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

<em>Abstract.—</em>Populations of Baltic salmon <em>Salmo salar </em>and cod <em>Gadus morhua </em>are facing acute threats because of poor reproduction. The salmon is afflicted with high yolk sac fry mortality, and the incidence of cod larvae mortality is high. There are also indications that anadromous Baltic brown trout <em>Salmo trutta </em>populations are affected by reproductive disorders. These top predators have significant ecological, economic, and socioeconomic importance. Other species are also suffering from poor reproductive success and declining populations. Burbot <em>Lota lota </em>populations are locally affected by inadequate sexual maturation, resulting in a failure to spawn; gonad anomalies have also been described in roach <em>Rutilus rutilus</em>. High egg mortality has been recorded for whiting <em>Merlangius merlangus</em>, flounder <em>Platichtys flesus</em>, and herring <em>Clupea harengus</em>. Attempts have been made to discover the cause of reproductive disorders in Baltic fish species, but the available data suggest several possible causes, both abiotic and biotic. Species with pelagic eggs such as cod and flatfish are dependent on salinity and oxygen concentrations, factors that often limit the volume of reproduction in the Baltic Sea. A variety of biotic causes (i.e., infectious diseases, parasitism, and toxic algae) have been shown to affect species such as roach and herring. There are indications that nutritional factors (i.e., thiamine and astaxanthin) are involved in the cause of the yolk sac fry mortality syndrome affecting the Baltic salmon. Furthermore, anthropogenic activities causing both local point sources (i.e., metals and persistent organic pollutants) and long-range transport and deposition of acidic rain and pesticides must also be considered as potential threats to Baltic fish species.