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

1. Bottom trawl fisheries are wide-spread 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. 2. We assessed the impact of bottom trawling on the food availability, condition and stomach contents of three flatfishes and 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 homogeneous environmental conditions. 3. For plaice, prey biomass initially decreased slower 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 y-1. This pattern was mirrored in both the condition and stomach contents of plaice, and for Long-rough dab. 4. 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. 5. 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 of the food intake, condition and growth rates of the predator. Synthesis and application. Our study indicates that fish production may be maximized by keeping 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 that affect prey availability to a lesser extent, such pots/creels.1. 底拖网渔业（bottom trawl fisheries）分布广泛，会造成底栖无脊椎动物（benthic invertebrates）死亡，进而可能导致目标鱼种的猎物可获得性下降。捕捞活动同时还会降低目标鱼种自身的种群丰度。建模研究表明，底拖网捕捞既可能使鱼类生产量上升，也可能使其下降，但迄今为止验证这一假说的实证证据（empirical evidence）仍十分有限。我们提出假说：底拖网捕捞对鱼类摄食量与健康状态的影响，取决于猎物与消费者的生物量比值随捕捞压力（fishing pressure）升高的变化趋势。 2. 我们在卡特加特海峡（Kattegat）的一片区域评估了底拖网捕捞对3种比目鱼（flatfishes）和挪威海螯虾（Norway lobster）的猎物可获得性、健康状态及胃含物的影响；该区域因设立了全面禁捕保护区而形成了陡峭的商业捕捞压力梯度，且其余环境条件均一。 3. 针对欧鲽（plaice），其猎物生物量随捕捞强度提升的下降速率最初慢于鱼类自身生物量的下降速率，因此当捕捞频率高于5次/年时，单只欧鲽可获得的食物量先上升后下降。这一模式在欧鲽和长粗糙菱鲽（Long-rough dab）的健康状态与胃含物中均得到了印证。 4. 未发现捕捞对菱鲽（dab）的猎物资源及健康状态存在影响。与之相反，随着捕捞强度提升，挪威海螯虾的种群生物量下降，但其自身健康状态却有所提升。 5. 综合来看，上述结果支持这一观点：当猎物丰度随捕捞活动下降时，猎物与消费者的生物量比值将决定捕捞活动最终会使捕食者的摄食量、健康状态及生长速率上升还是下降。 综合分析与应用启示。本研究表明，将拖网捕捞强度维持在相对较低水平可使鱼类生产量最大化，但这可能会对经济价值更高的挪威海螯虾（Nephrops）渔业产生负面影响。底拖网的生态影响可通过更换为对猎物可获得性影响更小的渔具（如笼具/虾笼（pots/creels））加以缓解。