Trawling disturbance can modify benthic production processes

Abstract

Summary 1.Trawling disturbance has wide-ranging impacts on the marine environment and is well known to modify benthic habitat and community structure. This has led to speculation about the positive and negative impacts of trawling on ecosystem processes such as production. 2. Existing theory suggests that frequent trawling disturbance may lead to the proliferation of smaller benthic species, with faster life histories, because they can withstand the mortality imposed by trawling and benefit from reduced competition or predation as populations of larger species are depleted. Since smaller species are more productive, trawling disturbance may 'farm the sea', with knock-on benefits for consumers, including fish populations. 3. We conducted the first large-scale studies of trawling effects on benthic production across quantified gradients of trawling disturbance on real fishing grounds in two regions (Silver Pit and Hills) of the North Sea. There were 27-and 10-fold differences in levels of beam trawl disturbance among the Silver Pit and Hills sites, respectively. 4. Size structure was described using normalized size-spectra, and the slopes and intercepts of these spectra were related to levels of trawling disturbance. Production was estimated from the size spectra, using a new allometric relationship between body mass and the production to biomass (P:B) ratio of marine invertebrates. The general validity of the relationship was confirmed using a phylogenetic comparative approach. 5. In the Silver Pit region, trawling led to significant decreases in infaunal biomass and production. The abundance of larger individuals was depleted more than smaller ones, as reflected by the positive relationship between the slope of the normalized size spectra and trawling disturbance. The effects of trawling disturbance were not significant in the epifaunal community. In the Hills region, where the range of trawling disturbance was lower, trawling disturbance did not have significant effects on biomass or production. 6. In the Silver Pit, relative infaunal production (production per unit biomass) rose with increased trawling disturbance. This was attributable largely to the dominance of smaller animals in the disturbed communities. The increase in relative production did not compensate for the loss of total production that resulted from the depletion of large individuals. There was some evidence for the proliferation of small polychaetes at moderate levels of disturbance, but at higher levels of disturbance their biomass and production fell. 7. We conclude that reported increases in the biomass and production of small infaunal invertebrates in the North Sea are attributable largely to recent increases in primary production that were driven by climate change, and not to the effects of trawling disturbance.

Impacts of Traditional Shore Seine Operation along the Tuticorin Coast, Gulf of Mannar, Southeast India

“…It has been reported that bottom trawling is the most damaging fishing activity 9,10 , whether mechanized or not. Trawling primarily reduces the surface roughness of the seabed 13 and destructive harvesting by bottom trawls could potentially reduce future harvests through destruction of essential habitats for commercial species or their prey 11 . Shore seines are said to be the least likely gear to maintain sustainable yields in artisanal fisheries 14,15 , and are referred to as a destructive gear 16 .…”

Mobile fishing gear reduces benthic megafaunal production on Georges Bank

“…One might expect that a degree of trawling would enhance benthic megafaunal productivity, as opportunistic species would most likely be favored in a moderately disturbed environment (Dayton & Hessler 1972, Jennings et al 2001. Smaller-bodied animals with high intrinsic rates of increase may benefit from the reduction in biomass of larger-bodied benthic fauna through a reduction in competition .…”

“…Jennings et al (2001) compared production at 2 sites in the North Sea that each experience a range of beam-trawl disturbance. An allometric relationship, based upon a suite of variables, was used to estimate community production.…”

Towards a meaningful assessment of marine ecological impacts in life cycle assessment (LCA)

“…Permanent community shifts from dominance by high biomass organisms, including bioturbating macrofauna, towards small-bodied opportunistic species, such as polychaetes, have been documented for heavily trawled areas, such as the Gulf of Mexico and the North Sea (Crowder et al, 2008;Jennings et al, 2001a;Jennings et al, 2001b).…”