Science Advisory Report  2011/028

Recovery Potential Assessment for the Laurentian South Designatable Unit of Atlantic Cod (Gadus morhua)

Summary

• The Laurentian South DU of Atlantic Cod consists of three stocks or management units: southern Gulf of St. Lawrence cod (management unit 4T-4Vn(Nov-Apr)), 4Vn resident cod (4Vn(May-Oct)), and eastern Scotian Shelf cod (4VsW). Until recently, the southern Gulf stock was the largest of the components.
• Long term projections were undertaken for the southern Gulf and eastern Scotian Shelf stocks and at the DU level. Future productivity conditions are very uncertain. Thus, these projections should not be interpreted as forecasts of future stock status because they depend on assumptions about future productivity and fishing mortality. The probability of current conditions continuing for a long period of time is unknown. These projections are explorations of the consequences of particular productivity assumptions.
• Projections are illustrated using the median and the 2.5^th and 97.5^th percentiles. The full range of uncertainty should be considered when interpreting these projections.

Southern Gulf of St. Lawrence Cod

• The southern Gulf population is at the lowest level observed in the 61-year record and is declining. Abundance of mature cod in 2008-2010 is estimated to average 37% of the average level in the mid to late 1990s and 10% of the average level in the mid 1980s.
• Natural mortality of southern Gulf cod aged 5 years and older (5+) is estimated to be unusually high (averaging 0.66 in 1994-2010). Predation by grey seals is considered to be a major component of this high natural mortality.
• Most other components of productivity are also currently low for southern Gulf cod. Weight-at-age declined to a low level in the late 1970s and early 1980s and has remained low since then. Recruitment rate (recruits per unit of spawning stock biomass (SSB)) was exceptionally high in the mid to late 1970s but declined to a low level in the 1990s and 2000s, comparable to the rates observed in the 1950s and 1960s. There has been no evidence of a compensatory increase in recruitment rate at the recent low levels of SSB.
• During the small directed fishery for southern Gulf cod in 2007 and 2008, fishing mortality is estimated to have been 0.11 for fully-recruited ages (9-10), a small fraction of natural mortality, but still unsustainable given current stock productivity. With the closure of the directed fishery in 2009, fully-recruited fishing mortality dropped to 0.014, a negligible level.
• A conservation limit reference point (LRP) has been established for southern Gulf cod, based on the SSB below which the probability of poor recruitment is high. The LRP is estimated to be 80,000 t. Estimated SSB has been below the LRP since 2003. Estimated SSB at the beginning of 2010 is 39,500 t.
• Productivity of the southern Gulf cod stock has been very low throughout the past 20 years. If these conditions persist in the future, this stock is expected to continue to decline, even with no fishing. Under these conditions, the probability of reaching the LRP would be zero.
• In 2009, following closure of the cod-directed fishery, fishing mortality of southern Gulf cod due to scientific monitoring activities and bycatch in fisheries directing for other species was at a very low level. The effect of this level of fishing mortality on the projected population trajectory and thus on the probability of population survival is negligible. In contrast, fishing mortality at the level estimated in 2007 and 2008, when there were small cod-directed fisheries, accelerates the projected population decline and thus decreases the probability of population survival.
• The only additional action that can be taken to improve the chances for recovery of southern Gulf cod would appear to be action to reduce the rate of natural mortality on adult (5+) cod. Given that predation by seals is considered to account for a high proportion of this mortality, grey seal removal would be expected to reduce natural mortality, though large declines in natural mortality would likely require substantial seal removals.

Eastern Scotian Shelf (4VsW) Cod

• The SSB of 4VsW cod reached the lowest level observed in the 53-year record in 2003 at about 7,500 t. Recently, it has rapidly grown to 64,000 t and is approaching the long term mean (75,000 t).
• Natural mortality of 4VsW cod aged 5 years and older (5+) was estimated to be unusually high in the 1990s and early 2000s (peaking at approximately 1.1) but has recently declined to 0.36.
• For 4VsW cod, most other components of productivity were at their lowest values in the period 1990-2000 and then have shown some recovery since then. Weight-at-age, condition, and area occupied all show such a pattern. There have also been improvements in recruitment rate in some recent years.
• Since the closure of the directed fishery in 1993 fishing mortality (ages 5-15) is estimated to have been 0.035, a small fraction of natural mortality. More recently it has dropped further to about 0.01, a negligible level.
• A LRP has been established for 4VsW cod, based on 40% of the spawning stock biomass at Maximum Sustainable Yield (MSY) during the productive period before 1990. The LRP is estimated to be 50,000 t. Estimated SSB has been below the LRP since 1992 with the exception of the 2009 estimate of 64,000 t, 25% above the LRP.
• If 1994-2009 productivity conditions persist in the future, projections indicate that SSB of the 4VsW population would be expected to decline below the LRP and then stabilize in the long term at a low level, even with no fishing. Productivity conditions have improved in the past few years compared to the average of those used in the projections.
• Removals of 4VsW cod at the level of the bycatch fishery since closure of the cod-directed fishery in 1993 have no detectable effect on the probability of survival or recovery.
• The only additional action that can be taken to improve the chances for recovery of 4VsW cod would appear to be action to reduce the rate of natural mortality on adult (5+) cod. Predation by grey seals is considered to be a significant component of natural mortality but its relative contribution is of unknown magnitude. Even without establishing the degree of causality, it is noted that the Sable Island grey seal population was under 50,000 animals when 4VsW cod was productive; the current Sable herd size is around 300,000, six times larger.

4Vn Resident Cod

• Catches of the 4Vn resident component (referred to as 4Vn) of the DU declined from 1985 until closure of the fishery in September 1993. Currently, landings have been restricted to bycatch only, which is very small relative to natural mortality.
• Both DFO and industry led sentinel surveys indicate that the abundance of cod in the 4Vn stock has declined to low levels and is currently at or near the lowest levels seen.
• Natural mortality has been high during the recent period. The possible causes of this increased natural mortality are not understood.
• A LRP, based on the minimum SSB from which the stock has readily recovered, was determined based on area-expanded mature survey biomass of July. The LRP corresponds to the 1973-1978 period, with a mean value of about 8,400 t. Currently (2004-2009) the biomass is about 2,250 t, or about 25% of the limit.
• Recruitment was highest for the 4Vn stock from 1975 to 1988. Recruitment rate was low for the early part of the time series, but higher since 1998.

Laurentian South Designatable Unit (DU)

• The demersal juvenile stage (4 - 35 cm long) is the most habitat-dependant period in the life-cycle of Atlantic Cod. Physical disturbance of structural components of habitat can reduce its value and increase mortality of juvenile cod. Existing data lack the spatial resolution required to evaluate the amount of suitable habitat available to demersal juveniles and whether it has changed in the past three generations, especially in the offshore. However, there is no indication that the amount of suitable habitat is currently limiting recovery of cod in this DU.
• In the mid 2000s, the combined survey indices of abundance and biomass of mature cod from the three stocks in the Laurentian South DU were near or below the lowest levels previously observed. Since then, these indices have increased slightly due to increases in the 4VsW stock.
• DU-level projections could be done combining the southern Gulf and 4VsW stocks only. Based on the survey indices, these two stocks comprise greater than 90% of the abundance of mature cod in the DU. According to projections, the combined mature abundance of the southern Gulf and 4VsW stock components would be expected to decline if current productivity conditions were to persist. The probability of exceeding their mature abundance 36 years earlier (i.e., the probability of no decline over 36 years) falls to zero early in this projection.

This Science Advisory Report has resulted from a Fisheries and Oceans Canada, Canadian Science Advisory Secretariat, zonal advisory process meeting, Feb. 21-25, 2011 on Recovery Potential Assessment (RPA) of Atlantic Cod). Additional publications from this process will be posted as they become available on the DFO Science Advisory Schedule.

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