NOTE Forney Decline in the abundance of Phocoena phocoena 



745 



0.40- 



I 



R 0.30- 



■& 0.10- 



o.oo 



Area 1 



85 86 87 



89 90 



Year 



92 93 



94 



Figure 3 



Relative abundance of harbor porpoise, Phocoena phocoena, for 

 the period 1986-93. Relative abundance is defined as the num- 

 ber of porpoise observed per kilometer surveyed, adjusted for the 

 effects of sea state and cloud cover on sighting rates. Areas corre- 

 spond to those shown in Figure 1. The combined relative abun- 

 dance for all central California was calculated as an average of 

 the adjusted values of porpoise per kilometer for areas 1 and 2, 

 weighted by the proportion of the total study area encompassed 

 by each (33.6% of the total study area was in Area 1; 66.4% was 

 in area 2). 



in fact the population is stable) or /-error (detecting 

 a decline when the population is in fact increasing; 

 Forney et al., 1991) has occurred. The latter is ex- 

 pected to be virtually zero for this eight-year time 

 series (Forney et al., 1991). The former was set at 

 0.10 a priori in order to increase power. Assuming 

 symmetry, this should have resulted in a 5% prob- 

 ability of detecting a decline if the population were 

 in fact stable. 



Movement of animals out of the study area 



In recent years, an increasing body of evidence indi- 

 cates that long-term changes have occurred in the 

 California Current during the last few decades, in- 

 cluding an increase in surface water temperature and 

 sea level height (Roemmich, 1992), a decrease in zoop- 

 lankton abundance ( Roemmich and McGowan, 1995), 

 and changes in the size of seabird populations (Ainley 

 et al., 1994). There has also been a dramatic increase 

 in the abundance of short-beaked common dolphins, 

 Delphinus delphis, off California (Barlow, 1995; 

 Forney et al., 1995), possibly due to a northward shift 

 in the distribution of this tropical and warm-tem- 

 perate species (Anganuzzi and Buckland, 1994). A 

 northward range extension into central California 

 has also been documented for another tropical and 



warm-temperate species, the bottlenose dol- 

 phin, Tursiops truncatus, following the El Nino 

 event of 1982-83 (Wells et al., 1990). This spe- 

 cies now is seen regularly in nearshore waters 

 off central California, within the range of the 

 harbor porpoise. 



Similarly, harbor porpoise in central Califor- 

 nia may have shifted their distribution out of 

 the study area (either to the north or farther 

 offshore) in response to environmental changes, 

 causing an apparent decline in abundance in 

 the nearshore region. Unfortunately, no detailed 

 data on harbor porpoise distributions and 

 oceanographic conditions are available to test 

 this hypothesis. However, it is noteworthy that 

 the relative abundance of harbor porpoise in 

 central California (dashed line in Fig. 3) exhib- 

 its a significant negative correlation (or=0.05, 

 Pearson correlation coefficient r=— 0.79, 

 P=0.035) with 1986-93 September sea-surface 

 temperature anomalies off Monterey Bay, 1 de- 

 spite the coarse nature of these two measure- 

 ments. Thus, in years when sea-surface tem- 

 peratures were warmer, the relative abundance 

 of harbor porpoise (a temperate species) was 

 lower, and vice versa. This may be indicative of 

 movement of harbor porpoise in relation to 

 changes in sea-surface temperature (or to other 

 environmental factors that are correlated with sea- 

 surface temperature), but it is not known whether 

 harbor porpoise move northward or offshore in re- 

 sponse to such environmental changes. 



Studies of pollutant ratios in animals along the 

 U.S. West Coast suggest that harbor porpoise do not 

 move frequently between central and northern Cali- 

 fornia (Calambokidis and Barlow, 1991). Consistent 

 with these observations, a shift of animals from cen- 

 tral to northern California is not indicated in the 

 analysis of this aerial survey series because the de- 

 clining trend is still significant when northern Cali- 

 fornia data are included. However, power to detect 

 trends reliably in the northern portion of the study 

 area is probably low, given only four surveys in this 

 region. Additional surveys will improve the ability 

 to detect a northward shift within California, if one 

 is present. It is important to note, however, that har- 

 bor porpoise off northern California show a continu- 



The sea-surface temperature anomaly is defined as the devia- 

 tion of the mean monthly sea-surface temperature in a given 

 year from the long-term mean for that month. Thus positive 

 anomalies indicate warmer than average months and vice 

 versa. Oceanographic Monthly Summary, U.S. Dep. Commer., 

 NOAA, National Ocean Service, available from NOAA/NOS, 

 Ocean Products Branch, 5200 Auth Road, Room 100, Camp 

 Springs, MD 20746. 



