TIMING OF COD REPRODUCTION - INTERANNUAL VARIABILITY AND THE INFLUENCE OF TEMPERATURE

We examined interannual variability in the timing of spawning of femal e cod Gadus morhua from 1947 to 1992 in 3 regions off Newfoundland, Ca nada, in the northwest Atlantic. Maturity data, assessed by visual exa mination of dissected gonads of cod collected by research trawls, were analysed with probit regressions to identify the day of each year on which 50 % of females had ceased spawning (which we ref er to as spawn ing time) on northern [Northwest Atlantic Fisheries Organisation (NAFO ) Division 3L] and southern (3NO) Grand Bank and on St. Pierre Bank (3 Ps). We minimized the bias in our estimates by separating sampling var iability and interannual variation in age structure from true interann ual variability in reproduction. Among regions, average spawning time (mean +/- SD) varied from Days 157 +/- 18 (June 6) and 139 +/- 16 (May 19) on northern and southern Grand Bank, respectively, to Day 135 +/- 24 (May 15) on St. Pierre Bank. Interannual differences in spawning t ime were significant within 3L (1948-1991), 3NO (1947-1992), and 3Ps ( 1953-1987) (likelihood ratio tests; p < 0.001). Interannual variation in spawning time was significantly associated with variation in water temperature prior to spawning in 3L and in 3Ps although the signs of t he associations differed between regions, casting doubt on the hypothe sis that the timing of cod reproduction represents an adaptive respons e to temperature change. The negative correlation between temperature and spawning time in 3L can be explained by the positive influence of temperature on gonad development. In 3Ps, we attribute the early spawn ing dates in years characterized by cold bank temperatures to (1) a th ermal barrier imposed by sub-zero temperatures on spawning migrations from the continental slope to the shelf, and to (2) increased rates of gonad development, and an earlier readiness to spawn, experienced by cod 'forced' to prolong their residence in warm slope waters. Our anal yses indicate that cod spawning time varies significantly among years, demonstrate how the effects of temperature on cod reproduction depend on regional hydrography, and underscore the importance of separating variation in sampling protocol and age structure from true interannual variability in spawning time.