Improving species identification, age, and life history information for shortraker rockfish (Sebastes borealis) in Prince William Sound, Alaska, using sagittal otolith analyses

Abstract

Shortraker rockfish (Sebastes borealis) are a long-lived species found in the North Pacific Ocean. Limited life history information compels fishery managers to apply conservative estimates of allowable bycatch for this species. To improve the available life history information, we analyzed sagittal otoliths, targeting fish from Prince William Sound, to assess (1) age estimation errors, (2) species misidentification, and (3) relationships between environmental factors and annual growth. To assess errors in age data, otoliths were analyzed for core Δ¹⁴C activity. Those values were compared with bomb radiocarbon reference curves for other North Pacific species (i.e., known age Pacific halibut, Hippoglossus stenolepis, and yelloweye rockfish, S. ruberrimus). Shortraker rockfish Δ¹⁴C activity followed reference curves, partially validating current age estimation techniques. To identify species, otolith shape analyses were performed on putative shortraker and rougheye rockfish (S. aleutianus). Clusters of shortraker, rougheye, and blackspotted rockfish (S. melanostictus) data were identified using random forest algorithms and otolith wavelet estimates. Misidentification rates were examined using out-of-sample error estimates and otoliths from fish whose species was validated using single nucleotide polymorphisms. Model classification errors suggest that historically assigned species were accurate and shortraker rockfish can be identified with 94% success. To investigate potential drivers of fish growth, a chronology of shortraker rockfish otolith increment widths was developed and related to environmental variables using dendrochronology methods. Model comparisons showed significant correlations between annual increment widths (i.e., an indicator of fish somatic growth) and environmental variables, specifically negative correlations with winter temperature at depth, spring surface salinity, and the North Pacific Gyre Oscillation, and positive correlations with spring and winter upwelling and the Pacific Decadal Oscillation 2-years earlier. These three analyses validate and improve data and life history information and support incorporating age and growth data into stock assessment models used to calculate allowable bycatch levels for shortraker rockfish.