DOI: 10.5061/dryad.04dv4
title: Data from: Bottom trawling affects fish condition through changes in the ratio of prey availability to density of competitors

description: 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 &gt;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.
description: 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 (&gt;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