Association of low oxygen waters with the depths of acoustic scattering layers in the Gulf of California and implications for the success of Humboldt squid (Dosidicus gigas)

Abstract

The ecology in the Gulf of California has undergone dramatic changes over the past century as Humboldt squid (Dosidicus gigas) have become a dominant predator in the region. The vertical overlap between acoustic scattering layers, which consist of small pelagic organisms that make up the bulk of D. gigas prey, and regions of severe hypoxia have led to a hypothesis linking the shoaling of oxygen minimum zones over the past few decades to compression of acoustic scattering layers, which in turn would promote the success of D. gigas. We tested this hypothesis by looking for links between specific oxygen values and acoustic scattering layer boundaries. We applied an automatic layer detection algorithm to shipboard echosounder data from four cruises in the Gulf of California. We then used CTD data and a combination of logistic modeling, contingency tables, and linear correlations with parameter isolines to determine which parameters had the largest effects on scattering layer boundaries. Although results were inconsistent, we found scattering layer depths to be largely independent of the oxygen content in the water column, and the recent success of D. gigas in the Gulf of California is therefore not likely to be attributable to the effects of shoaling oxygen minimum zones on acoustic scattering layers.