ASA PRESSROOM

142nd ASA Meeting, Fort Lauderdale, FL


[ Lay Language Paper Index | Press Room ]


Energy: Converting from Acoustical to Biological Resource Units

Kelly J. Benoit-Bird - benoit@hawaii.edu
Whitlow W. L. Au
Hawaii Institute of Marine Biology
P.O. Box 1106
Kailua, HI 96734

Popular version of paper 4aAOb6
Presented Thursday morning, December 6, 2001
142nd ASA Meeting, Fort Lauderdale, FL


Acoustic scattering from sonar is often used as index of biomass, or available biological energy, however this relationship has not been directly tested.

The goals of this work were to determine if:
     bullet  acoustic measures are a good index of biomass
     bullet  acoustic measures can be directly converted to measures of biomass

The ability to remotely measure biomass with sonar is directly applicable to field studies of the distribution of the prey of spinner dolphins, deepwater fishes, and tunas around the Hawaiian Islands.

Biomass
   Any measure of available biological energy including:
      bulet  Weight
      bullet  Volume
      bullet  Chlorophyll a
      bullet  Protein
      bullet  Carbon
      bullet  ATP
 
    bullet  Calories

Mesopelagic animals

 

 

 

 

 

  


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.
Trawl
 
Methods
 
We measured the acoustic scattering characteristics of live animals from the mesopelagic boundary community using a 200 kHz sonar system. The animals were captured using a midwater trawl and kept alive aboard the ship until acoustic measures could be taken. The animals were then frozen and transported to the laboratory where we measured various units of biomass on the same animals including dry weight, ash-free dry weight, volume, and calories. The acoustic data was then compared with the biomass data from each fish, shrimp, and squid to determine if there were any predictable relationships.
 
Conclusion
 
There were strong relationships between all the variables measured, making it possible to approximate any one measure from the others within a class of animals. This means that for these midwater animals, acoustic scattering from sonar can be used as an index of biomass, a common practice which has not been previously validated. Acoustic energy predicted biomass measures at least as well as body length, a standard predictor. The relationships between acoustic energy and all biomass measures from Hawaiian mesopelagic animals were linear and additive. Consequently, it is possible to directly convert acoustic energy from these animals to organic biomass units without having knowledge of the size distribution of the populations being studied, reducing potential error.
 Spinner dolphin
  In summary, for the Hawaiian mesopelagic boundary community
      Bullet  Acoustic scattering from sonar can be used as an index of biomass
      Bullet  It is possible to directly convert acoustic energy to organic resource units - without an intermediate step

We do not yet know if these conclusions can be generalized to other animals. These types of paired measures of acoustic and biological energy need to become a routine part of controlled acoustic studies.

Most importantly, these measures give us a way to study the energetics of the mesopelagic boundary community around the Hawaiian Islands. Preliminary application of the results of this study show that the boundary community around the islands can have energetic densities as high as 186 kcals/m^3. This important food base links the nearshore and open ocean systems around the islands. Understanding the dynamics of the energy in this trophic level are crucial to our comprehension of the many fisheries that depend on this resource, as well as the biology of the spinner dolphins.

During the meeting, the presenter may be contacted at the Ft. Lauderdale Marina Marriott Hotel.

[ Lay Language Paper Index | Press Room ]