Gear Selectivity in the Moray Firth

November 5, 2019 by No Comments | Category Marine Directorate general, Marine Directorate Science, Research Vessel Surveys

MRV Alba na Mara
Survey 1919A

Duration: 4 – 12 November 2019

Trawl BT201; Net mounted camera system; Scanmar instrumentation; Seltra box incorporating a 300mm square mesh panel

To undertake catch comparison trials using a Seltra sorting box rigged into the extension of the BT201. Target species will be commercial gadoids, nephrops, commercial flatfish, and anglerfish.

The Seltra sorting box consists of a four panel extension piece incorporating an escape window (in this case a 300 mm square mesh panel) into the top section. This four panel design helps to maintain good clearance between the bottom panel and the escape window. The Seltra is designed to reduce unwanted bycatch of whitefish while maintaining catches of nephrops.

The Seltra box will be provided ready rigged into a suitable extension piece with an 80mm codend attached. A count of the meshes round will be made at the joining point of this piece. This will be joined 1:1 at the appropriate point located in the taper of the BT201 with any section requiring cut away being retained for re-joining at the end of the cruise. If possible following rigging onto the BT201 the Seltra box and extension will be laid out on the harbourside for initial measuring and photographing.

Preferred area of operation will be on mixed prawn/fish grounds in the Moray Firth. This however will be open to discussion with the skipper and exact grounds will be decided following these. Trawl area will focus on consistently obtaining a range of gadoid species (haddock, whiting cod) plus nephrops. Discussions will take into account the working practices of the vessel.

Initial operations (approx 2 days) will focus on obtaining camera footage of the rigging of the Seltra box and of whitefish behaviour at the escape window itself.

Following successful completion of the camera work MSS engineering staff will depart the vessel and operations will switch to trawl catch comparison. The trawl will be fished with the Seltra box in place where the square mesh panel is alternatively covered / uncovered with a section of 80mm diamond mesh. This will provide multiple sets of paired hauls allowing a combined comparison of catch rates over time. At the end of each haul the catch will be sorted by species and measured.

To better enable comparison, variance between hauls will be kept to a minimum. All hauls will be undertaken in full daylight, avoiding times of dusk and dawn, and as far as possible all hauls will be undertaken in the same area. Ideally two sets of two hauls will be undertaken each fishing day and projected haul duration will take both this and the amount of work associated with sorting and measuring the catch into account.

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