Why haven’t herring recovered?

By Amy Brodbeck for The Cordova Times

Courtesy Photo

“One medium school of sand lance — two small schools of 1-year herring.”

With a data sheet in my hands, I quickly jotted down the identity of the schooling fish 1,000 feet below our plane. This week, I joined Prince William Sound Science Center oceanographer, Scott Pegau on an aerial survey in Prince William Sound.

This research is part of a larger program aimed to solve a mystery — why haven’t the herring recovered in Prince William Sound?

In 1989, the oil tanker Exxon Valdez oil spill dumped 42 million liters of Prudhoe Bay crude oil into Prince William Sound. For several years after the spill, populations of Pacific herring, Clupea pallasii, seemed relatively unscathed until 1993, when herring populations declined below harvestable levels.

The fishery collapsed and has remained largely closed for the past 23 years. In Prince William Sound, herring act as a link in the chain that connects lower trophic levels (phytoplankton and zooplankton, free-floating plants and animals) with higher tropic levels (salmon and halibut).

Herring also play an important role in communities and economies that depend on them, driving researchers to investigate why their populations have not recovered. Among other post-oil spill issues, such as lingering oil residue, studying herring has been designated by the Exxon Valdez Oil Spill Trustee Council to be in the top five focal points that require restoration research funding.

Pegau has spent seven years working with the collaborative Prince William Sound Herring Survey Program and the Herring Research and Monitoring Program. From analyzing the effects of oceanographic conditions to the presence of disease among herring, scientists in these programs have conducted a wide range of studies that strive to solve the mystery behind depressed herring population levels.

Based out of the science center in Cordova, Pegau has helped orchestrate collaboration across various projects and also plays a role in the research via aerial forage fish surveys. Researchers have successfully used aerial surveys to determine herring population density since the 1950s, but the level of precision needed in this research calls for a second set of eyes.

Pegau has coordinated with a vessel from the United States Geologic Survey, which uses cameras, jigs, and cast nets to capture herring and determine the accuracy of aerial IDs.
On the recent survey that I joined, we collected data in several survey boxes, which are 2.5-mile by 5-mile areas designated for forage fish monitoring throughout the sound.

Pegau and his technician, Alexandra Wright, distinguish between schools of sand lance and herring based on size, shape and location the school, and can estimate herring age by whether or not they “sparkle,” as age two herring often do.

Accurate age identification matters because history has shown that collapsed herring stocks have recovered if a ‘recruitment’ class was large enough, which is the population of fish that reach spawning age.

It takes three years for herring to reach spawning age, and researchers have found that lower numbers of herring have reached recruitment levels in Prince William Sound since the 1990s. Thus, much effort has been focused on the early life stage of this species — to determine when and ultimately why they are being lost.

In his aerial surveys, Pegau determines population estimates of age 1 herring, and explains that these aerial surveys can essentially “tell us what the future holds.” If researchers can monitor the populations of the same age class over a span of three years, they can disentangle at which point herring are lost before they recruit into spawning stock.

These aerial surveys are only one part of the puzzle. Many researchers have worked together to fill these gaps. By combining the information gathered from these surveys with zooplankton and ocean conditions, researchers aim to gain a better understanding of how these factors affect herring growth, disease, predation and energetics.

Because sustainable fisheries support sustainable economies, the goal of this research is to understand why the herring populations have not recovered in Prince William Sound and to ultimately help inform future restoration efforts.

Amy Brodbeck is a science communication intern for the Prince William Sound Science Center and teaching assistant and Master’s candidate for the School of Environmental and Marine Affairs at the University of Washington. She can be reached at amyb2015@uw.edu.