Whale and Dolphin Surveys in the Pacific Islands
Surveys for cetaceans to assess abundance across broad areas of the Pacific.
Our program conducts visual and acoustic cetacean (pronounced "seh-tay-shen") surveys and monitoring around the main and Northwestern Hawaiian Islands, Mariana Archipelago, American Samoa, and Pacific Remote Islands. We search and listen for whales and dolphins from a NOAA ship going 10 knots along predetermined tracklines (line-transect methodology).
The visual team continuously watches for cetaceans using 25×150 "big-eye" binoculars, scanning 180° forward of the ship. When an observer spots whales or dolphins, the team identifies the species and each person independently estimates the number of animals in the group. Observers measure the distance to the group, using markings on the lenses of the big-eyes, and the angle of the group from the ship's heading. The combination of distance and angle, together with geometric calculations, provide the perpendicular distance from the group to the trackline. At the end of the survey, it is the collection of these trackline distances that allow us to estimate the detectability of cetacean groups. The length of the survey tracklines, as well as species detectability and average group size, are some of the data we use to estimate abundance.
Why use visual observations?
Cetaceans are mammals and therefore must breathe air to survive. Although they spend most of their lives underwater, they must come to the surface to breathe. It is this obligation to the surface that makes visual observation the primary mode for cetacean surveys. Visual observers can estimate the number of animals in a group, identify the presence of young animals, and describe the behavior of the group—characteristics of the sighting that are difficult to measure or observe in other ways.
How do we find cetaceans and what do we collect?
Observers find cetaceans by scanning through the big-eyes, looking for anything that looks different from water and whitecaps. Sometimes, the observer will see the animals right away, especially if they are in large groups or very active at the surface. Other times, the observers see a splash or a group of birds or fish that alerts them to the presence of cetaceans. Whatever the "cue," once the group is spotted, we collect the data necessary to make abundance estimates. We often take the opportunity to collect photographs and tissue samples from some groups, either from the ship itself or from a small boat launched from the ship. Photographs of the animals allow us to track individuals using identifying marks on the dorsal fin, tail flukes, or body. We use tissue samples for genetic analysis to determine the sex of an animal and the population to which it belongs. We also use them to understand disease, identify contaminants that accumulate in the animal's blubber, and determine pregnancy. Occasionally, we deploy satellite tags on a handful of animals to track how the species move over a longer period than we can observe from the ship. The satellite tags last for many months and help us to understand population range and migration.
Passive Acoustic Surveys
Although we most commonly study cetaceans through visual surveys, we are only able to count them when they surface to breathe. To improve our abundance estimates and provide additional information, we eavesdrop on the sounds produced by cetaceans below the surface.
Why use passive acoustic monitoring?
Sound travels much further in water than light, and cetaceans commonly use sound rather than vision to communicate with each other and to find food. Just as cetaceans listen for each other and for their prey, we can listen for their sounds to locate groups of whales and dolphins. Some cetaceans exhibit cryptic behavior and can be very sneaky, avoiding the boats and ships that we use to study them. Poor weather and rough seas can also make it difficult for the visual team to observe animals from the ship, but the acoustics team can still detect animal sounds in rough conditions. The act of quietly listening for marine mammal sounds is known as passive acoustic monitoring. By monitoring the ocean for cetacean sounds, the acoustics team can locate and track animals, including some that might otherwise go unseen by the visual team.
How does passive acoustic monitoring work?
We use an acoustic array to eavesdrop on vocalizing marine mammals. The acoustic array consists of a series of hydrophones (underwater microphones) that are towed behind the ship (a "towed array") while it is moving along the trackline. The hydrophones immediately transmit sounds back to the ship. Scientists monitor the sounds by listening with headphones and watching a visual representation of the sounds known as a spectrogram to show sounds that cannot be heard by the human ear. Using these two techniques, the acoustic team can determine when a whale or dolphin is present and what species it is based on various characteristics of its vocalizations. Once animals are detected, the time difference between the arrivals of their sounds to the hydrophones is used to locate and track them as they move. Depending on the species and its location, the acoustics team may alert the visual team to the presence of animals in the area and help guide the ship to them.
What other things can we do with passive acoustics?
Although the acoustic team's primary mission is real-time monitoring and tracking of vocal cetacean groups, periodically, we use other sensors to listen in on whales and dolphins. The towed array is designed to filter out very low frequency sounds, such as ship sounds and the noise of the array flowing through the water, so that they don't mask the dolphin sounds. However, many baleen whales species produce low-frequency sounds, meaning we cannot detect these species using the array. When the visual observers identify baleen whales, we often deploy a sonobuoy to record the sounds from these whales. A sonobuoy has a hydrophone suspended from a cable with a VHF radio floating at the surface. The sonobuoy hydrophone transmits sounds back to the ship over the radio frequency.
Long-term acoustic recorders are also used to listen for whales and dolphins over periods of months to years in a specific area. High-frequency acoustic recording packages (HARPs) record a very broad frequency range, allowing us to monitor the occurrence of all vocal species for more than a year before they are recovered. We must recover the HARPs to review their recordings and identify the cetaceans. The Pacific Islands Fisheries Science Center maintains a passive acoustic network with several long-term monitoring sites.
We study the ecosystem, including the oceanographic properties of the waters we traverse and the occurrence of seabirds and sea turtles. Seabirds are important indicators of overall ecosystem health. Many species of seabirds and all species of sea turtles in Hawaii are protected by federal laws. We try to understand as many components of the ecosystem as we can in order to get a well-rounded picture of the ecosystem as a whole and to make the most of the opportunity of our time on the research vessels.
Why do we study the ecosystem?
Knowing the distribution and abundance of cetacean populations only gives us part of the picture in understanding their overall health. For a fuller picture, we must assess the habitat of cetaceans and the greater health of their ecosystem. Oftentimes, changes in the ecosystem are tightly linked with the dynamics of cetacean populations. A better understanding of the habitat helps us determine why some species are thriving and others might be declining.
How do we study the ecosystem?
The large research vessel has a variety of oceanographic sensors that continuously record sea temperature, salinity, and fluorescence (a measure of the level of primary productivity). Each NOAA ship also carries a conductivity-temperature-depth (CTD) instrument that can be cast to depths of 1000 meters or more. The CTD measures temperature, salinity, oxygen, and other nutrients from the surface to depth. It collects water samples at specific depths for more detailed information on water properties. During long surveys, we conduct a CTD each morning before sunrise and each evening after sunset to help characterize the ecosystem of the cetaceans we are studying.
The occurrence and abundance of seabirds are excellent indicators of ocean conditions and health. During select projects, we collect seabird data from the ship’s flying bridge, sun-up to sun-down, using visual observers following strip-transect methods. Strip-transect methods mean that the observers record all seabirds flying or sitting on the water that are within a specified distance from the ship—usually a few hundred meters. We may note unique seabirds beyond that distance, but we don't use them for evaluating density or abundance. The seabird observers work alongside the visual cetacean observers.
Five species of sea turtles are known to occur in Hawaiian waters. To take advantage of our research platform and vast at-sea coverage within our study area, we record any sightings of sea turtles. When a sea turtle is encountered, we will record its location and collect photographs for species identification. If we find sea turtles entangled in debris, we will launch a small boat to capture the turtle, remove the debris, and release the turtle back to the ocean. We keep the debris onboard with us and dispose of it when we return to port so that other animals don’t become entangled.