Richard Hudd

Fish reproduction and the impact of acidification in the Kyrönjoki River estuary in the Baltic Sea

SynopsisSeveral freshwater species use the Kyrönjoki River estuary as a spawning and nursery area. The main reasons for this seem to be the morphology of the estuary, the abundance of shelter provided by aquatic macrophytes, high food production and favourable temperature conditions. Acidification of the estuary due to drainage from acidic soils has made part of the estuary unsuitable for fish reproduction. In addition, year to year fluctuations in the acidity of the estuarine water have affected the reproductive success of several species. The severity of the effects of the acidification at the population level is determined by the spatial and temporal distribution of the larvae and juveniles.

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.

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.