Mesoscale eddies stimulate biological production in the ocean, but knowledge of energy transfers to higher trophic levels within eddies remains fragmented and not quantified. Increasing the knowledge base is constrained by the inability of traditional sampling methods to adequately sample biological processes at the spatio-temporal scales at which they occur. By combining satellite and acoustic observations over spatial scales of 10 s of km horizontally and 100 s of m vertically, supported by hydrographical and biological sampling we show that anticyclonic eddies shape distribution and density of marine life from the surface to bathyal depths. Fish feed along density structures of eddies, demonstrating that eddies catalyze energy transfer across trophic levels. Eddies create attractive pelagic habitats, analogous to oases in the desert, for higher trophic level aquatic organisms through enhanced 3-D motion that accumulates and redistributes biomass, contributing to overall bioproduction in the ocean. Integrating multidisciplinary observation methodologies promoted a new understanding of biophysical interaction in mesoscale eddies. Our findings emphasize the impact of eddies on the patchiness of biomass in the sea and demonstrate that they provide rich feeding habitat for higher trophic marine life.

Fish-eating “resident”-type killer whales (Orcinus orca) that frequent the coastal waters off northeastern Vancouver Island, Canada have a strong preference for chinook salmon (Oncorhynchus tshawytscha). The whales in this region often forage along steep cliffs that extend into the water, echolocating their prey. Echolocation signals of resident killer whales were measured with a four-hydrophone symmetrical star array and the signals were simultaneously digitized at a sample rate of 500 kHz using a lunch-box PC. A portable VCR recorded the images from an underwater camera located adjacent to the array center. Only signals emanating from close to the beam axis (1185 total) were chosen for a detailed analysis. Killer whales project very broadband echolocation signals (Q equal 0.9 to 1.4) that tend to have bimodal frequency structure. Ninety-seven percent of the signals had center frequencies between 45 and 80 kHz with bandwidths between 35 and 50 kHz. The peak-to-peak source level of the echolocation signals decreased as a function of the one-way transmission loss to the array. Source levels varied between 195 and $\text{224\hspace{0.05em}}\mathrm{dB}\text{\hspace{0.05em}re:1\hspace{0.05em}μ}\mathrm{Pa}.$ Using a model of target strength for chinook salmon, the echo levels from the echolocation signals are estimated for different horizontal ranges between a whale and a salmon. At a horizontal range of 100 m, the echo level should exceed an Orcinus hearing threshold at 50 kHz by over 29 dB and should be greater than sea state 4 noise by at least 9 dB. In moderately heavy rain conditions, the detection range will be reduced substantially and the echo level at a horizontal range of 40 m would be close to the level of the rain noise.

Urmy, S. S., Horne, J. K., and Barbee, D. H. 2012. Measuring the vertical distributional variability of pelagic fauna in Monterey Bay. – ICES Journal of Marine Science, 69: 184–196. Temporal variability is an important feature of aquatic ecosystems that can be difficult to measure. A stationary, upward-facing scientific echosounder was used to record the vertical distribution of pelagic fauna in Monterey Bay, CA, for 18 months. To characterize the distributions, a suite of metrics, including measures of density, abundance, location, dispersion, occupancy, evenness, and aggregation, was developed and tested. An algorithm to detect and count the number of acoustic backscatter layers was developed using image-analysis techniques. The metrics recorded a strong seasonal cycle, with total backscatter reaching a minimum during the spring upwelling season and peaking in autumn and winter. Variability in the vertical distribution of animals was greatest at long time-scales and decreased as a power (−1.050 to −1.585) of signal frequency. There were significant peaks in the power spectrum at 12- and 24-h periods, corresponding to the semi-diurnal tide and diel vertical migration. The diel signal was strongest in late winter and weakest during the spring upwelling season. Active acoustics are a useful addition to ocean observatories, and the metrics presented provide a useful set of tools to quantify the distribution and temporal variability of pelagic fauna.