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Deep Mapping Teuthivorous Whales and Their Prey Fields
In the last decade, great progress has been made in understanding the behavior and biology of many deep diving marine mammals. Much of this progress has resulted from the development of new technologies such as tagging that were supported, in large part, by various components of the Department of Defense (DoD), including SERDP. Although studies of the physical habitat of these deep-diving predators have adequately kept pace with these advancements, understanding of the available prey, a key component in the biological habitat of these animals, has not. This lag has been driven by the difficulties in studying squid (cephalopods of the order Teuthida), the primary prey of particularly deep-diving toothed whales such as sperm (Physeter macrocephalus) and beaked whales (Family: Ziphiidae), due to the rapid speed, relatively large size, and depth of these prey animals. Recent advances in active acoustic measurements now allow us to use this powerful remote sensing tool to assess squid behavior and distribution in water depths up to 600 meters. Teuthivores or cephalopod feeding whales, however, including sperm and beaked whales typically feed at depths of 1000 meters. The objective of this work was to develop a new platform to carry the acoustic instruments needed to assess squid and utilize this new tool to understand the foraging ecology of deep-diving teuthivores.
Dual-frequency (38 and 120 kHz) split-beam echosounders were integrated into a REMUS 600 autonomous underwater vehicle (AUV), effectively doubling the range of quantitative, multi-frequency acoustic data into the mesopelagic zone (600 to 1200 meters). Data from the first set of missions in a range of conditions revealed that the AUV provided a stable platform for the echosounders and improved vertical and horizontal positional accuracy over echosounders towed by ships. In comparison to hull-mounted echosounders, elimination of ship noise and surface bubbles provided a 17 and 19 dBW decrease in the noise floor for the 38 and 120 kHz echosounders, respectively, increasing the sampling range by 30 to 40%. The extended depth range increased the horizontal resolution from 37 to 40 meters to 0.6 to 3.7 meters, enabling discrimination of individual targets at depth. The project also developed novel, onboard echosounder data processing and autonomy to enable sampling not feasible in a surface ship or towed configuration. The project demonstrated the effectiveness of a new tool for examining the biology of animals in the mesopelagic zone (600 to 1200 meters) in ways previously only possible in the upper ocean by making measurements of prey abundance, size, and distribution within known foraging habitat areas for two deep-diving marine mammal species, Cuvier’s beaked whales (Ziphius cavirostris) and Risso’s dolphins (Grampus griseus).
Study efforts targeted habitat used differentially by deep-diving, air-breathing predators to empirically sample their prey’s distributions on and off a United States (U.S.) Navy testing range, the Southern California Anti-Submarine Warfare Range (SOAR), west of San Clemente Island. Fine-scale measurements of the spatial variability of potential prey animals were made from echosounders aboard both the ship and AUV. Significant spatial variability in the size, composition, total biomass, and spatial organization of biota was evident over all spatial scales examined and was consistent with the general distribution patterns of foraging Cuvier’s beaked whales observed in separate studies. Striking differences found in prey characteristics between regions at depth, however, did not reflect differences observed in surface layers. These differences in deep pelagic structure horizontally and relative to surface structure, absent clear physical differences, change long-held views of this habitat as uniform. The revelation that animals deep in the water column are so spatially heterogeneous at scales from 10 meters to 50 km critically affects our understanding of the processes driving predator-prey interactions, energy transfer, biogeochemical cycling, and other ecological processes in the deep sea and the connections between the productive surface mixed layer and the deep water column.
The significant differences in deep water squid that were observed in neighboring pelagic areas were consistent with the general distribution of foraging beaked whales in these areas. The study combined measurements with published information to estimate the consequences of the environment on beaked whale foraging and found that beaked whales would have a difficult time meeting their energetic needs in areas outside of SOAR, providing information for mitigation efforts. The heterogenous nature of squid in the preferred habitat of beaked whales is a key feature that appears to lead to the success of these predators, likely because of the steep costs they face to access food and limited foraging time. This highlights the relevant prey metrics that must be considered to understand the ecology of deep-diving predators and the scales at which researchers must approach these important questions.
The project explored the behavior of Risso’s dolphins foraging in somewhat shallower scattering layers off Santa Catalina, California using a similar approach. Three distinct prey layers were identified: a persistent layer around 425 meters, a vertically migrating layer around 300 meters, and a layer intermittently present near 50 meters, all of which were used by individual animals tagged as part of a companion project funded by the U.S. Navy. Active acoustic measurements demonstrated that Risso’s dolphins dove to discrete prey layers throughout the day and night with only slightly higher detection rates at night. Dolphins were detected in all three layers during the day with over half of detections in the middle layer, 20% of detections in the deepest layer, and 10% falling outside the main layers. Dolphins were found less frequently in areas where the shallow, intermittent layer was absent, suggesting that this layer, though containing the smallest prey and the lowest densities of squid, was an important component of their foraging strategy. The deepest layer was targeted equally both during the day and at night. Using acoustic data collected from the AUV, layers were found to be made up of distinct, small patches of animals of similar size and taxonomy adjacent to contrasting patches. Squid made up over 70% of the patches in which dolphins were found and more than 95% of those in deep water. Squid targeted by dolphins in deep water also were relatively large, indicating significant benefit from these relatively rare, physically demanding dives. Within these patches, prey formed tighter aggregations when Risso’s dolphins were present.
Application of the new echosounder AUV developed by this project resulted in great progress in understanding cetacean behavior and habitat use and improvements in predictive capabilities of these factors. Careful integration of a suite of traditional and novel tools is providing insight into the ecology and dynamics of predator and prey in the meso- and bathy-pelagic, including in geographical areas in southern California commonly used for sonar by the US Navy where significant DoD resources have been expended on biological and behavioral studies of potential effect. This advancement in providing direct measurements of the ecological context and drivers of foraging behavior and distribution is essential for effective estimation and mitigation of noise effects, including those from military sonar systems, on these deep-diving marine mammals.