AFSC/RACE/MACE: Upward looking moored echosounders - Shelikof Strait

We have developed and deployed 3 trawl-resistant bottom moorings equipped with prototype upward-directed 70 kHz split-beam calibrated echosounders to generate a pollock index of abundance in Shelikof Strait during the 2015 spring spawning season. This work has demonstrated that the moorings can be easily and cheaply deployed/recovered from small commercial fishing vessels, and the mooring data can provide invaluable information on spawning walleye pollock distribution and abundance. The goal of this project is to develop an innovative and efficient method to use autonomous echosounders to augment traditional ship-based acoustic surveys to assess aggregations of fish or other organisms in a wide variety of situations. Our particular application focuses on walleye pollock that periodically concentrate in a restricted part of their habitat for spawning purposes. Given that many other species aggregate in predictable and restricted parts of their habitat, other situations likely exist where an autonomous echosounder approach could be used to augment ship-based assessment surveys.

The major outstanding question regarding the utility of moored echosounders for abundance estimation is the degree to which temporally averaged measurements from stationary instruments represent the density of fishes over a wider area. Specifically, stationary echosounders provide high temporal resolution at a single point in space compared to ship-based surveys, and the time-averaged measurements of fish movements past or around a stationary sensor represent the abundance over a broader area than the relatively small sampling volume of the stationary instrument. Thus, the primary objective of the proposed work is to establish the relationship (and associated uncertainty) between temporal averaging of the stationary measurements and the areal size of the local fish density estimates that the averaging represents. The GOA Shelikof Strait pre-spawning walleye pollock AT survey will be the test bed for the proposed work. The extensive annual survey data time series facilitates a retrospective analysis to initiate a mooring array design.

Survey timing is critical for spawning stock surveys such as those conducted on spawning pollock in Alaska. It is essential that the surveys are timed to coincide with peak spawning so that survey-based abundance estimates represent the total numbers of spawners for the area and not some fraction of the total. The time of peak spawning can be estimated by conducting multiple survey passes over the duration of the spawning season, but this is extremely costly, and is often not possible due to vessel scheduling conflicts. A better approach to obtain peak spawning information is now possible using recently developed autonomous echosounder technology. That is, trawl-resistant bottom moorings with next-generation upward-directed echosounders can be deployed on the spawning grounds over the entire spawning season to develop a time-series of spawner abundance indices to identify the time of peak spawning. These mooring deployments will be used to assess the period of peak spawning of pollock and their behavior in Shelikof Strait.