Understanding physiological and environmental variables that initiate sexually maturity would provide fundamental information on life history dynamics. The aim of this study was to test the usefulness of the common circumnuclear ring (CNR), an oocytic structure similar to the Balbiani body, which appears just prior to oocyte development as a predictor of first maturation in Atlantic herring (Clupea harengus) stocks. The relative roles of physiology (e.g. fat) and photoperiod as triggers of maturation were also investigated. Samples were collected in May 2008 (72°26’-73°84’N/11°26’-18°40’E) and February 2009 (56°12’-59°45’N/00°25’-03°06’W). These data suggested that thresholds in body size may influence the decision to mature. We also found that short days (winter solstice) may be the photoperiod trigger for a first decision window for both Norwegian spring-spawning (NSS) and North Sea autumn-spawning (NSAS) herring. The second decision window for NSAS herring maturation appears to be triggered by longer days (spring equinox) while a decreasing rate of day lengthening may trigger NSS herring maturation. So photoperiodic cycle is a key determinate of the timing of maturation in Atlantic herring.