The recovery of twaite shad (Alosa fallax) population in Lithuania

Abstract

The abundance and biological characteristics of twaite shad (Alosa fallax), and the the reasons of its population recovery in Lithuania is presented in this study. Migratory fish are extremely susceptible to changes in environmental conditions. For a long time twaite shad have been enjoying the status of important object of commercial fishing in Lithuania waters: in the Curonian Lagoon and the lower reaches of the Nemunas River. The commercial catches of this fish species reached several hundred tons in the first half of XX century. However, 50–60 years ago the abundance of twaite shad started noticeably decrease due to increased pollution and marked deterioration of reproduction in rivers and Lagoon (dams, reclamation). For a long time twaite shad almost disappeared. It was included to Red Data Book of Lithuania since 1992. Lately twaite shad have noticeably increased in abundance as a result of rivers and Curonian Lagoon pollution abatement (several wastewater treatment plants were built, the amount of industrial and agricultural pollutants decreased) and deepening of the narrow Klaipėda Strait. Now, nitrogen and phosphorus levels are 2–3 times lower in rivers and Curonian Lagoon in comparison with the period 20–25 years ago. Twaite shad appeared in experimental and commercial catches in the Curonian Lagoon after a long period only in 1994. The abundance of twaite shad begin to grow and it was excluded from Red Data Book of Lithuania since 2005. Commercial catches of twaite shad in Baltic coastal zone and Lagoon have considerably grown up and reach more than 100 tons in recent years. On further replenishment of stocks, twaite shad is expected to grow in commercial importance. The abundance, migration routes, spawning grounds and biological characteristics of twaite shad have been investigated in the recent years. Key words: twaite shad, coastal waters, Curonian Lagoon, abundance, anthrophogenic factors.