Supervisor: Senior Research Fellow Markus Vetemaa, University of Tartu
Opponent: Professor Karin E. Limburg, PhD, State University of New York College of Environmental Science and Forestry
Otoliths or earbones are calcium carbonate structures located in the inner ear of all teleost fishes. Similar to the growth rings of trees, age and growth history of the fish is recorded in the otoliths. Otoliths also incorporate different chemical elements from the surrounding water, which can be used to study the migrations and natal origins of the fish.
The objectives of the present thesis were to use the information stored in otoliths to study spawning habitat preferences, migrations and age of freshwater fishes inhabiting the brackish Väinameri Sea (Baltic Sea). Study species were pike, burbot and ide, which are all important target species in commercial and/or recreational fisheries.
Freshwater spawning strongly prevailed amongst pike, burbot and ide sampled from the Väinameri Sea. While previous studies have suggested that multiple pike and ide spawning areas exist in the Väinameri Sea, the results of the present study do not confirm it. The quality of these brackish water spawning areas is most likely deteriorated for pike and ide. With ide it was also demonstrated that riverine spawning has practically ceased in Käina and Saunja bays, and most of the fish spawn in seasonally freshwater bays. Almost all ide sampled from Matsalu Bay were spawned to rivers or adjacent wetlands. In order to conserve and/or increase the population abundances of the studied species, most important freshwater spawning areas should be protected and/or restored.
Migration patterns of pike, burbot and ide were diverse, and no single species-specific movement pattern exists. Chemical profiles of otoliths demonstrated that the study species use various life history strategies – an example of phenotypic plasticity, which allows the species to withstand environmental disturbances. The gathered knowledge can be used in fisheries management and conservation actions.
The growth rates of burbot from Matsalu and Saunja bays were some of the highest ever recorded in the Baltic Sea and its freshwater basins. Minimum allowable catch size (e.g. total length of 40 cm) should be put to use in the sea and lower reaches of coastal rivers. Age structure of ide varied significantly amongst Käina, Matsalu and Saunja bays spawning stocks. In Käina Bay, young, 5–7 year old individuals dominated. In Matsalu and Saunja bays, old, 11–16 year old individuals dominated. The results of the present thesis demonstrated that serious recruitment problems occur in Matsalu and Saunja bays ide spawning stocks.