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

• Published in: ICES Journal of Marine Science
• Main Authors: Suthers, Iain M., Sundby, Svein
• Format: Text
• Language: English
• Published: Oxford University Press 1996
• Subjects: Articles; Canada; Norway; Gadus morhua; North East Atlantic; North West Atlantic; Northern Norway; midnight sun
• Online Access: http://icesjms.oxfordjournals.org/cgi/content/short/53/5/827; https://doi.org/10.1006/jmsc.1996.0104

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, r2=0.42), but an ANCOVA model comparing the AN and 4X regions was highly significant (r2=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°N latitude off Norway compared with 43°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.