Research Document 2017/025

Arrowtooth Flounder (Atheresthes stomias) Stock Assessment for the West Coast of British Columbia

By Grandin, C. and Forrest, R.

Abstract

Arrowtooth Flounder (Atheresthes stomias, Turbot) are an important component of the bottom trawl fishery in British Columbia. They are managed as a coastwide stock, with a TAC of 15,000 t and catch of 10,679 t in 2014. Prior to the introduction of freezer trawlers in the mid-2000s, most of the historical catch of Arrowtooth Flounder is understood to have been discarded at sea. This was largely due to proteolysis, which occurs in the muscle tissue of this species a short time after it is caught, making the flesh unpalatable. In the past five years, markets have been established for fillets that have been frozen at sea, and the freezer trawl fleet has taken an increasing proportion of the coastwide catch.

This assessment fits a female only Bayesian age-structured model to catch, survey and age-composition data from the years 1996-2014, for management areas 3CD (West Coast Vancouver Island), 5AB (Queen Charlotte Sound), 5CD (Hecate Strait), and 5E (West Coast Haida Gwaii). Catch data prior to the introduction of at-sea observers in 1996 were considered too unreliable for inclusion in the assessment due to unknown quantities of discarding at sea.

The Reference Case model presented in this assessment estimates the female spawning biomass to have been on a flat to increasing trajectory since 1996, consistent with the catch and survey data. A set of sensitivity analyses was done to test the effects of fixed parameters and assumed prior probability distributions on model outcomes. In all scenarios, there was strong confounding among parameters representing productivity and scale of the population, indicating that there was limited information in the short time series of available data to resolve the population scale.

Management advice is provided in the form of decision tables which forecast the impacts of a range of 2015 catch levels on Arrowtooth Flounder stock status relative to these reference points. The Reference Case decision table suggests that a 2015 catch equal to any of the catch levels tested, ranging from 0 t to 30,000 t (twice the 2014 TAC), would result in a 2016 biomass above all of the candidate biomass-based reference points tested, and 2015 harvest rate below the estimated UMSY. Large uncertainty in the assessment is however, emphasised. The magnitude of catch and discards prior to 1996 is a major source of uncertainty in this assessment that could provide critical information about the scale and productivity of this stock. Ageing of archived otoliths from the freezer trawl fleet is a high priority recommendation.

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