Abstract
Managing marine species effectively requires spatially and temporally explicit knowledge of their density and distribution. Habitat-based density models, a type of species distribution model (SDM) that uses habitat covariates to estimate species density and distribution patterns, are increasingly used for marine management and conservation because they provide a tool for assessing potential impacts (e.g., from fishery bycatch, ship strikes, anthropogenic sound) over a variety of spatial and temporal scales. The abundance and distribution of many pelagic species exhibit substantial seasonal variability, highlighting the importance of predicting density specific to the season of interest. This is particularly true in dynamic regions like the California Current, where significant seasonal shifts in cetacean distribution have been documented at coarse scales. Finer scale (10 km) habitat-based density models were previously developed for many cetacean species occurring in this region, but most models were limited to summer/fall. The objectives of our study were two-fold: 1) develop spatially-explicit density estimates for winter/spring to support management applications, and 2) compare model-predicted density and distribution patterns to previously developed summer/fall model results in the context of species ecology. We used a well-established Generalized Additive Modeling framework to develop cetacean SDMs based on 20 California Cooperative Oceanic Fisheries Investigations (CalCOFI) shipboard surveys conducted during winter and spring between 2005 and 2015. Models were fit for short-beaked common dolphin (Delphinus delphis delphis), Dall’s porpoise (Phocoenoides dalli), and humpback whale (Megaptera novaeangliae). Model performance was evaluated based on a variety of established metrics, including the percentage of explained deviance, ratios of observed to predicted density, and visual inspection of predicted and observed distributions. Final models were used to produce spatial grids of average species density and spatially-explicit measures of uncertainty. Results provide the first fine scale (10 km) density predictions for these species during the cool seasons and reveal distribution patterns that are markedly different from summer/fall, thus providing novel insights into species ecology and quantitative data for the seasonal assessment of potential anthropogenic impacts.
Highlights
Habitat-based density models or species distribution models (SDMs) are increasingly used for marine management and conservation applications, including the assessment of potential impacts from a wide range of anthropogenic activities (Louzao et al, 2006; Benson et al, 2011; Gilles et al, 2011; Goetz et al, 2012; Hammond et al, 2013; Redfern et al, 2013)
A total of 21,206 km of on-effort data from 20 California Cooperative Oceanic Fisheries Investigations (CalCOFI) surveys conducted during January to April in 2005–2015 were used to develop the winter/spring habitat-based density models
The number of sightings varied between years, for humpback whales that had as many as 21 sightings in 2013 and no sightings during the winter/spring surveys in 2007, 2009, and 2012 (Table 1)
Summary
Habitat-based density models or species distribution models (SDMs) are increasingly used for marine management and conservation applications, including the assessment of potential impacts from a wide range of anthropogenic activities (Louzao et al, 2006; Benson et al, 2011; Gilles et al, 2011; Goetz et al, 2012; Hammond et al, 2013; Redfern et al, 2013). SDMs are effective conservation management tools because they can be used to predict spatial and temporal changes in species distribution patterns. Gilles et al (2016) used SDMs to capture the seasonal distribution shifts of harbor porpoise (Phocoena phocoena) in the North Sea to assist in international marine spatial planning efforts. Hobday et al (2010) described a temporally flexible spatial approach to managing the bycatch of southern bluefin tuna (Thunnus maccoyii) off Australia using predictions from a habitat model. Such models are important in the marine environment because variability in ocean conditions can result in changes in species distribution. This is especially true in the California Current Ecosystem, where high interannual and seasonal variability in oceanic conditions result in marked shifts in cetacean distribution (Dohl et al, 1978; Forney and Barlow, 1998; Forney, 2000; Becker et al, 2012, 2014; Douglas et al, 2014; Henderson et al, 2014; Campbell et al, 2015)
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