MEPS 192:277-285 (2000)  -  doi:10.3354/meps192277

ABSTRACT: More than 3000 juvenile flatfishes from 118 different locations along the coasts of the North Sea were checked for biological tags (hyaline zones) surrounding the nucleus (primordium + larval phase) of their otoliths. All flounder Platichthys flesus specimens had developed such tags in their otoliths. In contrast, tags were rarely found in sole Solea solea. Many North Sea plaice Pleuronectes platessa from subpopulations inhabiting areas with extended tidal flats had pronounced hyaline zones, but lacked these tags along sandy beaches without tidal flats. In long bights with low water exchange in the German Wadden Sea and in the inner parts of the Westerschelde, multiple hyaline zones were frequently found in the otoliths of 0-group plaice. In previous investigations, it had been possible to experimentally induce early hyaline zones surrounding the nucleus of plaice otoliths by UV-B radiation, high temperature, or starvation treatment. In this study, strong indications are presented that UV-B mediated oxidative stress (hydrogen peroxide) in coastal areas, particularly in tidal pools during the pool-dwelling period of the 3 species, was the most important mechanism in the formation of the early hyaline zones in both the field and the laboratory. The occurrence or lack of hyaline zones surrounding the nucleus can be explained by the different patterns in settlement of the 3 species, by the prevailing weather conditions during and after settlement, and by differences in the hydrographic conditions. In the Dutch Wadden Sea, the lack of such early hyaline zones can be attributed to the lower tidal range and the weather conditions during the period of settlement. Due to their highly rheotactic behaviour, early flounder postlarvae almost exclusively inhabit tidal pools directly at the shoreline. Until the tide comes in, flounder may also be exposed to radiation much longer than plaice, since plaice are much more widespread over the tidal flats, depending on the prevailing tidal regime. For sole, tidal pools are less important habitats as compared to plaice and flounder. The distinct multiple hyaline zones in 0-group plaice otoliths are most likely also linked to oxidative stress. Other possible factors like diseases and injuries during the sorting out of by-catch on shrimp vessels are discussed.

KEY WORDS: Pleuronectes platessa · Platichthys flesus · Solea solea · Tidal flats · Environmental stress · UV-B radiation · Hydrogen peroxide · Otoliths · Biological tags