Shifts in spawning phenology of cod linked to rising sea temperatures

Warming temperatures caused by climate change have the potential to impact spawning phenology of temperate marine fish as some species have temperature-dependent gonadal development. Inter-annual variation in the timing of Atlantic cod (Gadus morhua) spawning in the northern North Sea, central North Sea and Irish Sea was estimated by calculating an annual peak roe month (PRM) from records of roe landings spanning the last three decades. A trend towards earlier PRM was found in all three regions, with estimates of shifts in PRM ranging from 0.9 to 2.4 weeks per decade. Temperatures experienced by cod during early vitellogenesis correlated negatively with PRM, suggesting that rising sea temperatures have contributed to a shift in spawning phenology. A concurrent reduction in the mean size of spawning females excluded the possibility that earlier spawning was due to a shift in size structure towards larger individuals, as large cod spawn earlier than smaller-sized individuals in the North Sea. Further research into the effects of climate change on the phenology of different trophic levels within the North Sea ecosystem should be undertaken to determine whether climate change-induced shifts in spawning phenology will result in a temporal mismatch between cod larvae and their planktonic prey.