Ainley and others 



Chapter 34 



Offshore Occurrence Patterns in Central California 



offshore. We believe that under these conditions, the murrelets 

 were distributed linearly along the shore within a few hundred 

 meters of the beach, a pattern often displayed by murrelets in 

 some regions to the north (Briggs and others 1987; Strong 

 and others, this volume). We would have been unable to 

 detect these birds because the ship could not venture close 

 enough to shore. Under these conditions, in a distributional 

 pattern likely similar to that of March 1993, some parents 

 would have to fly farther than others to bring food to their 

 chicks. This explanation for the variation in murrelet numbers 

 and distribution is hypothetical, of course. An adequately 

 designed regional study could easily test its validity. 



Among years when we saw few murrelets, ocean 

 anomalies may have limited food supply. In particular, El 

 Nino- Southern Oscillation conditions (deep thermocline, 

 warm water, low ocean productivity; see review in Ainley 

 and Boekelheide 1990) occurred during 1986 and from late 

 spring 1992 to early spring 1994 (see fig. 1). The response of 

 ether coastal alcid species to these conditions, and to winter 

 conditions when food is also sparse, is to spread out linearly 

 along shore where feeding opportunities are more diverse 

 than in the open ocean (Ainley and Boekelheide 1990; Ainley, 

 unpubl. data). At the least, the frequency of ocean anomaly 

 in the California Current region dictates that any investigation 

 of seabird natural history in this region should span at least a 

 5-year period (Ainley and others, in press). 



The negative effect of cooler sea-surface temperature (or 

 a positive one of salinity) on murrelet distribution during late 

 spring is consistent with the oceanography of this area and 

 the high availability of prey. A plume of cool, salty water 

 frequently upwells southward from Point Ano Nuevo (see 

 fig. 1 ), moves offshore and then curls back to the north in an 

 anti-cyclonic eddy (Schwing and others 1991). Besides 

 providing nutrients to this region centered on Ascension 

 Canyon, the eddy may concentrate and maintain prey in 

 place as alluded to above. The plume was often indicated by 

 surface measurements of temperature, salinity, and thermocline 

 characteristics revealed by the CTD ("conductivity and 

 temperature with depth" probe). However, as shown by 

 Schwing and others (1991), surface manifestations of this 

 plume and eddy disappear rapidly, i.e. within a day, following 

 cessation of upwelling-favorable winds. Such a rapid change 

 in ocean characteristics in this region precludes further analysis 

 in our study; a more directed investigation including 

 oceanographic measurements is required. 



It is possible that other murrelet populations along the 

 West Coast also occupy small at-sea ranges in proximity to 

 nesting areas during spring and summer. Repetitive, regional 

 surveys are needed to identify these habitats. On the other 

 hand, some populations apparently vary much more in the 

 choice of waters to frequent, as indicated by temporal variation 

 in numbers within Puget Sound (Speich and others 1992). 

 Strong and others (1993) noted late spring and summer shifts 

 off Oregon in murrelet clumping and hypothesized that it 

 may be a response to the appearance ofAmmodytes hexapterus, 

 an important prey species there but rarely found in central 



California. In these broader scale investigations in regions of 

 more closely spaced nesting populations, the movements of 

 non-breeders and adjacent breeding populations may confuse 

 interpretation of factors affecting distribution patterns. 



Many studies have found that Marbled Murrelets occur 

 very close to shore, usually within a few hundred meters and 

 in depths <15 m. Our results indirectly confirm this pattern, 

 but also indicate that under certain circumstances, the species 

 can occur much farther offshore. Analysis of the along- 

 beach surveys of the USDA Forest Service in the Ano Nuevo 

 area during the past few years (Ralph and Miller, this volume) 

 will be helpful in further interpretation of our results. The 

 timing of our surveys and those of the Forest Service, however, 

 did not correspond closely (their surveys were later in the 

 summer). On the other hand, consistent with the finding of 

 Strachan and others (this volume), we too detected highest 

 numbers in the vicinity of Ano Nuevo during the late spring, 

 as compared to earlier in the year. 



The fact that we did see significant numbers of muirelets 

 well offshore in some years indicates that surveys near the 

 coast to estimate murrelet populations (Carter and others 

 1990a), especially in the Ano Nuevo region, need to account 

 for the possibili ty that significant numbers of birds may be 

 far offshore. Either the surveys, as recommended above, 

 need to be repeated for several years to assess spatial variability 

 (and then choose the survey in which the murrelets are 

 distributed most linearly alongshore), or the surveys need to 

 include closely spaced, inshore-offshore segments that extend 

 well off the coast (at least to 12 km). 



Acknowledgments 



We thank the officers and crew of NOAA Ship David 

 Starr Jordan for logistic support. Chief Scientists W. Lenarz, 

 S. Ralston, and D. Woodbury provided substantial ship time 

 when the prime business of the ship was finished each day. 

 We also benefitted from their insights into fish distribution 

 and thank them for the use of their trawl data, the derivation 

 of which was no small task. D. Roberts crunched much of 

 the trawl data. We also appreciate discussions with C. Strong 

 on murrelet biology. Several persons assisted on the cruises: 

 C. Alexander, R. Ferris, I. Gaffney, M. LaBarr. P. Pyle, C. 

 Strong, P. Ryan, and J. Tweedy. Our time was supported by 

 the Point Reyes Bird Observatory, and in some years by the 

 Gulf of the Farallons National Marine Sanctuary, National 

 Marine Fisheries Service (Southwest Fisheries Center Marine 

 Mammal Division), the U.S. Fish and Wildlife Service, U.S. 

 Department of the Interior and the Environmental Protection 

 Agency. Data analysis was funded by EPA (a by-product of 

 an analysis to locate a dredged material disposal site offshore 

 of central California), and the USDA Forest Service. R. 

 Barrett, University of California, Berkeley, kindly provided 

 GIS facilities, C.A. Ribic provided statistical consultation, 

 and K. Briggs, G. Hunt, B. Tyler, and J. Baldwin provided 

 helpful comments on the manuscript This paper is contribution 

 number 613 of Point Reyes Bird Observatory. 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



369 



