Project Description
NOAA Fisheries has conducted shore-based counts of the Eastern North Pacific stock of gray whales (Eschrichtius robustus) most years since 1967 at or near Granite Canyon in central California. All of these counts were done during the two-month southbound migration (mid-December to mid-February). Abundance estimates showed an annual increase of 2.5% from the late 1960s until the end of the 1990s. The 1997/98 estimate (30,000) was the highest ever; however, the next two abundance estimates, in 2000/01 and 2001/02, dropped to 18,000 and 17,000, respectively, much lower than expected. It seems there had been a large decline in this population, because, in addition to the lower estimates, reported mortalities between Mexico and Alaska rose to over 270 and 300 in 1999 and 2000, respectively, well above the average of 38/yr. Also, calf counts were unusually low in 1999, 2000, and 2001. Since then, mortality rates have dropped to normal, and calf counts have been high, but there is still concern about the abundance. In 2006/07 another census was conducted, and though the abundance estimate is not yet available, the summary counts were very similar to those collected in 2000/01 and 2001/02. During January 2007, the Southwest Fisheries Science Center (SWFSC) began taking charge of the gray whale census at Granite Canyon. In January 2008, it is proposed that SWFSC will conduct a census applying their techniques while at the same time, NMML will provide experienced observers to help calibrate the previously used counting system.
Issues & Justification
In 2006/07, an additional population estimate was needed to verify the apparent drop of 43% between 1997/98 and 2001/02. As the SWFSC takes over the direction of the gray whale census in 2007/08, it is critical to maintain the monitoring of this population and continue the option for inter-year trend analyses. Without sufficient information on the compatibility of the counting regimes applied by AFSC and SWFSC, there will always remain a question as to how well the resultant abundance estimates can be compared. Without being able to resolve how counts in the next few years relate to counts conducted in the past, a valuable time-series of information will be compromised. Gray whales of the Eastern North Pacific population are one of the few stocks that have been removed from the list of endangered species without going extinct, and this is the only stock of large cetaceans that has been well documented through most of its recovery from commercial whaling to carrying capacity.
Goals
- Obtain systematic counts of gray whales during their southbound migration past the standard counting station at Granite Canyon, near Carmel, California.
- Provide a calibration of counting methods applied by AFSC in the past relative to new methods to be applied by SWFSC in the future.
- Establish an estimate of population size with an appropriate CV.
Methods
Typically, the gray whale census has been conducted from mid-December to late February to include virtually all of the southbound migration. Counts are done from small sheds (eye height 22.5 m above sea level) on the edge of a sea cliff at Granite Canyon. Single observers independently search for whales and record data on the time, location of sightings, count, direction-headed, and environmental conditions.
In 2006/07, the transition of this census from AFSC to SWFSC began. An overlap in personnel and technique is necessary to maintain the option for inter-year trend analyses. The SWFSC conducts the watch with pairs of observers working together instead of single observers as per the current protocol. One of the observers continuously looks for whales while the other records sightings and environmental data on a computer with a map showing sighting locations. If there are significant differences in the two techniques, a correction factor needs to be established. In January 2008, both SWSFS and AFSC will have observers at Granite Canyon conducting counts applying both the new and old methods. This might be the last opportunity to have this kind of overlap in effort, so it is critical to collect sufficient data for a comparative analysis of techniques and observers' performances.
Population abundance calculations from the observer counts account for: 1) whales passing during periods when there is no observational effort (prior to and after the census season, at night or when visibility is poor); 2) whales missed within the viewing range during on-effort periods; 3) differential sightability by observer, pod size, distance offshore, and various environmental conditions; 4) errors in pod size estimation; 5) covariance within the corrections due to variable sightability by pod size; and 6) differential diel travel rates of whales.