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Fishery Bulletin 93(4). 1995 



ous distribution into waters off Oregon (Barlow, 1988; 

 Barlow et al., 1988), where they are managed sepa- 

 rately because of jurisdictional boundaries. Without 

 simultaneous surveys in Oregon, a more widespread 

 shift in distribution cannot be ruled out. 



Alternatively, harbor porpoise may have changed 

 their distribution in relation to distance from shore 

 or water depth. The surveys extended only to approxi- 

 mately the 92-m (50-fa thorn) depth contour. Although 

 the majority of harbor porpoise are expected to be 

 found inshore of this depth (Barlow, 1988), an in- 

 crease in the proportion of animals found in deeper 

 waters could cause an apparent decline within the 

 0-92 m (0-50 fathom) study area. This hypothesis 

 cannot presently be tested because detailed surveys 

 for harbor porpoise in waters deeper than 92 m have 

 not been conducted off California, and inferences for 

 offshore areas cannot readily be drawn on the basis 

 of distribution patterns inshore of 92 m depth. 



Effects of fishery-related mortality 



If the detected decline in harbor porpoise abundance 

 along the California coast is a real phenomenon, then 

 one possible cause for the decline may be incidental 

 mortality of this species in California set gillnet fish- 

 eries. Although annual mortality is thought to have 

 gradually declined from about 200-300 animals per 

 year in 1980-87 to about 30-50 animals annually in 

 the last few years (Barlow and Forney, 1994), there 

 was no observer program to monitor mortality in set 

 gillnet fisheries from April 1987 to June 1990. Total 

 mortality estimates for these unmonitored years are 

 based on kill rates for the 1990-91 fishing season 

 and on estimated 1987-90 fishing effort (Perkins et 

 al. 2 ). These estimates are accurate only if the mor- 

 tality observed in the 1990—91 fishing season is rep- 

 resentative of 1987-90 rates. If fishery-related mor- 

 tality of harbor porpoise was in fact higher during 

 1987-90, this may have contributed to a decline in 

 abundance greater than is apparent from the mor- 

 tality estimates (see Perkins et al. 2 ). 



Absolute abundance estimates for central Califor- 

 nia harbor porpoise have recently been updated on 

 the basis of pooled data from the 1988-93 aerial sur- 

 veys, yielding an estimate of 4,120 (CV=0.22) har- 

 bor porpoise (Barlow and Forney, 1994). Assuming 

 an otherwise stable population, 9.3% of the popula- 

 tion, or roughly 350-450 animals, would have had to 

 be removed during each year of the study period in 



order to cause the decline indicated by the analysis 

 for central California. The fishery-related mortality 

 estimates range from 12 animals in 1993 to 197 ani- 

 mals in 1986 and have large standard errors, but 

 the upper 95% lognormal confidence limits of the 

 mortality estimates are lower than 9.3% of the popu- 

 lation estimate in all years since 1984 (derived from 

 data in Perkins et al. 2 ; Hanan et al. 3,4 ; Konno 5 ; 

 Julian 6,7 ). Although these mortality estimates as- 

 sumed fishing effort was known without bias, when 

 in fact it may have been underestimated (Julian 7 ), 

 fishery-related mortality alone does not appear to 

 be responsible for the decline in harbor porpoise 

 abundance within the central California study area. 

 However, potential effects of age or sex bias in har- 

 bor porpose mortality and potential time lags in the 

 impact of such takes are unknown. 



Change in natural mortality and net 

 reproduction 



An increase in natural mortality (e.g. due to increased 

 predation, disease, or a reduced food supply) could 

 contribute to a decline in abundance. Natural mor- 

 tality rates for harbor porpoise are not known. The 

 small size of harbor porpoise may make them vul- 

 nerable to shark and killer whale predation, but there 

 is no evidence (i.e. from strandings) to suggest that 

 predation rates may have been higher during the past 

 eight years than in prior time periods. Harbor por- 

 poise appear to be opportunistic feeders: market 

 squid, Loligo opalescens, spotted cusk eel, Chilara 

 taylori, and northern anchovy, Engraulis mordax, are 

 known to be prominent components of their diet in 

 the Monterey Bay region (Sekiguchi, 1987; Dorfman, 

 1990); however, both studies indicate that the north- 

 ern anchovy is the most important of these prey 



2 Perkins, P., J. Barlow, and M. Beeson. 1994. Report on pin- 

 niped and cetacean mortality in California gillnet fisheries: 

 1988-90. Admin. Rep. LJ-94- 11 . Southwest Fisheries Science 

 Center, Natl. Mar. Fish. Serv., P.O. Box 271, La Jolla, CA92038. 



3 Hanan, D. A., S. L. Diamond, and J. P. Scholl. 1986. An esti- 

 mate of harbor porpoise mortality in California set net fisher- 

 ies April 1, 1984 through March 31, 1985. Admin Rep. SWR- 

 86-16, 38 p. Available from Southwest Region, 300 S. Ferry 

 St., Terminal Island, CA 90731. 



4 Hanan, D.A., S. L. Diamond, and J. P. Scholl. 1987. An esti- 

 mate of harbor porpoise mortality in California set net fisher- 

 ies April 1, 1985 through March 31, 1986. Final rep. to Na- 

 tional Marine Fisheries Service, Southwest Region, 300 S. Ferry 

 St., Terminal Island, CA 90731. 



5 Konno, E. S. 1990. Estimates of sea lion, harbor seal and 

 harbor porpoise mortalities in California set net fisheries for 

 the 1987-88 fishing year. Draft rep. available from California 

 Department of Fish and Game, P.O. Box 271, La Jolla, CA 92038. 



6 Julian, F 1993. Pinniped and cetacean mortality in Califor- 

 nia gillnet fisheries: preliminary estimates for 1992. Int. 

 Whaling Comm. Working Paper SC/45/022, 29 p. 



7 Julian, F 1994. Pinniped and cetacean mortality in Califor- 

 nia gillnet fisheries: preliminary estimates for 1993. Int. 

 Whaling Comm. Working Paper SC/46/OH, 28 p. 



