How does liming of watercourses affect fish and invertebrates?

NEWS | 2012-06-01

In Sweden, there are ample domestic data clarifying how liming affects the fauna of acidified watercourses. A British research group has now summarised studies of liming effects from other parts of the world as well.

Altogether, the researchers found 34 independent studies providing usable data on the effects of liming on fish or invertebrates in watercourses. Seven of these studies had been performed in Sweden. One of them was composed of data from a substantial number of watercourses, collected within the framework of the ISELAW (Integrated Studies of the Effects of Liming Acidified Waters) initiative. The other studies had been made in Norway, Finland, UK and North America.

Usually more fish after liming – but not always

According to this review, mean fish numbers were 1.7 times higher in limed than in unlimed watercourses. However, there was substantial variation and in some 20% of cases there were, on the contrary, fewer fish after liming. This inconsistency may be partially explained by the varying duration of liming operations in the watercourses, but other factors played a part as well.

It also emerges clearly that different species are affected to a varying extent by liming. In watercourses where both salmon and trout are found, the salmon benefited more than the trout. Only in waters where salmon were lacking, in fact, was a significant increase in trout abundance after liming demonstrable.

Effect of liming on fish numbers in watercourses

The figure above shows how the British review summarised the effects of liming on the fish fauna in flowing waters. The data are taken from studies of Nordic, British and North American watercourses. For each individual study, the mean effect of the liming on fish abundance (combined with a measure of uncertainty) was specified. At the bottom, the individual results are combined in an estimated weighted mean that implies an average increase in fish stocks by a factor of 1.7 after liming.The figure above shows how the British review summarised the effects of liming on the fish fauna in flowing waters. The data are taken from studies of Nordic, British and North American watercourses. For each individual study, the mean effect of the liming on fish abundance (combined with a measure of uncertainty) was specified. At the bottom, the individual results are combined in an estimated weighted mean that implies an average increase in fish stocks by a factor of 1.7 after liming.

It was not feasible to demonstrate any general effect of liming on numbers of invertebrates. The results vary and are partially dependent on study design. On the other hand, the review suggests that the number of invertebrate species rises as a result of liming (and in groups that are particularly vulnerable to acidification, both individual and species numbers increase after liming). However, relatively few studies of invertebrates have been carried out, and the conclusions are therefore more uncertain than those concerning fish.

Important to compare with unlimed watercourses

The following are among the general conclusions drawn by the authors of this systematic review:

• If the main purpose of the liming is to boost the salmon population, the prospects of success are good.

• Not enough high-quality information on the impact of liming on invertebrates is available.

• Since acid deposition has decreased over time, it is hard to isolate the effects of liming in studies based solely on measurements taken before and after liming. The measurements need to include unlimed control sites as well.

• Future studies should, to a greater extent, be so designed as to permit estimation of variance and uncertainty, with improved statistical rigour.

• Future studies should report on outcomes in terms that are better aligned with the Water Framework Directive. Individual and species numbers are rough measures of ecosystem quality.

• When liming decisions are taken, the parameters of natural or desirable conditions in the aquatic system concerned should be defined.

In a similar systematic review, two of the British researchers have also clarified the impact of liming on lakes.