Where herring go after spawning in Prince William Sound (PWS) remains a mystery. Do they remain in PWS or migrate out into the Gulf of Alaska? A pilot project that started in 2012 demonstrated it was possible to implant acoustic tags in Pacific herring and then detect them on acoustic arrays as they traveled to the entrances of PWS. Over the last 7 years, we have put together a network of acoustic arrays at key areas around PWS in order to better understand the movement of these critical forage fish.
With the help of historical knowledge and aerial surveys, researchers set out into PWS in search of herring to tag. Adult herring are captured on the spawning grounds using jigs and transferred to a seawater holding tank aboard a research vessel. Individually, the herring are sedated, measured, and a small acoustic tag is surgically implanted. The tags then transmit pings that can be “heard” by portable hydrophones and groups of acoustic receivers installed in PWS which “listen” for tagged fish. These receivers are placed on the spawning grounds and at the major entrances to PWS including Hinchinbrook Entrance, southern Montague Strait, and the four southwest passages. Receiver data are uploaded once or twice a year. Data collected from the receivers are then pooled into a Multistate Cormack-Jolly-Seber (CJS) Model. Click here for a full report on the locations of PWS acoustic arrays.
We have already learned that after spawning, the majority of adult herring move to the entrances of PWS. When the herring migrate seasonally out of PWS into the Gulf of Alaska in the spring and summer there is a much higher probability they will leave using Hinchinbrook Entrance and then return in the fall and winter through Montague Strait. Among the acoustic arrays, Hinchinbrook Entrance was the most important and was used by nearly 3/4 of the herring tagged in 2019. Preliminary analyses of this migration through Hinchinbrook Entrance into the sound during fall and winter suggests that Orca Bay and/or Port Gravina may support primary herring overwintering area(s) within PWS. We continue to learn what other routes and areas in PWS the herring are using. To aid in this effort additional receivers were deployed in 2018 & 2019 near southern Knight Island Passage, Red Head (near Port Gravina), along the west side of Hawkins Island, near Johnstone Point (Hinchinbrook Island), and across the north end of Montague Strait. Since April 2017, 726 herring have been tagged on their spawning grounds. Data will be uploaded from the receivers in February of 2021 with the hope of better illuminating the paths these herring take. By knowing where the fish are, we can improve the design of studies that examine factors such as predation, food availability, and stock genetics that affect herring populations.