Scottish Marine and Freshwater Science Vol 8 No 5 | An Investigation into the Commercial Viability of Fish Traps and Jig Fishing in the Scottish Demersal Fishery

The primary aim of this project was to investigate the commercial viability of fishing with demersal fish traps and automated handlines in the Scottish demersal fishery. Establishing a demersal fishery with traps and automated handlines would provide the inshore sector with additional options for undertaking an environmentally friendly fishing method. A similar project was successfully undertaken by NAFC Marine Centre in 2005, investigating the commercial viability of automated handline fishing for demersal species such as pollack (Pollachius pollachius), saithe, cod and ling (Molva molva). In this study, two separate projects ran concurrently in the North Sea and the west coast of Scotland.

Shetland trialsFourteen commercially available fish traps were purchased from Carapax Marine Group AB, Sweden. The traps were originally developed by the Norwegian Institute for Oceanic Research and further developed by Carapax. The foldable parlour traps measured 150 cm(L) x 100 cm(W) x 120 cm(H) and were constructed from 12 mm aluminium for the top and middle frame and 14 mm hot dip galvanised steel for the bottom frame. Traps were covered with 30 mm twisted black nylon netting and had two large soft eyes constructed from monofilament mesh. The bottom frame of each trap was roped for added durability and a zipper was incorporated into each section of the trap for fish removal. Each trap was fitted with a detachable bait bag.Correspondence with the trap manufacturer revealed that the traps worked ideally in collapsible form in water currents of <1 knot. It was felt that this could potentially affect the efficiency of the traps around Shetland as the current speeds around the coast can increase to more than 3 knots. In order to determine whether the current had an effect on fishing capability, half of the traps were rigidified within a frame constructed at NAFC from 16 mm rod iron to compare fishing efficiency of collapsible and rigid traps. Rigidified traps were fitted with 6 x 450 g buoyancy floats on the top of the frame to help the traps maintain their upright orientation when sinking to and resting on the seabed. Traps were deployed in sets of two (here referred as a leader). Each leader had 60-100 ftm of 12 or 14 mm rope on the end (depending on the depths fished) that was held to the seabed with an anchor made from 40 kg chain link. The first trap in the leader was located 5 fathoms from the anchor (referred to as the anchor trap) and the second trap was located ten fathoms from the anchor trap (referred to as the end trap). Each leader consisted of one collapsible and one rigid trap. Collapsible and rigid traps were alternated between the anchor and end position to account for any potential bias in trap position on the leader. Traps in the same leader were baited with the same bait. A number of different bait types were used during the study including herring, mackerel, squid, mussel, dog whelk, sand eel and crab. Leaders were hauled using a Spencer Carter 1 ton hauler. Due to their lightweight design, both collapsible and rigid traps were lifted over the gunwale of the vessel by hand.Fishing trials were undertaken on NAFC Marine Centre vessel Atlantia II for a six week period in July-August 2015 at various locations around Shetland. Soak times for traps were recorded. Upon retrieval of traps, all saleable fish caught were recorded and length measured to the nearest cm. Catch weights of each species were also recorded. Any unmarketable by-catch was recorded and its general condition noted prior to release. A scale, modified from a catch-damage-index developed for cod (Esaiassen, et al., 2013) and another for invertebrates (Depestele, et al., 2011), was used to assess the condition of any bycatch that was returned to the sea. Catch composition, in relation to trap type, trap position and bait was analysed. Spatial variation in catch rates was examined and discard and bycatch composition and condition were evaluated. Finally, an assessment of the commercial viability of the fishery was made.West of Scotland trialsFour triple parlour fish traps were designed and constructed by Jim Mair, Marine Scotland. The mesh size in each of the traps was 12 mm. Each trap had three vertical entrances in the first compartment, which was baited (Figure 7). A large funnel entrance led to the first parlour section and a second large funnel entrance led from the two bottom sections to an upper chamber. When fish entered the top panel they had at least two entrances to negotiate before potentially escaping. Each trap was foldable for storage purposes but was made rigid prior to deployment.Traps were deployed in sets of two (here referred to as a leader), as illustrated in Figure 2. Each leader had 60-100 ftm of 12 or 14 mm rope on the end (depending on the depths fished) that was held to the seabed with an anchor made from 40 kg chain link. The first trap in the leader was located five fathoms from the anchor (referred as the anchor trap) and the second trap was located ten fathoms from the anchor trap (referred to as the end trap). Traps were baited with the same bait, mackerel and crushed shellfish, throughout the entire study. Mackerel were caught fresh daily with jigging machines. Traps were hauled with the vessel’s creel hauler and lifted aboard by hand or using the vessel’s landing derrick when necessary.Fishing trials were undertaken on Sea Spray OB 140 for a six week period in June 2015 to July 2015 at various locations on the west coast of Scotland. Soak times 17were recorded for each deployment. Due to the limited budget (there was only enough finance avail