Abundance and biomass from grab samplesAbundance and biomass of benthic invertebrates from the Smith-McIntyre grab samples per species and per station. Benthic invertebrates were sampled by taking five 0.1 m2 Smith-McIntyre grabs at haphazard locations within the station box. For each grab, a 50ml sediment sample was retained for grain size analysis and the rest of the sample was sorted over a 1 mm sieve and preserved in 4% formalin for identification. All invertebrates were identified to the highest practicable taxonomic resolution (mostly genus or family) and the wet weight of each individual organism was estimated after blotting.Infauna Kattegat Biomass all stations!!.csvStation positions and sediment compositionLatitude and longitude of the station and their sediment composition from particle size analysis. Fishing effort is given in the paper. Particle size distributions were determined using a Malvern laser diffraction particle sizer (Blott & Pye 2001).Kattegat treatments.csvFish stomach content dataStomach content data for plaice and dab have been submitted to the fish stomach database DAPSTOM that is maintained by CEFAS, UK. This contains the species, length, and weight of each fish, and the identity, number and weight of each prey item. The digestion state of each prey item is also given.Otter trawl catchesThe catch composition of all species per trawl for the otter trawl catches. Two tows of 30 min were carried out at each station with an otter trawl (distance across mouth of the net 25 m, 80 mm diamond mesh cod-end) at a speed of 3 knots between 07.45h and 17.00h. The total catch number and weight per species was measured.Kattegat otter trawl catches.xlsFish species Length-weight dataThe length and weight for individual fish that was used to estimate the condition of the fish. The length (to the nearest mm) and weight (to the nearest g) of the individual fish in the catch was recorded. If the catch of a species in a haul was large (>50 individuals), a subsample of ≥50 fish was measured and weighed. The BMI (body mass index) column is given here for quality assurance purposes only and not used in the analyses. Some values outside the possible range and have been marked as such.Kattegat Length weight data.xlsNephrops norvegicus length and weight dataNephrops norvegicus length and weight data that was used to estimate their condition. The carapace length and the total weight was measured, and their sex recorded.Kattegat_Nephrops.xls

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.