2023

Karjalainen, Juha; Keskinen, Tapio; Ruokonen, Timo J.; Marjomäki, Timo J.

Knowledge about the fish abundance and community composition is important in fisheries management, assessment of the exploitation potential, ecological state of lake ecosystems and impact of intensive fishing in lake restoration. In small, shallow lakes with no commercial fishing, estimating the fish stocks is challenging. Catch data are usually not collected, and direct counting of fish, e.g. by echo sounding is imprecise. Standardized test gill net fishing is widely used in Europe in assessment of ecological status in water management work. In this study, the changes in the fish communities of four central Finnish lakes (Jyväs-, Tuomio-, Alva- and Patajärvi) were monitored for more than 20 years by test gill net fishing. Fishing was done with standard multi-mesh gill nets, and the catches were sampled for age and growth determinations. In 2004–2006, a biomanipulation campaign with fyke nets was carried out in Jyväsjärvi, enabling the estimation of the roach and perch population size using cohort analysis. In every lake, the dominant species in the gill net catches were roach and perch. The average share of perch was the highest in the brownish Patajärvi and roach in Tuomio- and Alvajärvi. The share of perch increased in Tuomio-, Alva- and Patajärvi during the monitoring period. In total, 16 species were caught from the four lakes. The gill net catch per unit effort (CPUE) was regularly the highest in Tuomiojärvi, where net fishing is prohibited. The three-year intensive fyke net fishing catch of Jyväsjärvi was 104 tons (308 kg ha-1), mostly bream, roach and perch. Intensive fishing targeted 2-year-old and older fish, and based on the cohort analysis, the roach and perch abundance decreased to less than 10 % of the original. The share of old and large perch and roach decreased. The gill net CPUE of perch and roach varied strongly between-years in all lakes. In Jyväsjärvi, the gill net CPUE of roach decreased during the intensive fishing period, but not statistically significantly. With the abundant year-classes hatching during that period, the roach and perch populations quickly recovered after the intensive fishing ended, and the gill net CPUE of the roach returned to the pre-intensive fishing level. With the applied gill net fishing effort, the CPUE provided an index of the abundance of fish stocks and the proportions of the most abundant species. The selectivity of the fyke nets and gillnet made it difficult to use uncorrected CPUE as a comparable index of abundance, fyke nets targeting older age groups.