NORWAY - For decades, scientists have wondered what regulates changes in salmon when they transform from being freshwater to saltwater fish, and now researchers at Norway's Uni Research Ltd may be moving towards an answer.
A new study shows that light signals from increases in day length in the spring affect developmental processes in the fish's brain during smoltification.
In the recently published study, Lars Ebbesson and colleagues found that light increased the production of a special enzyme, type 2 deiodinase, activating the thyroid hormone in the smolt brain.
This enzyme stimulates the fish to prepare itself before it moves from freshwater out into salt water.
Mr Ebbesson said that previously: "We have presumed that changes in thyroid hormones have been important for normal smolt development, but we have not known how the hormone is activated and it specific roles."
They also found an important change in a similar enzyme in the gills. The gills are important for regulating the salt balance in the fish.
In the study, they found that this gill enzyme that activates the thyroid hormone in the gills only increases when the fish reaches saltwater.
The present study may explain why previous work on thyroid hormones and gill development in smolts, which have focussed on the freshwater parr-smolt transformation, have found thyroid hormones to have a minimal role.
"Taken together, these results provide new insight into the ways in which the smoltification is regulated and the ways in which the environment affects this special transition from fresh water to salt water," said Mr Ebbesson.
The research group is now striving to become a Centre of Excellence in Integrated Fish Neurobiology.
“The brain is the central regulator of most biological processes, yet only a few scattered research groups study how the fish brain works in Norway.
"In a country where fish plays such an important role in society, a centre that can give new insight to important fundamental questions on brain function will also impact how we manage wild fish populations and improve aquaculture practices,” said Mr Ebbesson.
Ebbesson also emphasises that knowledge about how the fishes' brains function will be important for the aquaculture industry. Among other things, they will be able to predict and regulate how the fish will be affected by environmental changes.