The study of cetaceans at sea is primarily the study of their ecology - how organisms interact with each other and with the rest of their environment. At the level of the individual, this involves investigating foraging, social and reproductive behaviour, which influence how well animals grow, survive and reproduce in relation to elements of their physical and living environment. At the population level, we are interested in factors that influence the distribution of species in space and time, their abundance or rarity in particular areas, and fluctuations or trends in populations over time. Ecology involves the study of physiology, natural history, behaviour, evolution and population dynamics, and requires experimentation, field studies and mathematical modelling.

Cetaceans are particularly difficult targets for field studies. They typically range widely over large areas and, because their prey lives underwater, they spend a large proportion of time foraging beneath the surface. In general, cetaceans are difficult to observe and even more difficult to catch for ‘hands-on’ studies, and the methods of study that have been developed reflect these restrictions.

These methods include a variety of ‘observation’ techniques from land or at sea, visual and acoustic surveys, the recognition of individual animals using photo-identification, telemetry (remote data collection), and tissue sampling. The application of these and other methods over the last 30 years has led to a dramatic increase in our knowledge of the distribution, abundance, movements, reproduction, survival, foraging behaviour, genetics and diet of a range of cetacean species. The diagram in the Figure below illustrates how the research techniques are used to study the various aspects of cetacean ecology. This is by no means a comprehensive list but it does convey an important message: a variety of methods can be used to investigate each topic, each providing a different window through which to view it. The study of cetacean ecology is relatively recent and the use of a range of research methods has proved beneficial to the rapid improvement in our knowledge.

Visual surveys (or sightings surveys) are one of the main research methods for investigating the distribution and abundance of cetaceans. They are valuable because they can be conducted at a wide range of scales: from small studies with a minimum of equipment in a small area to provide local information; to major surveys over large areas of ocean to provide accurate and precise estimates for use in a specific conservation or management context.

Sightings surveys use so-called line transect sampling techniques for data collection and analysis. These methods were first developed to estimate the abundance of terrestrial animals but are now widely applied to cetacean populations via shipboard or aerial surveys. In these sightings surveys, the study area is sampled by the survey platform searching along predetermined transects. The distance and angle to sighted animals (or groups) are measured (or estimated) allowing the calculation of perpendicular distance from the sighting to the transect line. These data are used to estimate the effective width of the strip searched so that sample density can be estimated. If transects are placed in the study area to ensure that everywhere in the area is equally likely to be surveyed (so-called equal coverage probability), sample density can simply be extrapolated to the whole area to provide an estimate of abundance.

Visual surveys have been used to estimate the abundance of minke whales in very large areas of the North Atlantic, North Pacific and Southern Ocean. Other large-scale surveys have been conducted in the eastern North Atlantic for harbour porpoises, in the eastern tropical Pacific for spotted and spinner dolphins, and in the Mediterranean for striped dolphins and fin whales. Smaller scale surveys have generated estimates of abundance for many other species around the world.

There are an increasing number of visual surveys which do not achieve equal coverage probability but nevertheless generate good line transect sampling data. For these surveys, the recently developed method of spatial modelling provides a way to obtain estimates of abundance and to investigate the relationships between cetacean distribution and environmental features such as the depth of the sea bed, water temperature and measures of ocean productivity.

Up

Sound propagates effectively through seawater and many cetacean species (especially the odontocetes) have evolved a highly developed acoustic capability as a result of this. Sound is used for foraging, in reproductive behaviour and for general communication. Early work included the discovery that the “songs” of humpback whales could be heard over hundreds of kilometres during migrations to their breeding grounds. More recent studies have shown how dolphins and porpoises use series of high frequency “clicks” to search for and hunt down fish prey, and how dolphins use “whistles” to communicate with each other.

Because sound is the primary sense used by cetaceans, acoustic methods can be very effective in the study of cetacean ecology and behaviour. Acoustic surveys are increasingly being used to investigate the distribution and abundance of sperm whales, porpoises and dolphins. In these surveys, a hydrophone (underwater microphone) is towed behind a ship on the end of a long cable. The sounds heard are recorded on computers onboard the ship and used to calculate measures of relative abundance. Hydrophones attached to so-called sonobuoys moored in remote locations are used to record the low frequency sounds of baleen whales, and an acoustic recording device known as a POD (porpoise detector) is a commonly used tool to monitor the use of coastal areas by porpoises and dolphins.

Many cetacean species possess distinctive natural markings (such as pigmentation patterns or nicks on dorsal fins) that vary from one animal to another so that individuals can be recognised at sea. Photographs of these distinctive markings on individuals (so-called photo-identification) form the basis for a widely used research method for studying cetaceans. Long-term photo-identification studies of recognisable individuals provide information on population size and survival (through mark-recapture analyses), movements and reproduction.

Mark-recapture estimates of population size are based on the simple idea that if a proportion of the whole population is marked in a first sample, then an estimate of this proportion can be obtained by observing the number of marked animals in a second sample. If various assumptions about the nature of the data are made, these two proportions can be equated to give an estimate of abundance. In most studies, records of recaptures (or re-identifications) from a series of samples are built up into so-called capture histories, which are then used to estimate population size.

Clearly, photo-identification can only be used for those species whose individuals possess lasting, recognisable natural markings. But many species do have such markings and abundance has been estimated using mark-recapture methods applied to photo-identification data for a range of cetaceans including bottlenose dolphins, killer whales, humpback whales and blue whales.

An interesting development is to use genetic identity determined from the analysis of DNA obtained from tissue samples as the mark. It is more difficult to collect the data but advantages of genetic markers include definitely not changing over time (which natural markings can), and providing information on the sex of sampled animals. This method has been applied to humpback whales in the North Atlantic.

Mark-recapture methods are also commonly used to estimate survival rates. The idea here is that the marked animals form a cohort that is followed for a period of time; the data on whether or not each animal is recaptured on subsequent occasions providing information on survival. The basic assumptions are the same as for estimating abundance. Survival has been estimated using these methods for humpback whales, gray whales, right whales, killer whales and bottlenose dolphins.

Individual recognition data are also valuable for examining reproductive parameters such as age at first birth, the interval between births and reproductive rate, as has been done for humpback whales. Intensive sampling is necessary to obtain such reproductive information but the effort is worth it because it is extremely difficult to obtain in other ways.

The ability to track cetaceans at sea depends critically on being able to attach a suitable device to the animal. This is difficult for cetaceans because, unlike seals, they are very difficult to catch and restrain and they do not have fur to which transmitters can be glued. However, the number of cetacean studies using telemetry is increasing. Transmitters have been deployed on harbour porpoises, bottlenose dolphins and even killer whales by physically capturing the animals and attaching the transmitter to the dorsal fin with pins. Transmitters have also been fired into the blubber of humpback, right and blue whales. For short-term behavioural studies, transmitters or data loggers can be attached to the animal’s skin with a suction cup.

The ability to collect data remotely from animals at sea via telemetry has dramatically improved our knowledge of the movements and behaviour of cetaceans at sea. The most valuable telemetry technique is satellite-relay telemetry, in which a transmitter on the animal sends data via a satellite orbiting the earth to a receiving station on land from where the data are recovered via a computer link. The data allow the location of the animal to be calculated and the great advantage of this method is that an animal can be tracked anywhere in the world for as long as the transmitter is working. Other data on, for example, dive depth, swimming speed and water temperature can also be relayed so that environmental factors influencing distribution, movements and foraging behaviour can be investigated.

Knowledge of diet is important not only in ecological studies but also in the context of interactions between cetaceans and fisheries. The contents of the stomachs and intestines of dead animals can sometimes be the only material available and much of what we know about cetacean diet has been obtained in this way. Faecal analysis can sometimes be used to study cetacean diet if faeces can be scooped from the water. Biochemical methods to study diet include analysing the fatty acids in the blubber, or stable isotopes ratios in skin. Tissues for these analyses are obtained from free-swimming animals using a biopsy dart fired from a crossbow or rifle.