Influences of spatial variability in pelagic scattering layers on sperm whales behavior

Abstract

The central slope of the Gulf of Mexico (GOM) is home to over 20 species of marine mammals. Prior to the Deep Horizon Oil Spill in 2010, sperm whales were the predominately sighted large whale in the area. This study investigated the distribution and utilization of the north-central GOM by sperm whales two years after the spill by comparing sperm whale presence to micronekton distribution. Micronekton, organisms 2–20 cm, might be a key component in understanding sperm whale behavior as they are potentially an essential link in the food web between primary producers and higher trophic levels, including cephalopods—primary prey items of sperm whales. A Simrad EK60 echosounder operating at 38 kHz recorded the backscatter of micronekton throughout the northern GOM. The spatial distribution of micronekton backscatter was highly variable and may be driven by proximity to the Mississippi Delta. Sperm whales were sighted by trained visual observers and foraging was detected using a towed hydrophone array. The sperm whales were sighted most often in shallow slope waters less than 900 meters in areas with higher micronekton backscatter, suggesting that the waters above sloping bottoms with dense patches of mesopelagic micronekton support the prey of sperm whales.