In the decades prior to 1993 there was a robust Pacific herring population in Prince William Sound (PWS). Not only are these forage fish a key link in the complex food web of PWS, but they supported a lucrative early season commercial fishery that brought the communities of the Sound to life each spring. By 1994, that fishery was closed and only briefly reopened for two years in the late 1990s. The current, approximately 10,000 ton biomass is tiny compared to the peak value of 130,000 tons or the long-term average prior to the collapse of around 65,000 ton.
The cause of the dramatic decline in the herring population is still hotly debated. Was it the 1989 Exxon Valdez oil spill, disease, climate change, predation, natural cycles, or some combination of these factors? While the reason for the decline remains debated, it is more important to understand what is preventing the herring population from recovering.
The goal of our research program is to improve our ability to predict changes in the herring population. In order to predict the population we will need to improve our understanding of the factors controlling the recovery of the herring in PWS. This program is a continuation of research funded by the Exxon Valdez Oil Spill Trustee Council (EVOSTC). The first integrated program was the PWS Herring Survey program that occurred between 2009 and 2013. That was followed by the first phase of the Herring Research and Monitoring (HRM) program (2012-2017) and now the second phase of the HRM program (2017-2021).
The HRM program is a mix of monitoring studies that provide data necessary to understand changes in the PWS herring population, and process studies that address particular aspects of herring. These process studies help us understand why populations may change or address assumptions in the population model. The first phase of the program focused on the overwinter survival of young herring and addressing assumptions in the model and measurements. We also explored several other aspects of herring in PWS. The second phase is focused more on adult herring and the connections between herring condition and recruitment and environmental conditions. This work is connected to the Gulf Watch Alaska program that examines other important environmental factors in the region.
Work from the previous programs included two syntheses. One in 2013 that focused on examining the factors influencing survival in the first year of life of herring. The second in 2014 that connects our knowledge of herring to environmental conditions. A special issue of Deep-Sea Research II in 2018 contains manuscripts from the various projects. More details can be learned from the reports on the EVOSTC website. Highlights from the first phase include determining the collapse of herring in PWS is unusual in how low the population reached and the duration of the crash when compared to other herring populations around the world. We found that juvenile herring must reach a critical length before they convert their energy allocation from growth to fat storage needed to survive the winter. Checking scales of fish that reached spawning age showed the fish that reached spawning age had all reached that critical length. We learned that there are differences in diets and energy content among years, and that the fish were in the best condition when more of their diet was from organisms outside PWS. The genetic structure of PWS herring was found to be like those populations to the east of PWS, but different from those to the west. The herring population had a further collapse starting in 2015 to a new record low level. New tools were developed for detecting antibodies for viral hemorrhagic septicemia, an extremely deadly disease for herring, that showed an increase in the presence of those antibodies in 2015 that may indicate a disease outbreak had occurred.
In the existing program we will continue to collect the measurements necessary to determine changes in the PWS herring population, observe where herring go after spawning, and determine when herring mature and become part of the spawning population. We continue to work on examining diseases and their role in limiting the herring population. Additional effort is being spent examining how the herring condition and recruitment is dependent on environmental factors, such as food availability and predator populations.
Data from this program is available on the Alaska Ocean Observing System Gulf of Alaska portal at https://portal.aoos.org/gulf-of-alaska. Once there search for the EVOS herring program.