Artificial reefs and substrata in the coastal zone of the Baltic — The problems and decizions

Abstract

Examination of the coastal zone of the Baltic Sea along the Latvian, Lithuanian and Kaliningrad region coasts showed a worsening of environmental conditions there during 1980s–1990s, including a sharp decrease of the main macro benthos communities — Mytilus edulis — Furcellaria lumbricalis (in shallow water) and Dreissena polymorpha (in estuaries). The total amount of Furcellaria, the main spawning substrate for the Baltic herring, in Klaipeda — Venspils region decreased from 80000 t till 15000 t in this period. Large areas with Dreissena polymorpha in the Lielupe River, the Kisezers Lake and spawning grounds of herring in the Kaliningrad Bay happened under the layer of ooze. Since 1982 with the aim of environmental restoration a different kind of spawning substrata, artificial reefs and collectors for Furcellaria lumbricalis, Blue Mussels and Dreissena polymorpha cultivation were constructed and examinated in the coastal zone of the Baltic Sea, the Riga Bay, the Parnu Bay, the Lielupe River, the Kisezers Lake, the Kursiu Marios Lagoon and the Kaliningrad Bay. The results, obtained in 1980s and 1990s, showed that the main types of hydrobionts could be settled on the artificial substrates and reefs. The best-tested artificial substratum in the Baltic Sea coastal zone was a permanent concrete frame with a kapron net cover. The artificial reefs constructed from utilized tires and fishnet, had a low resistance to the storm loads and after 2–3 years the destruction of these reefs was observed. These constructions could be used only in the estuaries and lagoons with a low level of hydrodynamics forces. In the Lielupe River, the Kisezers Lake and the Kursiu Marios Lagoon the best results could be acquired by light artificial substrata, constructed of tires with kapron mesh, and different types of collectors, providing a solid base for sedimentation and attachment of periphyton filters. In some cases the biomass of Dreissena polymorpha on artificial substratum in the Lielupe River reached 5 kg/m2. The experiments on artificial spawning substrata for the herring gave the best results with the removable constructions of kapron net. The maximal densities of herring eggs on the net surface of the artificial spawning substrata were observed in the Vistula Bay in 1984–1985 2 (42.000 to 5.000.000 eggs per 1 m). The mortality of eggs on the net surface varied from 20 to 50%. The experiments used similar substrata in the Klaipeda District in 1986 showed the lower efficiency of application of the artificial spawning grounds. The density of clutches 2 varied from 98.000 to 360.000 eggs per 1 m, with the mortality of eggs close to 20%. Experiments with artificial substrata in the Gulf of Riga were performed in 1987. Only single clutches were observed, with the density about 1000 eggs per 1 m2 and mortality more than 90%. These low amounts could be caused by a high degree of silting of the substrata, and the subsequent pollution, which caused the development of saprolegnia. The experiments with artificial substrata in the Parnu Bay and coastal waters of Estonia in 1987–1988 did not present the positive results. Such factors as resistance to storms, cleanliness of the surface and terms of installation were considered to be the main requirements for the efficient applying of spawning substrata.