|Fisheries-induced evolution and impaired productivity of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence.
Doug Swain, Fisheries and Oceans Canada, Gulf Fisheries Centre, Moncton, New Brunswick, Canada
There is accumulating evidence of fisheries-induced evolution in life-history traits. These changes may not be readily reversible by limiting fishing and, through their effects on productivity, may contribute to the slow recovery observed for many collapsed fish populations. These issues are examined here for the case of Atlantic cod (Gadus morhua) in the southern Gulf of St. Lawrence (SGSL).
The fishery for SGSL cod intensified beginning in the late 1940s, and the biomass of commercial-sized cod declined by 80% from the mid 1950s to the mid 1970s. The stock recovered quickly in the late 1970s despite on-going fishing, but collapsed a second time in the late 1980s and early 1990s. Despite severely restricted fishing mortality for nearly twenty years, the stock has shown no sign of recovery, instead declining further. The difference in stock recovery between the mid 1970s and the early 1990s reflects a change in productivity. Size-at-age of SGSL cod declined sharply in the late 1970s and early 1980s and has remained low since then. Natural mortality of adult cod increased in the 1980s and early 1990s and remains very high. At its current level of productivity, the SGSL cod population does not appear to be viable.
Age and size at maturation of SGSL cod declined rapidly in cohorts produced from the mid 1950s to the early 1970s, likely reflecting an evolutionary response to intensified fishing. There has been little recovery in age and size at maturation despite very low fishing mortality since 1994. A predicted consequence of early maturation is increased natural mortality due to higher costs to reproduction. However, the current high natural mortality of SGSL cod appears to be primarily a cause, rather than a consequence, of the continued early maturation in this population, now replacing fishing mortality as the agent of selection favouring early maturity. This high natural mortality may reflect a predation-driven Allee effect or ‘predator pit’.
The decline in size-at-age of SGSL cod in the late 1970s and early 1980s reflected a density-dependent decrease in growth rate combined with the phenotypic response to a change in the direction of size selection by the fishery. Size-at-age remains low despite good conditions for growth (warm water temperatures, high per-capita prey biomass) and little size-selective fishing mortality. This may partly reflect a genetic response to the strong selection against fast growth imposed by the fishery in the 1980s and early 1990s. However, size-selective natural mortality favouring slow-growing phenotypes and/or reduced foraging success due to increased risk of predation may also contribute to the lack of recovery in size-at-age.
It has been suggested that fisheries-induced life-history changes may not be readily reversed by limiting fishing. For example, reversal of fisheries-induced evolution may be slow because, compared to the selection imposed by fisheries, natural selection in the reverse direction is expected to be weak. However, in this case, lack of reversal appears to reflect strong natural selection acting in the same direction as fishery selection had acted. This strong natural selection may itself be partly a consequence of overfishing (e.g., the result of a predation-driven Allee effect). More precautionary management regimes are needed in order to moderate fisheries-induced evolutionary changes and to avoid stock collapses that are not readily reversed be stopping fishing.