• Pacific herring juvenile winter survival and recruitment in Prince William Sound

      Sewall, Fletcher; Norcross, Brenda; Mueter, Franz; Kruse, Gordon; Heintz, Ron; Hopcroft, Russ (2020-05)
      Small pelagic fish abundances can vary widely over space and time making them difficult to forecast, partially due to large changes in the number of individuals that annually recruit to the spawning population. Recruitment fluctuations are largely driven by variable early life stage survival, particularly through the first winter for cold temperate fishes. Winter survival may be influenced by juvenile fish size, energy stores, and other factors that are often poorly documented, which may hamper understanding recruitment processes for economically and ecologically important marine species. The goal of this research was to improve understanding of recruitment of Pacific herring (Clupea pallasii) within Prince William Sound (PWS) through recruitment modeling and by identifying factors influencing winter survival of young-of-the-year (YOY) herring. Towards this end, my dissertation addresses three specific objectives: 1) incorporate oceanographic and biological variables into a herring recruitment model, 2) describe patterns in growth and condition of PWS YOY herring and their relationship to winter mortality risks, and 3) compare the growth, condition, swimming performance, and mortality of YOY herring that experience different winter feeding levels. In the recruitment modeling study, annual mean numbers of PWS herring recruits-per-spawner were positively correlated with YOY walleye pollock (Gadus chalcogrammus) abundance in the Gulf of Alaska, hence including a YOY pollock index within a standard Ricker model improved herring recruitment estimates. Synchrony of juvenile herring and pollock survival persisted through the three-decade study period, including the herring stock collapse in the early 1990s. While the specific mechanism determining survival is speculative, size-based tradeoffs in growth and energy storage in PWS YOY herring indicated herring must reach a critical size before winter, presumably to reduce size-dependent predation. Large herring switched from growth to storing energy, and ate more high-quality euphausiid prey, which would delay the depletion of lipid stores that compelled lean herring to forage. Lipid stores were highest in the coldest year of the seven-year field study, rather than the year with the best diets. With diets controlled in a laboratory setting, spring re-feeding following restricted winter diets promoted maintenance of size and swimming ability, but had little effect on mortality rates compared to fish continued on restricted rations. Declines in gut mass, even among fully fed herring, and low growth potential suggest limited benefits to winter feeding. Mortalities due to food restriction compounded by disease were highest among herring that fasted through winter months, and among small herring regardless of feeding level. Taken together, these findings illustrate the importance of achieving a critical size and high lipid stores in the critical period before winter to promote YOY herring winter survival and ultimately recruitment.
    • Spatial resilience and the incorporation of traditional ecological knowledge in mapping Sitka herring

      Shewmake, James W. II; Greenberg, Josh; Verbyla, Dave; Holen, Davin (2013-05)
      This project assesses the utility of Traditional Ecological Knowledge (TEK) in conducting research on herring stocks within Sitka Sound. By considering ethnographic data of the marine environment it is possible to identify key spatial attributes associated with the resource. This information was used to construct a social-ecological systems model (SES) for analysis within a spatial resilience framework. From this SES model, resilience surrogates were identified to analyze effort and success within the fishery. These indicators provided valuable insight into how subsistence users relate to the marine environment when they participate in the harvesting of herring spawn. To collect TEK data, the researcher, employed as a graduate intern with the Division of Subsistence, Alaska Department of Fish and Game (ADF & G) worked cooperatively with the Sitka Tribe of Alaska (STA). TEK data was used to identify marine habitat types, subsistence harvest locations (mapping), customary and traditional practices, and changing trends in accessibility to the resource. This information was supplemented with quantitative data including spatial habitat mapping and herring spawn distribution. A Geographic Information System (GIS) was used to display, analyze, and understand these variables and their measured outcomes to construct the SES model.