The blue rod reproduces precisely at the full moon, but it is also required that it is bright and warm.
– I guess that the bluefish in the Stockholm archipelago will reproduce precisely on Midsummer’s Eve, provided it is not windy, says Lena Kautsky, professor of marine plant ecology at Stockholm University’s Baltic Sea Center to Swedish Radio’s Science Radio.
However, how the blue bar knows that it is a full moon is a mystery.
It also seems that the bluefish are releasing their eggs and sperm earlier in the year due to climate change.