Analyses/Maps
Birds
XANTUS'S AND CRAVERI'S MURRELET
ABOUT THESE MAPS
Maps a, b and c show the density (birds/km2) of Xantus's and Craveri's murrelets (combined) in the Upwelling, Oceanic, and Davidson Current seasons, displayed in five minute latitude x 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 none of these murrelets 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.
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 (2001), California Department of Fish and Game, Office of Spill Prevention and Response (CDFG-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.
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 x five minute longitude cells using CDAS, a custom geographic information system for analyzing marine bird and mammal surveys (MMS-CDAS 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
Xantus's murrelet, a California state-listed threatened species (and likely a few Craveri's murrelets, in general perhaps occurring farther at sea) frequents waters overlying the continental slope and deeper depths. It is seen most frequently in the study region during the Upwelling (post-breeding) and to a lesser degree the early Davidson Current (pre-breeding) seasons. This occurrence pattern represents post-breeding and pre-breeding movements of this species. Xantus's murrelet nests on islands in the Gulf of California and the Channel Islands during the late Davidson Current Season. Craveri's murrelet breeds in the same general region but on average much farther south (Gulf of California).
These seabirds do not appear to be well detected by aerial surveys, which would explain their concentration off the Gulf of the Farallones where ship-board surveys have been more frequent. Even unidentified murrelets appear to be sparse in suitable habitat lying to the north and south (where aerial surveys have been conducted).
No information is available on their diet in the study area, but likely they eat small fish and euphausiids. See Tables 15 and 16 for related summary information.