Spawning and hatching temperatures of whitefish (Coregonus lavaretus (L.)) in the Northern Baltic Sea
Veneranta, Lari; Urho, Lauri; Koho, Jorma; Hudd, Richard
Catches of sea-spawning whitefish (Coregonus lavaretus (L.)) have decreased and stocks of this diverse species have become vulnerable in the Baltic Sea area. The objectives of this study were 1) to find out whether whitefish can still hatch in the wild in the Archipelago Sea of the southern Gulf of Bothnia, and 2) to define the differences in spawning and hatching times as well as hatching success between areas at different latitudes with diverging temperature loads. To estimate the spawning and hatching temperatures and times of the sea-pawning whitefish, both field and laboratory experiments were carried out in 2007 and 2008. Fertilised whitefish eggs were incubated in spawning areas and at two different temperatures in the laboratory. Naturally-spawned whitefish larvae were also sampled in the vicinity of these test areas and from a more northern reference area. The study indicated that the spawning of whitefish in the northern Baltic Sea starts in late autumn when the day length decreases to approximately 8 h 15 min and the seawater temperature decreases to a level of 7.2 to 3.5 °C, regardless of latitude. The hatching period of sea-spawning whitefish larvae is not only dependent on temperature day-degrees but also on the temperature increment. Whitefish larvae hatch in spring after the ice breakup when the water temperature reaches a level of 2 to 4 °C. The hatching experiment revealed that some natural reproduction can still take place in the southern parts of the Gulf of Bothnia in the Baltic Sea, specifically the Archipelago Sea and the Bothnian Sea, although whitefish larvae are more frequent in the northern areas.