Effects of different analysis techniques and recording duty cycles on passive acoustic monitoring of killer whales

Abstract

Killer whales in British Columbia are at risk, and little is known about their winter distribution. Passive acoustic monitoring of their year-round habitat is a valuable supplemental method to traditional visual and photographic surveys. However, long-term acoustic studies of odontocetes have some limitations, including the generation of large amounts of data that require highly time-consuming processing. There is a need to develop tools and protocols to maximize the efficiency of such studies. Here, two types of analysis, real-time and long term spectral averages, were compared to assess their performance at detecting killer whale calls in long-term acoustic recordings. In addition, two different duty cycles, 1/3 and 2/3, were tested. Both the use of long term spectral averages and a lower duty cycle resulted in a decrease in call detection and positive pod identification, leading to underestimations of the amount of time the whales were present. The impact of these limitations should be considered in future killer whale acoustic surveys. A compromise between a lower resolution data processing method and a higher duty cycle is suggested for maximum methodological efficiency.

Effects of subsampling of passive acoustic recordings on acoustic metrics

“…The acoustic presence of the focal species was increasingly underestimated with decreasing duty cycles. These findings match the trends observed by Riera et al (2013), showing that a duty cycle of 1 3 resulted in a 24% decline in encounter detections of killer whale calls and a consequent underestimation of the time whales were acoustically present in the data set compared to a larger duty cycle of 2…”

Effects of duty-cycled passive acoustic recordings on detecting the presence of beaked whales in the northwest Atlantic

“…Predictably, any reduction in recording effort over a given monitoring period will lead to underestimation of species presence, except for those that are continuously present and acoustically active (Riera et al, 2013;Thomisch et al, 2015). The results presented here demonstrate that frequent, short listening periods provide a more accurate assessment of daily presence than longer, less frequent periods, even when the overall amount of recording effort is lower.…”

Evaluating autonomous acoustic surveying techniques for rails in tidal marshes

“…Shortening subsampling periods resulted in increasingly underestimated detection rates of rails. Previous studies reported similar trends where subsampling one‐third of the killer whale ( Orcinus orca ) calls led to a 24% decline in detections (Riera et al ) and subsampling less than one‐tenth of the North American right whale ( Eubalaena glacialis ) calls resulted in a reduction in acoustic presence up to 26% (Thomisch et al ). These findings demonstrate the importance of understanding tradeoffs between subsampling duration and estimation biases prior to the analysis of acoustic data.…”