FISH MASS MORTALITY IN THE ETOLIKON LAGOON, GREECE: THE ROLE OF LOCAL GEOLOGY b Y Ioannis LEONARDOS and Apostolos SINIS (1) \ ABSTRACT. - Mass mortalities of fish in the Etolikon lagoon have been observed sporadically. In the mass morality which occurred in November 1990 it has been estimated that a large part of the flrh stock was lost. As a result, fish catches were greatly reduced in the lagoon in the following years. Hydrobiological studies suggest that the deaths are due to the high concentration of H2S in the hypolim- nion. The continual enrichment of the water with H,S is attributed to the presence of gypsum deposits in the drainage basin and to the anaerobic breakdown of the accumulated organic matter. The presence of &S in combination with the physiography of the ecosystem, the influxes of fresh and salt water, the runoff of agricultural chemicals and the strong prevailing southerly winds are responsible for the observed phenomenon. &UM& - Mortalith de masse de poissons dans la lagune d'Etolikon, G r h : le r61e de la gbologie locale. Des mortalit& de masse de poissons ont kt6 observ6es de f a ~ o n sporadique dans la lagune d'Etolikon. Une grande partie du stock de poissons a kt6 considb& comrne perdue apks celle p i est survenue en novembre 1990. En cons&quence, les captures de poissons dans la lagune ont btb considb- rablement kduites pendant les ann&s suivantes. Des dudes hydrobiologiques suggbrent que ces mor- talith sont dues B une forte concentration de H2S dans I'hypolimnion. L'enrichissement permanent de I'eau en H2S est attribue B la prhence de dkp6i.s de gypse dans le bassin versant et B une @node anab- robique caus& par I'accumulation de matibre organique. La prksence de H2S combine avec la physio- graphie de I'&osystbrne, I'afflux d'eau douce et d'eau salk, le recours aux engrais agricoles et la pddominance des vents de secteur sud expliquent ce phbnombne. Key-words. - Fish population, Greece, Lagoon, Mass mortality. The Mesolongi and Etolikon lagoons (38°18'36"N-21032'00"E) in Western Greece are among the largest lagoonal ecosystems in the Mediterranean (Fig. 1). The Etolikon lagoon with a surface area of 16 km2 makes up the northern part of the ecosys- tem, while the Mesolongi lagoon with a surface area of 110 km2 comprises the central and southern part. The Mesolongi lagoon is connected to the south through sandbanks to " the Gulf of Patras (Fig. 1). The lagoons themselves are connected by two narrow ope- -4 ; nings, with a length of 50 m and depth of 0.5 m, which occur on either side of the Eto- - likon Island. The Etolikon lagoon is a meromictic lagoon with a mean depth of 12 m and 4 maximum depth of 30 m. It has an atypical orientation and is a tectonically formed la- goon (Hatzikakidis. 1952; Day et al., 1989; Danielidis, 1991). The Mesolongi lagoon is shallow (mean depth 0.5 m; maximum 1 m) and lies with its long axis parallel to the shore line and is a typical lagoon (Day et al., 1989; Danielidis, 1991). The Acheloos and (1) University of Thessaloniki, Department of Zoology, P.O. Box 134, 54006 Thessalo- niki, GREECE. Cybium 1997, 21(2): 201-206.