Research Document - 2012/108
A biomass Limit Reference Point for the NAFO 4T American plaice (Hippoglossoides platessoides) fishery
By R. Morin, D.P. Swain, and S.G. LeBlanc
Abstract
Several indicators of the status of NAFO 4T American plaice suggest that the stock is presently at an all-time low abundance level. The spawning stock biomass (SSB), estimated using an age-structured population model, underwent a major decline from the late 1970s to the early 2000s. SSB was at its lowest level in 2009 at less than 31,000 tonnes and the latest estimate (2012) was 41,676 tonnes. Age-4 recruits peaked in abundance in the late 1970s, reaching levels that up to 13 times their average abundance over the past five years. The dynamics of the 4T plaice stock are dominated by a pattern of high natural mortality on adults, combined with chronic poor recruitment. Several models of stock-recruit relationships were examined, but Bayesian formulations of the Beverton-Holt and Ricker models formed the basis for determining the median SSB that produced 50% of maximum recruitment (Blim). Blim was estimated at 68,000 tonnes from the Beverton-Holt model and 60,000 tonnes from the Ricker model. An intermediate level of the two estimates, 64,000 tonnes, was proposed as Blim for 4T American plaice. The SSB for the 4T plaice stock has met or exceeded Blim only once since 1996. The recent low stock performance was observed during a period of low fishing mortality. Fishing mortality (F) was negligible on 4 to 9-year-old plaice throughout the time series, but for older plaice, estimated F varied between 0.1 and 0.2 from the mid-1970s to the early 1990s, dropping to below 0.1 and reaching less than 0.01 in recent years.
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