Population dynamics of juvenile brown trout (Salmo trutta L.), recruitment, mortality, biological production and smolt yield in two danish baecks

The dynamics of sea trout populations in two small shallow tributaries of the River Gudenaa in mid‐Jutland, Denmark, were analyzed by electrofishing surveys 1978–2008 and capture of brown trout smolts in traps (Wolf type) 1979 to 1997. Most sea trout enter the baecks in the period November to January and spawn throughout the baecks and leave soon after spawning.

Electrofishing took place every year in spring and autumn on several sites along the baecks. The recruitment in mid‐April in Brandstrup Baeck varied from 37 to 820 recruited fry per 100 m2. The recruitment in Tjaerbaeck in 1981–1996 varied from 9 to 83 recruits per 100 m2. Because of lower recruitment the growth rate of parr was higher in Tjaerbaeck because of density dependent growth, either through feeding rates and/or agonistic behaviour.

Ageing from scales of parr and smolt enabled separation length (TL) into five (parr) and four (smolt) age classes, with two and three year smolt dominating. In Brandstrup Baeck the annual smolt yield varied from 4.5 to 34.5 smolts per 100 m2 with a mean number of 17 smolts per 100 m2. In Tjaerbaeck it varied from 4.0 to 13.1 with a mean of 7.1 smolts per 100 m2.

In both baecks the smolt yields depend on recruitment, and the percentage smolt yields from recruits or 0+ declines with increasing recruitment. The growth rate from recruits to 0+ was dependent on densities of 0+, 1+, 2+ and 3+, respectively, and a growth model showed that the growth rate of 0+ was about 90% of maximum growth rate for densities of ∑(0+, 1+, 2+, 3+, 4+) below 30 trout per 100 m2 and 60% of maximum growth rate for densities of ∑(0+, 1+, 2+, 3+, 4+) above 134 trout per 100 m2.

From recruitments, densities, mortalities and growth rates the biological production rate in wet weight was calculated for each year class (1979–1997) from recruitment and up to 4½ year after recruitment. The annual productions in Brandstrup Baeck vary from 11.3 to 22.2 g wet wt. per 1 m2 with a mean of 15.9 g wet wt. per 1 m2.

The productions in Tjaerbaeck (1981–1989) varied from 5.4 to 9.3 g wet wt. per 1 m2 and with a mean of 6.9 g wet wt. per 1 m2. Total mortality Z was divided into smolt loss, true natural mortality (mass dependent) and emigration loss. The high migration rate of parr outside the smolt run period in spring is explained by the possibility that the baecks are too shallow for up growing trout.

Whether these parr stay in the main stem Gudenaa or migrate to sea later is not known.