Research on whitefish has a long history at the Department of Environmental and Biological Sciences in Joensuu. For fish researchers, late spring is a busy time, as whitefish eggs that have survived the winter are starting to hatch. Juvenile whitefish are used in various experiments: currently, the researchers are studying the effects of environmental toxins and elevated water temperatures on fish fertilisation, as well as the role of microplastics in the food chain of fish.
“Fish research is one of the lines of our aquatic research,” Professor Raine Kortet says.
Together with Professor Anssi Vainikka, he leads the university’s large and multidisciplinary research group on Aquatic Ecology and Behavioural Ecology. The group has a long history of research into aquatic biology, aquatic ecology, evolutionary ecology and behavioural ecology, among other things. One of the fish species frequently used in their studies is the whitefish.
“Due to its external fertilisation, the whitefish is a good model species. It produces large quantities of eggs, which can be divided into smaller portions. Moreover, the number of juvenile whitefish is often large, which makes it a good species for various studies,” Senior Researcher Hannu Huuskonen says.
He has studied the whitefish since the 1990s, using multiple whitefish populations, such as those originating from the rivers Oulujoki, Koitajoki and Kokemäenjoki. In Finland, the whitefish has six different ecomorphs, some of which are endangered, such as the migratory whitefish.
“For the lake-spawning whitefish, however, things are looking brighter. The situation of the migratory whitefish is complicated by a lack of fish passages, which have not been taken into consideration in many construction projects,” Huuskonen notes.
The whitefish study currently ongoing at the Joensuu Campus includes 900 recently hatched juvenile whitefish. During the study, the fish will spend three weeks in oxygen-rich water in the laboratory’s new fish tanks. The study involves a controlled family setting with five female and ten male fish that have produced a total of 50 fish families.
“The fish will be anaesthetised, after which we will collect their eggs and sperm to undergo dry fertilisation. As in many other studies, we are collaborating with the Natural Resources Institute Finland in this one, too.”
This is the first growth experiment using the new system, but, according to the researchers, the same fish tanks can also be used for studies involving crayfish and aquatic insects in the future.
“We study the effects of cadmium, an environmental toxin released into waterways in connection with mining, on whitefish fertilisation and offspring. We exposed fish sperm – i.e. not eggs – to two different toxin levels for a period of ten seconds during fertilisation. A swim test performed on juvenile whitefish showed that there are differences in growth and mobility, and these differences were visible already in newly hatched fish still carrying a yolk sac. Our preliminary analyses indicate that juvenile fish exposed to the environmental toxin have a poorer ability to swim.”
Climate change weakens the ability of fish to swim
Studies going into the next generation of fish usually last for a long time, typically at least six months. The standardised settings of the fish tanks make the studies easy to reproduce. Before embarking on a study involving fish, permission to conduct research on animals is required. All animals, including embryo-stage fish inside eggs, need proper care on a daily basis.
“Whitefish, like all endemic fish species here in Finland, reproduce only once a year, and this means that we only have one shot in our study. If we used non-native aquarium fish such as the zebra fish, we’d have access to multiple generations over the year,” Huuskonen explains.
The fish tanks are filled with cool water, +10 degrees Celsius, and this reduces the occurrence of various fish diseases. In any case, the whitefish spawns in cold water in the autumn, and eggs develop under ice, hatching around the time the ice cover melts in early spring.
The researchers are also studying whitefish fertilisation when exposed to elevated water temperatures. This is an important line of research, as the whitefish is expected to lose the battle to climate change – and to warmer waters that will follow.
“We noticed that a short-term and a very moderate exposure of fish sperm to an elevated water temperature prior to fertilisation weakened the swimming ability of juvenile fish when compared to a control group. Since the fish in both the groups were genetically identical, and exposure to an elevated temperature did not affect sperm quality as such, the weakened swimming ability seems not to be associated with genetics,” Associate Professor Jukka Kekäläinen says.
The swimming ability of fish is tested by putting one juvenile fish at a time in a swimming tube with a standardised water current. With 900 juvenile fish and one test lasting around one minute, a researcher might end up spending the day, or even weeks, by the pool.
In fish, the swimming ability is associated with their survival, ability to find food and ability to avoid predators. The better the swimming ability, the better the chances of a fish to make it into adulthood.
“There is great variation in the swimming ability of juvenile fish: some can only swim for seconds, while others can go on for half an hour. When a fish is done swimming, it ends up in a sieve to be put down and examined.”
Whitefish eat microplastics, too
Microplastics are currently being studied on both UEF campuses from slightly different angles. A study on the role of microplastics ingested by juvenile whitefish is currently ongoing in Joensuu, and an article on the results has just been submitted for review and publication in a scientific journal.
In whitefish studies, Artemia, a living zooplankton, is used as a standard food. It lays dry, dormant eggs from which living larvae hatch when exposed to salty water. This plankton belongs to the family of brine shrimp and can survive in dried-up salt lakes even up to ten years.
“Artemia and the form of microplastic we use in our study, i.e. industrially manufactured polystyrene, are roughly the same size: approximately 100 micro metres. Juvenile whitefish are eager to eat either,” Huuskonen explains.
“They are identically accumulated in the intestine. The fish doesn’t actually choose the plastic but eats it alongside plankton. This happens despite that fact that even juvenile fish carrying a yolk sac have already developed taste organs. The juvenile fish could spit the microplastic out if it wanted to.”
Microplastic takes up space in the intestine of a fish and causes a sensation of hunger. The juvenile fish then needs to use more time and energy for eating, and this slows down its growth. The survival of fish is dependent on how much they eat and how quickly they grow: 99.9 per cent of juvenile whitefish die before reaching adulthood.
“Round microplastics can pass through the intestine of a fish; they do not necessarily build up there. However, the possible absorption through cell membranes of chemicals and other substances present in microplastics may have adverse effects on fish health.”
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