MBNMS Ecosystems Observations 1999

Monterey Bay National Marine Sanctuary

MARINE MAMMALS

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Gray Whale Strandings

Based on the most recent counts of southbound gray whales by scientists from the National Marine Mammal Laboratory, the California population of gray whales numbers about 26,000 whales. This estimate is derived from shore-based counts conducted from a promontory at Granite Canyon, California (along the Big Sur Coast) during the 1997-1998 winter migration. It appears that the eastern north Pacific gray whale population is still increasing in size, but the rate of increase may have slowed from the 2.5 percent annual growth shown over the last decade. Examination of the growth trend for this population over the past thirty years suggests that carrying capacity for these whales may be reached at about 32,000 to 37,000 gray whales. Information collected during the most recent migration, however, has resulted in some speculation that this population may be nearer its environmental limit than previously suspected.

Gray Whale Calf Census Data*
Year	Calves	Est. % of total pop.
1999**	400	1.6
1998**	1316	5.1
1997	1439	6.5
1996	1141	5.1
1995	601	2.6
1994	1000	4.5
Preliminary results; data gathered from shore-based sighting surveys of northbound migrating gray whale calves passing Piedras Blancas. *Estimate Please note: these results are preliminary and still unpublished; they are not to be cited without the permission of the author. --Provided by Wayne Perryman Southwest Fisheries Science Center

In 1999 media attention was focused on the number of stranded gray whales found on beaches along their migration route from Mexico to Alaska. Roughly 270 gray whales stranded last year, compared to an average of about thirty strandings/year reported over the previous four years. Much of the increase in reported strandings came from Mexico and Alaska. However, there was also more research effort in these two areas, confounding comparisons with previous years. In spite of the increased effort, it is clear that many more gray whales stranded last year and we are not certain why. With a population size of about 26,000 whales, natural mortality is probably around 800 animals/year, so the number of stranded whales is not in itself cause for concern. However, the dramatic increase in strandings is a clear indicator that something was different within this population during the winter of 1999.

There were other anomalous findings concerning this population last year. The southbound migration of gray whales is led by near-term pregnant females, and these "wide-bodied" whales are easily distinguished from other adults (see Figure 1). During late December 1998 and early January 1999, scientists from the National Marine Fisheries Service's Southwest Fisheries Science Center conducting aerial surveys and photogrammetric studies (making measurements from photographs) along the California Bight noted very low numbers of pregnant females swimming southbound. In addition, preliminary analysis of measurements from vertical aerial photographs indicates that whales photographed in 1999 were narrower, relative to their length, than southbound gray whales photographed in 1997 and 1998. These findings suggest that pregnancy rates were lower and overall body condition may have been poorer last year.

Figure 1: These horizontal photographs of migrating gray whales illustrate how whales change shape as their condition changes. The upper image is of a near-term pregnant female headed southbound, while the lower image shows a much thinner northbound adult.

Last year was the sixth consecutive spring that scientists from the Southwest Fisheries Science Center have surveyed northbound gray whale cows and calves at Piedras Blancas, California. Preliminary analysis of those counts shows a dramatic drop in the number of northbound calves (see chart above). Because of the low counts of southbound pregnant females and relatively few reported strandings of neonates, we suspect that the reduction in numbers of northbound calves resulted from a reduction in pregnancy rates rather than an increase in calf mortality.

The combination of observations for 1998–1999 clearly indicates that last year was not a good one for gray whales. We don't know whether the changes we report are the result of a short-term shift in their environment or whether they reflect the first signal of a long-term change in this population. There have been reports from scientists studying the benthic amphipods—the gray whale's primary prey in the northern Bering Sea—that these populations of small crustaceans may be less abundant than they were a decade ago. The 2000 migration may provide the answer to many of these questions.

--Wayne Perryman
Southwest Fisheries Science Center

Elephant Seal Populations

		1997	1998	1999

Año Nuevo	Births¹	2684	2797	2362

	High count²	5358	4588	--

Other Big SurBeaches	Pups to weaning³	500	120	420

Piedras Blancas	Births⁴	1200	1650	1900
	High count⁵	5000	5000	4000

1 estimated production (for 1999: 87% of all females; for 1997-1998: 95% of all females); lower production in 1998 reflects an apparent adverse impact of the previous year's El Niño.
2 high counts occurred in February 1997 and April 1998; not taken in 1999
3 approximate number that survived to weaning
4 approximate number of births
5 occurred during the spring molt

Sources:
Año Nuevo: 1999—Dan Crocker, University of California Santa Cruz; 1997-1998—Pat Morris and
Guy Oliver, University of California Santa Cruz
Piedras Blancas and Other Big Sur Beaches: Unpublished data, provided by Brian Hatfield, USGS, Piedras Blancas Field Station; not to be cited without the permission of the author.

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Last modified on: March 31, 2000