Seasonal distribution and abundance of cetaceans off Southern California estimated from CalCOFI cruise data from 2004 to 2008

Abstract

Abstract-Documenting year-round diversity and distribution of marine mammals off Southern California is important for assessment of effects of potentially harmful anthropogenic activities. Although the waters off Southern California have been surveyed extensively for marine mammals over the past 18 years, such surveys have been periodic and were conducted primarily from summer to fall, thereby missing potential seasonal shifts. We examined seasonal abundance and population density of cetaceans off Southern California from 16 shipboard line-transect surveys conducted quarterly during 2004-08. The study area consisted of 238,494 km 2 of coastal, shelf, and pelagic oceanic habitat from nearshore waters to 700 km offshore.

Habitat-Based Density Models for Three Cetacean Species off Southern California Illustrate Pronounced Seasonal Differences

“…The overall study area abundance estimates are similar to those derived previously using line-transect analyses (Campbell et al, 2015), but the habitat models provide fine-scale detail in spatial density patterns that are more useful for conservation and management applications. Given the increased use of SDMs to help evaluate and reduce potential risks to cetaceans (e.g., Redfern et al, 2013; U.S. Department of the Navy, 2015), and the documented high degree of seasonal variability in the distribution of cetaceans in the study area (Dohl et al, 1986;Green et al, 1992Green et al, , 1993Forney and Barlow, 1998;Becker et al, 2014;Douglas et al, 2014;Henderson et al, 2014;Campbell et al, 2015), it is critical to develop seasonally-explicit SDMs. Campbell et al (2015) were able to provide separate linetransect abundance estimates for winter and spring seasons; however, in our study sighting data were not sufficient to develop separate habitat models for the two cool seasons.…”

“…Five different research vessels were used during the survey period, but Douglas et al (2014) found no significant difference in perpendicular distance to transect line for vessels with varying platform height. All surveys were conducted in passing mode (i.e., when a cetacean/cetacean group is sighted the ship continues on course and is not diverted to the vicinity of the sighting) with two dedicated observers searching for cetaceans FIGURE 1 | CalCOFI sampling stations (yellow circles) and the approximate 385,460 km 2 study area (black bold line) used to develop the winter/spring species distribution models presented in this study.…”

“…The study area was clipped at 125 • W longitude to reflect the lack of systematic survey effort west of this line in winter/spring. Also shown is the approximate 238,495 km 2 southern portion of the CalCOFI study area (hashed area encompassed by the dashed blue line) used by Douglas et al (2014) and Campbell et al (2015) to develop line-transect density estimates. The southern boundary roughly follows the Unites States/Mexico border and is consistent with that used by Becker et al (2016) to develop the summer/fall estimates used for comparison in this study.…”

“…Sighting, group size estimates, effort, and weather data were collected and recorded on paper forms and later entered into an electronic record. Detailed descriptions of the survey protocol can be found in Douglas et al (2014).…”

“…Such models are particularly 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, 2012Becker et al, , 2014Douglas et al, 2014;Henderson et al, 2014;Campbell et al, 2015).…”