• Inter-decadal change in sablefish, Anoplopoma fimbria, growth and maturity in the Northeast Pacific Ocean

      Howard, Katy B.; Adkison, Milo D.; Hillgruber, Nicola; Sigler, Michael F. (2008-08)
      Errors in growth and maturity estimates can drastically affect the spawner-per-recruit threshold used to recommend commercial fish catch quotas. Growth and maturity parameters for Alaskan sablefish, Anoplopoma fimbria, have not been updated for stock assessment purposes for 20 years, even though sablefish aging has continued. In this study, the old length-stratified data set (1981-1993) was updated and corrected for bias. In addition, newer, randomly collected samples (1996-2004) were analyzed, and new length-at-age, weight-at-age, and maturity-at-age and length parameters were estimated. A comparison of the two datasets showed that in recent years, sablefish are growing larger and maturing later and that growth and maturity differ somewhat among regions. The updated growth information improves data fits in the sablefish stock assessment model. It also provides results that are biologically reasonable. These updated and improved estimates of sablefish growth and maturity help ensure the continued proper management of this commercially important species in Alaskan waters.
    • 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.
    • Temporal and size-based patterns in juvenile sablefish energy allocation and diet

      Callahan, Matthew W.; Beaudreau, Anne H.; Mueter, Franz J.; Heintz, Ronald A. (2020-08)
      A recent marine heatwave in the Gulf of Alaska caused depressed growth, poor body condition, and low survival in many fish species, but juvenile sablefish (Anoplopoma fimbria) thrived. These fast-growing piscivores are the target of a valuable commercial fishery in Alaska as adults and have historically shown high variability in recruitment. The first winter is a period of nutritional stress and high mortality for many fish species and first winter survival may dictate year class strength, but the importance of the first winter for juvenile sablefish is understudied. We examined juvenile sablefish energy storage, growth, and diet during their first two years of life, specifically as newly settled juveniles in their first autumn, in late winter, and during their second summer and autumn. Sablefish grew rapidly in autumn and growth slowed but continued through winter. Mean energy density (kJ g⁻¹) declined over the winter but total energy (kJ individual⁻¹) increased significantly between October and March. Slopes of energy density and total energy versus length regressions were atypical for high latitude marine fish in that they were steepest in March. This indicates that large fish grew during winter with minimal energy depletion while small fish grew but depleted their energy stores. Stable isotope results revealed that larger fish were enriched in [delta]¹³C and [delta]⁻¹⁵N in March relative to smaller fish, suggesting diet differences may contribute to size-specific energy storage patterns during winter. Pacific herring (Clupea pallasii) dominated diets but consumption of herring and other prey varied seasonally and annually. Relative stomach content weights were highest in autumn 2018, which was a period of rapid growth. Results of this study show advantages for sablefish achieving large size prior to winter and broadly support the hypothesis that first winter is a life history bottleneck for juvenile sablefish. The generalist feeding strategy of sablefish and rapid growth early in life may provide the ability to adapt to a wide range of environmental conditions.