Bottom trawling affects fish condition through changes in the ratio of prey availability to density of competitors

Abstract

Summary Bottom‐trawl fisheries are widespread and cause mortality of benthic invertebrates, which in turn may lead to a decrease in the availability of prey for target fish species. Exploitation also reduces the abundance of the fish species themselves. Modelling studies have shown that bottom trawling could lead to both increases and decreases in fish production, but so far empirical evidence to test these ideas has been very limited. We hypothesize that the effect of bottom trawling on the food intake and condition of fish depends on how the ratio of prey to consumers changes with increasing fishing pressure. We assessed the impact of bottom trawling on the food availability, condition and stomach contents of three flatfishes and the Norway lobster in an area in the Kattegat that is characterized by a steep commercial bottom‐trawling gradient due to the establishment of an area closed to all fisheries, but otherwise has homogeneous environmental conditions. For plaice, prey biomass initially decreased at a slower rate with trawling than the biomass of fish, and as a result, the amount of food available per plaice increased before decreasing at trawling frequencies >5 times year−1. This pattern was mirrored in both the condition and stomach contents of plaice and for long‐rough dab. No effect of trawling on dab prey and condition was found. Conversely, the condition of the main target species – Norway lobster – increased as its biomass decreased with increased trawling intensities. Together, these results support the idea that when the abundance of the prey declines in response to exploitation, the ratio of the prey to consumer biomass will determine whether exploitation will result in an increase or a decrease in the food intake and condition of the predator. Synthesis and applications. Our study indicates that fish production may be maximized by keeping bottom‐trawling intensities relatively low, although this may negatively affect the economically more important Nephrops fishery. The effects of bottom trawls may be mitigated by switching to gears, which affect prey availability to a lesser extent, such as pots or creels.

Stable isotopes reveal the effect of trawl fisheries on the diet of commercially exploited species

“…Due to the closeness to significance and the contextual indications from the Kattegat data, GAMM condition predictions were used to investigate the relationship between δ 13 C and δ 15 N values and condition. The relationships between trawling intensities and body condition for Nephrops , dab and plaice in the Kattegat were not presented within the present publication as they formed part of a previous publication 25 . Summarizing these results, a significant negative linear trend of Nephrops condition was observed with increasing trawling intensity in the Kattegat, while for plaice a domed relationship with its peak at approximately 5 times trawled per annum was detected.…”

“…The relationship between isotopic signatures and predicted species condition were investigated using least square regression analysis of δ 13 C (untreated and lipid normalized) and δ 15 N versus species condition (previously published data 25 for the Kattegat and the present publication for Nephrops for the Irish Sea see above) revealed a significant negative relationship for plaice with δ 13 C. Nephrops condition in the Kattegat and the Irish Sea showed significant positive relationships with δ 13 C (Fig. 4A,B,D, Table 3).…”

“…GAMs were used as the relationships were not expected to necessarily follow a linear relationship 25 . The response variable ‘biomass’ was total benthic infaunal biomass sampled per station, excluding the biomass of rare large infauna species such as large echinoderms and bivalve species (e.g.…”

Global analysis of depletion and recovery of seabed biota after bottom trawling disturbance

“…Bottom trawling resuspends sediments (10,11); reduces topographic complexity and biogenic structures (12)(13)(14); reduces faunal biomass, numbers, and diversity (15,16); selects for communities dominated by fauna with faster life histories (17); and produces energy subsidies in the form of carrion (18). These effects lead to changes in community production, trophic structure, and function (19,20). Given the patchy and dynamic distribution of bottom fishing (21), fished seabeds comprise a mosaic of undisturbed, recently impacted, and recovering benthic communities and habitats (22).…”

Distribution of Bottom Trawling Effort in the Yellow Sea and East China Sea

“…The bottom trawling intensity was stated as swept area ratio [16, 17]. It was the ratio of the area trawled to the total area in each cell.…”