Julie N. Oswald – jno@st-andrews.ac.uk
Sam F. Walmsley – sjfw@st-andrews.ac.uk
Scottish Oceans Institute
School of Biology
University of St Andrews, UK

Caroline Casey – cbcasey@ucsc.edu
Selene Fregosi – selene.fregosi@gmail.com
Brandon Southall – brandon.southall@sea-inc.net
SEA Inc.,
9099 Soquel Drive,
Aptos, CA 95003

Vincent M. Janik – vj@st-andrews.ac.uk
Scottish Oceans Institute
School of Biology
University of St Andrews, UK

Popular version of paper 1pAB6 Oscillatory whistles—The ups and downs of identifying species in passive acoustic recordings
Presented Tuesday afternoon, June 8, 2021
180^th ASA Meeting, Acoustics in Focus

Many dolphin species communicate using whistles. Because whistles are produced so frequently and travel well under water, they are the focus of a wide range of passive acoustic studies. A challenge inherent to this type of work is that many acoustic recordings do not have associated visual observations and so species in the recordings must be identified based on the sounds that they make.

Acoustic species identification can be challenging for several reasons. First, the frequency contours of dolphin whistles are variable, and each species produces many different whistle types. Also, whistles often exhibit significant overlap in their characteristics between species. Traditionally, acoustic species classifiers use variables measured from all whistles, regardless of what type they are. An assumption of this approach is that there are underlying features in every whistle that provide information about species identity. In human terms, we can tell a human scream or grunt from those of a chimpanzee because they sound different. But is this the case for dolphin whistles? Can a dolphin tell whether a whistle it hears is produced by another species? If so, is species information carried in all whistles?

To investigate these questions, we analyzed whistles produced by short- and long-beaked common dolphins in the Southern California Bight. Our previous work has shown that the whistles of these closely related species overlap significantly in time and frequency characteristics measured from all whistles, so we hypothesized that species information may be carried in the shape of specific whistle contours rather than by general characteristics of all whistles. We used artificial neural networks to organize whistles into categories, or whistle types. Most of the resulting whistle types were produced by both species (we called these shared whistle types), but each species also had distinctive whistle types that only they produced (we called these species-specific whistle types). Almost half of the species-specific whistles produced by short-beaked common dolphins had oscillations in their contours, while oscillations were very rare for both long-beaked common dolphins and shared whistle types. This clear difference between species in the use of one specific whistle shape suggests that whistle type is important for species identification.

We further tested the role of species-specific whistle types in acoustic species identification by creating three different classifiers for the two species – one using all whistles, one using only whistles from shared whistle types and one using only whistles from species-specific whistle types. The classifier that used whistles from species-specific whistle types performed significantly better than the other two classifiers, demonstrating that species-specific whistle types collectively carry more species information than other whistle types, and the assumption that all whistles carry species information is not correct.

The results of this study show that we should re-evaluate our approach to acoustic species identification. Instead of measuring variables from whistles regardless of type, we should focus on identifying species-specific whistle types and creating classifiers based on those whistles alone. This new focus on species-specific whistle types would pave the way for more accurate tools for identifying species in passive acoustic recordings.

Daniel Noreña – d.norena@uniandes.edu.co
Kerri D. Seger
Susana Caballero

Laboratorio de Ecologia Molecular de Vertebrados Marinos
Universidad de los Andes
Bogotá, Colombia

Popular version of paper 3aAB2
Presented Wednesday morning, December 9 , 2020
179th ASA Meeting, Acoustics Virtually Everywhere

Growing ship traffic worldwide has led to a relatively recent increase in underwater noise, raising concerns about effects on marine mammal communication. Many populations of several dolphin species inhabit the eastern Pacific Ocean, particularly along the Chocó coast of Colombia. Recent research has confirmed that anthropologic noise pollution levels in this region are one of the lowest in any studied area around the globe, allowing an opportunity for scientists to listen and analyze a relatively undisturbed soundscape in our oceans.

Figure 1. Vessel traffic in the Americas (a) and in (b) Colombia in particular. Red indicates high traffic and blue areas have no traffic. Note the gap in traffic in the Colombian Pacific coast where the Gulf of Tribugá is located (inside black/red box) as compared to all other coastal regions.

Currently, the CPC is slated for the construction of a port in the Gulf of Tribugá, pending permits. Previous port construction projects in other countries have shown that this will change the acoustic environment and could compromise marine fauna, such as dolphin communication. This is the first study to document the whistle acoustic parameters from several dolphin species in the region before any disturbance. Opportunistic recordings were made in two different locations alongside the coast: Coquí, Chocó, and a few hundred kilometers north Bahía Solano, Chocó.

Figure 1. (a) The Colombian Pacific coast and (b) whale-watching locations and ports of the Pacific coast of Colombia. Ports are red markers and whale-watching spots are blue markers.

Five different delphinid species were recorded: Common bottlenose dolphin (Tursiops truncatus), Pantropical spotted dolphin (Stenella attenuata), Spinner dolphin (Stenella longirostris), False killer whale (Pseudorca crassidens) and Short- beaked common dolphin (Delphinus delphis). Comparing these recordings to those made from dolphin populations in more disturbed areas around the globe showed that the repertoires of four of the five species were different. These differences could be because the Chocó dolphins represent populations that use whistles with more natural features while the other, more disturbed, populations may have already changed their whistle features to avoid overlapping with boat traffic noise.

However, avoiding overlap with other conspecifics or other species in the same habitat is natural, too. This is called the acoustic niche hypothesis (ANH). The ANH states that geographically sympatric species should occupy specific frequency bands to avoid overlapping with each other. A Linear Discriminant Analysis (LDA) was done to explore whether the five different species have already adjusted their whistle features to avoid overlapping with other species. Frequency band separation is not the only feature of whistles that dolphins could adjust. The LDA used nine different features to observe if there is any natural division between any of the features.

Figure 2. LDA plot for nine whistle variables among the five species.

Tracking these whistle features in Chocó over time will help determine whether the different whistle features between the Chocó dolphins and dolphins from more disturbed areas are a result of the natural acoustic niche hypothesis or a result of noise pollution avoidance. If constructed, the port could force species to adjust their whistle features like populations from noisier habitats already have, and that could disrupt the acoustic niches that already exist, some of their whistles may still be interrupted by boat noise. Such disturbances could increase their stress levels or could lead to area abandonment, which would cause economic and ecological disasters for the region that relies on artisanal fishing and ecotourism.