|Title||A comparison of acoustic and visual metrics of sperm whale longline depredation|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||Thode A.M, Wild L., Mathias D., Straley J., Lunsford C.|
|Journal||Journal of the Acoustical Society of America|
|Type of Article||Article|
|Keywords||Alaska; behavior; click; diving behavior; echolocation; foraging; high-latitude habitat; orcinus-orca; physeter-macrocephalus; prey-capture; south georgia|
Annual federal stock assessment surveys for Alaskan sablefish also attempt to measure sperm whale depredation by quantifying visual evidence of depredation, including lip remains and damaged fish. A complementary passive acoustic method for quantifying depredation was investigated during the 2011 and 2012 survey hauls. A combination of machine-aided and human analysis counted the number of distinct "creak" sounds detected on autonomous recorders deployed during the survey, emphasizing sounds that are followed by silence ("creak-pauses"), a possible indication of prey capture. These raw counts were then adjusted for variations in background noise levels between deployments. Both a randomized Pearson correlation analysis and a generalized linear model found that noise-adjusted counts of "creak-pauses" were highly correlated with survey counts of lip remains during both years (2012: r(10) = 0.89, p = 1e-3; 2011: r(39) = 0.72, p = 4e-3) and somewhat correlated with observed sablefish damage in 2011 [r(39) = 0.37, p = 0.03], but uncorrelated with other species depredation. The acoustic depredation count was anywhere from 10% to 80% higher than the visual counts, depending on the survey year and assumptions employed. The results suggest that passive acoustics can provide upper bounds on depredation rates; however, the observed correlation breaks down whenever three or more whales are present. (C) 2014 Acoustical Society of America.