Distribution and Abundance of Marine Birds in Nearshore Waters of Monterey Bay, California
- Laird Henkel
Moss Landing Marine Laboratories, California State University
End Date: March 20, 2001
Monterey Bay, California is a site of regional significance for marine birds, particularly during winter. I studied the seasonal abundance and factors affecting the distribution of nearshore (<1 km from shore) marine birds in Monterey Bay during 1999 and 2000. Total abundance was greatest during migration periods in spring and fall, and total diversity was greatest during winter.
Factors that I investigated with respect to the distribution of marine birds included sea-surface temperature, gradients in sea-surface temperature (fronts), and water clarity. I tested for effects of these factors at several spatial scales, down to 100 m. Water temperature alone did not affect the distribution of marine birds, but plunge-diving species were found in areas with greater sea-surface temperature gradients, indicative of fine-scale fronts.
Summary to DateSpecies assemblages were fairly consistent between the same season in different years. Mean density of all species (363 birds per square kilometer) was considerably greater than density reported for Monterey Bay as a whole, indicating that the nearshore environment should receive unique consideration in abundance and distribution studies of marine birds.
Several species were found at greater densities in different specific ranges of water clarity. Contrary to my hypothesis, plunge-diving species were found more frequently than expected by chance in turbid (less clear) water.
- Total marine bird abundance was greatest during migration periods in spring and fall.
- Total marine bird diversity was greatest during winter.
- Mean density of all species (363 birds km-2) was considerably greater than density reported for Monterey Bay as a whole.
- Water temperature alone did not affect the distribution of marine birds.
- Habitat association
Study MethodsI conducted 34 at-sea surveys for marine birds between 11 February 1999 and 19 March 2001. Transects were located parallel to shore, between 400 m and 800 m offshore (distance to shore varied as a result of surf conditions), and extended from Capitola (Santa Cruz Co.) south to Monterey (Monterey Co.; Fig. 1). This distance was chosen for sampling based on opportunistic observations indicating that this zone would contain the greatest densities of marine birds. The northern (Capitola to Moss Landing) and southern (Moss Landing to Monterey) sections of Monterey Bay were sampled in two consecutive days. Combined length of north and south bay surveys was approximately 48 km. Surveys were conducted parallel to shore to avoid the confounding factors of water depth and distance from shore. Approximately three weeks were allowed between bay-wide transects to avoid temporal autocorrelation.
Surveys were conducted from a 17-foot (5.2 m) open motorboat, traveling consistently at 15 km hr-1 (8 knots). Observers divided the field of view from the centerline of the boat to perpendicular on either side, each recording birds within 50 m of the vessel (for a 100 m strip), and the exact time (hour, minute and second) of each sighting. All birds on the surface of the water were recorded, but flying birds were recorded only if they were plunge-divers (App. A). Surveys were conducted only in conditions of Beaufort 3 or less. Location and speed were determined using a hand-held Magellan Global Positioning System (GPS). Sea-surface temperature was measured every 5 s approximately 0.5 m below the surface, using an Onset Tidbit XT temperature logger.
Mean sea-surface temperature (SST) was calculated for each survey, and mean monthly SST was calculated as the mean of all survey means during that month. I calculated a mean monthly upwelling index, from daily upwelling index values measured at 36° N, 122° W, southwest of Monterey (Pacific Fisheries Environmental Lab 2001). Means were calculated by month for 1999 and 2000 separately, and grouping both years.
I computed mean density of each seabird taxon during the course of the study, by survey, by month, and by season. I chose four seasons that corresponded to approximate oceanographic seasons: spring, or early upwelling (March, April, and May); summer, or late upwelling (June, July, and August); fall (September, October and November); and winter (December, January, March). These seasons also corresponded to the biological life cycles of many seabird species, which breed in summer and migrate to and from wintering sites in fall and spring. Seasonal seabird abundance data were compared graphically, and using the Percentage Similarity index, in which Percentage Similarity was the sum of all the minimums of either 1) percentage of a given species (out of the total) in sample 1, or 2) the percentage of that species in sample 2.
At-sea surveys from a 17' boat (100 m strip transect) between Capitola and Monterey, 500 m offhsore. 34 surveys between Feb. 99 and March 01. More info available upon request.