Research Document - 2011/040
Natural mortality and projected biomass of southern Gulf of St. Lawrence cod (Gadus morhua)
By D.P. Swain
Abstract
The population of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence collapsed in the early 1990s and has shown no sign of recovery since then despite severely limited fishing mortality. One of the main reasons for this lack of recovery is elevated M, the instantaneous rate of natural mortality. Survey-based estimates of total mortality and relativefishing mortality indicate that M was very low in the 1970s and had increased to a higher level (0.4 or higher) by the 1990s, possibly increasing to even higher levels since then. Population models which estimate age-aggregated (2+ years) or age-disaggregated (2-4 and 5+) trends in M were compared. Models which estimate a single age-aggregated time trend in M are not consistent with the data for this stock. Models which estimate separate time trends in M for young (age 2-4 yr) and older (5+) cod provide a better fit to the data. Estimated trends in M differ between the two age groups. Estimated M declined in the early 1990s for cod aged 2-4 years. For cod aged 5+ yr, M was low (near 0.1) in the early 1970s, increased slightly in the late 1970s and early 1980s, then increased sharply in the late 1980s and early 1990s, and has remained at a high level since then. The rapid increase in M of 5+ cod as their abundance declined is consistent with the hypothesis that their current high M is at least partly a predation-driven Allee effect. At the current levels of M and other components of productivity, spawning stock biomass (SSB) is projected to decline steadily even with no fishing, reaching levels near extirpation (SSB<1000 t) in about 40-50 years. Based on the preferred model, in order to have a high probability (P) of reaching the limit reference point (SSB=80,000 t) in 20 years, 5+ M would have to decrease from the estimated current level of 0.63 to 0.4 or lower if other components of productivity remain unchanged (P = 70% for M = 0.4, 100% for M= 0.35).
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