Prince William Sound (PWS) Pacific herring (Clupea pallasii) population estimates are generated annually by Herring Research and monitoring (HRM) team with a Bayesian age-structure-assessment (BASA) model. The model inputs include aerial surveys of mile-days of spawn (total number of miles of spawn observed each spring), acoustic surveys of spawning biomass, age-sex-size, historical egg deposition, and disease prevalence data. The PWS surveys used by the model are conducted each spring. The survey area covers traditional spawning regions within Prince William Sound and occasional surveys in the Kayak Island area, although Kayak Island is not included in the inputs to the BASA model.
The mile-days of milt surveys collected by ADF&G extend back to the early 1970s, making this the longest abundance time series used in the model. Acoustic surveys collected by the Prince William Sound Science Center started in the mid 1990s. ADF&G has collected herring age, sex, and size data from PWS commercial fisheries and fishery independent research projects since 1973. Egg deposition scuba surveys were conducted by ADF&G in 1983, 1984, and 1988-1997. Historical harvests from commercial pound, seine, and gillnet fisheries from 1980-1998 are also included as inputs to the model, although there have been no commercial herring fisheries in PWS since 1998. An output of the model is the annual estimate of the biomass.
There is not complete agreement about the cause of the decline of the herring population in the early 1990s, but an outbreak of viral hemorrhagic septicemia (VHS) is one mechanism thought to be possibly responsible for the decline. Contact with crude oil during the early life stages of herring may have had negative effects on the overall fitness of the population making them more susceptible to outbreaks of disease. In 2014, the BASA model estimated a declining trend in herring biomass. The observed mile-days of milt estimate is the lowest level on record (Figure 1). The weight and length at age values have been low since 2015 meaning the population is composed of smaller than normal fish (Figure 2). There are very few older fish being observed in the population as well. Disease researchers on the HRM team observed high levels of antibodies to the VHS virus in 2015 (Figure 3). This suggests that an outbreak of the pathogen occurred between 2014 and 2015. The output of the BASA shows this as the lowest estimated biomass on record (Figure 4). The warm waters associated with the “Blob” are likely to have caused nutritional stress that may have contributed to the outbreak of the virus. Herring are categorized as a forage fish and are near the bottom of the food chain. Low abundance of herring may have negative impacts on predators that consume them including whales and salmon.
Figure 1. Mile-days of milt in Prince William Sound based on aerial surveys and biomass estimates from acoustic surveys.
Figure 2. Weight and length at age data collected by ADF&G.
Figure 3. VHS antibody levels observed by the disease researchers.
Figure 4. Modeled biomass (shaded) with acoustic biomass estimates (bars) corroborating the observed decline from the aerial milt survey.