Effect of age and temperature on spawning time in two gadoid species

Large inter-annual differences in spawning time and duration have been reported in a number of fish stocks. This study considers the relative importance of age and temperature on this variability in cod (Gadus morhua) stocks from both sides of the Atlantic and haddock (Melanogrammus aeglefinus) in the North Sea. Age related differences in spawning time were evident from the proportion of spawning or spent fish. These effects were not linear but resembled a step function. The effect of age was opposite between the Northeast and Northwest side of the Atlantic, with younger fish spawning later than older fish in the North Sea and the opposite occurring in the Northwest Atlantic. Such differences in regional trends may be related to differences in energy acquisition and allocation. Temperature effects were considered both as an effect on the developing gonad and as cue to spawning. There was significant inter-annual variation in spawning time but there was little evidence of an effect of temperature on this variation. In Div. 3L and Div. 3NO, cod spawned later in recent years. The fact that mean age of spawners has declined in both these areas and young fish in these regions tend to spawn earlier than old fish suggests that the later spawning in recent years is unlikely related to changes in age structure. Analyses by age suggest fish of all ages are now spawning later than in previous years in these stocks. Such findings point to the underlying complexity of the processes governing spawning time.