Project Overview

The goal of this project is to develop improved mathematical models of the Target Strength of Bering Sea euphausiids. These models are needed to improve the accuracy of acoustic estimates of krill and fish biomass done in the Bering Sea. As part of this research, we participated in a pollock assessment research cruise aboard the RV Oscar Dyson conducted by the MACE program at the Alaska Fisheries Science Center which is part of NMFS. The Research Team measured several parameters of live euphausiids which will be used as input data for the scattering models. In addition to the shape and mass of the animals, we focused on measuring the density and sound speed of the animals which have a large effect on how much sound they scatter. In addition to the euphausiids, we also collected measurements on several other types of zooplankton found in this region including: copepods, amphipods, jellyfish, siphonophore bracts, fish larvae, and pteropods. Funding for this project is from NOAA and the NPRB ???.

The Research Team

SoMAS Professor Joe Warren and graduate student Joy Smith are the research team for this project and we’re working with Dr. Patrick Ressler at the Alaska Fisheries Science Center. Joe is a bioacoustician who is interested in improving our ability to convert acoustic backscatter data into biologically meaningful information. Joy’s thesis project will be based (in part) on the data collected during this cruise. While leaving Long Island in the summer for a month was difficult to do, the opportunity to travel to Alaska, sail out of Dutch Harbor (the home port of “Deadliest Catch”), and participate in the pollock stock assessment cruise were too much of an opportunity to pass up. In three weeks at sea we collected a multitude of data on several euphausiid species as well as other zooplankton. We are working on the analysis of these data post-cruise and will be presenting our preliminary results at a science symposium in Anchorage in January 2009.

Pollock and zooplankton

Walleye pollock is the largest fishery in the United States (in terms of landings, that is lbs of fish caught) and accounts for roughly 1/3 of all fish landings in the US. So why have you not seen pollock on your restaurant menus ? The majority of pollock is processed and made into fish sticks, fast food fish sandwiches, and surimi (imitation crabmeat). Annual surveys (using acoustics and net trawls) are used to estimate the Bering Sea pollock stocks which in turn are used to set fishery catch limits. We’re interested in zooplankton in these waters for two reasons: acoustically and ecologically. Acoustically, zooplankton scatter sound and may cause uncertainty in the acoustic estimates of pollock stocks so better understanding how zooplankton scatter sound can improve the assessors ability to differentiate between zooplankton and fish schools in the water column. Ecologically, zooplankton serve as a primary food source for many of the fish (including pollock) in these waters as well as other animals such as baleen whales, seals, and sea birds.

Density Measurements

The density of an individual zooplankton helps determine the amount of acoustic energy the animal scatters.  Typically, the more dense the organism, the more energy the animal will reflect acoustically.  We would like to define the ranges in density measurements for specific zooplankton taxonomic groups in order to improve population estimates made by models that use density of individual organisms as a variable. The density of individual organisms is measured by placing the animal in seawater and adding saltier, more dense seawater to the solution untill the buoyancy of the animal changes.  By knowing various properties of the seawater and salty solution (such as temperature, conductivity, and volume used), the density of the actual organism can be derived from a simple calculation.  We also use a dual-balance method where we directly measure the mass and indirectly measure the volume of the animal to calculate its density.

Soundspeed Measurements

We are using a time-of-flight measurement technique using a 500 kHz waveform that is transmitted through a chamber that is either empty or packed with zooplankton. By looking at the change in the arrival time of the wave form travelling through the empty and full chambers we can estimate what the speed of sound is through the animals.