Jakob Kjellman

Bad matching between hatching and acidification: a pitfall for the burbot, Lota lota, off the river Kyrönjoki, Baltic Sea

Abstract The episodic acid runoff from the catchment areas of rivers with sulphide-bearing fine-grained sediments caused recruitment failures of burbot ( Lota lota ) in the Gulf of Bothnia. In the episodically acid river Kyronjoki, emptying into the Gulf of Bothnia, the Baltic Sea, the burbot spawn in winter and the eggs were calculated on average to hatch on 10 May. This date coincided with the period observed to hamper the burbot year-class formation in 1983–1993. The number of days with pH lower than 5.3 during 7–27 May correlated negatively ( r p =−0.68, n =11) with year-class strength. The timing of the acid period was more harmful to recruitment than the severity of the episodic environmental disturbance. Successful recruitment was possible even if pH had been below 5.3 for several weeks previous to the hatching. We concluded that low pH, within one month after hatching, caused mass larvae kills, subsequently leading to burbot recruitment failures.

Field estimations of temperature-dependent processes: case growth of young burbot

In this study, the growth of young burbot (Lota lota) was temperature-dependent, but varied between age-groups. The growth of age-0 increased with temperatures 4–18 °C. Growth of age-groups 1 and 2 was not significantly related to surface water temperatures and varied between the 2 years studied. This study was a re-evaluation of burbot sampled between 1978 and 1980 by plankton net, beach seine, and electric fishing in Lake Kuohijärvi, the Finnish Lake District. In the re-evaluation, growth was studied both as mm day−1 and as absolute growth (mm), whereas the original study did not include absolute growth. Though the methods should be equivalent, they are not. In our example, time between measurements, temperature, and growth were negatively correlated. When analysing mm day−1 the estimate variability, therefore, increased with temperature, whereas the variability was homogenously spread on the two axes, growth and degree-day, when analysing absolute growth. Field rate analyses contain an error risk that should be recognised when assessing the temperature-dependency of any processes, e.g. growth or mortality.

Changed length-at-age of burbot, Lota lota, from an acidified estuary in the Gulf of Bothnia

SynopsisThe burbot population off the River Kyrönjoki has suffered from acidification in its reproduction area, causing a reduction of the population throughout the 1980s. During the same period of time the mean length-at-age of burbot was found to increase, with a significant increase in length-at-age for age groups 3–8 from 1979 to 1993. The length-weight relationship however was not found to have changed during this same period. The decreasing burbot population was found to influence the changed length-at-age. An increase in potential prey species population was also documented during this period of increasing length-at-age. The increased lengths were associated to the changed availability of food caused by the simultaneous fluctuations in both burbot and potential prey species populations.

Responses of Ruffe (Gymnocephalus cernuus (L.)) Abundance to Eutrophication

The responses of ruffe (Gymnocephalus cernuus (L.)) abundance to eutrophication were studied in the brackish water of the Baltic Sea off Helsinki. The productivity gradients in the area permitted an analysis of the dependence of ruffe catches taken with gill nets in 1969 to 1972 on spatial differences in eutrophication level. There appeared to be a relationship between the level of primary productivity and the catch per unit effort of ruffe. The gill net catches showed that ruffe were more abundant in eutrophic than oligotrophic areas but less abundant in the most eutrophic areas. Due to the better purification of municipal sewage and the relocation of effluent discharge sites, primary production has declined in the most eutrophicated inner bays during recent decades. The responses of ruffe reproduction to these changes were studied on the basis of catches taken with small beach seines in 1982 to 1996. The number of zero-catches of 0+ ruffe increased significantly after oligotrophication, but decreased again in 1996 after the accidental reintroduction of waste waters.

Ecological and Life History Characteristics of Ruffe (Gymnocephalus cernuus) in Relation to Other Freshwater Fish Species

Abstract Ecological and life history characteristics of ruffe (Gymnocephalus cernuus) were studied in relation to 33 other native freshwater fish species in Finland. The descriptive ecological characteristics included 1) trophic guild, 2) species interaction, 3) adult habitat, 4) spawning habitat, 5) reproductive behavior, and 6) reproductive guild. The life history variables included 1) age at maturity, 2) length at maturity, and 3) fecundity as a number of eggs. Three different clusters of fish species were found in cluster analysis based on the ecological characteristics. The most distinct cluster of 7 species was comprised of predators. Ruffe was grouped into the largest cluster together with 20 other species and was closest to the cyprinid, gudgeon (Gobio gobio). In multidimensional scaling species were placed in two hypothetical dimensions firstly by their interaction with other species, trophic guild and adult habitat and secondly by their spawning habitat. Ruffe was placed near the center of the plot, suggesting that it has no special ecological requirement among the characteristics and species studied. Subsequent analysis of the life history characteristics revealed that ruffe was similar to the most typical r-selected species in Finland, showing high fecundity and low length at maturity, but with slightly slower growth rate. We, therefore, concluded that adult ruffe is a potential invader species showing no special ecological requirements and with life history characteristics typical for r-selected species.

Post-stocking perturbations in a population of subarctic whitefish, Coregonus lavaretus (L.): Effects on growth, condition and cestode infection

Lake Kilpisjärvi was stocked with a total number of 440 000 migratory and benthic whitefish fry during the period of 1959–1964. Thereafter, the Diphyllobothrium ditremum plerocercoid infection became a problem decreasing the feasibility and commercial exploitation of the whitefish stock. Since dense fish populations may cause stunting, all stockings in the study area were stopped in an attempt to enhance the fisheries. The aim of the present study was to explore the effect of the cessation of the stocking on the growth, condition, catches and D. ditremum infection of the whitefish population. A total of 1594 whitefish were sampled with benthic gill nets. Growth was estimated from length-at-age and mass–length relationship was estimated with a non-linear regression in 1982–1983, 1992–1993 and 1997. Parasite infection was studied in 1992–1993 and 1997. Growth and condition were poor in the beginning of the 1980s. In 1992–1993, the growth and condition of the fish improved as the numerical catch per unit effort decreased from 1982 to 1992. The mean catches of other fish species were low. There were no significant changes in whitefish growth or condition from 1992–1993 to 1997. From the 1970s to 1982–1983, during slow growth and poor condition of the fish, the median gillraker count was decreasing to 19 (min–max 14–26). In 1992–1993, the gillraker count had increased significantly to 23 (min–max 19–26). However, the improved growth and condition did not cause decrease in the D. ditremum burden. On the contrary, based on our own results and literature notes, the parasite abundance seems to increase during the observation period of 1964–1997.