Early Life Stage Mortality Syndrome in Fishes of the Great Lakes and Baltic Sea

<em>Abstract</em>.—Feral stocks of Baltic salmon <em>Salmo salar </em>suffer from a yolk sac fry mortality syndrome designated M74. This study showed that M74 is family dependent, with a 100% mortality within affected family groups. Differences between family groups were noted with regard to the age when the disease was manifested, and accordingly, the groups were categorized into those with early, intermediate, or late development of disease. Family groups with early development of disease had a short survival time, and after 5–10 d the whole family group was dead. Family groups with late development of disease survived for a longer period. Three consecutive stages of M74, preclinical, clinical, and terminal, are described. No clinical symptoms of disease can be seen in the newly hatched fry. During development, the yolk sac fry progressively pass through the preclinical stage and enter the clinical stage, which is characterized by aggravating neurological symptoms. In the terminal stage, the majority of symptoms might be secondary to the low heart rate that develops during the disease. The gross pathological characteristics of M74 include a distended gallbladder, a pale spleen, and a yolk sac precipitate. Yolk sac fry with M74 also have diminished yolk absorption and faster consumption of the pigments in the yolk sac fat droplet. To further elucidate the pathogenesis of M74 and to determine the involvement of nutritional and toxicological factors, future work must include both histological and functional studies of tissues with regard to the symptoms and gross pathological characteristics of M74. However, because there is heterogenicity between family groups that develop M74, careful selection of experimental samples is necessary and should include categorization of each family group and definition of the stage of disease at sampling.