Traps and netting, better together than alone: an innovative approach to improve

The red swamp crayfish Procambarus clarkii is the most widespread invasive crayfish in Europe, and responsible for a plethora of negative impacts on aquatic ecosystems. Most capture methods used for controlling crayfish populations have a bias towards the capture of adults, however, the removal of the young-of-the-year crayfish (YOY) may be essential for achieving effective control of invasive populations. This paper analysed the crayfish caught during a management campaign carried out in five permanent stream pools from southern Spain. We compared size structure, CPUE and sex-ratio obtained with two control methods: cylindrical traps (a method commonly used in crayfish management) and horizontal hauls using a fine-mesh net (inspired by zooplankton sampling techniques). Horizontal hauls showed a higher selectivity for catching YOY and higher efficiency (eight-fold) than traps. The combined use of both gears increased total catch by 46%. Our results suggest that YOY may be sharply underestimated if only cylindrical traps are used. The YOY cohort represented 60% of the total catch during the management campaign. Therefore, active netting with a fine mesh may be a complementary method to the use of traps in order to manage invasive populations of P. clarkii and may provide a better understanding of the structure and dynamics of invasive crayfish populations