survey. Collected squids and other pelagic prey were identified at sea (where possible), measured and weighed, and/or preserved for later identification and evaluation. Vertical profiles of hydrographic parameters were conducted at trawl stations including temperature, salinity, oxygen content, and fluorescence. | ||
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Products: The primary objective of the trawling portion of this survey was to develop the methods for quantitative sampling of mid-water prey and to develop preliminary data on the relationships between acoustic backscatter, prey composition and biomass, and sperm whale distribution. The project deliverables will focus on the results of the prey sampling efforts and have resulted in both a cruise report summarizing survey effort, hydrographic sampling, sperm whale sightings, and trawling efforts and a summary report which includes detailed summaries of trawl stations during both the testing and operational phases of the cruise. Following the pilot cruise, a study plan was produced for the winter 2010 survey to assess the relationships between sperm whales and their prey based upon the lessons learned during this pilot study. Data analyses and final report preparation has been delayed due to the DWH event. | ||
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Importance to BOEMRE: The potential for acoustic disturbance resulting from seismic industry activities in the GOM is the subject of much debate. The potential effects of seismic exploration on marine vertebrates (cetaceans and sea turtles) and their prey species are largely unknown. The most recent NMFS Biological Opinions (2002, 2003) for lease sales in the GOM have included recommendations that “BOEMRE should support investigations into the effects of seismic noise on the distribution of cephalopods and fish (i.e., sperm whale prey items) near seismic vessels, including diel vertical migration, startle effects, distribution and abundance.” The possible impacts of seismic noise on prey species (e.g. squid) are an information gap that may influence future mitigation. Before this can be addressed, methodology needs to be developed to accurately identify squid stocks in the GOM. | ||
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Current Status: |
ongoing | |
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Final Report Due: |
December 2011 | |
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Publications: |
none | |
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Affiliated WWW Sites: |
none | |
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Revised date: |
June 2011 | |
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ESPIS |
ESPIS
- All completed ESP Studies: | |