Latitudinal variability in lunar spawning rhythms: absence of a lunar pattern in the northern mummichog Fundulus heteroclitus macrolepidotus

In this study, the natural spawning season of the northern mummichog Fundulus heteroclitus macrolepidotus was examined from May to July in the lower St John River, New Brunswick, Canada (45 degrees N), through measurements of gonado-somatic index (I(G)), liver-somatic index (I(L)) and condition factor (K). I(G) increased during the prespawning phase (mean +/-s.e. 5.49 +/- 0.20%), peaked during the spawning phase (14.68 +/- 0.51%) and dropped during the regression phase (3.23 +/- 0.29%). A single peak of spawning was observed in F. h. macrolepidotus, contrasting with multiple peaks observed in studies south of 41 degrees N in North America. Within the prespawning phase, there were significant differences between sampling dates in female body mass, gonad mass, liver mass and K. On the basis of results from this study, water temperature is believed to be the predominant factor controlling initiation of spawning in F. h. macrolepidotus.