Herring schools can extend for miles and be deep enough that they cannot be seen from the air. By using acoustics, we can estimate the biomass of these schools. The acoustic estimate is then provided to the age-structure-analysis model to help constrain the model output. The Prince William Sound Science Center has annually conducted surveys of the spawning biomass using hydroacoustic techniques.
Surveys are conducted using a towed hydroacoustic sensor. Schools are located during the day, often by using the presence of whales. The surveys are conducted at night when the school is further off the bottom. The acoustic return, known as echograms, are analyzed to determine the biomass of fish observed.
This project provides the data required to estimate the herring population in Prince William Sound. The observations are provided to all of the other projects for use in their analysis. It is only by collecting the information included in this project that we can determine the status of herring recovery. During the spring of 2020 (24 March - 3 April 2020) we completed acoustic surveys for Pacific herring in Port Gravina, Hawkins Island (Windy Bay and Canoe Pass), Double Bay, Rocky Bay, and Stockdale Harbor. Because of some scheduling complications related to the coronavirus pandemic, our survey occurred a bit earlier than in the past few years. We observed 18,000 tons of herring (Fig. 1) in eastern PWS, but our survey was too early and did not overlap with aggregations observed in bays in northeast Montague Island and in Port Etches on Hinchinbrook Island. As a result, our overall abundance estimate in 2020 is probably a bit low.
Figure 1. Time series of acoustic biomass estimates (MT, metric tonnes) of Pacific herring in Prince William Sound. The survey conducted during 2014 did not yield a biomass estimate due to adult herring occupying water too shallow to survey effectively with acoustics.