Are magnetic and electromagnetic fields of anthropogenic origin potential threats to early life stages of fish?

•Rainbow trout embryos and larvae exposed to static magnetic field of 10 mT and 50 Hz electromagnetic field of 1 mT.•No significant effect on embryonic or larval mortality, hatching time, larval growth, or the time of larvae swim-up from nest.•MF and EMF enhanced the yolk-sac absorption rate.•Larvae with absorbed yolk-sacs by the time of swim-up were less efficient in body Wt increase after first feeding. The number of underwater cables transferring electric current in sea and freshwater environments is constantly increasing. As a result, the risk of negative effects of magnetic fields generated in the vicinity of those cables on fish eggs and larvae is also growing. This is especially the case for species that settle on the bottom for certain periods of time during early development. To study those effects, eggs and larvae of rainbow trout, Oncorhynchus mykiss, were subjected under experimental conditions to a static magnetic field (MF) of 10 m T and a 50 Hz electromagnetic field (EMF) of 1 m T for a period of 36 days (i.e., from eyed egg stage to approximately 26 days post hatching, dph). Neither MF nor EMF had significant effect on embryonic or larval mortality, hatching time, larval growth, or the time of larvae swim-up from the bottom. However, both MF and EMF enhanced the yolk-sac absorption rate. Although it was not related directly to magnetic field effect, it was also shown that larvae with absorbed yolk-sacs by the time of swim-up were less efficient in taking advantage of available food at first feeding (i.e., obtained smaller weight at age). That indicates the importance of processes affecting yolk-sac absorption rate.