• Exploring the potential role of late stage predation and Chinook salmon age structure

      Manishin, Kaitlyn A.; Seitz, Andrew C.; Westley, Peter A. H.; Cunningham, Curry J.; Goldman, Kenneth J. (2018-12)
      Chinook salmon (Oncorhynchus tshawytscha) populations across the North Pacific have displayed a decrease in body size-at-return resulting from declines both in age- and body size-at-maturity. These changes have precipitated the loss of the oldest age classes in some populations and have occurred throughout the range of this species, suggesting a shared - yet currently unknown - driver in the common marine environment. A hypothesis for the cause of these changes is intense and/or selective predation marine mortality after the first winter in the ocean, potentially from predators selectively removing relatively large sub-adult Chinook salmon. Here I consider the question: under what circumstances could predation on large sub-adult individuals by salmon sharks (Lamna ditropis) change the age structure of a Chinook salmon population? To address this question, I first estimated total per capita prey consumption by salmon sharks - an increasingly acknowledged predator of salmon on the high seas - using three methods: 1) daily ration requirement, 2) bioenergetic mass balance, 3) and a von Bertalanffy growth model. Second, I examined the effects of additional predation on an indicator Chinook salmon population from the Yukon River by simulating alternative predation scenarios with a stage-structured life cycle model. Scenarios described the strength and selectivity of predation, and the resulting simulated age structure was then compared to observed demography. The selectivity and intensity of removals required to produce this change in age structure were considered in the context of top predators, focusing on salmon sharks. The daily ration method yielded individual salmon shark consumption estimates of 1461 and 2202 kg·yr-1, the mass-balance method produced estimates of 1870 kg·yr-1, 2070 kg·yr-1, 1610 kg·yr-1, and 1762 kg·yr-1, depending on assumed diet, and the growth model output estimates of 16,900 kg·yr-1 or 20,800 kg·yr-1, depending on assumed assimilation efficiency. The per capita prey consumption estimates from the mass-balance method may be the most realistic because they incorporated life history data specific to salmon sharks and did not produce extreme values. Taken as a whole, these estimates suggest salmon sharks have energetic requirements similar to those of piscivourous marine mammals and corroborates conclusions of previous research suggesting that endothermic fishes exhibit metabolic rates similar to marine mammals. The simulated mortality scenarios that most closely mimicked observed shifts in age structure of the indicator Chinook salmon population focused intense and selective predation on the third year of Chinook salmon residence in the ocean. This simulated predation is corroborated by emerging results from an independent electronic tagging study in which tagged Chinook salmon experienced high predation rates, and research suggesting that killer whales (Orcinus orca) selectively prey upon Chinook salmon in their third year at sea. In summary, salmon sharks likely have high energetic requirements that could result in a large biomass of prey consumed, Chinook salmon populations are sensitive to predation during the third ocean year, and salmon sharks and other predators appear to frequently consume fish at that ocean stage. Taken together, these lines of evidence point to a potentially important mechanism for top down pressure on Chinook salmon populations that may explain observed changes in age-at-return, which in turn can affect population productivity. Future work and more robust data on predator distributions and abundances are needed to explore this finding further.
    • Patterns and environmental drivers of juvenile sablefish movement in Southeast Alaska

      Ehresmann, Rhea K.; Beaudreau, Anne H.; Seitz, Andrew C.; Green, Kristen M. (2018-08)
      Sablefish Anoplopoma fimbria are a long-lived, deep-dwelling groundfish that inhabit the North Pacific Ocean, ranging from northern Mexico to the Gulf of Alaska to Japan, supporting one of Alaska's most valuable commercial fisheries. After decades of heavy fishing, declines in the Sablefish population led to significant fishing restrictions but few strong year classes developed in recent years. Most Sablefish research has focused on the larval, near-surface juvenile, or adult life history stages, but few studies have examined post-settlement juvenile Sablefish in nearshore areas. This study used acoustic telemetry to understand the presence and movement of juvenile Sablefish in a nursery area in Southeast Alaska. Throughout the summer and fall of 2015 and 2016, 40 juvenile Sablefish implanted with acoustic transmitters were monitored using an array of eight fixed receivers in St. John Baptist Bay, Baranof Island, Alaska. We quantified the movement patterns of 28 juvenile Sablefish using displacement from the head of the bay, daily distance traveled, daily duration within the bay, unique movement types among individuals, and movement in relation to environmental variables. From these analyses, we show that juvenile Sablefish exhibit fidelity to the middle-head region of the bay, display relatively high rates of daily movement and residence, demonstrate three distinct movement patterns, and are influenced by environmental variables like water temperature, diel state, moon phase, and day of year. Our results show that juvenile Sablefish exhibit seasonality in movements as they progressively emigrate from the bay throughout the summer and fall. Certain factors were found to increase the likelihood of movement for juvenile Sablefish, perhaps allowing them to remain in suitable environmental conditions. This study fills a gap in our knowledge of Sablefish early life history and reinforces the importance of nursery areas like St. John Baptist Bay for juvenile Sablefish prior to recruitment into commercial fisheries.