• 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.
    • The seasonal dynamics of coastal Arctic lagoons in Northwest Alaska

      Tibbles, Marguerite; Seitz, Andrew C.; Falke, Jeffrey A.; Prakash, Anupma; Robards, Martin D. (2018-12)
      Lagoons are zones of habitat transitions between freshwater and marine ecosystems, providing safe and productive feeding habitats for whitefishes in Northwest Alaska, important to subsistence users in the region. However, many important lagoon processes are not understood. Therefore, the goal of this thesis was to gain a baseline understanding of two important seasonal processes of lagoons in Northwest Alaska. First, I attempted to identify environmental processes correlated with Arctic lagoon breaching for three indicator lagoons that represent a range of environmental characteristics using generalized linear models (GLM) in an information theoretic approach and model averaging. Second, I developed a habitat suitability (HS) model to identify the range of physical conditions that whitefishes may experience if overwintering under ice of these lagoons during the Arctic winter, for the same three lagoons. The GLM model suggested that lagoon breaching day of year was slightly negatively related to day of year of river break-up, but other unconditional confidence intervals for the covariate parameters overlapped zero indicating considerable uncertainty in these estimates. Further data collection and monitoring in the region is needed to improve and verify lagoon breaching modelling results. The HS model indicated that lagoons have reduced suitability as whitefish habitat in winter due to loss of habitat due to the presence of bottomfast ice and a reduction of liquid water quality due to cold temperatures, high salinities and low dissolved oxygen levels. Importantly, small lagoons without freshwater inputs were potential sinks for fish populations. The results from this research will help the National Park Service and the Native Village of Kotzebue in a joint effort to understand and manage these important habitats that are critical for subsistence fisheries as the Arctic faces an uncertain future with climate change, oil spill threats, and increased coastal development.