Selection for fast growth during the larval life of Atlantic cod Gadus morhua on the Scotian shelf

We tested the hypothesis that fast growing Atlantic cod Gadus morhua survived better than their population of orlgin in the winters of 1991-1992 and 1992-1993 on the Scotian Shelf (northwest Atlantic). Survivors were defined as fish >90 d of age at capture which comprised eplbenthic juveniles >20 mm sampled mainly near the bottom. The majority of larvae and juveniles aged 590 d at capture were sampled with a midwater trawl and were assumed to be representative of the pelagic population (survivors + non-survivors) from which epibenthic survivors originated. Standard length corrected for shrinkage was linearly correlated to lapillar radius (r2 = 0.97). lndividual growth histories were reconstructed from the width of lapillar increments. Selection for fast growth was weak in the winter of 1991-1992 and back-calculated growth and length a t age of survivors were not significantly larger than that of the population (repeated-measures MANOVA). In winter 1992-1993, a strong selection for fast growth was evident in late larvae 41 to 80 d old. The divergence in length a t age between survivors and the population reached 4 mm at an age of 70 d , corresponding to a 13 d reduction in the duration of the larval phase. Survivors in the winter of 1992-1993 had larger hatch marks than the population, suggesting that the potential for fast growth may be reflected in traits present at hatching. Our results support the hypothc.sis that fast growth increases the surv~vorship of Atlantic cod dunng larval 11fe in the plankton and Indicate that the intensity of slze-selective mortality may vary considerably from year to year.

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