Abstract
The 2018 California Current Ecosystem Survey (CCES) was conducted between 26 June and 4 December 2018 as a joint project of the Marine Mammal and Turtle Division (MMTD) and the Fisheries Resources Division (FRD) of NOAA’s Southwest Fisheries Science Center (SWFSC). One of the primary objectives of this line-transect survey was to collect marine mammal sighting data to support the derivation of cetacean density estimates for the California Current Ecosystem (CCE) study area. Given the heterogeneity of the 2018 survey coverage in the CCE study area (Henry et al. 2020), density estimation required model-based (rather than design-based) analytical approaches for updating population size estimates for US West Coast marine mammal stocks. This report summarizes the results of the cetacean habitat modeling effort.
Habitat models, or species distribution models (SDMs), have been recognized as valuable tools for estimating the density and distribution of cetaceans and assessing potential impacts from a wide range of anthropogenic activities (e.g., Abrahms et al. 2019; Gilles et al. 2011; Goetz et al. 2012; Hammond et al. 2013; Redfern et al. 2013). SDMs for cetaceans have been developed for US West Coast waters from systematic ship survey data collected by SWFSC since 1991 (Barlow et al. 2009; Becker et al. 2010, 2014, 2016, 2018, 2020; Forney 2000; Forney et al. 2012). The most recent models provide spatially-explicit density predictions at a 0.1˚ (approximately 10km x 10km) grid resolution (Becker et al. 2020), and multi-year average density surfaces have been used by the US Navy to assess potential impacts on cetaceans as required by US regulations such as the Marine Mammal Protection Act and Endangered Species Act (U.S. Department of the Navy 2013, 2015, 2017).
The overall goal of this study was to include the 2018 survey data in the previous 1991–2014 modeling dataset in order to improve SMDs for the CCE study area. Specific objectives included:
- Generating multi-year average density surfaces for the Navy and others to use in their long-term (2–7 year) environmental planning efforts; and
- Providing updated abundance and “minimum population size (Nmin)” estimates as defined in the Guidelines for Assessing Marine Mammal Stocks (National Oceanic and Atmospheric Administration 2016).
To develop improved SDMs and to update US West Coast cetacean stock abundance estimates, sighting data from CCES 2018 were combined with previous line-transect survey data collected within the CCE to create a robust modeling database spanning more than 25 years (1991–2018). Habitat models were developed based on previously established methods that allow for the incorporation of segment-specific estimates of detection probability and included dynamic covariates from an ocean model calibrated to the CCE study area (Becker et al. 2016). In addition, recently-developed techniques for deriving more comprehensive estimates of uncertainty in SDM predictions (Miller et al. In Prep.) were used to provide variance estimates for the model-based abundance estimates. SDMs were developed for long-beaked common dolphin (Delphinus delphis bairdii), short-beaked common dolphin (Delphinus delphis delphis), Risso’s dolphin (Grampus griseus), Pacific white-sided dolphin (Lagenorhynchus obliquidens), northern right whale dolphin (Lissodelphis borealis), striped dolphin (Stenella coeruleoalba), common bottlenose dolphin (Tursiops truncatus), Dall’s porpoise (Phocoenoides dalli), sperm whale (Physeter macrocephalus), blue whale (Balaenoptera musculus), fin whale (B. physalus), humpback whale (Megaptera novaeangliae), Baird’s beaked whale (Berardius bairdii), and a “small beaked whale guild” that included Mesoplodonts (Mesoplodon spp.) and Cuvier’s beaked whale (Ziphius cavirostris). Sample sizes were also sufficient to develop the first model-based density estimates for minke whale (B. acutorostrata) in this study area.
The habitat-based models of cetacean density developed in this study represent an improvement over the previous models described by Becker et al. (2020) because they included additional The habitat-based models of cetacean density developed in this study represent an improvement over the previous models described by Becker et al. (2020) because they included additional sighting data over the continental shelf and slope that were surveyed more sparsely in previous years, providing better representation of these important habitat regions. In addition, the model- based abundance estimates more accurately account for uncertainty than prior iterations owing to methodological improvements.