Effect of mesh size on baited trap catch composition for noble crayfish (Astacus Astacus)

Abstract

Key-words: trap selection, minimum size, noble crayfishMinimum size is a common management regulation in noble crayfish (Astacus astacus) fisheries. The most used catching method baited traps, catch crayfish down to 50 mm total length (TL). In Norway the minimum size is 95 mm TL and this regulation allow female crayfish to spawn a least once before being harvested. To reduce the fraction of non-legal sized crayfish in trap catches, mesh-size were increased from 17.5 to 21 mm (knot to knot) in Lake Steinsfjorden (1983) and nationwide (2001). We compared catch per unit effort data in traps with different mesh sizes from the two most important noble crayfish fisheries in Norway to evaluate the effects of the regulation. Traps with mesh size 21 mm catch significantly fewer crayfish compared to traps with 17.5 mm and 12 mm. As intended, traps with 21 mm caught significantly less non-legal sized crayfish compared to 17.5 mm. Increased mesh size thus reduce the catch-induced mortality of non-legal sized crayfish, resulting in a larger spawning population. Mesh size regulations is an important management tool, especially for recruitment limited crayfish population, and is rather easy to enforce compared to minimum size. RÉSUMÉEffet de la taille des mailles de nasses appâtées sur la composition des captures de l'écrevisse à pattes rouges (Astacus astacus)

A novel ‘triple drawdown’ method highlights deficiencies in invasive alien crayfish survey and control techniques

“…All traps were modified with an extra 5‐mm mesh in place to increase their efficiency in retaining smaller individuals (e.g. Johnsen, Skurdal, Taugbøl, & Garnås, 2014). Sets of 10 traps were baited with fresh oily fish and deployed nightly over four nights, totalling 40 trap nights for each study site, with the exception of CON2016, where only 25 trap nights were possible.…”

Environmental DNA (eDNA) Monitoring of Noble Crayfish Astacus astacus in Lentic Environments Offers Reliable Presence-Absence Surveillance – But Fails to Predict Population Density

“…The relative density or CPUE is given as the number of crayfish per trapnight. The number of crayfish caught per trapnight is regarded as a reliable measure of crayfish density with sufficient effort, and even at efforts as low as 15 trapnights (Zimmerman and Palo, 2011;Johnsen et al, 2014).…”

“…The red list status and its importance in freshwater ecosystems has led to the development of surveillance programs aiming to monitor distribution and relative density of noble crayfish (Johnsen et al, 2019). In Norway, as in other countries, estimates of relative density are obtained by trapping crayfish with baited traps (Johnsen et al, 2014). This is relatively time consuming, and in order to increase the number of monitored populations, environmental DNA (eDNA) methodology has recently been included in the Norwegian surveillance programs for both crayfish plague and freshwater crayfish (Johnsen et al, 2019;Strand et al, 2020).…”

Size-mediated, density-dependent cannibalism in the signal crayfish Pacifastacus leniusculus (Dana, 1852) (Decapoda, Astacidea), an invasive crayfish in Britain

“…Therefore, management interventions that target smaller crayfish or a wider range of size classes should be considered. For example, manipulation of crayfish catch size distribution was achieved by the modification of baited trap mesh size (as in Johnsen et al, 2014). Other removal methods should be considered such as netting, electrofishing (Price & Welch, 2009) and microhabitat traps (Parkyn et al, 2011), that have also been shown to select for smaller crayfish compared to baited traps.…”