Role of the midnight sun: Comparative growth of pelagic juvenile cod (Gadus morhua) from the Arcto-Norwegian and a Nova Scotian stock

Size-at-age of pelagic juvenile cod from the north-east Atlantic off northern Norway was approximately twice that of cod from the north-west Atlantic, off south-western Nova Scotia, Canada. Arcto-Norwegian cod (AN, 17-48 mm standard length, SL, 34-90 d post-hatch), were sampled in July 1988 with a capelin pelagic trawl, while south-west Nova cod (NAFO region 4X, 7-32 mm SL and 32-105 d post-hatch) were sampled in May-June 1985-1986 with a Tucker trawl. Growth over the previous 14 d, back-calculated from otolith daily growth increments was 0.71 mm d(-1) and 0.33 mm d(-1) for AN and 4X cod respectively. Within and between stocks, water temperature and zooplankton biomass were significantly correlated with the 14 d index (linear model, r(2)=0.42), but an ANCOVA model comparing the AN and 4X regions was highly significant (r(2)=0.71), indicating additional factors. Gear selection was not found to be responsible. While genetic factors could produce this result, there is 48% more time during May-July for visual feeding north of the 71 degrees N latitude off Norway compared with 43 degrees N off eastern Canada, using the light intensity threshold for larval cod feeding. Our hypothesis of light-limited feeding opportunity is consistent with a size- and temperature-dependent consumption model, and with aquaculture methods, as well as the necessity for fast growth in the short northern summer for over-winter survival. (C) 1996 International Council for the Exploration of the Sea