Gulf of Alaska beaked whale density report

Posted on September 14, 2015

From Clicks to Counts: Using Passive Acoustic Monitoring to Estimate the Density and Abundance of Cuvier’s Beaked Whales in the Gulf of Alaska (GoA) - Download PDF

The Gulf of Alaska (GoA) is home to three known species of beaked whales, Baird’s (Berardius bairdii), Cuvier’s (Ziphius cavirostris), and Stejneger’s (Mesoplodon stejnegeri). Beaked whale distribution and abundance is poorly understood with only limited sightings within the GoA (Allen and Angliss 2013). To address gaps in knowledge regarding the distribution and abundance of beaked whales and other marine mammals in the GoA, a 26-day visual and acoustic line-transect survey was conducted during the summer of 2013. One of the main objectives of the acoustic survey was to obtain acoustic-based density and abundance estimates for Cuvier’s beaked whales in the study area. The survey area was divided into four strata to reflect distinct marine mammal habitat types; ‘inshore’, ‘slope’, ‘offshore’ and ‘seamounts.’ Passive acoustic monitoring was conducted around the clock for 23 days over 6,304 kilometers (km) of trackline using a towed hydrophone array system. There were 93 acoustic encounters of beaked whales (32 Baird’s, 47 Cuvier’s and 14 Stejneger’s), of which 79 (85 percent) were localized during post-processing. Of these localized encounters, 18 Baird’s, 40 Cuvier’s and 10 Stejneger’s occurred during line-transect effort. Cuvier’s beaked whales were the only species of beaked whale with a sufficient sample size to reliably estimate density and abundance using line-transect distance sampling methods. Comparatively, visual survey methods resulted in only one sighting of Cuvier’s beaked whale (one individual), six sightings of Baird’s beaked whale (49 individuals) and five unidentified beaked whale encounters (nine individuals) during 4,155 km of visual effort. Line-transect distance sampling methods were used to estimate the density and abundance of Cuvier’s beaked whales using the acoustic data. Encounter rate varied by strata, and was highest in the seamount stratum (10 animals/1,000 km), followed by the offshore (7 animals/1,000 km) and slope strata (3 animals/1,000 km), respectively. An acoustic-based density and abundance estimate was obtained for each stratum Offshore: D̂/1,000 km2 = 0.20; N̂=122; CVb = 48 percent; Seamount: D̂ /1,000 km2 = 0.30; N̂ =138; CVb = 30 percent; and Slope: D̂ /1,000 km2 = 0.08; N̂ = 31; CVb = 74 percent) and for the entire survey area (A =142,204 km2; D̂ /1000 km2 = 0.21; N̂ = 291; CVb = 28 percent) Results from this study represent the first estimates of density and abundance for Cuvier’s beaked whales within the GoA, and the first acoustic-based estimates for any species of beaked whales using a line-transect survey design.

Department of the Navy. 2015. From Clicks to Counts: Using Passive Acoustic Monitoring to Estimate the Density and Abundance of Cuvier’s Beaked Whales in the Gulf of Alaska (GoA). Prepared by T.M Yack, T. Norris, E. Ferguson, S. Coates, and B.K. Rone for Assistant Secretary of the Navy. Submitted to Naval Facilities Engineering Command Pacific, Pearl Harbor, Hawaii, under Contract No. N62470-10-3011, Task Order 22, issued to HDR, Inc., San Diego, CA. 31 August 2015. 43 Pages.