Article written by Mel Cosentino.
Humans have been fascinated by whales and dolphins for thousands of years, evidenced by the numerous paintings, sculptures, and myths that have existed about them throughout history. Maybe it is because they spend most of their lives underwater and far from shore that we are so attracted to them. But that is also why there is still so much we don’t know about their biology, ecology, and natural behaviour.
To study these evasive creatures, we scientists make the most of each encounter with them, collecting a wide range of data. The standard method is to collect ‘visual’ data, anything we see: species, group size, behaviour, and, if lucky, photographs of their dorsal fins and/or tails to be used for individual identification purposes. Relying solely on visual data, however, is limiting. Detecting cetaceans at sea is a difficult task in itself, and detection rates are highly affected by weather conditions, particularly sea state, which is especially problematic when studying inconspicuous species, such as the harbour porpoise. And clearly it is not possible to conduct visual surveys at night, or during heavy fog or rain.
Whales and dolphins produce a wide range of sounds that researchers study to learn more about the animals.
Luckily for us, cetaceans produce a wide range of sounds that we can study to help us understand of how they interact with the environment, as well as how anthropogenic activities that produce underwater noise affect their natural behaviour. For example, baleen whales can vocalise at frequencies < 10 Hz, while toothed whales can vocalise at frequencies above 130 kHz, and we can hear none of them! Cetaceans use these sounds for foraging, navigation, and finding predators, as well as to communicate with each other. For communication purposes they use many different sounds, including whistles, moans, shrieks, knocks, thumps, creaks, buzzes, pulses, up or down calls, ratchets, and trumpets. A whole range that are species and context specific. Odontocetes (i.e. toothed whales) also produce another type of sound: echolocation clicks. These clicks are used to obtain an assessment of its environment by listening to echoes as the sound waves reflect on different objects.
Harbour porpoises (Phocoena phocoena)
The harbour porpoise is one of the six extant porpoise species in the world. It is dark grey (lighter on the belly) and has a small, rounded head with no noticeable beak. The dorsal fin is small and triangular. They are quite small, and although there are variations between populations, they reach a maximum of 189 cm in length and 72 kg in weight, females being larger and heavier than males. Harbour porpoises in Norway and Sweden are smaller than in the rest of the North Atlantic, reaching a maximum of 153 cm, while in Scottish waters, they reach a maximum of 173 cm in length and 50 kg in weight.
There is high variation in the maximum longevity for harbour porpoises in the North Atlantic. In Canada, they do not live longer than 10 years, while in the UK they can reach up to 24 years of age, living a mean of about 12 years. Males reach sexual maturity at 6 years, while females at 3.5 years of age, and become pregnant every 2.5 and 3 years, which is more often than other cetacean species, such as killer whales that give birth every 5 years or more. Gestation lasts between 10 and 11 months, and the baby porpoise is less than 80 cm and 7 kg at birth, which takes place between May and August.
The harbour porpoise occurs exclusively in the northern hemisphere, in coastal and offshore waters in the Atlantic and Pacific oceans. Depth and slope are the most important variables determining harbour porpoise distribution, which are related to the distribution of their prey, such as sandeels that dwell at the sea floor Harbour porpoises are mostly found in waters between 50 and 200m deep. Other factors affecting harbour porpoise distribution are salinity, tidal state, and distance from land. Porpoises move within the same geographic area, and although individual movements vary greatly, as some porpoises travel dozens of km a day moving to far areas, while others stay in the same area for weeks, no coordinated migrations have been identified.
The harbour porpoise is one of the six extant porpoise species in the world.
As a species, their main predator is the killer whale, and it has been suggested that harbour porpoises produce high frequency vocalisations to them, as these frequencies are way above killer whale hearing. In UK waters, the killer whale population is very small and therefore likely of little concern for harbour porpoises. In recent years, a new, unexpected predator has been reported for the North Sea and adjacent waters: the grey seal (Halichoerus grypus). Observations of grey seals feeding on harbour porpoises have been made in many European countries, including Belgium, France, the Netherlands, and the UK. These observations include reports of scavenging as well as active predation!
Listed under ‘Least Concern’ on the IUCN Red List of Endangered Species, the harbour porpoise faces nonetheless considerable threats due to anthropogenic activities, especially in certain regions. The main threat is being caught in fishing nets, which is driving the Baltic Sea population to extinction. Other threats include anthropogenic noise, pollution, and marine debris. Additionally, these little critters are being killed by bottlenose dolphins in coastal waters off California, in the US, and off Scotland, in the UK. The reasons for these non-predatory deadly interactions remain unknown for the scientific community, although several hypotheses have been postulated.
The main threat to harbour porpoises is incidental catches in fishing gear.
Harbour porpoises are protected by international legislation throughout their distribution range. In Europe, for example, the parties of the Agreement on the Conservation of Small Cetaceans in the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS) are obliged to develop and adopt measures to protect harbour porpoises, as well as to acquire knowledge about their occurrence, abundance, and distribution. Additionally, the harbour porpoise is listed in the Annex II of the Habitats Directive (Council Directive, 1992) and member states are required to set up Special Areas for Conservation (SACs) for listed species.
Studying harbour porpoises, however, is not easy. They are especially difficult to observe at sea, even at short distances and in good weather conditions. They are quite small, approximately 1.5 m long, and surface only a few seconds at a time, typically travelling in groups of 3 or fewer animals, or alone. However, they are highly vocal, producing entirely stereotyped narrow-band high-frequency (NBHF) echolocation clicks. Because of this, Passive acoustic monitoring (PAM) systems are increasingly used alongside visual methods as well as independently to study cetaceans that are difficult to detect visually, such as the harbour porpoises. PAM systems have several advantages over visual surveys as they can be used during bad weather conditions (e.g., high waves, fog) and at night.
Their vocal activity can be used to detect them acoustically, even when visual detection is not possible.
Temporal (a) and spectral (b) characteristics of a typical click of a harbour porpoise click (approximate duration: 100µs, peak frequency: ~131kHz)
What are they talking about?
Harbour porpoises can be easily detected using PAM systems as their echolocation clicks are quite singular. These echolocation clicks emitted in click trains are used for foraging as well as communication purposes, and they do so by altering the amplitude and inter-click intervals (ICI).
Acoustic behaviour and communication in odontocetes have been studied using two main methods: fixed PAM that are moored to the seabed and acoustic tags that are attached to individuals. To date, studies on acoustic behaviour of wild harbour porpoises have focussed on foraging, using both fixed PAM and acoustic tags. Acoustic tags are the most reliable tool for acoustic behaviour studies, especially when carrying other sensors. However, these are invasive and can only be deployed on a small number of individuals, for short periods of time.
Harbour porpoises emit characteristic echolocation clicks in click trains to forage and communicate.
Communication, on the other hand, has only been studied in captive or semi-captive individuals. These studies confirmed that harbour porpoises communicate and transmit information using only echolocation clicks, through variations of the ICI within a series of clicks, known as a click train. By studying the patterns of these variations we can understand their behaviours. An amazing study conducted recently, studying the interaction between a female and her calf, as well as between adults, found that some behaviours can be easily identified when plotting clicks per second (estimated as the inverse of ICI) versus time. For example, contact calls click trains look like a long S, while for aggressions they look like a vertical line. The duration of the click trains also varies markedly, for example contact calls are less than 1 second long, while clicks trains during grooming can last up to 16 seconds. Moreover, the duration of the click itself also varies, although not significantly. There are also indications that the temporal (e.g., duration) and spectral (e.g., peak frequency) characteristics of calves differ to that of adults, yet it remains widely understudied.
Our understanding of wild harbour porpoise communication is virtually unknown. The aim of my project is to develop a methodology that can help us detect and identify different acoustic behaviours and thus increase our understanding of harbour porpoise ecology.