Spawning site fidelity by Atlantic cod (Gadus morhua) in the Gulf of Maine: implications for population structure and rebuilding

Rebuilding the Gulf of Maine stock of Atlantic cod (Gadus morhua) has been much slower than expected. An important source of scientific uncertainty contributing to the difficulties in managing rebuilding has been the lack of understanding of cod population structure. Previous research indicates that the stock functions as a metapopulation that is made up of multiple subpopulations and many finer-scale spawning components. This study investigated fine-scale, multiyear spawning site fidelity by a spring-spawning component of Atlantic cod in the western Gulf of Maine. Movements of acoustically tagged cod (n = 63) with respect to a known spawning site were tracked using passive acoustic telemetry. A large proportion (38-67%) of tagged cod exhibited spawning site fidelity between 2010 and 2012. After adjusting for fishing mortality, natural mortality, and skipped spawning, the estimated rate of spawning site fidelity ranged between 47 and 95% in 2011. Multiyear spawning site fidelity was also observed, with individuals being tracked for up to four consecutive spawning seasons. Spawning site fidelity serves as one of the multiple mechanisms that contribute to the formation and maintenance of the observed metapopulation structure. Spawning site fidelity also reduces the reproductive connectivity among spawning sites, thus delaying both recolonization of abandoned spawning sites and stock rebuilding. Future stock assessment models and fishery management plans that incorporate the metapopulation structure of cod in the Gulf of Maine are expected to be more effective at preventing continued declines in spawning diversity and promoting rebuilding.