Density of Cetaceans and Turtles in the Gulf of Mexico

The goal of this research was to develop Gulf-wide cetacean and sea turtle spatial density models (SDMs) based on line-transect surveys conducted in the U.S. waters of the Gulf of Mexico. Surveys used to develop the SDMs for species occupying continental shelf and oceanic waters of the Gulf of Mexico were conducted during the Gulf of Mexico Marine Assessment Program for Protected Species (GoMMAPPS) project and comparable-prior-year surveys. Aerial survey data from seasonal surveys conducted during 2011/2012 and 2017/2018 (GoMMAPPS Surveys) were used to develop SDMs for cetacean and sea turtle species over the continental shelf. Data collected from vessel surveys, including the two-team surveys conducted during summer 2017, winter 2018, and summer/fall 2018 (GoMMAPPS Surveys) and 2003, 2004, and 2009 (that included only one survey team), were used to develop SDMs for cetaceans in oceanic waters. In addition, for Rice's whale, surveys conducted in 2018 and 2019 were also used in developing the SDMs specific for this species.

Habitat-based SDMs were developed using a generalized additive modeling (GAM) framework to determine the relationship between cetacean and sea turtle abundance and environmental variables. Samples for modeling were created by summarizing survey effort and environmental variables with a hexagon grid developed by the Environmental Protection Agency expanded to fit the entire Gulf of Mexico. The grid was created in a Lambert azimuthal equal area projection and the area of each hexagon is 40 km2. For all hexagons that contained survey effort segments, cetacean and sea turtle density was calculated using total number of animals observed, segment effort length and average sighting condition covariates in the hexagon, and the parameters estimated in distance sampling abundance models. A total of 19 SDMs were developed for individual or groups of species. For each modeled taxon, predictions were made for the period 2015-2019 on the hexagon grid, summarized into mean monthly densities for the 5-year period, and then resampled into traditional raster grids. Models were extrapolated beyond the U.S. waters of the Gulf of Mexico to provide insight into potential high density areas throughout the Gulf. However, extrapolations of this type should be interpreted with caution. This website contains the predictions as raster grids. The hexagon predictions are available as shapefiles in NCEI Accession 0256800. These models can be used by managers and federal agencies to assist with population assessments, environmental impact analyses, and conservation of these species.

As of October 2022, these are the latest available models for the Gulf of Mexico produced from NOAA Southeast Fisheries Science Center (SEFSC) surveys. The prior series of Gulf of Mexico models, produced by a collaboration between the Duke Marine Geospatial Ecology Lab (MGEL) and SEFSC and published as Roberts et al. 2016, are archived here. As of October 2022, SEFSC and MGEL consider the Roberts et al. 2016 models obsolete and recommend the GoMMAPPS models available here be used instead.