Yield, fishing and movements of introduced and wild brown trout and introduced landlocked Atlantic salmon in Lake Päijänne and its sidewaters in 1990–2007

Carlin tags were used to study yield, fishing and movements of stocked and wild brown-trout and stocked landlocked Atlantic salmon introduced in 1990–2007 in Lake Päijänne and its sidewaters/ watercourses. The total average tag return rates were 8% and 36% for 2-year-old and 3-yearold stocked brown trout, 4% for wild brown trout smolts and 7% and 20% for 2-year-old and 3-year-old stocked landlocked salmon, respectively. For 2-year-old brown trout, 53% of returns arrived during the first lake year (stocking year) and 34% during the second lake year. Corresponding proportions for landlocked salmon were 78% and 17%, respectively.

The average yield of stocked brown trout was slightly higher than the yield of landlocked salmon. Total average yields per thousand fish were 66 kg, 336 kg and 469 kg for 2-, 3- and 4-year-old brown trout, and 24 kg and 135 kg for 2- and 3-year-old landlocked salmon, respectively. Pure relative yield, i.e. yield mass vs. stocking mass, was low in both species: 0.46 for brown trout and 0.26 for landlocked salmon. It was higher for 3-year old fish (0.47) than 2-year-old fish (0.23). All marked groups produced less yield mass compared to stocking mass. During the first and second lake year, growth rate was higher for stocked brown trout than for landlocked salmon. By contrast, landlocked salmon caught during the third year had grown faster than stocked brown trout.

In Lake Päijänne, stocked fish moved mainly to the Tehinselkä open lake area. However, most stocked fish were caught in the waters in which they were introduced. Wild trout smolts from rivers flowing to northern Päijänne spread throughout the lake, but most of them were already caught in the nearest open area to river mouths, Ristinselkä. Most of the killed stocked and wild fish were harvested in lakes. Most wild trout caught in rivers were released. Over half of total individuals were taken with gillnets and around 25% with rod. In Lake Päijänne, around 25% of stocked fish were caught using fishing gear that only professional fishers use (different kind of trapnets, trawl), while the proportion of fish released by these methods was higher compared to the gear of recreational fishers.

Drawing conclusions about real stocking profitability using estimations based purely on low tag returns carries some difficulty, especially with regard to 2-year-old fish. However, low yields may be the consequence of stocked fish being caught all too early, presaging an alarming future for endangered wild lake-migrating brown trout, with difficulty in renewing hatchery stocks. A more organised programme for tagging and stocking will be needed for better yield monitoring in future.