Cover • Figures & Tables • Introduction • Materials & Methods
Discussion • Results • Acknowledgements • Literature Cited
Introduction
|
A fundamental goal in ecology is to understand the factors that determine the distribution and abundance of organisms. While a vast literature exists relating factors important in determining the distribution and movement patterns of terrestrial and near-shore predators to spatial and temporal patterns in the distribution of their food resources, similar studies for pelagic marine species are rare. This largely stems from the expense and difficulty of simultaneously measuring the distribution and abundance of predators, prey, and related environmental factors in the open ocean over the larger temporal and spatial scales they forage. As a result, most studies correlate distribution and abundance patterns of oceanic predators to direct or indirect indices of prey abundance and environmental parameters over small spatial scales and short time periods. This is particularly true for studies of the foraging ecology of large predators. The blue whale (Balaenoptera musculus) is the largest organism that has ever existed. This combined with its mammalian metabolic rate, leads to the highest average daily total energy requirements of any species. Blue whales feed almost exclusively on euphausiids, at rates of up to 2 metric tons per day (Rice 1978). They satisfy this high energy demand by feeding in patchily distributed seasonal but ephemeral aggregations where euphausiids are found at high densities (Tomilin 1967, Yochem and Leatherwood 1985, Schoenherr 1991, Tershy 1992, Croll et al. 1998, Fiedler et al. 1998). Thus, blue whale foraging appears to only occur in regions of exceptionally high productivity. Traditionally, it has been speculated that blue whale distribution and movement patterns consist of a seasonal migration from high latitudes where foraging takes place to low latitudes where they mate and give birth (e.g. Mackintosh 1965; Lockyer 1981). However, data from the Pacific indicate that feeding also takes place at low latitude, "upwelling-modified" waters (Reilly and Thayer 1990, Fiedler et al. 1998, Croll et al. 1998), and data from both the Pacific and Indian Oceans indicate that some individuals remain at low latitudes year-round (Yochem and Leatherwood 1985). Detailed information on the movement patterns of blue whales off California is not available, but they are believed to migrate annually between foraging areas in Central/Southern California (May-September), the west coast of Baja California (September-December) and the Gulf of California (January-April) (Calambokidis 1995). A number of studies have found that blue whales off California seasonally forage upon dense euphausiid schools in highly productive coastal upwelling regions (Schoenherr 1991, Croll et al. 1998, Forney and Barlow 1998, Fiedler et al. 1998). Croll et al. (1998) proposed that: 1) the distribution of blue whales in the coastal California current region is defined by their attraction to areas of predictably high prey density; 2) the preferred prey of these whales are several species of euphausiids (Euphausia pacifica , Thysanoessa spinifera, and Nyctiphanes simplex) that are abundant in the California Current region; 3) blue whale foraging efforts are concentrated on dense aggregations found at discrete depths in the water column; 4) these localized areas of high euphausiid densities are predictable and sustained by upwelling regions of seasonally high primary production; 5) topographic breaks in the continental shelf located downstream from these regions work in concert with euphausiid behavior to collect and maintain large concentrations of euphausiid swarms; and 6) despite seasonal and inter-annual variability, these processes are sufficiently consistent that the distribution of Balaenoptera whales can be predicted. In this study, we test these predictions for California blue whales feeding in a well-defined foraging area (Monterey Bay, California) over several years. Since at least 1986 blue whales have been reported seasonally foraging in Monterey Bay on dense aggregations of euphausiids (T. spinirfera, E. pacifica) (Schoenherr 1991, Calimbokidis 1995). Due to their high prey demands, we hypothesized that blue whales in Monterey Bay forage on dense aggregations of euphausiids that result from the processes we proposed above. To test this hypothesis, we used a combination of concurrent ship- and mooring-based oceanographic sampling, hydroacoustic sampling, net sampling, opportunistic cetacean sighting records, visual surveys, and time-depth recorder deployments to describe the foraging ecology of whales within the seasonal upwelling context of Monterey Bay between 1992-1996. Specifically, we: 1) define prey patches and whale foraging behavior within prey patches, 2) determine spatial and temporal patterns in the distribution and abundance of whale prey patches, and 3) examine physical and biological factors important in creating whale foraging patches. |
 |
 |
Home | Introduction | Visitors | Education | Research | Protection | Calendar | Foundation | Search
Credits
For comments or question please refer to the Webmaster
Last modified on: Jan 15,
2000