DISTRIBUTION, ABUNDANCE, AND IMPLICATIONS FOR CONSERVATION OF WINTER WATERBIRDS ON TOMALES BAY, CALIFORNIA JOHN P. KELLY AND SARAH L. TAPPEN, AudubonCanyon Ranch, Cypress Grove Preserve,Marshall, California 94940 TomalesBay, on the Marin Countycoast, is one of California'slargest and least disturbedestuaries, yet little has been publishedon the statusof its waterbirdsor their vulnerabilityto environmentalthreats. Shuford et al. (1989) reportedthe resultsof waterbirdcensuses of up to 10 consecutive years in the adjacentPoint Reyes area between 1965 and 1982 but providedlimited informationon waterbirduse of TomalesBay. Other anecdotalaccounts and aerial survey work over the last30 yearssuggest that TomalesBay supports unusually high abundances of somewaterbird species (Moffitt1943, Grinnelland Miller 1944, Calif.Dept. Fishand Game unpubl. data,U.S. Fishand Wildlife Service unpubl. data, National Audubon Society ChristmasBird Counts)and is worthyof protection(National Oceanic and AtmosphericAdministration 1987, Neubacheret al. 1995). In thispaper, we presentresults from sevenwinters of baywidewaterbird censuseson TomalesBay, compareresults with other studiesto providea historicaland geographicalperspective on the importanceof TomalesBay to waterbirds,evaluate species distributions within the bay with regardto possiblehabitat relationships and the importanceof particularhabitat areas, and, on the basisof theseevaluations, identify needs for conservationof winterwaterbird populations on TomalesBay. We addressthese objectives with regardto all waterbirdspecies associated with TomalesBay and immediatelyadjacent ponds and marshes, with the exceptionof sandpipers (Scolopacidae),plovers (Charadriidae), and large and medium-sizedgulls (Laridae;Table 1). Most gullswere excludedfrom the studybecause of difficultiesin countinglarge numbersof moving birds and identifying species.Sandpipers, plovers, and gullswere alsodifficult or impossibleto observefrom surveyboats because these speciesoften concentratedon exposedtidal flats or in alternativehabitats away from the bay.Abundances of wintershorebirds and gulls on TomalesBay have been reported elsewhere (Kelly 1993, Kellyet al. 1996). STUDY AREA The longstraight expanse of TomalesBay floodsthe lower20 km of the fault-generated Olema Valleyon the centralCalifornia coast, about 45 km northwestof SanFrancisco (Figure 1; Galloway1977). The baydiffers from other, generallyshallower, Pacific coast estuaries and lagoonsin havinga muchgreater area of open water at low tide and thereforemore waterbird habitatthrough the tidalcycle. Approximately 91% of the bay's28.5-km z areais subtidal. With hillssurrounding the narrow(1-2 km)bay, and little fetch whenwinds blow from directions other than northwest,surface waters remain relativelycalm duringwinter. Tomales Bay contains37 eelgrass(Zostera marina)beds covering a total area of 392 hectares,predominanfiy in the Western Birds 29:103-120, 1998 103 •••••••00•00•00 •00•0•00000•000000•00000 0•0•0•0•0•0•0•0000••0• •000•0••000•000000•00000 •••0•0•0•00••0• ••0•00•0•000•000000•00000 •0•00•0••0• •000•00••0• ............................ 000000000•0•00•0000000000•00 00000•000•00•00000000•0•00 00•0•0•00••0•0••0•0000 00•0•••0••••0•0•0 00•00•0•000•00000000•000000• ••••••••oooooo• 00•000•0•000•000•00000•00000• 00•00•00•0•00000••000• •••000••0•••000• •0•0••00•0•0•00•00•000• •••00••0••0•0000• •0•0••00••0•00•0000000 WINTER WATERBIRDS ON TOMALES BAY, CALIFORNIA ;and Point Walker Tom's Point oint Pacific Routes of Ocean ObservationBoats PointReyes NationalSeashore Point 0 2 4 km I,,,, I I Lagunita., Creek Figure1. TomalesBay studyarea. Shadingsdenote winter waterbird count sections; parallellines denote three boat transects. Supplementary counting points (circled) are (1) Walker Creek, (2) LivermoreMarsh, (3) MillertonGulch to Bivalve,(4) Bivalve,and (5) Inverness. northernhalf of the bayfrom Pelican Point northward to Tom'sPoint (Figure 1, Spratt 1989). Water depthsin the bay averageabout 3 m below mean lowerlow water (MLLW) and vary from intertidal shoals along the eastshore 106 WINTER WATERBIRDS ON TOMALES BAY, CALIFORNIA to areasup to 20 m deepin the mainchannel along the west shore. In general, sedimentsgrade from primarily fine to coarsesand in the northernreaches of the bay to muddiersubstrates in southernportions (Daetwyler 1966). The two primarypoints of freshwaterinflow, and the largesttidal fiats and marshes,are at LagunitasCreek at the southend of the bay and Walker Creek near the north end (Figure 1). Numerous smaller perennial and ephemeralstreams enter the bay alongits perimeter,each associated with a small,generally insular delta marsh. Rainfall is highlyseasonal, with 95% of annualrain fallingfrom October throughApril and 55% from December throughFebruary, based on 42-year averagesat CypressGrove Preserve. Spatialand temporal variations in the bay'ssalinity are influencedby variably high levelsof freshwaterinflow duringwinter, low flows in summer,and constraintson tidalexchange with the oceanimposed by the linearshape of the bay and the narrow mouth at its northwestend. Thereforesalinities in the southernreaches of the bay are highlyvariable, ranging from nearly freshafter heavywinter runoff to slightlyhypersaline in summer,whereas regulartidal mixing at the north end of the bay maintainssalinities that consistentlyreflect those of the outercoastal waters (Hollibaugh et al. 1988). The differencebetween mean high and mean low tidesis about1.1 m, with an averageannual maximum tide swingof about2.5 m, basedon National Oceanic and AtmosphericAdministration predictions. Tidal currentsare strongestalong the west shorewithin about6 km of the northwestend but are generallygentle. METHODS We conducted21 (mean3/yr, minimum1, maximum4) baywidecounts of waterbirdson TomalesBay overthe sevenwinters from 1989-90 to 1995- 96. Eachyear, consecutive counts were made at 2- to 3-weekintervals from mid-Decemberto lateFebruary each year. Each baywide survey encompassed simultaneouscounts conducted from three 17- to 21-ft (5.3- to 6.6-m) Boston Whalersor similarboats. The boatstraveled in formation,following parallel 18-kmtransects from MillertonPoint near the southend of the baynorth to the SandPoint Buoy (Figure 1). Boatsfollowing the east-shoreand west-shore transectseach carried one team of observers.The midbayboat carriedtwo (portand starboard)teams. Each team consistedof two competentwaterbird observers,using binoculars, and one data recorder.Observers on the east- shoreboat countedall waterbirdsin a transectbounded by the path of their boat and the eastshore. Similarly, observers on the west-shoreboat counted all waterbirdsin a transectbounded by their boat and the west shore. Observerson the midbayboat countedall waterbirdsbetween the other two boats.Drivers of the east-and west-shoreboats continually adjusted their positionsvisually to maintain roughlyequal distancesbetween boats and transectboundaries. Therefore, shoreline transects each represented approxi- matelya fourthof the arealextent of the bayand the midbaytransect covered approximatelyhalf of the bay'sextent. We maintainedcontinual radio communication among observation boats to facilitateadjustments of interboat and boat-to-shoredistances and to clarify counts of confusingbirds or flocks along the transectborders. 107 WINTER WATERBIRDS ON TOMALES BAY, CALIFORNIA Observerson each boat countedonly those birdsthat passedsouthward throughan imaginaryvertical plane defined by the bowsof the threeboats. The boatstraveled at velocitiesof about4 knots,but occasionally we stopped to countdense aggregations of birds.Birds flushing ahead of the moving boatswere not counteduntil they eventuallypassed southward through the observationplane. Birdspassing northward over or aroundthe boatswere subtractedfrom the counts,but this seldomoccurred as long as boats maintaineda constantvelocity. Countswere generallyconducted during rising neap tides, with at leasta 4.5-ft tideheight (MLLW) when the boatsarrived at the shallowWalker Creek Delta/Tom'sPoint area, abouttwo hoursafter starting each count. We began the countsbetween 0930 and 1000 to avoidearly morning fog and rough afternoonseas, and each count was completed in about3.5 hours.Conditions duringcounts were generally calm (Beaufort scale 0-2). In areasthat could not be reachedby boats,we conductedsupplementary counts immediately prior to the boatcounts, using telescopes from shore(Figure 1). Duringeach count, we partitionedthe rawtransect data into four sections: (1) southof TomasiniPoint; (2) TomasiniPoint to PelicanPoint; (3) Pelican Pointto Tom'sPoint; (4) Tom'sPoint to SandPoint (Figure1). The sections correspondto intervalsdistinguished by estuarineconditions that might influencethe composition,distribution, or availabilityof foodfor waterbirds. Suchconditions include differences in waterturbidity and color(pers. obs.), eelgrassdistribution (Spratt 1989), substratetexture (Daetwyler 1966), water temperature, freshwater inflow, and ocean-water exchange (Hollibaughet al. 1988). Therefore,data from eachcount were partitioned into 12 subsets,delineated by threetransects each divided into four sections. Supplementalshore-based data were pooled with the appropriatesection and transect counts. We recorded all waterbirds in the shallows at the extremesouth end of the bay(south of MillertonPoint) as occurring in either east-shore or west-shore transects. We identifiedwaterbirds to specieswhenever possible. Undifferentiated loons,Horned/Eared grebes,Western/Clark's grebes,