UNIT 23 

 MARINE MAMMALS OF THE PACIFIC REGION INCLUDING HAWAII 



well below its optimum sustamable population 

 (OSP) and, theretore, is characterized as a strate- 

 gic stock. 



HARBOR PORPOISE: 

 CENTRAL CALIFORNIA STOCK 



is limited, harbor porpoise in central California is 

 considered to be a separate stock. Other Pacific 

 coast stocks of harbor porpoise include: 1 ) a north- 

 ern California stock, 2) an Oregon/Washington 

 coastal stock, 3) a Washington inland-waters stock, 

 and 4) an Alaska stock. 



Left page: Hawaiian monk 

 seal. Northwestern Hawai- 

 ian Islands NationalWildlife 

 Refuge. 



Stock Definition and 

 Geographic Range 



In the Pacific, harbor porpoise are found in 

 coastal and inland waters from Point Conception, 

 California, to Alaska and across to Kamchatka and 

 Japan. Harbor porpoise appear to have more re- 

 stricted movements along the west coast of the con- 

 tinental United States than along the U.S. east 

 coast. Regional differences in pollutant residues 

 from harbor porpoise tissue samples indicate that 

 the species does not mix freely between Califor- 

 nia, Oregon, and Washington (Calambokidis and 

 Barlow, 1991). The study also showed some re- 

 gional differences within California (although the 

 sample size was small). This pattern stands in sharp 

 contrast to the east coast of the United States and 

 Canada where harbor porpoises are believed to 

 migrate seasonally from as far south as the Caroli- 

 nas to the Gulf of Maine and Bay of Fundy. Early 

 genetic analyses did not show any significant dif- 

 ferences between samples from California and 

 Washington, but more recent analyses with larger 

 sample sizes do show significant differences. These 

 studies show that porpoises on the west coast are 

 not freely mixing or migratory, and movement is 

 sufficiently restricted that genetic differences have 

 evolved. 



In its harbor porpoise assessment (Barlow and 

 Hanan, 1995), the NMFS and the California 

 Department of Fish and Game recommended that 

 the animals inhabiting the central California coast 

 (from Point Conception to the Russian River) be 

 treated as a separate stock. The justifications for 

 this were: 1) fishery mortality of harbor porpoise 

 is limited to central California, 2) movement of 

 individual animals appeared to be restricted within 

 California, and consequently 3) fishery mortality 

 could cause the local depletion of harbor porpoise 

 if the central California coast stock was not man- 

 aged separately. Because the recent genetic studies 

 have confirmed that movement on the west coast 



Population Size 



A 1994 review (Barlow and Forney, 1994) of 

 previous estimates of harbor porpoise abundance 

 along central California resulted in a new estimate 

 of 4,120 animals (CV = 0.22)' based on a series of 

 aerial surveys from 1988 to 1993. This recent es- 

 timate is not significantly different from the pre- 

 vious estimate of 3,274 animals (CV = 0.31) but 

 is more precise (owing to the greater number of 

 kilometers surveyed). Both of these estimates only 

 include the region between the coast and the ')] 

 m (50 fathom) isobath. In California, the vast ma- 

 jority of harbor porpoises are sighted within this 

 depth range; however, 24% of the harbor porpoises 

 seen during aerial surveys of Oregon and Wash- 

 ington were between the 100 m and 200 m 

 isobaths (55-109 fathoms). Thus, these abundance 

 estimates are likely underestimates of the total 

 abundance by a non-trivial amount. The current 

 minimum population estimate of 3,431 animals 

 in central California is based on aerial surveys con- 

 ducted between 1988 and 1993 (Barlow and 

 Forney, 1994). 



Current Population Trend 



An analysis of a 1986—95 time series of aerial 

 surveys was conducted to examine trends in har- 

 bor porpoise abundance in central California 

 (Forney, 1996). After controlling for the effects of 

 sea state, cloud cover, and area on sighting rates, a 

 negative trend in population size was evident. The 

 trend was not statistically significant, but statisti- 



' Coefficient of varidiion (CV) is a statistical measure used to 

 calculate confidence intervals (CI), which gauge the accuracy 

 of population estimates. An accurate population estimate is 

 characterized by a low CV and a narrow CI. CI is often given 

 a percentage likelihood of being correct (e.g. 9S% means that 

 it the data were resampled and the CI were recalculated 100 

 tmies, then ^)'S times it would contain the true value. 



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