Catching invasive Chinese mitten crabs while releasing the endangered European eel and other fish by‐catch: The implications of fyke net design

Abstract

Modifications of a traditional fyke net design were trialled in 42 hauls over 5 months at 42 locations in the Thames estuary, London. These trials were to determine whether the modified nets could be used to catch invasive mitten crabs while at the same time releasing endangered eels, back into the river. The modifications included rings of different diameters fixed into the netting to provide escape apertures and also a variation in mesh size. A standard, unmodified net was included as a control. Captured mitten crabs, eels and other fish by‐catch were measured and recorded for all deployed nets. Mitten crabs and eels were caught in all nets except those of the largest mesh size (70 mm) which caught no eels. This may have been the combined effect of the mesh size and it being set on the square, versus the normal diagonal netting which may become increasingly constricted in one axis, under tension. Such a square mesh net could be used to trap crabs of carapace width > 65 mm, while releasing all eels. The smallest rings, 22 mm internal diameter, inserted into the mesh may have allowed the escape of eels < 35 cm length, but retained larger, market legal, individuals. This suggests that a slightly smaller escape ring could potentially be used to release eels of ≤ 30 cm in length, in line with current regulations.

Design and testing of a trap removing Chinese mitten crabs (Eriocheir sinensis, H. Milne Edwards, 1853) from invaded river systems

“…A few other methods were described to obtain an indication of the size of a crab population in river or estuarine systems. Methods used were calculating the density of crab holes in riverbanks (Rudnick et al, 2003), visual inspection of the presence of crabs combined with manual sampling of habitats like rocks (Gilbey et al, 2008), bottom trawling (Cabral & Costa, 1999; Rudnick et al, 2003), mark and recapture experiments (Bell, Eaton, Bannister, & Addison, 2003; Gíslason, Jónasson, Pálsson, Svavarsson, & Halldórsson, 2017; Munch‐Petersen, Sparre, & Hoffmann, 1982) and the use of fyke nets and/or (baited) pots (Anastácio, Marques, Águas, Wójcik‐Fudalewska, & Normant‐Saremba, 2018; Clark, 2011; Clark et al, 2017; Garcia‐de‐Lomas et al, 2010; Normant, Wiszniewska, & Szaniawska, 2000; Soes, van Horssen, Bouma, & Collombon, 2007). The first three of these techniques were used in tidal areas and were unsuitable for (small) rivers because of practical difficulties.…”

“…Although the technique has proven effective, there are some concerns about the number of bycatch of fish (Clark, 2011; Clark et al, 2017). Modifications of the net (e.g., larger mesh size) can mitigate this, but comes with the drawback that small crabs can escape (Clark et al, 2017). In addition, the placement and emptying of the fyke nets can be tricky, which is labor intensive and time consuming.…”

Editorial: Introduced species challenges and opportunities for marine conservation ecology and management practices: Notes inspired by a recent MSC certification

“…For a long time fishing has been considered as a way to regulate some already established non‐indigenous aquatic species; however, the environmental, economic, and technical problems related to the application of harvesting methods are significant (Boudouresque & Verlaque, ; Clark et al, ; Morris & Whitfield, ; Nuňez, Kuebbing, Dimarco, & Simberloff, ). This way of treating introduced species is also reflected in the MSC Certification Requirements (Marine Stewardship Council, ).…”