Our objectives are to confirm recent observations derived from electronic data storage tags of cyclical spawning behavior associated with a semi-lunar cycle in Atlantic cod and assess both its adaptive significance and potential effects on cod mating systems. We will evaluate whether circulating levels of sex steroids and the histology of gonadal tissue demonstrate a pattern consistent with semi-lunar cycles over a full spawning season in a captive population of cod held in net pens in Stöðvarfjörður. We will use telemetry and both active and passive hydroacoustics to evaluate how this cyclical behavior affects the dynamics of a wild cod spawning aggregation in Kollafjörður. Oceanographic and particle tracking models to assess the adaptive significance of cyclical spawning to early life history stages. We will employ an individual-based eco-genetic model to investigate evolutionary explanations for this behavioral pattern. These studies will provide a better understanding of how the timing of reproductive events evolved in marine fishes and demonstrate a methodology for identifying and confirming their existence. Our results will have practical implications to the management, conservation, and commercial production of Atlantic cod.