Yield, fishing and movements of introduced brown trout in watercourses north of Lake Päijänne in 1990–2005

Yield, fishing and movements of brown trout introduced in 1990–2005 in watercourses flowing from the north into Lake Päijänne were studied using Carlin-tag returns.

The total average tag return rate was 14%. Trout introduced at 2 years old in 1990–1999 produced a return rate of 12%, but in 2000–2005 only 2%. The corresponding return rates for trout introduced at 3 years old were 22% and 12%. In all five water systems the return rates were higher for 3-year-old fish than for 2-year-old. Stockings in lakes rather than streams produced a higher return rate, as much as twice as high in the case of 2-year-old fish. The Äänekoski-Vaajakoski watercourse produced the highest, with the lowest in the Kivijärvi area and in the Saarijärvi watercourse. For 2-year-old fish, 44% of returns arrived during the first lake year (stocking year), and 39% during the second year. For 3-year-old fish, the proportion of first-year returns was 80%, with 88% for 4-year-old fish.

Total average yield was 136 kg per thousand fish. The factors significantly affecting yield were water system, decade, fish age and type of stocking site. The highest yield was detected in the Äänekoski-Vaajakoski and Rautalampi watercourses, the former perhaps due to the preponderance of 3-year-old fish and effective fishing on the narrow watercourse, while the abundant vendace stock probably favoured the latter. The yield for 2-year-old trout dropped to less than one fourth in 2000–2005 compared to 1990-1999. Yield was higher for fish introduced in lakes rather than in streams. Growth was fastest in 2-year-old fish, although no difference was detected between water systems in this respect. Pure relative yield, i.e. yield mass vs. stocking mass, was less than one in both decades: 0.43 in 1990–1999 and 0.16 in 2000–2005.

With regard to the Rautalampi-watercourse, correcting the figures using the coefficient of 3.5 – estimated for unreturned tags for Lake Konnevesi in the 1980s – reveals that stocking was productive in 81% of marked groups in 1990–1999, but only in 10% of groups in 2000–2005. Profitability was lower in other water systems.

Stocked trout stayed mainly in the waters in which they were introduced, especially in the latter half of the 1990s and beyond, probably owing to recovered vendace stocks. Of 2-yearold fish, 55% were taken in the same water in which they were introduced, compared with 70% of 3-year-old fish. Most, individuals, 74%, were harvested in lakes and 26% in streams, while 58% were taken with gillnets and 37% with rod.

Making conclusions about real stocking profitability using estimations based purely on tag returns carries a certain amount of risk. However, in terms of yield potential it is clear that stocked trout are caught all too early, presaging an alarming future for endangered wild lakemigrating brown trout.