Abstract
The effect of anthropogenic magnetic field on water ecosystems has been constantly growing as a result of an increasing number of underwater cables transferring electric current. Northern pike were subjected to a static magnetic field (emitted by DC cables) of 10 milliteslas (mT) during the embryonic phase and in the first six days post-hatching (DPH), in the period when larvae of this species in the natural environment are attached to artificial substrate. No statistically significant effect of a magnetic field was recorded on the hatching success (87.1% in the magnetic field and 83.3% in the control) or on larvae mortality (54.5% in magnetic field and 54.3% in control). The size of larvae at hatching (9.84 mm SL in a magnetic field and 9.86 mm SL in a control) and their growth rate during the first six days of life (0.52 mm·day−1 in a magnetic field and 0.53 mm·day−1 in a control) were also almost the same – no statistically significant differences. A significant effect of a magnetic field, however, was observed on the time of hatching (one day earlier in a magnetic field than in a control), yolk-sac size on 1 DPH (smaller in a magnetic field), and yolk-sac absorption time (faster in a magnetic field). Faster yolk-sac absorption time in a magnetic field was interpreted as an indication of increased metabolic rate. Even if some negative consequences may be expected as a result of shorter time until first feeding, the actual risk for increased Northern pike larvae mortality due to those factors seems to be negligible. Although it cannot be excluded that higher than 10 mT magnetic field values are hazardous for fish larvae, such values do not occur in the natural environment (e.g. along underwater cables).