Science Advisory Report  2011/043

Recovery Potential Assessment of the Maritime Designatable Unit of American Plaice (Hippoglossoides platessoides)

Summary

• The American Plaice (Hippoglossoides platessoides) Maritime Designatable Unit (DU) was designated as Threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in April 2009 due to significant declines in stock abundance.
• COSEWIC defined the Maritime DU as encompassing three stocks: the northern Gulf of St. Lawrence (NAFO 4RS), southern Gulf of St. Lawrence (4T), Scotian Shelf and Bay of Fundy (4VWX).
• COSEWIC designated this DU as threatened based on a rate of decline of equal or greater than 30% of the abundance of mature individuals over three generations. None of the plaice stocks in the Maritime DU has recovery targets defined under the precautionary approach framework. Reduction in the decline rate to less than 30% over three generations (the COSEWIC criterion for threatened status) was considered a reference level indicating a reduced risk of extinction.
• This RPA evaluates the risk of attaining the reference level for stock status by projecting the abundance of adult female for each stock and for the DU over 48 years (three generations). These long term projections, rather than predicting future stock status, describe abundance trends under current conditions of productivity and mortality. The probability that these conditions will change over the projected time period is unknown. Most projections have extremely large confidence intervals, reflecting a large degree of uncertainty.

Northern Gulf of St. Lawrence (NAFO 4RS)

• Trawl surveys in this area since 1985 indicate widely fluctuating abundance but with an increasing trend over time for both the total and adult population.
• A Bayesian state-space model was applied to plaice grouped in three size stages. The model describes an increasing trend during the late 1980’s and early 1990’s for the first two stages, at lengths <35 cm, followed by relative stability in abundance. Plaice of lengths 35 cm and longer appear to follow an increasing trend since the mid-1990’s.
• Natural mortality (M) on 4RS plaice of 35 cm and longer has increased over time, but is lower than other stocks in the Maritime DU (median M of 0.4 for 1999-2009 period).
• The adult female stock of American Plaice in 4RS is projected to increase over the next 48 years (median trend); however, due to uncertainty in the projection, there is an 18% probability of the stock declining to the reference level, even without any harvests.
• The northern Gulf stock of American Plaice is currently exploited at a low level with a small directed fishery in 4R and limited bycatch in other fisheries. Current harvest levels have minimal impact on the projected abundance of female plaice (19% probability of declining to the reference level).

Southern Gulf of St. Lawrence (NAFO 4T)

• Trawl surveys of 4T, conducted annually since 1971, indicate that the stock attained peak abundance in the late 1970’s, but has declined since then to its lowest level in recent years. Similar trends are observed for the total and adult populations.
• An age-based population model (VPA) was applied to this stock. The spawning stock biomass has declined over time to its lowest level in 2009 at 28 thousand tonnes. M has increased over time and is estimated at 0.45 on plaice of both sexes, age 4 years and older. Year-class strength was particularly high in the 1970’s, but has been at a low level for the past 20 years.
• A 48-year projection was done using the VPA model results. Under productivity conditions observed over the past 20 years and with no harvesting, spawning stock numbers (SSN) will decline below the reference level with a 55% probability (59% with harvesting continued at current levels). Assuming that future stock productivity (growth, recruitment and mortality) may encompass conditions observed throughout the 34-year time period of the VPA model, SSN is projected to decline below the reference level with a probability of 31% without harvests (34% with current harvest levels).
• A Bayesian state-space model was applied to plaice grouped in three size stages. All stages have declined over time and are currently at their lowest level. M on plaice of 35 cm and longer, composed mainly of females, has increased over time and is currently greater than 0.6.
• Projections were conducted with the Bayesian 4T model to estimate the female spawning stock numbers (SSN) over 48 years, 2010-2057. Without any exploitation, the median SSN was projected to increase, but with a probability of 36% of declining to below the reference level. The projected SSN was similar with current harvests continuing over the next 48 years (37% probability of declining to the reference level).
• The high rate of natural mortality in recent years appears to be the major cause of the lack of recovery of 4T plaice, as fishing mortality is currently estimated to be very low (mean F=0.03 on age-10+ plaice in 2000’s, based on VPA model).

Scotian Shelf and Bay of Fundy (NAFO 4VWX)

• Annual summer trawl surveys since 1970 in 4VWX, indicate that the stock was most abundant in the 1970’s, but has declined since then. Trends in mature abundance depend on assumptions concerning size at maturity after 1985. Maturity was not measured from 1986 to 2009. Sampling in 2010 indicated a decline in the size at maturity relative to 1985, but the extent of the decline is uncertain. Assuming constant size at maturity after 1985, there has been a decline in the abundance of adult plaice. If the size at maturity is assumed to have declined after 1985, the abundance of adult plaice has remained stable.
• A Bayesian state-space model was applied to plaice grouped in stages based on sex and maturity. When size at maturity is assumed to decline after 1985, abundance remained stable for adult females but increased for adult males. When it is assumed that there has been no change in maturity after 1985, abundance declines for adult females and remains stable for adult males. Natural mortality estimates (M) were dependent on the maturity assumptions; however, M increased over time on adult females under either assumption.
• Projections of female spawning stock numbers (SSN) were made over 48 years under three levels of exploitation. Assuming a decline in size at maturity from 1985 to 2009, the probability of SSN declining below the reference level is 25% with no exploitation, 28% at recent exploitation, and 39% with a 750 t harvest. Assuming no decline in size at maturity after 1985, projections were more pessimistic (probabilities of 32%, 36% and 44%, respectively, of declining below the reference level).

Maritime Designatable Unit

• Combining trawl survey abundance estimates, American Plaice in the Maritime DU have declined by 28% since 1985 and by 39% to 43% for adult plaice since 1987, depending on the maturity assumption in 4VWX. The decline in abundance in the areas surveyed since at least 1971 (NAFO divisions 4T and 4VWX) has been 75% (78%-81% for mature plaice).
• Plaice are widely distributed throughout the Maritime DU. In summer trawl surveys, plaice occupy between 66 and 84% of the area surveyed. The area occupied by the total plaice population appears to be stable or slightly increasing over time; however, the area occupied by the upper size range of adult plaice appears to be in decline.
• The stock projections based on Bayesian state-space models were combined to provide a view of future trends of abundance of female spawning stock numbers. The median SSN will increase over the next 48 years with a 29% probability of plaice in the DU declining to the reference level without a fishery and 36% probability with current harvests.
• For each stock in the Maritime DU, the difference in probability of declining below the reference level without fishing or with current harvests is small (1% to 4%).

This Science Advisory Report has resulted from a Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, zonal advisory process meeting, March 24-25, 2011 on Recovery Potential Assessment (RPA) of the Maritime Designatable Unit of American Plaice. Additional publications from this process will be posted as they become available on the DFO Science Advisory Schedule.

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