Tapio Sutela

Relationship between annual variation in reservoir conditions and year-class strength of peled (Coregonus peled) and whitefish (C. lavaretus)

We estimated year-class strength (YCS) of whitefish and peled by VPA in two boreal reservoirs Lokka and Porttipahta with mean regulation of water level amplitudes of 2.34 and 2.49 m, respectively. YCS of peled Coregonus peled(Gmelin) was related to winter drawdown most probably via egg desiccation. In the relatively shallow Lokka dissolved oxygen deficiency in late winter also had a negative impact on survival of eggs and year-class strength. In contrast to peled, whitefish Coregonus lavaretus(L.) indicated no significant relationship with winter drawdown or dissolved oxygen level. The obvious reason for the pattern is that peled spawn on the reservoir bottom, but whitefish spawn in the ascending rivers, which are mainly unaffected by reservoir regulation.

Short sea migration and precocious maturation in reared Atlantic salmon post-smolts in the northern Baltic Sea

Short sea migration and precocious maturation in reared Atlantic salmon post-smolts in the northern Baltic Sea Panu Orell*, Jaakko Erkinaro, Mikko Kiljunen, Jyrki Torniainen, Tapio Sutela, Mikko Jaukkuri, and Aki Mäki-Petäys Natural Resources Institute Finland, University of Oulu, PO Box 413, Oulu FI 90014, Finland Department of Biological and Environmental Science, University of Jyvaskyla, PO Box 35, Jyvaskyla FI 40014, Finland Natural History Museum, University of Jyvaskyla, PO Box 35, Jyvaskyla FI 40014, Finland *Corresponding author: tel: þ358 405305830; e-mail: panu.orell@luke.fi

Seasonal shift in boreal riverine fish assemblages and associated bias in bioassessment

Collated electrofishing data from wadeable riffles of boreal rivers in Finland revealed a substantial shift in the fish assemblage composition, accompanied by a decline in total fish density and a reduction in species richness from early summer to late autumn. As the major changes in fish assemblages, the density of cyprinids decreased sharply from June towards autumn, whereas salmonids peaked in September. These shifts were considered to originate mainly from spawning migrations, fish movements to winter refuge habitat and other temperature-related responses of rheophilic versus eurytopic fish species. Temporal change in fish assemblages induced a prominent variation in the monthly fish-based index values used for bioassessment. The proportion of sampled sites classified as high or good in ecological status within the Water Framework Directive (WFD) based on fish increased from 25.9% in July to 68.3% in October. These results, combined with the observed timing of young of the year (0+) fish recruitment to electrofishing catch, suggest that sampling of fish in boreal rivers for WFD monitoring should be restricted to a considerably shorter period than the prevailing practice in order to avoid temporal bias.

Impacts of Water Quality and Hand-Picking of Dead Eggs on the Survival of Brown Trout and Atlantic Salmon Eggs

Abstract The effect of water source and removal of dead eggs by hand on egg survival of brown trout Salmo trutta and Atlantic salmon S. salar was studied experimentally in a hatchery trial, which started from fertilization in October and lasted until egg eye-up in early March; no antifungal chemical treatments were used. The experimental treatments were water source (surface water, filtered surface water, or groundwater), species, and hand-picking of dead eggs (used, not used); we used four replicates per treatment, resulting in 48 batches. Incubation of eggs in groundwater yielded higher egg survival to eye-up than incubation in surface water or filtered surface water. Hand-picking of dead eggs enhanced survival of eggs incubated in surface water but not those incubated in groundwater. The best combination for egg survival in both species was the use of groundwater incubation without hand-picking of dead eggs.