Fish eggs susceptible to chemicals
Anne Berit Bjørken
Low concentrations of certain chemicals in seawater injure fish eggs and larvae, which are more susceptible to chemicals than we supposed," says Leif Jørgensen, a SINTEF researcher.
Fish larvae are highly susceptible to the effects of chemicals
Most marine fish eggs drift freely in the upper layers of the sea, unlike salmon eggs, which lie on the bottom. The eggs float because they are a little "fresher" than seawater, i.e. they contain just a little less salt, making them buoyant. Round each egg lies a thin shell and inside the shell is a membrane, which is extremely sensitive to injury or pollution. Chemicals can destroy the membrane, allowing the fresh water to leak out. The result is that the egg becomes as salty as the seawater, whereupon it sinks to the bottom and dies. Eggs need to hatch near the surface of the sea, where the larvae can find food. The chemicals that have been tested are dispersants that are used to dissolve oil-spills.
Eggs
"We have carried out studies in order to determine how much eggs and larvae can tolerate. The eggs were exposed to water containing various concentrations of different chemicals, and we measured the leakage of fresh water through the membrane. Our studies show that the membrane tolerates only very low concentrations of chemicals before it begins to leak."In the worst case, we might actually lose a large proportion of a year-class of cod in a particular area, if such chemicals are used in the spring after the cod have spawned."
The studies showed that, while some eggs lose their ability to float in the water, the surviving eggs may undergo abnormal cell division, leading to the development of abnormal larvae.
Larvae
Cod larvae are even more vulnerable than eggs. The researchers found a high rate of mortality in larvae exposed to water that contained low concentrations of chemicals. The metabolism of the larvae is especially affected when they are exposed to chemicals."The larvae have a very low tolerance threshold," says Jørgensen. These experiments were a collaborative effort that involved the Aquaculture and Environmental Technology Group at SINTEF Applied Chemistry, NTH's Dept. of Environmental Technology and the MR Center.
Fish
The scientists have now evolved a new technique for studying the effect of chemicals on fish. This is based on a special chamber which can be placed in the MR tomograph at the MR Center in Trondheim. The MR tomograph uses nuclear magnetic resonance to study the processes that take place within a fish, allowing its inner life to be studied while it swims around. Previously, it was usually necessary to kill the fish in order to study any effects of chemicals."This is a completely new, groundbreaking technique that enables us to study the energy budget of the fish during even the slightest exposure to chemicals," says Jørgensen.
"As well as monitoring the metabolism of a fish and any metabolic changes that occur, we can also measure its energy uptake, ammonia concentration, etc. ", he adds.