During the first days after their arrival in the Wadden Sea, newly settled plaice Pteuronectes platessa L., flounder, Platichthys flesus L., and sole, Solea solea L., stay at low tide in the residual waters on the tidal flats. Here they can be exposed to high levels of solar radiation and extreme temperature conditions. As a result of the extreme temperatures, all settlers eventually leave the shallows and start to exhibit tidally-phased feeding migrations from the tidal channels on to the submerged fiats, In contrast to warm summers, for example 1982, no formation of a hyaline zone could be detected in the otoliths of surviving O-group plaice in 1990, 4 weeks after such a mass emigration, which could be directly observed on particular days in both hot and cold summers. Analysis of UV-B radiation data indicated that in 1990 cloud cover and wind conditions during the settling period protected the post-larvae from high temperature and radiation stress which is necessary to induce hyaline zones in the otoliths. Nevertheless, histological examination of the dorsal skin revealed cellular changes characteristic of sunburn damage in part of-the O-group plaice population in 1990.
Mortality of O-group plaice during and after such a mass emigration, which was directly observed in the drainage gulleys in 1982, was estimated at approximately 14–27%. This rate is assumed to be close to the maximum mortality which can be expected from abiotic factors for O-group plaice in the Wadden Sea.