Andre presfaktorer end næringsstoffer og klimaforandringer – vurdering af omfanget af stenfiskeri i kystnære marine områder

Abstract

As part of a larger project funded by the Danish Environmental Protection Agency (“Effects on the quality elements defined by the EU Water Framework Directive (WFD) of other pressure factors than excess nutrient load and climate change”) a number of environmental pressure factors other than excess nutrient loading and climate change have been identified as potential risks to all of the quality elements of the WFD (Petersen et al. 2018). In Denmark, boulder reefs in shallow waters have been extensively exploited habitats targeted for their high concentration of easy-to-collect large boulders for constructing sea defences and harbour jetties. This has destroyed important habitats for the EU WFD quality element macroalgae. Extraction of boulders has for all practical matters been prohibited since 1999 but there is no record of the extent of the volume of stones extracted. However, in order to assess how the historical extraction may potentially have affected attached macroalgae vegetation, an analysis based on historical records have been carried out.  For the assessment of the amounts of boulders that have been removed, we have partly assessed the few available direct sources of boulder extraction from recordings from the extraction vessels as well as estimated consumption of the boulders for major construction work, primarily in relation to the construction of harbour jetties. We have then applied these amounts to the total number of boats that have been recorded performing boulder extraction in the period 1900-1999. There are no complete inventory of all boats extracting boulders and we have only had access to a few logbooks of the ships. Based on these data, it was estimated that 8.3 million m3 m of boulders (corresponding to 83 million stones) have been extracted from coastal Danish waters (<10 m water depth, predominantly 4-7 m). In the analysis, only stone fractions from 150-600 mm in diameter (type 5) and 340-2000 mm in diameter (type 6) have been considered. Gravel extraction is thus not included in the analysis. It was estimated that the extraction corresponds to a loss of 55 km2 hard substrate suitable for macroalgae colonization. The estimates are associated with major uncertainties and no data from before 1900 are included. There are only a few studies that have investigated the ecological effects of Danish stone reefs, but knowledge from the re-establishing of a stone reef at Læsø Trindel in the Kattegat have been used to elucidate the effects of boulder extraction on the occurrence of macroalgae. If it is assumed that Læsø Trindel data are representative, the removed boulders correspond to a potential loss of macroalgae biomass of 14,980-15,790 t of ash-free dry weight (AFDW). As data from Læsø Trindel may underestimate the real loss, as the reef was not fully covered by the end of the study period, it is estimated that a realistic loss of standing biomass of macroalgae associated to the boulder extraction more realistic in in the order 15,000-75,000 t AFDW. In Danish waters, the extent of stable hard substrate in marine Natura 2000 areas has been estimated to 2080 km2. It is thus reasonable to assume that it is <5% of stable hard substrate that has been lost due to boulder extraction. This will on a national level not constitute a major environmental problem in relation to the quality element macroalgae. Locally, in specific water bodies the loss may have had a higher impact and have caused a relatively higher loss of suitable stable substrate. In addition, as most the extracted boulders can be assumed to have occurred as cavernous reefs, it is valuable habitats for various heterotrophic organisms that have disappeared.