Marine Birds in Nearshore Waters of Monterey Bay

Seabirds are the most visible fauna of the Monterey Bay National Marine Sanctuary and are often the only marine organisms that many visitors to the sanctuary encounter. Given the abundance and visibility of seabirds in Monterey Bay near shore, it is surprising that they have received relatively little scientific research attention. Monterey Bay is a hot spot for a considerable diversity of seabirds, especially during winter, the non-breeding season for most birds. Seabirds are especially abundant near shore, probably because nutrients from wave action and river input support a year-round food supply. I conducted at-sea surveys from 1999 through 2001 just outside the surf zone between Capitola and Monterey, to quantify the seasonal abundance of seabirds in the nearshore zone and to study several oceanographic factors that I thought might affect where each species was likely to occur.

Not surprisingly, overall density of seabirds within the nearshore zone (less than one kilometer from shore) was more than double the density reported for Monterey Bay as a whole. I recorded a mean density of more than 360 birds per square kilometer, whereas John Mason, a previous student at Moss Landing Marine Laboratories, found a density of 173 birds per square kilometer in a study area that included nearshore as well as offshore areas in Monterey Bay.

Figure 1. Mean distribution of Western/Clark’s Grebes in Monterey Bay by month. Circles are scaled to mean abundance per one-kilometer transect segment, shown from north (top; Capitola) to south (bottom; Monterey) on the y-axis.

More than 50 percent of all birds in my study were Western or Clark’s Grebes. These closely related grebe species nest during summer at freshwater lakes, primarily in northeastern California and the Great Basin. They migrate annually to coastal regions and winter in considerable numbers in Monterey Bay. In fact, as many as 10,000 grebes may winter locally, making Monterey Bay an area of regional significance for this species. Interestingly, Western and Clark’s Grebes do not completely vacate Monterey Bay during summer. Numbers peak in spring, indicating that Monterey Bay is used as a pre-migration staging area, and some grebes (probably young non-breeders) remain through summer (Figure 1).

Other abundant winter visitors included California Gull, Surf Scoter, and Marbled Murrelet, all species that breed primarily north of Monterey Bay. During summer and fall, abundant species included Brandt’s Cormorant, Western Gull (both local breeders), and Sooty Shearwater (visitors from New Zealand, during their non-breeding season). Also abundant during the fall were California Brown Pelican, Elegant Tern, and Heermann’s Gull, all species that breed primarily in Mexico and disperse north after breeding, presumably to take advantage of abundant northern anchovies in Monterey Bay.

Of the oceanographic factors investigated with respect to seabird distribution, the most interesting is water clarity. Water clarity is extremely variable in nearshore Monterey Bay – sediment input from the Pajaro and Salinas Rivers results in plumes of very turbid water during the winter rainy season. Other researchers have hypothesized that plunge-diving species (like terns and pelicans, which forage from the air) should prefer clearer water than pursuit divers (like cormorants and grebes, which swim underwater in pursuit of prey). Theoretically, plunge divers need to be able to see their prey below the surface, whereas pursuit divers may benefit from increased turbidity, which prevents the prey from seeing them coming. I used a transmissometer (an instrument that sends a beam of light through the water to measure light transmittance) mounted of the side of a 17-foot skiff to monitor water clarity and seabird distribution simultaneously.

Contrary to the hypothesis, plunge-diving species occurred more often in the most turbid water and less often in the clearest water available. Pursuit divers combined also occurred in more turbid water, but this pattern was not consistent among all pursuit-diving species: Brandt’s Cormorants occurred more often in the clearest water available. Why? In reality, shallow plunge divers probably only need to be able to see about one meter below the surface of the water, since they rarely dive deeper than that. Both shallow plunge divers and most pursuit divers may benefit from some turbidity, preventing prey fish from detecting the predators at a distance. But Brandt’s Cormorants, which forage on more sessile bottom fish, probably require greater water clarity to detect their prey – they are unlikely to encounter prey by diving blindly in murky water. A missing piece of the puzzle is prey abundance. It would be useful to know whether prey abundance is greater in turbid river plumes than in clearer water.

There is still much to learn about nearshore marine birds in the sanctuary. For example, the diet of Western and Clark’s Grebes in marine waters of California has never been studied. We also don’t know where many of the abundant seabirds in Monterey Bay nest – do the Western Grebes wintering here nest at Clear Lake in northern California, or at Great Salt Lake? Opportunities abound for additional research on seabirds in nearshore waters of Monterey Bay.

Laird Henkel
Moss Landing Marine Laboratories and H.T. Harvey& Associates