Assessing Marine Mammal Populations

Fin whales. Photo: S Lyday

By Shannon Lyday

Do you ever wonder how scientists know how many whales are left?  How do you determine if an endangered species, such as the blue whale, is recovering? 

By law, the Marine Mammal Protection Act requires that the National Marine Fisheries Services (NMFS) assess the stocks of marine mammals in U.S. waters.  This assessment is designed to determine sustainable population levels to develop conservation and management strategies for different species.

The Southwest Fisheries Science Center is the NMFS office responsible for estimating the number of cetaceans (whales and dolphins) in the Eastern Pacific.  This daunting task is undertaken at least every three years by a team of scientists who examine abundance and distribution as well as human-caused mortality (e.g. fisheries bycatch). 

In 2008, the ORCAWALE (Oregon, California and Washington Line-transect and Ecosystem) cetacean surveys are currently underway aboard the NOAA vessel McArthur II.  This five-month research cruise is surveying waters along designated transect lines which run from the Canada border to the Mexico border and out to 300 nautical miles off of the west coast. 

Although the primary objective of ORCAWALE is to estimate the abundance and to understand the distribution of cetaceans, the cruise also characterizes the open ocean ecosystem through physical and biological oceanographic sampling, studies of mid-trophic level organisms, and seabird research.

To survey marine mammals a team of six observers rotates through three positions. Two observers constantly scan the waters through “big-eyes”, which are mounted 25×150 binoculars.  The third observer records data as well as scans the water close to the ship through hand-held binoculars.  When mammals are sighted the group stops to identify the species and obtain group size estimates.

NFMS has permits to approach cetaceans to obtain photographs and biopsy samples.  These techniques allow the team to identify individual animals, which help determine population structure, track migration, and distinguish sub-populations.    Individual animals can be identified with photographs – for example humpback whales have unique patterns on their flukes.   Skin and blubber samples are collected for genetic analysis using a biopsy dart. 

In addition to observing from the flying bridge, there are two acousticians aboard that tow an acoustic array behind the boat, constantly listening for whales and dolphins.  Cetaceans use sound to communicate and to echolocate, especially the odontocetes (toothed whales). 

Using acoustics helps the team detect and locate ceteaceans - for example sperm whales vocalize approximately 66% of every hour, making them more likely to be detected by the acousticians than the observers.  Some species such as killer whales have distinctive dialects that distinguish a family group, allowing the biologists to identify a particular pod by sound alone.

The data from the ORCAWALE cruise are collated with data from other sources to generate stock assessments for each species that occurs in the Pacific.  NMFS produces reports that outline each species’ population size, population trend, and potential biological removal.  To learn more click here.

Shannon Lyday was a researcher aboard the McArthur II for the recent ORCAWALE cruise. To read about Leg 3 of the cruise, visit Shannon’s blog.