Thermal stratification influences maturation and timing of spawning in a local Clupea harengus population

Maturation and timing of spawning in relation to temperature were studied in a local Atlantic herring Clupea harengus population inhabiting a small semi‐enclosed ecosystem (7 km²) separated from the larger outer fjord system by narrow sills on the west coast of Norway. Ambient temperatures varied annually up to 4° C during both the pre‐spawning and spawning periods from February to April, but without affecting the spawning time. Instead, the timing of spawning was found to be related to thermal stratification in response to spring warming, which occurred about the same time every year regardless of initial temperatures.