| Summary: | In the early 1990s, commercial and recreational fishermen in the Kalmar Sound of the Baltic Sea reported decreasing stocks of Eurasian perch (Perca fluviatilis) and northern pike (Esox lucius). Test fishing surveys confirmed regionally declining abundances of both species. Both pike and perch spawning were observed annually, but virtually no young-of-the-year were captured in surveys from 1997–1999. The rapid decrease of the stocks was probably a result of the widespread recruitment failure. Studies of potential causes of the recruitment failure were conducted in 1998–2000. Field experiments excluded suboptimal water quality in brackish water recruitment areas as regulatory factor. Laboratory tests did not indicate reproductive disturbances in parental fish that could be related to chronic toxicity or endocrine disruptions, as hatching success and yolk-sac larvae survival were not negatively affected. Exposure to filamentous algae (Pilayella sp.) and diatoms, common in the spawning areas, had only a slight negative effect on pike egg hatchability. Two hypotheses that could not be rejected are food deficiency during the post hatching period and egg predation.
|
|---|