Impact and distribution of bottom trawl fishing on mud-bottom communities in the Kattegat

Abstract

The Kattegat in the inner Danish waters has been trawled for at least 80 yr, but so far only few attempts have been made to quantify the trawl effort, its spatial distribution and its potential ecological impact on the benthic fauna. GIS-analyses of VMS-data from trawling in the Kattegat by both Danish (2005−2009) and Swedish (2007−2009) vessels show that 95% of all trawling occurs below 22 m depth. Most activity takes place on homogeneous benthic habitats with muddy sediment at depths below the halocline and with almost the same salinity across the entire area. Furthermore, the estimate of trawled area demonstrated that the habitats are nearly 100% impacted, and frequencies of trawling beneath 100 m depth can reach 20 events per year. Multivariate analysis of community composition could not discriminate between lightly trawled and heavily trawled areas. However, a strong habitat selectivity of the trawl activity and inter-correlation between trawl activity and depth-related community structures complicated interpretation of the results. Species with biological traits previously categorized as sensitive to physical disturbance showed higher abundance in areas with low trawl activity compared to areas with higher activity. Thus, the Kattegat has been impacted to an extent where areas with reference conditions for certain habitats below 22 m no longer exist. Consequently, it is unknown how the benthic communities would have appeared without trawl disturbance and, thus, difficult to determine the impact of continued disturbance.

Mediation of macronutrients and carbon by post-disturbance shelf sea sediment communities

“…In cohesive sediments, we find that there are few differences between sediment communities that have experienced different frequencies of fishing (as found in Pommer et al 2016), but there are fundamental changes in species composition, abundance and biomass among post-fishing disturbance levels in non-cohesive sediments. Importantly, these alterations to infaunal biodiversity are not necessarily transformative in terms of biogeochemical functioning as theory might predict (Cardinale et al 2012), rather there is little to differentiate the relative contribution of post-disturbance communities in moderating organic carbon and macronutrient dynamics (Wrede et al 2017).…”

“…Alterations in community biomass associated with fishing pressure, as observed here, are known to decrease the strength of interspecific interactions and minimise the effects of biodiversity on ecosystem processes (Caliman et al 2012), yet the lowest species abundances documented here were not in communities that had experienced the greatest frequency of fishing. This apparently anomalous finding can be explained, however, because the areas under study likely represent a permanently disrupted state (Collie et al 2000;Kaiser et al 2000;Pommer et al 2016), with recovery times measured in years (Kaiser et al 2006). In such environments there tends to be a predominance of small species with opportunistic reproductive modes that are able to respond quickly post-disturbance and can reach high abundances, but these species are more likely to have a low bioturbation potential (Solan et al 2004a;Queirós et al 2013).…”

Substantial changes in the depth distributions of benthic invertebrates in the eastern Kattegat since the 1880s

“…As pointed out by Pommer et al. (), chronic trawling has occurred in the deeper parts of the Kattegat for at least 80 years and it is therefore difficult, not to say impossible, to find suitable spatial reference areas suitable to trawling. In our study, we use a temporal reference from the time long before intensive trawling in the Kattegat and we notice substantial changes in the depth distribution of species particularly in the area trawled in recent times.…”

Biodiversity and ecosystem services in Nordic coastal ecosystems

“…The historic trawl ban decision represents an indirect driver that may partially explain the relatively high diversity of the benthic fauna (Hansen & Blomqvist, in prep.). Exactly how much this ban has contributed to the benthic diversity in the Sound is still uncertain, as the Sound may not function to serve as a reference for the nearby and continually trawled Kattegat (Pommer et al, 2016).…”