The Navy and other users of the marine environment need the ability to estimate the density of marine mammals within their operational areas to conduct environmental assessments and to comply with environmental regulations. Using traditional methods of analysis (e.g., line-transect modeling), quantitative estimates of marine mammal density are available only for broad regions of the world's oceans. Recently, new methods of analysis have been developed that enable finer scale estimates of density from the same type of survey data. This new approach, called geo-spatial habitat modeling, uses additional information about the relationship between marine mammals and their habitat to estimate marine mammal density on a geographic map.
As a result, the objective of this project was to improve the ability to estimate cetacean abundance in certain geographic areas to better guide the location of Navy activities using habitat modeling. In a fundamental way, habitat modeling enables cetacean density to be interpolated between transect lines and between the relatively rare sightings of each species.
The approach for this project consisted of four tasks: (1) further development of analytical tools for spatial modeling from line-transect data, (2) development of spatially explicit models based on the collection of cetacean survey and environmental data from 1986 to 2002, (3) validation of these models using new survey and environmental data collected along the U.S. west coast and in the eastern tropical Pacific, and (4) evaluation of the predictive power of these models across seasons using aerial survey data collected in the California Current region in winter and spring. Additional environmental variables were added that for some cetaceans better represented their habitat by analysis of existing net-tow samples and acoustic backscatter data.
In particular, cetacean survey data were used to build spatially explicit models that could predict cetacean density in the eastern tropical Pacific based on geographically fixed factors and environmental variables. Generalized Additive Models (GAMs) were used to define habitat relationships for cetaceans seen on past surveys in the eastern Pacific. A software interface was written for the spatially explicit models and made accessible to the Navy.
Data were used from ship-based cetacean and ecosystem assessment surveys to develop habitat models to predict density for 15 cetacean species in the Eastern Tropical Pacific (ETP) and for 12 cetacean species in the California Current Ecosystem (CCE). All data were collected from 1986-2006 surveys and included over 17,000 sightings of cetacean groups on transects covering over 400,000 km. Three modeling approaches to predict cetacean densities from habitat variables were evaluated with final acceptance of the GAMs with nonparametric smoothing functions. Ordinary kriging was chosen as the preferred method of interpolating oceanographic measurements because it is widely used and because, qualitatively, it did not produce unrealistic “bull’s eyes” in the continuous grids. A combination of in situ and remotely sensed data gave the best predictive power for the modeling effort.
Model results were supplied to the Duke University developed (SERDP project RC-1390) software product, called the Spatial Decision Support System (SDSS). The SDSS enables a user to view the model outputs as color-coded maps of cetacean density as well as maps that depict the precision of the models (expressed as point-wise standard errors and log-normal 90% confidence intervals). Quantitative information can be obtained about cetacean densities, including the coefficients of variation. A specific operational area can be defined by (1) choosing one from a pull-down menu, (2) uploading a shape file defining that area, or (3) interactively choosing perimeter points. Density estimates for a user-selected area are produced along with estimates of their uncertainty. As with all models, limitations exist and include the issue that warm-season data in the CCE should not be used to predict cetacean densities for the cold season.
The software product and embedded habitat models will enable the Navy to make more accurate and defensible estimates of cetacean abundance within Navy ranges and operational areas. These estimates will improve the quality of environmental investments to enable more accurate appraisal of the potential impacts of military activities on cetaceans.