Marine scientists at the Norwegian Institute of Marine Research (HI) tricked Norwegian spring-spawning herring into believing that it was autumn when it was actually spring, which in turn then changed the herring’s spawning period.
In the first quarter of the year during February – March, large quantities of Norwegian spring-spawning herring, also known as NVG herring, come to the coast to spawn. Most of the herring in the North Sea, on the other hand, spawn in the autumn.
Autumn and spring spawning herring are the same species, but different populations. An international team of Swedish, Norwegian, Danish and Irish researchers used genome sequencing (genetic markers) to characterize 53 herring populations from the Atlantic and Baltic Seas. Genome sequencing makes it possible to analyse and compare several million markers of the fish. Each fish has a unique genome, but among the population, some markers are always the same. It gives us the opportunity to separate populations from each other.
But what exactly is it that makes the spring-spawning herring spawn in the spring? And what does it take for the spawning time to change? Marine scientists are now in the process of finding out.
“We wanted to test whether the spawning season is affected by the length of the day, which increases in the spring and decreases in the autumn,” says researcher Florian Berg, who led the study.
To find out, Berg and his colleagues conducted an experiment over three and a half years. They raised larvae of spring-spawning herring in two similar tanks:
- In one tank, they controlled the light so that it followed the natural daylight variation in the sea area outside Bergen where the herring was caught.
- In the second tank, the light regime was postponed by six months – so that the “days” became longer in the tank in parallel with them becoming shorter in nature, and vice versa.
- Temperature, food supply and other environmental conditions were otherwise identical in the two tanks, so that the researchers could study the effect of mere variation in light.
Wait for spring
“By manipulating the photoperiod, we let herring that had hatched in the spring grow up with autumn light. Therefore, we were interested in seeing if they became spawning mature after three years, as normal – or after three and a half years, when they experienced their third “spring”,” Florian Berg explains.
The result? The herring that had the photoperiod manipulated waited until they experienced that the days got longer before they became ready for spawning. The spawning period was thus forested by six months.
In other words, it seems that it is the photoperiod – the length of the day – that controls the biological clock of the herring.
Do not be affected by spring odor
The results enabled the researchers to refute an alternative hypothesis: that the “smell of spring” in the sea gives the herring a signal that it is time to spawn.
The tanks were supplied with water from the fjord outside, where the salinity and nutrient content of the water changes with the seasons. The sign of spring in the lake came while the days became shorter for the herring.
“If we saw that the herring spawned while it was spring in nature, even though it was autumn in the tank, we could have explained it with such a chemical signal. But that was not what we saw,” Berg explains.
Shows that the spawning season is climate robust
Research can shed light on how herring will cope under changing environmental conditions in the future.
“This shows that Norwegian spring-spawning herring will always spawn in the spring, even if climate change causes increased temperatures. Herring spawning depends on the light – which will always follow the seasonal variation, even with climate change,” says Florian Berg.
This is the first time that such an extensive attempt to manipulate day length has been made with herring. The researchers are already well underway with a new experiment, where they also have different temperatures in the two herring tanks. This will be able to confirm whether the temperature also affects the spawning of the herring, in addition to the length of the day.
Researchers have previously found that herring grow less in warmer seas .