|A Cassin's Auklet runs across a calm sea surface to gain flight. This small seabird, abundant near the Farallon Archipelago, is helping PRBO map productivity "hot spots." © wwwEricPreston.com|
Southeast Farallon Island, where PRBO has been studying breeding seabirds for more than 30 years, is merely the most visible feature of an extraordinary marine realm. Ocean waters surrounding the Farallon Archipelago--which includes the Cordell Bank seamount northwest of the islands and Point Reyes to the northeast--are uncommonly productive. Here, seasonal upwelling of cold water infuses the sunlit surface layer with nutrients, resulting in massive blooms of plankton and diverse, abundant predator populations.
Within the California Current System (CCS), reaching from British Columbia to Baja California, the most vigorous upwelling is localized. Where the Point Reyes headland juts into the current's north-to-south flow, a dense plume of upwelled waters rises and spreads westward. Oceanographic features then form, such as fronts--large changes in temperature or salinity within small areas. Infrared satellite views confirm dramatic seasonal concentrations of chlorophyll within the plume, evidence of microscopic plantlife capturing energy for an entire marine ecosystem.
Just west of the Farallon Islands (and about 30 miles from the Golden Gate), the continental shelf drops off a submarine precipice, the so-called Farallon Escarpment, into a 6,000-foot abyss. This shelf break and the steep flanks of the Cordell seamount are near-vertical surfaces where upwelling also occurs, and plant and animal plankton concentrate.
Together, these features often draw consumer organisms across great oceanic expanses to feast in the feeding shadow of the Farallones--a "rain shadow" of food resources below the sea surface.
The salmon, seabirds, whales, dolphins, sea lions, seals, sharks, and sea turtles in the food web's upper echelons are primary reasons why three National Marine Sanctuaries have been designated offshore of central California: the Gulf of the Farallones in 1981, Cordell Bank in 1989, and Monterey Bay in 1992. While affording top marine predators of these areas protection from petrochemical exploration or drilling, and from hazardous waste dumping, sanctuary status leaves marine life vulnerable to the effects of other human activities. These range from commercial and recreational fishing to shipping in some of the busiest lanes along the West Coast.
Are there distinct ocean areas where marine predators of the CCS concentrate, in association with their prey, and where added protection might be warranted? To investigate this, PRBO has led collaborative planning (see Observer 127, Winter 2002) and has recently undertaken new research, reported here. Marine Protected Areas (MPAs) in pelagic--open-ocean--waters may be the next major conservation step needed to ensure the continued abundance of marine wildlife, including fish.
MPAs can range from no-take zones, completely off-limits to fishing (one at Georges Bank in the Atlantic has begun to resupply adjacent waters with groundfish, depleted by past overfishing), to Marine Parks and Reserves, which set forth site-specific conservation and management goals (see "Seven Potential Benefits"). Their aim is to balance human uses of ocean wildlife--fishes-- with the needs of other organisms in the marine ecosystem--ecologically dependent creatures such as seabirds. This represents an ethic at the heart of emerging ecosystem-based management strategies for the 21st century.
To assess the potential for marine protection based on scientific evidence, we are conducting novel at-sea research around the Farallon Archipelago. Typical of PRBO, we use birds to obtain information about ecosystem dynamics. Seabirds, along with a few other predators at the top of the food web, are numerous and conspicuous: they can help us identify ocean "hot spots," areas of elevated marine productivity where predators aggregate to feed on dense prey concentrations. Tiny non-swimming animal prey (zooplankton) collect in areas where water masses converge, or fronts. Often marked by foam lines at the surface, fronts are pockets of concentrated productivity. Swimming prey such as small fishes and squid also select these areas, where they find abundant plankton prey. Top predators, including highly mobile salmon, marine mammals, and seabirds, arrive at these well-defined locales to exploit the foraging opportunities.If we can demonstrate predictability, or recurrence between seasons and from year to year, in the locations of such strong food web interactions, our work will be of great value in any initiative to establish new Marine Protected Areas.
Sampling the Sea's Abundance
|PRBO biologist Ben Saenz (left) and project leader Jaime Jancke, PhD, collect oceanographic data at one of the set points on a transect line across the Gulf of the Farallones. Photo by Sophie Webb|
To understand how we gather the needed data, picture the following scenario. Five PRBO scientists board a research vessel at dawn on a May morning and embark on a three-day research cruise. Rather than plotting a simple course to the west or north, we travel seven parallel transects, from near shore into the deep waters beyond the Farallon Islands, crossing the shelf break and other important features such as oceanic fronts. One observer counts all the seabirds, assisted by another who's also responsible for the whales and dolphins. One technician monitors instruments that continually sound for ocean depth as we travel, tracing the profile of seafloor ridges and ravines, and echolocating masses of large and small animal life where it concentrates above the unseen seascape. At set points along the transect lines, we stop and lower sensors and nets into the water, reading the ocean's temperature, salinity, and chlorophyll content, and sampling the zooplankton found at various depths. Our data will show where numbers of seabirds occur in relation to all these other variables.
In five such expeditions to date (starting in May 2004), we have confirmed the key role of two Farallon breeding seabirds as indicators of important foraging areas. The Cassin's Auklet and Common Murre each comprised some 40% of the total number of birds (roughly 11,000) that we counted on four cruises in 2004. Brandt's Cormorant, Pigeon Guillemot, and Sooty Shearwater (the latter a Southern Hemisphere migrant to the CCS) were the other principal players.
Plankton-eating Cassin's Auklets occur in great localized concentrations, as shown by the large dots in the map graphic at left (click on caption for larger view). During strong upwelling events, we found that auklets consistently forage in water about 200 meters deep, fairly close to the Farallon Islands, Cordell Bank, and the shelf break. When the winds calm down and upwelling slacks off, they tend to move north to the deeper waters of the Cordell Bank and shelf break.
Fish-eating Common Murres, in contrast, forage in relatively shallow waters near the mainland, away from the Cordell Bank and shelf break, and where semi-stable layers of water have formed (warm and fresh above cold and salty). Stratified water often attracts zooplankton to the zone near the surface; then swimming prey of the size favored by murres may also cluster there, more accessible to diving birds.
These associations--some ephemeral, some predictable--are promising, and we aim to continue amassing the information needed to understand them as possible persistent hot spots. With four more research cruises scheduled for 2005 (including one under way in April, as this Observer goes to press), we also hope to extend funding for this project beyond this year.
A thorough knowledge of the foraging areas and oceanographic processes important for seabirds and other top marine predators will be invaluable to management agencies and conservation interests weighing possibilities for an MPA in the feeding shadow of the Farallones (see "Fundamental Questions").
Compared with the ease of mapping seabird breeding colonies and seal and sea lion rookeries--and protecting them within reserves like the Farallon Islands National Wildlife Refuge--charting the watery locales most important to wildlife is extremely challenging. But many unanswered questions may now be yielding to our best efforts to see the ocean's food supplies through the eyes of seabirds.