Analyses/Maps

Birds

ASHY STORM-PETREL

ABOUT THESE MAPS
Maps a, b and c show the density (birds/km²) of ashy storm-petrel in the Upwelling, Oceanic, and Davidson Current seasons, displayed in five minute latitude by five minute longitude cells. Densities are based on the combined data sets of several studies (see “Methods” and “Data Sources” below). The color and mapping intervals were customized to show the most structure and to highlight significant areas, while allowing comparisons among marine bird species. Cells that were surveyed but in which no Ashy storm-petrels were observed have a density of zero. Areas not surveyed appear white; no information is available for these areas. Blue lines indicate the boundaries of the National Marine Sanctuaries in the study area: Cordell Bank, Gulf of the Farallones and Monterey Bay.

In order to provide one map for the species that integrates the patterns of its spatial and temporal occurrence and abundance in the study area, map d shows seasonal high-use areas, displayed in 10 minute latitude by 10 minute longitude cells, and breeding colonies. The seasonal high use map provides a further synthesis of densities presented in maps a, b and c, and portrays the relative importance of various areas to the species. Areas with consistently high use are highlighted on this map. To provide a relative reference for the “high use” areas, cells are also shown where the species were absent (i.e., the cell was sampled but the species was not recorded there) or present but at lesser concentrations in any particular season. See the Methods section below for further explanation of seasonal high-use areas. Breeding colonies are also shown; the relative size of the symbols indicates the colony size.

Because the sighting data for this species extends beyond the western extent of the standard map frame shown here, additional maps were made that include a greater western extent. These maps (with the word "pelagic" in the filename) are included on the CDROM.

DATA SOURCES
Densities for marine birds at sea are based on data from eight survey programs conducted between 1980 and 2001, which were combined into a new MMS-CDAS data set (MMS, 2001) using software (CDAS) developed for the Minerals Management Service. Of the data sets on the original MMS-CDAS CD-ROM, four aerial survey data sets contained data in the study area from Pt. Arena to Pt. Sal. Of these, the OSPR survey program is ongoing and data from recent years were added to this data set. In addition, data from four ship-based survey programs were converted to a compatible format for analysis (see section overview for details on individual data sets).

Data sources for aerial, at-sea data include MMS-CDAS (MMS, 2001), California Department of Fish and Game, Office of Spill Prevention and Response (CDF&G-OSPR, unpublished data). Early data were collected using methods described by Briggs et al. (1983, 1987b); more recent data were collected using updated technology but using the same general method. Data sources for ship-based survey data include David Ainley of H. T. Harvey and Associates and Carol Keiper of Oikonos, Inc., unpublished data (see Oedekoven et al., 2001 for details on methods); and Lisa T. Ballance, from the Ecology Program of the Southwest Fisheries Science Center, NMFS, NOAA, unpublished data. Data on Ashy storm-petrel colonies were obtained from Carter et al. (1992) supplemented by Sydeman et al. (1998), Whitworth et al. (2002).

Although the at-sea data span the years from 1980 to 2001, data are not available for all seasons in all years. For the Upwelling Season, data are from 1980-1982 and 1985-2001. For the Oceanic Season, data are from 1980-1982, 1991 and 1994-2001. For the Davidson Current Season, data are from 1980-1986 and 1991-2001.

METHODS
At-sea densities are the result of a synthesis of data from eight shipboard and aerial survey programs conducted in the study area in the years 1980-2001 (see “Data Sources” below). Bird observation data and trackline data from these studies were converted to a common format. All aerial data were continuous; ship-based data were converted separately into a continuous transect to the extent possible. From the digitized survey data, the distributions of effort and of species were mapped into five minute latitude by five minute longitude cells using CDAS, a custom geographic information system for analyzing marine bird and mammal surveys (MMS, 2001). The length and width of the survey trackline in a given cell (estimated trackline width varied by platform, depending on speed and height above water) were used to estimate the area sampled. The number of birds of each species seen in a cell was then divided by the area sampled in the cell to estimate density. If a cell was surveyed more than once, densities were averaged, with an adjustment made for effort.

The seasonal high-use areas on map d were developed using a similar approach as for Maps a, b and c, but the data were binned into 10’x10’ cells. For each season, the cells with densities in the top 20% of non-zero values were designated “high use” for that season. Cells were scored for “high use” in one, two, or three seasons and are depicted by color. To provide a relative reference for the “high use” areas, cells are also shown where the species were absent (i.e., the cell was sampled but the species was not recorded there) or present (but densities were never in the top 20% for any season).

RESULTS AND DISCUSSION
Ashy storm-petrel is endemic to the California Current and is considered by the State to be a “Species of Special Concern”; a major colony is at the Farallon Islands. It is common in the study area and the most abundant Storm-petrel in waters of the central California national marine sanctuaries. Surveys recorded 1,472 sightings of 4,339 individuals.

A multiple regression model of nine variables explained 17.3% of variation in cell density, with important explanatory variables for this species being ENSO period (i.e., periods of unusually warm or cold ocean temperature), season, and year; see Table 19. The species was more abundant during the Oceanic Season and during years of La Niña, indicating that when ocean temperatures were cold, Ashy storm-petrels were more concentrated closer to the Farallon breeding colony, which they visit only at night. During nesting (Upwelling Season), this species occupies waters mainly over the outer slope (mean depth of occurrence 1,615 ± 52 m), mostly outside of National Marine Sanctuary boundaries. During the period of molt (Oceanic Season), Ashy storm-petrels move inshore to frequent shallower slope waters (mean depth of occurrence 1,144 ± 61 m) and a large concentration occurred over the Monterey Bay canyon.

In recent years, however, this post-breeding concentration has shifted to the area around Cordell Bank (not shown on the maps). As the species begins its seasonal return to the Farallon nesting colony (Davidson Current Season), they shift to the north again to deeper waters of the outer slope (mean depth of occurrence then was 2,579 ± 121 m). The species seems to be most dispersed during the Davidson Current Season, but in all seasons the Farallon Escarpment is by far its most important area.

Overall, ashy storm-petrel numbers increased 1985-2002 in a curvilinear fashion: steeper increase in numbers between 1985 and 1992, followed by a less steep increase to 2002.

Ashy storm-petrel feeds on invertebrates and larval fish found at the surface. See Tables 15 and 16 for related summary information.