Histopathological responses of newly hatched larvae of whitefish (Coregonus lavaretus s.l.) to UV-B induced toxicity of retene

Positively phototactic fish larvae may be exposed to increased ultraviolet-B (UV-B) radiation alone or, potentially and in addition, to polycyclic aromatic hydrocarbons (PAHs) such as retene (7-isopropyl-1-methylphenanthrene) at the egg or larval stages. Suspended and sedimental particulate material near pulp and paper mills may act as sources of retene in chemically contaminated lake areas. In laboratory conditions whitefish larvae were pre-exposed to retene (10, 32 and 100 microg/l), with relevant controls, and irradiated in semi-static tests for 3 h once a day (2 consecutive days) with two UV-B doses (CIE-weighted 2.8 or 5.4 kJ per m(2) per day) or with visible light only. These UV-B doses correspond with slightly subambient and 80% increases relative to the natural maximum daily doses of the solar UV-B in Finland in early May. The UV-B radiation alone increased mortality only slightly (2.7 and 4.0%, respectively). Similarly, no mortality was observed due to retene alone. On the contrary, simultaneous UV-B and retene exposure caused very high mortality to whitefish and all larvae died in the highest retene concentration. The photoinduced acute LC(50) for retene was 13.3 microg/l. Retene treated fish exhibited signs of behavioral irritation and hypoxia during and after the exposure to UV light. Severe skin damages were detected in larvae exposed simultaneously to retene and UV-B. The structural signs of sunburn could also be seen in UV-B and solvent controls (DMSO) with UV-B. Even at the lowest retene concentration, the number of mucous cells increased significantly in simultaneous chemical and UV-B treatment. We consider the tissue reaction as protective response against UV induced retene toxicity. Further, regarding liver parenchyma, fish exposed to retene with UV-B had lesions, revealing hepatotoxicity. We suggest that synergism of the UV-B radiation and the photoactivating pollutants such as retene is a potential risk factor to be taken into consideration in lake areas chemically contaminated by the pulp and paper industry.