NOTE Forney. Decline in the abundance of Phocoena phocoena 



747 



items. In recent years, California Cooperative Oce- 

 anic Fisheries Investigations (CalCOFI) surveys have 

 revealed a decline in California anchovy populations 

 (Jacobsen et al., 1994), and the occurrence of this 

 species in the diet of California sea lions, Zalophus 

 californianus, off southern California has decreased 

 markedly since 1991. 8 Based on these considerations, 

 an alternate hypothesis for the observed decline in 

 harbor porpoise abundance is that the population has 

 been affected by a decline in a major food source, the 

 northern anchovy. However, too few stranded or in- 

 cidentally caught harbor porpoise have been avail- 

 able recently for examination of food habits ( Peltier 

 et al., 1993, 1994; Lennert et al., 1994;), and data 

 are presently insufficient to test the possibility of 

 such a relationship. 



Conclusions 



Despite a reduction in fishery-related mortality, the 

 harbor porpoise population in nearshore waters off 

 central California appears to have declined between 

 1986 and 1993. Potential causes of this decline are 

 poorly understood, and further studies are impera- 

 tive. Recent restrictions on coastal gillnet fisheries 

 have substantially reduced harbor porposie mortal- 

 ity in central California. If the decline was caused 

 primarily by fishery-related mortality, then the popu- 

 lation should exhibit a stable or increasing trend over 

 the next several years. Continuation of the aerial- 

 survey time series will provide a means of detecting 

 such a trend. Additional research should also address 

 possible relationships between the density and distri- 

 bution of harbor porpoise and environmental conditions 

 (such as water temperature and prey availability). 



Acknowledgments 



Financial support and personnel were provided by 

 the National Marine Fisheries Service (Southwest 

 Regional Office and Southwest Fisheries Science 

 Center) and the California Department of Fish and 

 Game (CDFG). The observers and data recorders 

 were J. Barlow, M. Beeson, J. Carretta, D. DeMaster, 

 S. Diamond, T. Farley, D. Fink, T. Gallegos, D. Hanan, 

 A. Hudoff, L. Jones, E. Konno, S. Kruse, M. Larson, 

 J. Lecky, T. Lee, M. Lowry, R. Pitman, J. Scholl, J. 

 Sisson, and B. Taylor (and K.A.F). Aircraft were char- 

 tered from Aspen Helicopters, Inc. and contracted 



Lowry, M. 1994. Southwest Fisheries Science Center, Natl. 

 Mar. Fish. Serv., P.O. Box 271, La Jolla, CA 92038. Personal 

 commun. 



from CDFG. Pilots were B. Cheney, B. Cole, J. Drust, 

 B. Hansen, D. Hansen, L. Heitz, K. Lankard, E. 

 Schutze, J. Turner, R. Riediger, R. Throckmorton, and 

 R. Van Benthuysen. The computer data acquisition 

 software was written by J. Cubbage and modified by 

 J. Barlow. Aerial surveys were conducted under Ma- 

 rine Mammal Protection Act Permit 748 and NOAA 

 Sanctuary permits GFNMS/MBNMS-03-93 and 

 MBNMS-11-93. Drafts of this manuscript were re- 

 viewed by J. Barlow, S. Benson, R. Brownell, J. 

 Carretta, P. Dayton, D. DeMaster, J. Enright, T. 

 Gerrodette, F. Julian, R. Neal, and two anonymous 

 reviewers. I thank all for their useful comments and 

 constructive criticism. 



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