Effects of fluctuations in sea-surface temperature on the occurrence of small cetaceans off Southern California

Abstract

Abstract-The link between ocean temperature and spatial and temporal distribution patterns of 8 species of small cetaceans off Southern California was examined during the period 1979-2009. Averages and anomalies of sea-surface temperatures (SSTs) were used as proxies for SST fluctuations on 3 temporal scales: seasonal, El Niño-Southern Oscillations (ENSO), and Pacific Decadal Oscillations (PDO). The hypothesis that cetacean species assemblages and habitat associations in southern California waters co-vary with these periodic changes in SST was tested by using generalized additive models. Seasonal SST averages were included as a predictor in the models for Dall's porpoise (Phocoenoides dalli), and common dolphins (Delphinus spp.), northern right whale dolphin (Lissodelphis borealis), and Risso's dolphin (Grampus griseus). The ENSO index was included as a predictor for northern right whale, long-beaked common (Delphinus capensis), and Risso's dolphins. The PDO index was selected as a predictor for Dall's porpoise and Pacific white-sided (Lagenorhynchus obliquidens), common, and bottlenose (Tursiops truncatus) dolphins. A metric of bathymetric depth was included in every model, and seafloor slope was included in 5 of the 9 models, an indication of a distinctive spatial distribution for each species that may represent niche or resource partitioning in a region where multiple species have overlapping ranges. Temporal changes in distribution are likely a response to changes in prey abundance or dispersion, and these patterns associated with SST variation may foreshadow future, more permanent shifts in distribution range that are due to global climate change.Cetaceans are higher-trophic-level marine predators whose movement patterns and habitat preferences are typically related to the distribution of their prey (Wishner et al., 1995;Gowans et al. 2007). Unlike baleen whales, small cetaceans (porpoises, dolphins, and small-toothed whales) generally do not undertake oceanscale annual migrations to track prey or to move between breeding and feeding grounds. Rather, small cetaceans may display a high degree of site fi delity, or they may move seasonally inshore and offshore or along regional-scale coastlines (Leatherwood et al., 1984;Dohl et al., 1986;Shane et al., 1986;Forney and Barlow, 1998).Although many small cetacean species may overlap in any one region of their total range, they often differ in their occurrence or habitatuse patterns, perhaps refl ecting competitive exclusion or niche partitioning. This separation of habitat and resources often occurs along depth, slope, and sea-surface temperature (SST) gradients (Reilly, 1990;Forney, 2000;Ballance et al., 2006;MacLeod et al., 2008). Habitat preferences likely refl ect differences in preferred prey. Dolphins may follow prey habitats as they shift not only seasonally but through large-scale climatedriven changes such as the El Niño-Southern Oscillation (ENSO) or the Pacific Decadal Oscillation (PDO) (Shane, 1995;Defran, 1999;Benson et al., 2002;Ballance et al., 200...

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

“…In recent years, interest has increased in the development of predictive models to forecast near realtime marine mammal distribution as a way to inform, mitigate, and decrease the effect of potentially harmful human activities in the marine environment Forney et al, 2012;Thompson et al, 2012;Henderson et al, 2014). Although our data set spans a 5-year period that ends in 2008, visual and acoustic data on detections of marine mammals continue to be collected with corresponding oceanographic data, both physical and biological, during CalCOFI cruises.…”

Estimation of population size and trends for highly mobile species with dynamic spatial distributions

“…Dall's porpoise are probably opportunistic foragers, but feed primarily on small schooling fishes, squids and lanternfish (Jefferson, ). Previous studies have found evidence for interannual and seasonal shifts in distribution both north–south and inshore/offshore following the distribution of cool surface waters (e.g., Becker et al., ; Forney & Barlow, ; Henderson et al., ; Tynan et al., ), although the scale of individual movement patterns is not well known (Jefferson, ).…”

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.…”

“…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).…”

“…Systematic cetacean monitoring was initiated in July 2004 (Soldevilla et al, 2006), providing seasonal line-transect density and abundance estimates within the CalCOFI study area Campbell et al, 2015). These estimates do not explicitly consider variability in ocean conditions, which can affect the distribution and density of cetaceans in this region (Henderson et al, 2014). Further, these studies provide uniform cetacean density estimates for broad regions (i.e., minimum area = 71,407 km 2 ) with no information on spatial patterns; finer-scale estimates of species density are needed to better assess potential impacts of marine activities that may adversely affect cetaceans and to increase our ecological understanding of species distribution patterns.…”