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 2021 (3/2821 – 4/6/21, 4/15/21 – 4/18/21) we completed surveys for Pacific herring in Port Gravina, Fidalgo, Hawkins Island (Windy Bay and Canoe Pass), Double Bay, Rocky Bay, Stockdale Harbor, and Porth Etches. The timing of our cruises is always weather-dependent. This past spring, we might have been a bit late getting into Gravina to survey, but our timing at Stockdale was just right. We observed 4500 tons of herring in Gravina and 800 tons in Stockdale harbor (Figure 1). The observed aggregation at Redhead in Gravina was very close to shore in shallow water making it difficult to survey with acoustics.
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. 2020 estimates are preliminary and will be updated in the coming months. The 2021 estimate is preliminary and will be updated in the coming months.