Research Document - 2000/022

Success and Failure of Atlantic cod, Gadus morhua : a case study from coastal Newfoundland.

By I.R. Bradbury, G.L. Lawson, D. Robichaud, G.A. Rose, and P.V.R. Snelgrove

Abstract

Despite a century of study, ecologists still struggle to understand fluctuations in marine populations, including many commercial species whose fisheries have collapsed despite considerable management effort (Hjort 1914, Sissenwine 1984). We present data on Northwest Atlantic cod that encompasses the adult spawning to juvenile settlement stage and demonstrate that timing of spawning is critical, particularly as it relates to spawning location and coastal circulation in regulating recruitment success. In what is presently the largest commercially-exploited Atlantic cod (Gadus morhua) stock remaining in the Northwest Atlantic, we found that spawning occurred at consistent, discrete locations that were generally favorable for propagule retention within the bay. However, when cod spawned relatively early in the year (April - May), colder water led to slower development, and currents swept most eggs offshore before hatch. Spawning later in the year (June-August) resulted in faster hatching, thereby reducing duration of early and vulnerable life stages and the probability of being swept offshore. Elevated densities of pelagic larvae and settled juveniles were associated with late spawning. Successful recruitment to the coastal region therefore requires spawning at sites where propagules are likely to be retained, and at times when egg development rates are maximized. This 'right time, right place hypothesis' suggests that in tandem with more traditional variables such as spawner biomass and condition, timing and location of spawning may be important to spawning success in marine populations that live in highly seasonal environments.

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