Monitoring programme of the Tornionjoki salmon and trout Monitoring programme of the Tornionjoki salmon and trout
Monitoring of Tornionjoki salmon and trout was initiated in the 1970s. Tornionjoki salmon has encountered the same decades-long period of serious recession due to over-fishing and later also due to so-called M74 syndrome as most of the other Baltic salmon stocks. In the 1980s, the stock was considered to be near extinction. However, the stock showed a quick revival parallel with the most other northern Baltic stocks in the 1990s. Sea trout stock is still seriously endangered because of over-fishing and habitat degradation in the tributaries.
Supplementation stocking of salmon and trout juveniles for conservation purposes was started in the late 1970s. Adipose fin clipping of stocked juveniles has enabled close follow-up of the effects of supplementation. Because of the recovery of salmon stock, stocking of salmon juveniles was ceased in 2002.
Monitoring programme of the Tornionjoki salmon and trout includes annual
Monitoring is organised in cooperation with the Swedish National Board of Fisheries. Occurrence of the parasite Gyrodactylus salaris is studied in cooperation with the Finnish Food Safety Authority. In addition, there have been periodic studies on habitat use of juveniles, population genetics etc.
Catch enquires
Currently 5,000-8,000 licenced rod fishers visit annually the river via Finland. 1,500 of them are interviewed by a postal enquiry after the fishing season. The relatively high response rate (65%) and the enthusiasm of the fishers ensure the quality of the catch statistics.
Scale sampling
Local fishers and owners of fishing camps are instructed to collect scale samples from the rod fishing catch. Hundreds of scales attached with detailed information about the fish are compiled annually along the river valley. Smolt age, sea age, spawning marks etc. can be recognized from the scales by an expertised scale reader.
Electrofishing
Parr densities are annually monitored by standardized electrofishing on permanent, wadeable sites along the river system. The Finnish-Swedish network covers about 100 surveyed sites on the salmon nursery habitats, and about 40 sites in the trout spawning tributaries. As most of the salmon nursery habitats are too deep for wading, sites do not represent a random sample from the salmon habitat, but the electrofishing results merely serve as indices of abundance.
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| Electrofishing in a typical salmon nursery habitat of the Tornionjoki. Photo: Ville Vähä |
Smolt trapping
A large fyke-net type of smolt trap is annually set up in late May, after the ice break-up. The trap covers almost 100 meters of the 800-meter wide river, 5 km upstream from the estuary. Daily catches are monitored, wild and reared-origin smolts are identified, and continuous mark-recapture experiments are conducted to estimate the catching efficiency of the trap. Sub-samples of smolts are aged and sexed. Thousands of wild salmon smolts are tagged with external tag and released in order to study migration routes and harvest of the Tornionjoki salmon. Periodically, samples are taken for genetic, parasitological and physiological studies etc. Smolt migration period normally ends in the early July.
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The smolt trap near the mouth of the Tornionjoki. Catches are handled and treated on the attached raft. Tens of thousands of salmon smolts are caught each year, which is about 5% of the total smolt run. Photo: Ville Vähä |
Hyrdoacoustic counting of ascending spawners
Dual-frequency Identification Sonar (DIDSON) has been available on the market since mid-2000s. DIDSON uses sound to produce video images of underwater areas. The research institute has purchased four DIDSON units for monitoring of the salmon spawning runs in the rivers Tornionjoki and Simojoki. Long range (LR) DIDSON acoustic imaging systems are operated at low frequency mode, which enables detection of salmon-sized targets up to about 80 meters distance. In the Tornionjoki, a suitable monitoring site is found about 100 km upstream from the sea. The large majority of salmon reproduction habitats of the river system are located upstream from this counting site, and roughly nine out of ten salmon which ascend the river pass the counting site.
Tagging programme
Carlin tag has been extensively used around the Baltic Sea for decades. This visible external tag is injected in about 5,000 wild-born and 2,000 reared smolts annually in the Tornionjoki. Wild smolts are tagged at the smolt trap. Reported recaptures vary between 1% and 10% of the amount of tagged fish. With the appropriate incorporation of reporting rates of recaptures, tag shedding etc., recapture data is used in the salmon population abundance model. Recaptures also facilitate detailed studies on migration routes, life history traits etc.
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Wild trout smolt and Carlin tag. Photo: Ville Vähä |
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