MESA: Hydroacoustic Surveys
Hydroacoustics is used to survey speciesí distributional patterns and abundance as well as predator-prey relationships and prey availability for top predators, such as Steller sea lions, harbor seals, and whales. Hydroacoustic data produces echograms, pictures of the fauna in the water column. From the acoustic data, estimates of speciesí biomass can be calculated. Different frequencies are used to target different types of fauna: 38 kHz is typically used for herring because they produce strong backscatter due to their swim bladders; 200 kHz targets organisms that produce a weaker signal, such as plankton.
The example of the 38 kHz echogram shown in the top photo depicts dense schools of herring in red with pollock and other forage species closer to the substratum (blue/green). Trawl surveys are conducted in coordination with Hydroacoustics as a means of groundtruthing the echogram to the species captured in the trawl.
Hydroacoustic surveys have been used to track forage fish populations around marine mammal haulout and feeding areas. The map above depicts two Steller sea lion haulout areas near Benjamin Island and Brothers Island where we conducted surveys to assess prey availability. We also provide equipment and scientific support to other state and federal agencies to help study a variety of topics, such as prey availability for harbor seals in Glacier bay, Alaska.
Hydroacoustics can be applied to studying animal behavior, as species (such as herring) can be identified and tracked over space and time. The echograms below show seasonal patterns of herring schooling behavior: herring gather in large dense schools associated with the substratum in fall, spread out to a thinner layer in early winter, and then form a dense consistent layer in close proximity to the substratum for winter. From acoustic studies, we have found that herring are quite active in small schools during fall, but associate with deep trenches and do not move for 2-3 months during winter.
Auke Bay Laboratories
Alaska Fisheries Science Center, NOAA Fisheries
Ted Stevens Marine Research Institute
17109 Pt Lena Loop Rd
Juneau AK 99801