Sooty Shearwaters Off California: Distribution, Abundance and Habitat Use’

The Condor 88:355-3&l 0 The Cooper Ornithological Society 1986

SOOTY SHEARWATERS OFF CALIFORNIA: DISTRIBUTION, ABUNDANCE AND HABITAT USE’

KENNETH T. BRIGGS Institute of Marine Sciences,University of California, Santa Cruz, CA 95064

ELLEN W. CHU A.I.B.S.-Bioscience, 730 1 lth Street, NW, Washington,DC 20001

Abstract. Sooty Shear-waters(Puffinus griseus) migrate to waters of the North Pacific Ocean each spring after nesting on colonies in the South Pacific. Their numbers and habitat affinities off California were investigated during 60 aerial and 24 ship surveys in 1975 to 1978 and 1980 to 1983. Population densities computed monthly indicated arrival throughout California waters in late March and departure of most birds from southern California one to two months earlier than in waters to the north. Peak densitiesexceeded 10 birds kmm2over the shelf and upper continental slope off central California for two to six months each year but reached these levels on only a few occasionsin northern and southern California. Densities in all areasgenerally were highest in late spring and reachedsecondary peaks in mid- to late summer after variable declinesfrom the spring peak. Populations are estimated to total 2.2 to 4.2 million birds in May, June, or July, depending on year and method of calculation. Population turnover may be rapid, total numbers of birds migrating through the area may be much higher than the “instantaneous” population. Shearwaters concentratedin relatively shallow, cool waters, especiallywhere strong thermal gradientsmarked the edgesof upwellings. Key words: Sooty Shearwaters;Puffinus griseus; California; seabirds;migration; population density.

INTRODUCTION of shearwater concentrations. Related studies Sooty Shearwaters (Pufinus griseus) nest on of diet and fat accumulation and results of offshoreislands of New Zealand, Australia, and computer modeling of Sooty Shearwater en- Tierra de1Fuego. Richdale (1963) studied their ergy demands are reported in Chu (1984) and breedingbiology in southernNew Zealand, but Briggs and Chu (in press). virtually nothing is known of numbers or nesting phenology of the South American popu- STUDY AREA AND METHODS lations (Murphy 1936, Palmer 1962). During Our data come from two studiesconducted at the austral winter, some Sooty Shear-watersre- sea: 1975 to early 1978 in the Southern Cali- main in cool waters of the southern hemi- fornia Bight, and 1980 to early 1983 in central sphere (Jehl 1973, Brown et al. 1975, Ainley and northern California. Additional observa- et al. 1983) but many migrate northward as tions were made in 1979 in Monterey Bay, far as Alaska in the Pacific Ocean and Green- where largenumbers of Sooty Shear-watershave land in the Atlantic (Bourne 1956, Phillips traditionally been reported (Loomis 1900, Beck 1963). Some banding has been done on New 1910, Stallcup 1976). Zealand colonies (Richdale 196 3, Warham 1964), but there are few returns and the mi- STUDY AREA gration routes remain controversial (Phillips 1963, Ainley 1976, Guzman and Myres 1982). The surveyed area extended westward 185 to Along the west coast of North America, Sooty 400 km from the California coast and covered Shearwatersare reported to be among the most about 2 15,000 km2 within the California Cur- numerous of all seabirds from May through rent System (Fig. 1). The California Current September (Sanger 1972; Wiens and Scott arises at about 40” to 44”N as the southward- 1975; Briggs and Chu, in press). flowing extension of the West Wind Drift (or During multiyear studies of seabirds and North Pacific Subarctic Current). As it flows marine mammals off California, we focused southward off California it carries a mixture attention on the Sooty Shearwater as a key of cool, relatively fresh subarctic waters and species in overall seabird trophic dynamics. cool, salty, nutrient-rich waters advected from This paper presents our estimates of shear- upwelling regions along the coast. A limb of water population density throughout the year the current recurves counterclockwiseat about and examinesenvironmental conditionsat sites 33”N to flow northward as the Southern Cal- ifornia Countercurrent. This large, quasi-per- manent eddy centers along the undersea ridge I Received 12 September 1985. Final acceptance 21 extending 200 km southeastfrom Point Con- February 1986. ception and affects productivity off southern

[3551 KENNETH T. BRIGGS AND ELLEN W. CHU

cussbelow the relationship of shearwatercon- centrations to thermal fronts associatedwith upwellings. METHODS We made ship and aerial surveys 48 times in southern California and 36 times in central and northern California. To estimate bird density (individuals km-*) we used standardized strip-transecttechniques (50-m-wide transects at 60 to 65 m altitude for aerial counts, and 400-m transectson both sidesof a vesselmov- ing at 18 to 22 km hr-I). Our techniques are described and analyzed in Briggs et al. (198 1, 1983, and 1985a, 1985b). For birds as large as shearwaters, the ship and aerial protocols yielded density estimates that were statistically homogeneous. That is, although observers on neither platform could count or identify all birds present,on a regional basis(1 O,OOO+km*) mean density values calculated from ship and aerial data did not differ significantly(Briggs et al. 1985b). Thus, for this 126~ 1231 ,201 lli” paper we pooled ship and aerial results for our FIGURE 1. Map of the California coast showing shelf and slope topography and surveyedareas (shaded). Total southern California studies. In many months, coverageeach year (linear km) is indicated for each type however, we had done both types of survey. of survey. For thesemonths, we averagedall samplestak- en in each cell of a 5’ latitude-longitude grid. California (Owen 1980). Off southern Califor- On the other hand, the ship did not regularly nia, continental shelf topography is complex. visit the rough waters within 40 km of Point The seafloor consistsof a series of basins and Conception (a center of Sooty Shearwater ridges, some capped by islands, and is termed abundance), and no matching aerial data for the continental borderland to distinguish it southern California are available for the from less complex regions (Emery 1960). The months of July and September 1975, April and main continental slope (Patton Escarpment) August 1976, and August 1977. Because we lies some 200 km off San Diego. Farther north, undersampled this area of high bird density, off central and northern California, the shelf our population estimatesfor thesemonths may varies from 5 to about 75 km in width. be low. Waters are generally coolest and least saline Sampling tracks followed for the southern offshore, in the north. Coastal upwelling, California studiesappear in Briggset al. (198 1); brought on by pulsesof north winds, occursat aerial transectssurveyed in central and north- almost any time over the shelf, generally reach- em California are shown in Briggset al. (1983, ing maximum development in the south ear- 1984). The total coverage achieved by each lier in the year (April-May) than farther north type of survey is shown for three latitudinal (June-July; Bakun and Nelson 1977). Up- regions in Figure 1. For the most part, sam- wellings enhance local food web productivity pling took place during the last week of each (Hickey 1979, Cox et al. 1982). Major up- month and required three to six days. wellingsare associatedwith promontories such After calculatingshearwater density for each asCape Mendocino, Point Arena, Point Reyes, geographicgrid cell visited, we averaged den- Point Sur, and Point Conception. Waters up- sitiesto produce monthly meansfor five depth/ welled over the shelf may be entrained by ed- latitude regions. Offshore areas include only dies, injecting coastal plankton populations waters deeper than 2,000 m; waters shallower offshore into the California Current (Ham-y than 1,999 m we labeled shelf-slope. Depth is 1984, Mooers and Robinson 1984). Where this highly correlated with distancefrom the main- occurs,abrupt horizontal gradientsin physical land in central and northern California but and chemical properties often prevail. Coastal poorly correlated to about 150 km off southern upwellings, offshore eddies, and the often California. Our sampling routinely included strong, meandering thermal fronts lying be- offshore waters in central and northern Cali- tween persist for periods ranging from days to fornia but not in the south. weeks (Breaker 1983, Fiedler 1983). We dis- Numerous sightingswere logged at the ge- SOOTY SHEARWATERS OFF CALIFORNIA 357

neric level. During our study, Sooty Shear- California; they were calibrated by data from waters could be confusedwith Short-tailed and adjacent aerial lines, ship data, or NOAA buoy Flesh-footed shearwaters (P. tenuirostris and data (Piltz 1982, this study). P. carnapes)but, becauseof differencesin back Relationships among habitat variables were color, flight pattern, and size, generallynot with investigatedwith Principal Components Anal- co-occurring Pink-footed (P. creatopus) or ysis (PCA) applied to values for each sampled Buller’s shearwaters (P. bulleri). The Short- grid cell (SAS 1982). Because some environ- tailed may be the predominant shearwater in mental variables in central and northern Cal- central and northern California during winter ifornia were highly correlated, we preferred (Stallcup 1976), when shearwaternumbers are PCA to multiple regressionanalysis, which as- very low. During aerial surveys, sightingswere sumes uncorrelated, independent variables. recorded at the specieslevel primarily when Habitat variables included water depth, dis- underwing pattern and color were clearly vis- tance from the mainland, and distance from ible (which happened quite often, due to star- the nearest point on the shelf break (defined tling of flocks by approach of the aircraft). Ob- as 200-m depth). Surface water temperature servers on vessels used the field marks and gradients in temperature were also con- discussedin Stallcup (1976) to distinguishthe sidered. Gradients were calculated as temper- species. We observed no Short-tailed Shear- ature difference divided by distance measured waters during numerous vessel surveys off between center points of adjacent 5’ x 5’ grid southernCalifornia in springthrough early au- cells. The maximum gradient value was se- tumn and encountered none among the 160 lected for each combination of adjacent cells. collected there and in Monterey Bay in the This was a conservative index that changed same months. Similar paucity of the Short- more rapidly in the cross-shelfdirection than tailed has been reported for Monterey Bay col- parallel to the coast. Thermal features smaller lections comprising many hundreds of birds than 8 km generally were not resolved by this (Morejohn et al. 1978; D. Croll, pers. comm.). gradient index. Values of latitude in the north On thesebases, we assignedunidentified shear- and latitude-longitude in the south provided a water sightingsto the various speciesaccording general index to the pattern of transition of to the proportions of shearwaters positively surfacewater properties (temperature, salinity, identified at the same location, provided we and nutrients) occurring from north to south had identified more than half of the birds seen along the axis of the California Current (Bemal at that location. Where shearwaterswere most- and McGowan 198 1). Longitude was included ly unidentified (for example, if glare had been for the south becauseof the northwest-south- severe), we did not assignthem to speciesand east orientation of the southern California do not report them here. For this reason, our coastline. Components were rotated orthogo- figuresmay underestimate the actual densities nally and the criterion for significancewas set in central and northern California, where all at an eigenvalue 2 1.0. By this procedure, we data come from aerial counts (which are crit- identified a small number of consistently sig- ically affected by glare), and the numbers of nificant components (two or three) explaining unidentified birds accounted for a larger por- much of the variation in monthly environ- tion (3 1% of all sightings)than in the south mental data. In separateanalyses, we then as- (18% of all sightings). sessedthe correlation between shearwaterden- To examine relations of shearwater density sity and values of the first three principal to selectedenvironmental parameters,we used componentsat each sampled location. To con- hydrographic charts to characterize each sam- trol variance, shearwater density was trans- pled grid cell (approximately 80 km2) for water formed by taking the log, after adding 0.01 depth and distancefrom various points. About (Sokal and Rohlf 198 1). This emphasizedpres- 299 and 103 grid cells were visited per survey ence-absenceof the birds rather than varia- in central and northern California and in tions in density. These analyses focused on southernCalifornia, respectively. Surface tem- habitat characteristicswith scalescorrespond- peratures were obtained by through-hull and ing to aggregationsof bird flocks but are un- bucket thermometers aboard ship and by using suitable for determining the fine scale char- an infrared radiation thermometer mounted acteristicsof feedingsites (tens to a few hundred through the aircraft floor (Briggs et al. 1984). m). Where temperatures were not directly avail- RESULTS able (equipment malfunction), we have used satellite infrared images (NOAA 6 and 7 sat- REGIONAL AND SEASONAL VARIATION ellites) to contour thermal features onto the IN DENSITY surveyed track lines. Satellite-contoured tem- Throughout California, Sooty Shearwaterpop- peratures represent 5% of values for northern ulations were highest from May through Au- 358 KENNETH T. BRIGGS AND ELLEN W. CHU

by our averaging procedures. Local densities (in IO’ by 10’ latitude-longitude grid cells) exceeded 1,000 birds km-2 six times in the south and eleven times in central California. These concentrations occurred near San Miguel and Anacapa islands (at either end of Santa Bar- bara Channel), in Monterey Bay, and in the Gulf of the Farallones. Sightings of isolated flocks of 1,000 to 20,000 birds were logged on , 27 occasions(some of these away from tran- sectsused in density calculations), and in one instance (21 June 1981) we made 75 aerial photographsof a flock covering at least 4 km2 in Monterey Bay. Analysis of these photo- graphs led to an estimated 63 1,000 birds, the most spectacular sighting in our six years of study. Sooty Shear-waterdensity was low in offshore areas, attaining mean values above 1 km-2 in only six months in central California and only once in the north (June 198 1). Annual curves were bi- or trimodal in most cases;initial peaks in spring were followed by midsummer dips and one or more late summer or autumn peaks. The differences in density between peaksand troughswere greatestin the two offshore areasand over the shelf and slope FIGURE 2. Annual curves of Sooty Shearwater popu- in the north; seasonal curves were smoother lation density for five geographic/depthregions off Cali- over the shelf and slope in central California. fornia. For each region three curves represent the mean Abrupt peaks and troughs probably represent (center1& 1 SE. Shaded areas lie more than 1 SE below the mean. Hatched areason the map depict locationswhere passageof major segmentsof migrating pop- annual mean shearwaterdensity exceeded 20 birds km-2 ulations. Complexities such as multiple peaks during April through September. over the shelf in the north may indicate pulsed southward movements of different segments of the population summering in the Gulf of gust or September and were distinctly concen- Alaska or movement of some birds to the north trated in shelf and slope waters. Population from central California. The broad, unimodal density on the southern California shelf and curve in the shelf/slope region of central Cal- slope reached values as high as 1 bird km-2 in ifornia during 198 1 may have arisen by one June through September 1975, April through or more of a variety of processes:migration September 1976, and April, May, and July may have been very prolonged; birds migrat- through September 1977 (Fig. 2). Density there ing both north and south may have intermin- exceeded 10 birds kmm2in June 1975 and 1976 gled in summer; or the peaks associatedwith and in May 1977. Over the shelf and slope in migrations may have been masked by the pres- central California density exceeded 1 bird km-2 ence of a large, sedentary summering popu- during April through September each year; in lation. the same depth region of northern California, We can make regional comparisons of mi- density oscillated somewhat in late summer gration dates between 1975 and 198 1, which and exceeded 1 bird km-2 each April, May, had similar prevailing temperature regimes, and June. Density was high (> 10 birds kmm2) and 1976 and 1982, the two years coinciding in central California shelf and slope waters for with onset of El Nitio episodesin the tropical two to six months each year but reached these Pacific (Lynn 1983). Aligning data from these levels only twice off northern California. The years suggeststhat Sooty Shear-watersarrive highest density values in each shelf/slope re- about the sametime in eachregion (late March), gion were: 18 birds kmp2 in southern Califor- but the late summer departure from southern nia (June 1976), 50 birds km-2 in central Cal- California is more abrupt than elsewhere. ifornia (July 1981) and 26 birds km-2 in Over 10,000 records for flight directions ob- northern California (September 198 1). Each of tained throughout southern California in 1976 the shelf/slope regions is large (> 28,000 km2), and 1977 indicate no seasonalchange in the and much local variation in density is masked predominantly northwest (upwind) flight di- SOOTY SHEARWATERS OFF CALIFORNIA 359 rection (K. T. Briggs, unpubl.). This was true even as late as August, when other authors have supposedbirds to be migrating south out of California waters(Guzman and Myres 1982). In central and northern California, flight di- rection data were affected by potential startling of birds by the survey aircraft and predomi- nance of north winds. Nevertheless, over 79% of 6,621 sightingsin August and September show north or northwest flight. In recent studies, we have found Sooty Shearwaters to fly predominantly upwind, even if it means mov- ing south in June (K. T. Briggsand D. G. Ain- ley, pers. obs.).

NUMBERS OF BIRDS OFF CALIFORNIA Since Sooty Shearwaters are one of the most important avian predators in the North Pa- cific, it makes senseto estimate the numbers of birds that might be present off California at one time. This can be done in three general ways. First, we can compute monthly averages for each of the geographic-depthregions, mul- tiply these by the appropriate areas, and com- bine them to yield an average, so-called “instantaneous” population. Doing so gives a maximum of about 2.6 million Sooty Shear- waters off California in May, with about 80% FIGURE 3. Mean monthly sea surface temperatures of the population in the central and southern (small graphs; 20-year means) and monthly departures from the mean (large graphs)for each of three latitudinal shelf-slope regions. Combined numbers fall sectorsoff California. Means are adaptedfrom Auer f 198l- below 1.0 million after the end of July; most 1983) and monthly anomaliesare basedon measurements birds are still concentratedin the central coast- made during this study and adapted from Auer (198 1). al area. Persistentwarm anomalies in autumn 1976 and 1982 co- incided with onset of El Nirio conditions in the tropical A second procedure simply entails adding Pacific. the maximum values for each region for each month; this would give us population estimates corresponding to environmental con- shearwater population would normally have ditions conducive to large populations. Doing migrated south (Lynn 1983, Simpson 1983). so for our data yields maximum monthly pop- Combining population estimates from these ulations of more than 4.0 million in May and cool and warm years reveals that the popula- 2.7 million in June and July. These high added tion was greatest in May, June, or July of the populations derived from data taken in years cool years, reaching a level of about 2.3 to 2.7 of heavy upwelling and high plant, plankton million. In the warm El NiAo years (1976 and and fish productivity (cf. Smith and Eppley 1982), the population was high in May (3.5 1982). million), dropping quickly to lessthan 1.O mil- Third, we can match different years on the lion after June. Thus, warm environmental basisof similar environmental conditions, such conditions may have hastened shearwater mi- as temperature, that correlate with and prob- gration or at least departure from California. ably influence shearwatermigrational dynam- Although our systematic sampling ended be- ics (see discussion below). The summers of fore the normal arrival dates for Sooty Shear- 1975 and 198 1, for example, were character- waters in 1983, we noted that when El Niiio ized by strong upwelling and temperatures at conditions reached peak in summer 1983, or below 20-year mean values (Fig. 3). These Monterey Bay populations were greatly re- were followed by summers with anomalously duced compared with other years (A. Bald- warm water coinciding with El Nifio episodes ridge, pers. comm.; KTB, pers. obs.). in the tropical Pacific. In both episodes,while We stress that our estimates produce in- oceanographicanomalies, including tempera- stantaneoustotals for the state, not estimates ture, became evident off California during late of the total number of birds that might pass summersof 1976 and 1982, maximum anom- through. Since the rates of population turnover alies followed about four months after the during migration and the proportion of birds 360 KENNETH T. BRIGGS AND ELLEN W. CHU

TABLE 1. Principal components analysis of environ- Monterey Bay. Component III mostly com- mental variables in 294 grid cells sampled in July 1980. prised the unique variance in the gradients Significant loadings(multiple r) are underlined. variable, and generally explained about 19% of the total. Component Component component Variable I 11 III Variation in shearwater density correlated significantly (r = 0.35 to 0.45) with Compo- Water depth 0.948 0.090 -0.055 nent I in springto summer 1980 and 1982 and Distance to with Component III (multiple r = 0.23 to 0.68) mainland 0.862 -0.007 -0.107 in almost all months (April through Septem- Distance to ber) when birds were abundant. Bird density shelf break 0.924 0.196 -0.064 seldom correlated with variation in Compo- Latitude -0.059 -0.963 -0.010 nent II (latitude-temperature). Water tem- These same patterns also characterized the perature 0.232 0.917 -0.069 southern California data. On Component I, Temperature there was little significant contribution from gradient -0.180 -0.047 0.982 water temperature, but temperature gradients Variance ex- varied significantly as the inverse of depth and plained 43.2% 30.3% 16.5% distance from shore (i.e., strong thermal gra- Shearwater dients bordered upwellings in the shallow density 0.381 0.239 0.406 waters near the mainland and on the “aprons” of the islands).Component II consistedmainly that staysin California to feed during summer of the inverse variation of temperature on lat- are unknown, we cannot reliably estimate the itude-longitude, reflecting the influence of ad- total numbers of shearwatersthat may use Cal- vection of cool water from the north and heavy ifornia waters at some time during the year. upwelling in the San Miguel Island area. In 11 Given the flight speedstypical of this species of 18 surveys made in April through Septem- (20 to 50 km hr-l), and estimates of up to 12 ber, we found a significant inverse correlation to 35 million birds in the Gulf of Alaska during of shearwater density with Component I (up summer (Gould et al. 1982) the numbers of to Y = -0.59 in midsummer 1976 and 1977) shearwatersmigrating through or summering and in 9 of 18 surveys a significant positive off California might be an order of magnitude correlation with Component II (up to r = 0.77 higher than our instantaneous estimates of 2 in September 1975). to 4 million. Overall, these analyses point to an affinity of Sooty Shear-watersfor cool waters near the ENVIRONMENTAL CORRELATES OF coast. However, whereas Sooty Shearwaters DENSITY concentrated in the coolest waters of the Principal componentsanalysis of northern and Southern California Bight, they shunned the central California habitat data revealed the im- coolest waters off northern California. Coastal portance of two (occasionally three) habitat upwellings from Point Arena to Cape Men- gradients, or components, that together ex- docino, where surface temperatures were 7 to plained an average of 76% of standardized 10°C in summer, were little visited by shear- variance. For example, data from July 1980 waters; the birds instead concentrated in the show (Table 1) high cross-shelfcorrelation be- south in warmer waters from 30 to 70 km tween water depth, distance to the mainland, offshore (Fig. 2). Heaviest concentrations water temperature, and distance to the shelf- throughoutthe statewere encounteredin waters break (Component I). Becausewe sampled al- 11 to 14°C within or south of active upwell- most the same areas each month, these depth ings. and distancecorrelations did not vary, but there Interestingly, Sooty Shearwaters also made was seasonality in temperature. Water tem- little use of the vast expanseof the eastern half perature significantly correlated with distance of the California Current (seaward of coastal from shore (multiple Y = 0.23 to 0.64, yt = upwellings) for feeding or migration, or they 299) in April through November each year but did so only for short periods. Our sampling not in winter. Component II, explaining 30% routinely extended 185 km offshore; relatively of variance, reflected the prevalence of cool few birds ever were seen more than 100 km temperatures at higher latitudes. Temperature from the coast, even where sea temperatures gradients contributed significantly to Com- were low. During transectsto 400 km off Mon- ponent II in data from winter 198 1 and winter terey Bay in June,August, and September 1982, to spring 1982: large, cross-shelftemperature we found an average of only 0.04 birds km-2 gradients(averaging O. l-0.2”C km-2) prevailed more than 100 km offshore, and no birds from at these times in the warm waters south of 200 to 400 km. Unless our routine sampling SOOTY SHEARWATERS OFF CALIFORNIA 361 missed significantmigration much farther off- inantly juvenile rockfish (Sebastesspp.), eu- shore, or distribution among habitats in 1982 phausiids, market squid (Loligo opalescens), was extremely atypical, then the preferred hab- and northern anchovies (Engraulismordax). itat for Sooty Shearwatersincludes the coastal Despite the abundance of anchovies in south- upwelling zone, but not California Current em California, they were not a primary Sooty waters seawardof the central continental slope. Shearwater food in the samples reported by Additionally, however, presence of bird con- Chu (1984). We think this is due to anchovy centrationsat some distanceremoved from the behavior during the late spring peak of shear- coldest upwelled waters indicates that tem- water abundance; the fish occur in wary, dis- perature by itself is an unreliable indicator of persed spawningschools (Mais 1974). In sum- the suitability of feeding areas.It is much more mer, post-spawning anchovies occasionally likely that feedingshearwaters make useof such form large, near-surfaceschools in the day near optical cuesas slicks,color discontinuities,and the southern California coast. But, by the time shoals of bait, which in turn are differentially these schoolsbecome common, and thus sus- common within and near upwelling centers(cf. ceptible to heavy shear-waterpredation (the Briggs et al. 1984, Laurs et al. 1984). These peak occurs in October), most Sooty Shear- cuesmay prevail where physicalprocesses (e.g., waters already have left the northeastern Pa- current shearsor convergences)act to concen- cific for the breeding grounds. Rockfish juve- trate seabird prey. niles and euphausiids,which are both abundant in the northwestern Southern California Bight DISCUSSION (Brinton 1976, Parrish et al. 198 1) are the During spring, Sooty Shear-watersarrive in the mainstays of the Sooty Shearwater diet until eastern North Pacific at about the same time upwelling tapers off after June, and shearwater acrossa broad front. They arrive off California populations begin to dwindle. in late March and reach densitiesover one bird In contrast, shear-waterfood resources in km-2 a month later. Arrival dates for other central California may be more diverse, more north Pacific areas include late March to early persistentlyavailable through springand sum- April off Japan (Palmer 1962), mid-March in mer, and perhaps locally more abundant. the Gulf of Alaska (Gould et al. 1982), early Among primary shearwaterprey species,rock- April off Vancouver Island (Guzman and fish are the predominant resident pelagic Myres 1982), and February to April in the cen- spawners;euphausiids are abundant in spring tral Gulf of California (D. W. Anderson, in and summer (probably more so than to the litt.). Ring (1970) and D. G. Ainley (in litt.) south; Mais 1974, Brinton 1976), and squid reported that Sooty Shear-watersfly north past are exceptionally abundant and available in the Hawaiian Islands in March and April. midsummer at shallow water spawningcenters With some annual and geographicvariation, such as Monterey Bay (Morejohn et al. 1978, shearwater density off California peaked in Parrish et al. 198 1, Chu 1984). April through July, usually in May or June. In Contrasting with the situation in the south, southern California, this peak was followed by the ecology of anchovies in central California declines of 50% or more and a later (July, Au- invites heavy predation by shear-waters.Here, gust,or September),lower peak. Density curves post-spawning fish occur in dense daytime in central California were somewhat broader, schoolsvery close to shore, particularly from and the late summer populations were much Mot-r-0Bay to Point Arguello, in Monterey Bay, higher than in the south. In northern Califor- and in the Gulf of the Farallones; anchovies nia, substantial peaks were also observed in also average larger than in the south (Mais late summer and early autumn; in one case 1974). In late summer, shearwatersfatten and (September 198 l), fall densitieseclipsed those complete their premigratory molt at the same of spring. time they feed on energy-rich anchovies (Chu 1984). DISTRIBUTIONS OF SHEARWATER PREY North of Cape Mendocino, prey availability The pattern of early population declines in the may be greatestafter midsummer. Large num- south and persistenceof densepopulations lat- bers of Sooty Shearwaters occurring sporad- er in summer at higher latitudes parallels that ically off extreme northern California probably seenin several other seabird populations (e.g., make useof rockfishand squid,which are much Black-footed Albatross, Diomedea nigripes, more abundant there than off southern Cali- and Brown Pelican, Pelecanusoccidentalis) and fornia (Mais 1974). Additionally, an ecologi- correlates with several aspectsof food avail- cally distinct anchovy population spawns in ability and prey behavior (Sanger 1974, midsummer in the Columbia River Plume off Anderson and Anderson 1976). Chu (1984) Oregon (Richardson 198 1). In part, the large reported that Sooty Shearwaters eat predom- shearwater concentrations seen off Oregon in 362 KENNETH T. BRIGGS AND ELLEN W. CHU

August (Wiens and Scott 1975) and off north- some offshore species(e.g., Leach’s Storm-Pe- ern California in August through October (this trel, Oceanodromaleucorhoa, and Buller’s study) may form in response to feeding op- Shearwater).Phalaropes (Phalaropus jiilicarius portunities on these southward-drifting an- and P. lobatus),pelicans, and Cassin’s Auklet chovies. Comparative studies of diet north of (Ptychoramphusaleuticus) share the Sooty Monterey Bay would be most instructive. Shearwater’s affinity for the frontal zone per We believe that concentration of Sooty se (Briggset al. 1983, 1984). Recently, Cory’s Shearwaters over the shelf and upper conti- Shearwaters(Callonectris diomedea) have been nental slope rather than in waters farther off- shown to closely associate with shelf-break shore (particularly north of Point Conception) fronts of the Gulf Stream (Haney and Mc- relates to two general aspects of prey avail- Gillivary 1985). ability. First, three important prey speciesare distributed closeto shore in summer: rockfish, ANNUAL POPULATION VARIATIONS squid, and anchovies. Second,prey far offshore Ainley (1976) noted the occasional decline in in California Current waters may be more dif- abundanceof Sooty Shearwatersoff California ficult to find and capture. During April through during years of environmental warming. We September, offshore waters in the California surveyed populations during two such events: Current exhibit more stable vertical stratifi- the relatively weak 1976 to 1977 episode and cation than those in and near upwelling centers the early phase of the strong one of 1982 to (e.g., Huyer 1983, Brink et al. 1984). Concen- 1983. Sooty Shearwatersoccurred in fairly large trations of phytoplankton and associatedzoo- numbers during spring in 1976 and 1982, but plankton tend to be found near or below the densitiesthen declined unusually rapidly from thermocline by day (30 to 100 m); becauseof midsummer onward. Shearwaters were fairly nocturnal vertical migration of zooplankton abundant off southern California during the and nekton, prey are found nearer the surface height of the thermal anomaly in summer 1977. but are dispersed through a greater vertical We did not make standard surveys during range at night. While this in no way rules out summer 1983, when the ocean off central Cal- nighttime surface feeding by shearwaters, it ifornia was at its warmest and least productive does imply that prey will be found in lower in almost twenty years.However, our own brief concentrations. As noted by D. G. Ainley (in observations on Monterey Bay and conver- litt.) fur seals(Callorhinus ursinus) and other sations with other observers indicated that diving mammals are quite successfulpredators Sooty Shearwaterswere scarce.Off British Co- on small fish and squid in theseoffshore waters lumbia, Vermeer and Rankin (1984) found in some seasons.This reinforces the idea that these birds to be an order of magnitude more prey may be concentrated too deep for shear- numerous in 1983 than in limited previous or water exploitation. Surface waters are also subsequent surveys. This suggeststhat the much clearer seawardof the continental slope. population that normally summers off Cali- Thus, even where prey are concentrated at fornia may have shifted a thousand or more depthstypically reached by diving shearwaters km northward. (5 to 10 m, Brown et al. 1981), water clarity abets avoidance by the prey. UNRESOLVED QUESTIONS Beyond this, except where eddiesentrain up- Several questionsabout Sooty Shearwater dis- welled water, surface convergences, often tribution remain unresolved. Our data show marked by abrupt thermal gradients, are less primarily northwestward (upwind) flight in all common offshore than near the shelf break. seasons, including the period of presumed Convergent fronts are often found at the sea- southwardmigration (late summer). Except for ward edgesof upwellings and can concentrate the suggestionsof early authors and the ob- macroplankton and their fish (and bird) pred- servations of R. L. Pyle and R. L. DeLong ators. These features may be relatively long- (unpubl.), the route(s) of southward migration lived (several weeks or more for features of 50 thus remain unclear. It may be that most birds to 200 km extent), thus providing some con- depart across a broad front away (southwest) tinuity in feeding conditions. Outer Monterey from the coast rather than along it, thereby Bay and the shelf-breakregion for 100 km south avoiding the warm coastal waters of southern of Pt. Reyes are examples of areas where ther- California and Mexico (which average 18 to mal fronts are persistentand shearwaternum- 25°C during August and September). berswere large.We have found upwellingfronts Population turnover rates during migration to mark the offshore limits of distribution for are still unknown, yet we need them to deter- some coastal bird species (e.g., Common mine the total population that might pass Mm-i-e, Uria aalge, and Western Gull, Larus through California (Guzman and Myres 1982). occidentalis)as well as the near-coastlimits for Resolution of this problem may require mark- SOOTY SHEARWATERS OFF CALIFORNIA 363 ing or telemetric tracking at various stagesof Croxall [ed.], The feedingecology of seabirdsand their the annual cycle. role in marine ecosystems.Cambridge Univ. Press, London, England. The origin of birds that visit California is BRIGGS,K. T., K. F. DETTMAN,D. B. LEWIS,AND W. B. also uncertain (Ainley 1976, Guzman and TYLER. 1984. Phalarope feeding in relation to au- Myres 1982). Opinions range from solely South tumn upwelling off California, c 51-62. In D. N. American origin, to origin in the southwestern Nettleshiu. G. A. Sanaer, and P. F. Surinaer leds.1. Pacific, to mixtures of the two. Migrations over Marine bi;ds: their feeding ecology an‘d comierciai fisheriesrelationships. Canadian Wildlife Service, Ot- equatorial regions may be more or less rigor- tawa, Canada. ous (thus affecting numbers of migrants), and BRIGGS,K. T., D. B. LEWIS,W. B. TYLER,AND G. L. HUNT, final summer feeding locations may shift ac- JR. 1981. Brown Pelicans in southern California: cording to meteorological conditions or food habitat use and environmental fluctuations. Condor 83:1-15. availability along the route. In fact it is possible BRIGGS,K. T., W. B. TYLER,AND D. B. LEWIS. 1985a. that bimodal peaksin abundanceoff California Comoarison of shiv and aerial surveysof birds at sea. result from arrival of birds from different nest- J. Wildl. Manage. 49:407-413. . ing grounds.And California populations might BRIGGS,K. T., W. B. TYLER,AND D. B. LEWIS. 1985b. also vary in composition from year to year, Aerial surveys for seabirds: methodological experi- ments. J. Wildl. Manage. 49:414-419. depending on stagesof the El Nifio episodes. BRIGGS,K. T., W. B. TYLER,D. B. LEWIS,P. R. KELLY, Only analyzing several years of observations ANDD. A. CROLL. 1983. Brown Pelicans in central in the tropics (e.g., Ainley and Boekelheide and northern California, 1980-1982. J. Field Omi- 1983) and perhaps recoveries of banded birds thol. 54:353-373. BRINK,K. H., D. W. STUART,AND J. C. VANLEER. 1984. from different colony areas will provide an- Observations of the coastal upwelling region near swers. 34”30’N off California: spring 1981. J. Phys. Ocean- ogr. 14:378-391. ACKNOWLEDGMENTS BRINTON,E. 1976. Population biology of Euphausia pa- cij& off southernCalifornia. Fi& Bull. ?4:733-762. We thank D. Lewis, W. Tyler, D. Dettman, K. Dettman. BROWN.R.G.B.. S. P. BARKER.D. E. GASKIN.AND M. R. M. Pierson, M. Bonnell, aid G. Farrens for assistancein SANDEMAN.’ 1981. The foods of Great and Sooty the field. D. Arias. D. Carlson. and W. Fitler assistedwith shearwatersPM&US gravis and P. griseusin eastern computer analyses,and D. G&ison provided additional Canadian waters. Ibis 123:19-30. statisticalhelp. C. Hitchcock, C. Caldwell, and D. Briggs BROWN,R.G.B., F. COOKE,P. K. KINNEAR, AND E. L. assistedwith manuscript preparation. T. Dohl, M. Bon- MILLS. 1975. Summer seabirddistributions in Drake nell, M. Pierson,D. Ainley, M. T. Myres, and D. Anderson Passage.the Chilean Fiords. and off southern South offered valuable criticisms of earlier drafts. Our research AmeG& Ibis 117:339-356.’ was supported by contracts AA550-CT7-36 and 1412- 0001-20909 from the U.S. Minerals Management Service. CHU, E. W. 1984. Sooty Shearwatersoff California: diet and energygain, p. 64-7 1. In D. N. Nettleship, G. A. Sanger,and P. F. Springer [eds.], Marine birds: their LITERATURE CITED feeding ecology and commercial fisheries relation- AINLEY,D. G. 1976. The occurrenceof seabirdsin the ships. Canadian Wildlife Service, Ottawa, Canada. coastalregion of California. West. Birds 7:33-68. Cox, J. L., L. R. HAURY,AND J. J. SIMPSON.1982. Spatial AINLEY,D. G.. AND R. J. BOEKELHEIDE.1983. An eco- patterns of grazing-related parameters in California logical comparison of oceanic seabird communities coastal surface waters, July 1979. J. Mar. Res. 40: of the South Pacific Ocean. Stud. Avian Biol. 8~2-23. 1127-1153. AINLEY,D. G., E. F. O’CONNOR,AND R. J. BOEKELHEIDE. EMERY,K. 0. 1960. The seaoff southernCalifornia. John 1983. The marine ecology of birds in the Ross Sea, Wiley & Sons, New York. Antarctica. Omithol. Monogr. 32. FIEDLER,P. C. 1983. Satellite remote sensingof the hab- ANDERSON,D. W., AND I. T. ANDERSON.1976. Distri- itat of spawning anchovy in the Southern California bution and statusof Brown Pelicansin the California Bight. Calif. Coop. OceanicFish. Invest. Rep. 24:202- Current. Am. Birds 30:3-12. 209. AUER,S. [ED.]. 1981, 1982,1983. Oceanographicmonth- GOULD, P. J., D. J. FORSELL,AND C. J. LENSINK. 1982. ly summaries, Vols. 1 and 2, issued monthly. U.S. Pelagicdistribution and abundanceof seabirdsin the Dept. Commerce, NOAA, Washington, DC. Gulf of Alaska and eastern Bering Sea. FWS/OBS- BAKUN, A., AND C. S. NELSON. 1977. Climatology of 82148. U.S. Fish and Wildlife Service, Anchorage, upwelling related processesoff Baja California. Calif. AK. Coop. Oceanic Fish. Invest. Rep. 19:107-l 27. GUZMAN,J., AND M. T. MYRES. 1982. Chronologyof the BECK,R. H. 1910. Water birds in the vicinity of Point offshoremigration of Sooty Shearwatersin the eastern Piiios, California. Proc. Calif. Acad. Sci. 4:57-72. Pacific. Abstract. Pacific Seabird Group Conference, BERNAL.P. A.. AND J. A. MCGOWAN. 1981. Advection 6 to 9 January 1982. and upweiling in the California Current, p. 38 l-399. HANEY, J. C., AND P. A. MCGILLIVARY. 1985. Aggre- In F. A. Richards [ed.], Coastal upwelling. Amer. gationsof Cory’s Shearwaters(Callonectris diomedea) Geophys. Union, Washington, DC. at Gulf Stream fronts. Wilson Bull. 97: 19l-200. BOURNE,W.R.P. 1956. Migrations of the Sooty Shear- water. Sea Swallow 13:26-36. HAURY, L. R. 1984. An offshore eddy in the California BREAKER,L. C. 1983. The space-timescales ofvariability Current System. Part 4. Plankton distributions. Prog. in oceanicthermal structureoff the central California Oceanogr. 13:95-l 11. coast. Ph.D.diss., Naval PostgraduateSchool, Mon- HICKEY,B. M. 1979. The California Current System- terey, CA. hypothesesand facts. Prog. Oceanogr. 8: 191-279. BRIGGS,K. T., AND E. W. CHU. In press. California sea- HUYER, A. 1983. Coastal upwelling in the California birds: updating models of trophic impact. In J. P. Current System. Prog. Oceanogr. 12:259-284. 364 KENNETH T. BRIGGS AND ELLEN W. CHU

JEHL, J. R., JR. 1973. The distribution of marine birds itoring network. POCS Tech. Pap. 82-3. U.S. Dept. in Chilean waters in winter. Auk 90: 114-135. Interior, Minerals Management Service, Los Angeles. KING, W. B. 1970. The trade wind zone oceanography RICHARDSON,S. L. 1981. Spawning biomass and early pilot study. Part 7. Observations of seabirds, March life history of northern anchovy, Engraulis mordax, 1964 to June 1965. U.S. Fish and Wildl. Serv. Spec. in the northern subpopulationoff Oregon and Wash- Sci. Rep. Fish. 586. ington. U.S. Fish Wildl. Serv. Fish Bull. 78:855-876. LAURS,R. I%., P. C. FIEDLER,AND D. M. MONTGOMERY. RICHDALE,L. E. 1963. Biology of the Sooty Shearwater, 1984. Albacore tuna catch distributions relative to Pufinus griseus.Proc. Zool. Sot. Lond. 141:1-l 17. environmentalfeatures observed from satellites.Deep- SANGER,G. A. 1972. Preliminary standing stock and Sea Res. 31:1085-1099. biomassestimates of seabirdsin the subarcticPacific LQOMIS,L. M. 1900. Californian water birds. Proc. Calif. region, p. 589-6 11. In A. Y. Takenouti [ed.], Biolog- Acad. Sci. 3~227-322, 349-363. ical oceanographyof the northernNorth PacificOcean. LYNN, R. J. 1983. The 1982-83 warm episode in the Idemitsu Shoten, Tokyo, Japan. California Current. Geophys. Res. Lett. 10:1093. SANGER,G. A. 1974. Black-footed Albatross (Diomedea MAIS, K. F. 1974. Pelagic fish surveysin the California nigripes), p. 96-128. In W. B. King [ed.], Pelagic Current. Calif. Dept. Fish Game Fish. Bull. 162:1- studies of seabirdsof the central and eastern Pa&c 78. Ocean. SmithsonianContrib. Zool. 158. Washineton. . I MOOERS,C.N.K., AND A. R. ROBINSON.1984. Turbulent DC. jets and eddies in the California Current and inferred SAS INSTITUTE,INC. 1982. SAS user’s guide: statistics. cross-shoretransports. Science223:5 l-53. SAS Institute, Cary, NC. MOREJOHN,G. V., L. D. KF~ASNOW, AND J. T. HARVEY. SIMPSON,J. J. 1983. Large-scale thermal anomalies in 1978. The importance of Loligo opalescensin the the California Current during the 1982-1983 El Nirio. food web of marine vertebratesin Monterey Bay, Cal- Geophys. Res. Lett. 11:283. ifornia. Calif. Dept. Fish Game Fish. Bull. 169:67- SMITH, P. E., AND R. W. EPPLEY. 1982. Primary pro- 68. duction and the anchovy population in the So&em MURPHY,R. C. 1936. Oceanic birds of South America, California Bight: comparison of time-series. Limnol. Vol. 2. Macmillan, New York. Oceanogr.27: 117. OWEN, R. W. 1980. Eddies of the California Current SOKAL,R. R., AND F. J. ROHLF. 1981. Biometry. W. H. System:physical and ecologicalcharacteristics, p. 237- Freeman, San Francisco. 263. In D. Power [ed.], The California islands: pro- STALLCUP,R. W. 1976. Pelagic birds of Monterey Bay, ceedings of a multidisciplinary symposium. Santa California. West. Birds 7: 113-l 36. Barbara Museum of Natural History, Santa Barbara, VERMEER,K., AND L. RANKIN. 1984. Pelagicseabird pop- CA. ulations in Hecate Strait and Queen Charlotte Sound: PALMER,R. S. [ED.] 1962. Handbook ofNorth American comparison with the west coast of the Queen Char- birds. Yale University Press, New Haven, CT. lotte Islands. Can. Tech. Rep. Hydrogr. and Ocean PARRISH,R. H., C. S. NELSON,AND A. BAKUN. 1981. Sci. 52:1-40. Transport mechanisms and reproductive successof WARHAM,J. 1964. Marked Sooty Shearwaters,Pujinus fishes in the California Current. Biol. Oceanogr. 1: griseus,in the Northern Hemisphere. Ibis 106:390- 175-203. 391. PHILLIPS,J. H. 1963. The pelagicdistribution of the Sooty WIENS,J. A., AND J. M. SCOTT. 1975. Model estimation Shearwater,Procellaria grisea. Ibis 105:340-353. of energyflow in Oregon coastalseabird populations. PILTZ, F. 1982. Results of the Minerals Management Condor 77~439-452. Service funded west coast meteorologicalbuoy mon-