The Setting
The mesopelagic boundary community in the Hawaiian Islands is a land-associated community of small, midwater fishes, shrimps, and squids. This community migrates every night from deep water into the shallower waters around the islands. It serves as an important food resource for many predators including spinner dolphins, deepwater snappers, tunas, and billfish. The dynamics of the boundary community are likely to be important to the energetics of these predators, but the biological energy available in the boundary community remains unknown. As a result, it is difficult to determine if the boundary community can solely sustain large populations of many predators, or if the predators must rely on other resources as well. Monitoring this community has been difficult. Net trawling for these mesopelagic animals is time consuming and the number of animals captured is often artificially low because they can avoid the net.  Because the mesopelagic animals live so deep, there is no feasible way to determine how many animals are avoiding the tow net. Surveying the boundary community using remote techniques, such as sonar, is desirable because it reduces costs, facilitates the monitoring of greater areas, and dramatically reduces avoidance by the mesopelagic animals.

The Specific Problem

 

The data obtained from sonar systems are often converted into estimates of animal abundance. This requires information on the acoustic scattering properties of the animals in question and the size distribution of individuals in the population, which is often unavailable. Biologists, however, often need information on the amount of biological energy available to higher trophic levels (i.e., higher levels of the food chain), not just the numbers of prey. To convert acoustic measures to biomass, standard length-weight relationships for the species are applied to the abundance estimates. This requires two steps and consequently, compounds the error in the estimate of biomass. Alternatively, acoustic energy is taken as a direct index of biomass, allowing relative comparisons to be made, but not permitting estimates of the actual available biological energy in a population. This direct use of acoustic energy as an index of biomass has also not been directly validated.