Tracklines and associated observations were mapped and analyzed using ArcMap (ESRI, Redlands, CA). GPS data were recorded in NAD27 map datum and projected to an USGS Albers Equal Area Conic map projection for presentation and subsequent density analyses. Concatenated GPS and observation data were then used to generate point and line coverages in ArcMap (ESRI, Redlands, CA). We designed a custom analytic tool using ArcMap Model Builder that allows for the construction and export of user-specified and effort-adjusted spatial binning of species observations along continuous trackines. For the purposes of this report, we calculated seabird density estimates and marine mammal counts along continuous 3.0-kilometer and 7.7-kilometer trackline segments (i.e., 3.0 kilometer and 7.7 kilometer bins). Therefore, marine bird densities (at 3-kilometer scale, for example) are based on a composite strip area ranging from 0.15 per kilometer squared (one observer on effort) to 0.30 per kilometer squared (two observers on effort). We made no effort to adjust densities such that they would be proportional to variations in the area of buffered transect strip bin (i.e., weighted offset variable). These data are associated with the following publication: Mason, J.W., McChesney, G.J., McIver, W.R., Carter, H.R., Takekawa, J.Y., Golightly, R.T., Ackerman, J.T., Orthmeyer, D.L., Perry, W.M., Yee, J.L. and Pierson, M.O. 2007. At-sea distribution and abundance of seabirds off southern California: a 20-Year comparison. Cooper Ornithological Society, Studies in Avian Biology Vol. 33. References- ESRI. ArcGIS Desktop: Release 10. Redlands, CA: Environmental Systems Research Institute.