Objectivesof the project are to improve our knowledge on the whitefish population in theRingkøbing Fjord Lagoon and effects associated with the commercial exploitationof the population, i.e. to what extend the traditional gill-net (46 mm monofilnets) fishery for whitefish affect both the whitefish population and otherspecies of fish in the lagoon. Another goal is to establish how much naturalreproduction and stocking of hatchery reared fry contributes to the adultpopulation. These results will provide a much better basis for the managementof whitefish populations in Denmark in general and in Western Jutland inparticular.
Thenatural population of whitefish in the Ringkøbing Fjord Lagoon has been thesubject of an extensive fishery for more than 100 years. The fishery isprimarily performed by commercial fishermen, but estimated from the number ofrecreational fishers in the area, a substantial amount is caught by this groupas well. The lagoon holds the largest population of whitefish in Denmark. Theofficial landing statistics (only covering the commercial catches) shows thatthe catch through the 20th century typically has varied between 10 and 60 tonsper year (e.g. mean 1980-2000 25.1 tons per year). Since 2001 the landings haveincreased to a mean of 55 tons per year (range 14-94 t), with a mean value of1.2 m DKK. This constitutes 75-95 % of the total Danish whitefish fishery.
Since1986 ca. 4 million hatchery reared fry has been stocked in the lagoon eachyear. 3.6 million are stocked as newly hatched larvae in April. 0.4 million areraised to a size of 3-4 cm before stocking in late May.
Thepopulation of sea trout (Salmo trutta)in the main tributary of the lagoon, the River Skjern, is much smaller thanexpected, considering the environmental conditions of both the river and thelagoon and the size of the river. One possible reason is by-catch in thewhitefish fishery. The landing of sea trout and the endangered salmon (Salmo salar) from the lagoon isprohibited and the discard mortality for sea trout is considered to be veryhigh. Investigations on the subject of by-catch in gill-nets set for whitefishin the Baltic Sea supports this hypothesis.
In the project we estimate the catch of whitefishand the by-catch of other fish species in the whitefish gill-net fishery, withspecial emphasis on salmonids, by a combination of experimental fishery,monitoring selected commercial fishing trips and a questionnaire the fishermenon their effort. A number of different approaches re time and place of fishingand net construction is tested to describe how much by-catch can be minimized.
The result of natural spawning in River Skjern isinvestigated by a combination of catching newly hatched larvae with drift-netsand e-DNA analysis of water samples from the river. The latter method is a verynovel approach.
Through the experimental fishing, supplemental dataon the whitefish, salmonids as well as other species (less detailed) arecollected to describe population dynamic parameters (size and age distribution,growth, condition etc.), primarily of whitefish and salmonids.
The results shows, that by-catch of sea-trout in thewhitefish gill-net fishery is unavoidable, but also that the by-catch can bereduced substantially by employing specific gears and methods. The by-catch ofsalmon is insignificant while the by-catch of other species, especiallyflounder is substantial. These results will be reported in autumn 2016. Theinvestigation on natural reproduction in the River Skjern is still ongoing andwill be reported in 2017.
This project is coordinated by DTU Aqua.
The project is funded by the Danish Rod and Net Fishing License Funds.
|Period||01/01/2011 → 31/12/2016|
- Research areas: Freshwater Fisheries and Ecology & Population Genetics