Objectives of the project are to improve our knowledge on the whitefish population in the Ringkøbing Fjord and effects associated with the commercial exploitation of the population. Another goal was to establish how much natural reproduction and stocking of hatchery reared fry contributes to the adult population and to what extend the commercial fishery affects other species of fish. These results will provide a much better basis for the management of whitefish populations in Denmark in general and in the Western Jutland populations in particular.
The natural population of whitefish in the Ringkøbing Fjord has been the subject of an extensive fishery for more than 100 years. The main gear used is 46 mm monofil gill nets. The fishery is primarily performed by commercial fishermen, but estimated from the number of recreational fishers in the area, a substantial amount is caught by this group as well. The official statistics (only covering the commercial catches) shows that the catch through the 20th century typically has varied between 10 and 60 tons per year. Since 2001 the catch has increased and in most years been above 60 tons (highest catch 2009: 100 t).
Since 1986 ca. 4 million hatchery reared fry has been stocked in the fjord each year. 3.6 million is stocked as newly hatched larvae in April. The rest is raised to a size of 3-4 cm on a diet of artificial pellets before stocking in late May. Several subjects related to the whitefish population and fishery is of considerable interest to investigate.The population of sea trout (Salmo trutta) in the main tributary of the Ringkøbing Fjord, the River Skjern Aa, is much smaller than expected, considering the environmental conditions of both the river and the fjord and the size of the river. One possible reason is by-catch in the whitefish fishery. The landing of sea trout and the endangered salmon (Salmo salar) from Ringkøbing Fjord is prohibited and the discard mortality for sea trout is considered to be very high (in general salmon is not caught in the gear used in this fishery). Investigations on the subject of by-catch from the Baltic Sea indicate this to be true. The by-catch of sea trout will be estimated partly thorough experimental fishing and partly through surveys of commercial fishing.
It has never been estimated how much the stocking of whitefish larvae and fry contributes to the population in general or to the catchable fraction of the population (legal minimum size 34 cm). We will first establish if otolith microstructure analysis can be used as a method to distinguish between hatchery reared and naturally hatched individuals. If not chemical marking (most likely using strontium) in the hatchery will be used. We will then estimate how much the stocked larvae/fry contributes to the population. Finally, we will collect data on the age structure and growth of the whitefish. These parameters will be used to evaluate if fishing pressure in this fishery is a major mortality factor.
|Period||01/01/11 → 31/03/13|
- Research area: Freshwater Fisheries and Ecology