Growth histories derived from otolith microstructure of three Norwegian cod stocks co-reared in mesocosms; effect of initial size and prey size changes

Specific growth rates and survival were not significantly different among t he pelagic juveniles of three electrophoretically distinguishable stocks of Norwegian cod (Gadus morhua), cc-reared in four replicate 5.2 m(3) mesocos ms, from hatch to day 38 or day 43 post-hatch. Arcto-Norwegian cod (AC, the main commercial cod stock which spawn in Lofoten and are distributed in th e Barents Sea, mostly north of 70 degrees N), were compared with a nearby c oastal stock (Balsfjord cod, BC - from a fjord near Tromso at 69 degrees 30 'N), and a more southern coastal cod stock (CC, from near Bergen at 60 degr ees N). The increment width series of the lapillus - otolith growth history - and the relative daily growth rate were not significantly different amon g stocks within any mesocosm. Although AC larvae were larger, being derived from larger brood stock, they did not exhibit consistently greater otolith growth during the experiment. Larger size at harvest was correlated with g rowth during the latter part of rearing (>fourth week), which is when most differences in size at age are generated. Decreases in daily increment widt hs occurred in all stocks within 1-2 days, when the zooplankton food supply was changed from nauplii and copepodites to adult copepods, and when the a verage AC cod began to exhibit greater dairy growth. Increment widths of re ared AC cod at 37 days post hatch (>8 mu m) were 2-3 times greater, and ont ogenetically more variable than those of similar sized cod previously sampl ed from the Barents Sea (3-4 mu m). Average otolith growth histories of pel agic juvenile cod reveal a substantial range in growth potential that is no t apparent from more conservative comparisons of weight, and reveal a tempo ral sensitivity useful for the assessment of environmental fluctuations in the rearing process. (C) 1999 International Council for the Exploration of the Sea.