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A study in the Norwegian demersal seine fisheries showed that fish which escape the net closer to the surface are less likely to survive
A study has found that fish which escape from a seine net closer to the surface are less likely to survive than fish which escape during the towing process.
The report called “Surface selection of haddock and cod in the Norwegian demersal seine fisheries” by Ólafur Arnar Ingólfsson, Odd-Børre Humborstad, and Svein Løkkeborg from the Institute of Marine Research, Fish Capture Research Group, Bergen, Norway, examines the effect seine netting has on fish caught in the net during the hauling process.
One of the recommendations made by the paper is that selective fishing gear be designed to allow undersized fish to escape during the towing process which would give the fish a higher chance of survivability.
The Abstract of the report reads:
Size selection in active fishing gears is a continuous process and undersized fish may escape during the whole fishing operation. Fish that escape during the surface hauling operation are likely to experience higher mortality due to barotrauma-related stress than fish escaping at the fishing depth during the towing process.
A well-functioning selectivity device should therefore select mostly at depth for enhancing survival probabilities of escaping fish. The current gear regulation in the Norwegian demersal seine fishery is likely to cause large proportion of undersized fish to escape at the surface. In this study, we estimated surface selection of haddock and cod in demersal seine by using an automatic release system and a small meshed codend that collected fish escaping during surface hauling.
The collecting bag contained 19% undersized haddock compared to 10% in the conventional square-mesh codend indicating that about 50% of undersized haddock brought to the surface were released. The proportions of undersized cod were 8% for the collecting bag and 1% for the conventional square-mesh codend.
These results demonstrate that surface selection is significant for both haddock and cod. Based on this finding, we discuss methods to improve size selectivity at the fishing depth.
Experimental procedure
The fishing trials were conducted on fishing grounds off Finnmark in Northern Norway at depth ranging from 54 to 117 m. The trials were conducted on board the commercial seiner “Ballstadøy” (34.9 m length overall, 1350 kW main engine) from 2 to 16 May 2017. The vessel’s conventional demersal seine was used which had a fishing circle (stretched seine opening) of 146.4 m (732 meshes of 200 mm netting) and 100 m fishing line. The length of the four-panel tapered seine belly was 53.8 m, ending with 100 mesh circumference of 130 mm nominal mesh size. The seine ropes were 2000 m long combination ropes with steel cores, 50 mm in diameter.
In line with fisheries regulations, a square-mesh codend was used. The codend had a total length of 13.5 m, the square mesh section was cylindrical (single panel without selvedges), 12.5 m long, 8 m circumference (116 bars), with 130 mm nominal mesh size of knotless 8 mm PE twine. The square meshes were joined in front to a diamond section of 130 mm mesh size, constructed of double 5 mm PE netting, 1 m in length, and 100 meshes in circumference. A codend extension of 130 mm nominal mesh size and 21.3 m long was used where the foremost 99.5 meshes where of 3 mm braided PE and the rearmost 49.5 meshes of double 4 mm braided PE. Hauls were also taken with an additional 99.5 mesh long (15 m) extension of double 4 mm braided PE. Hauls were thus taken with both 21.3 m (short) and 36.3 m (long) extensions (Figure 1).
Schematic drawing of extensions and codend. (a) The shorter extension. (b) The longer extension. (c) The square-mesh codend with the small meshed codend attached.
The extensions had the same circumferences as the codend (100 meshes). Behind the codend, a 16 m codend of 75 mm mesh size was connected to the codend’s codline. Midway through the experiment, two rows of 20 meshes were measured wet for opposite sides of the codend with an Omega mesh gauge (www.marelec.com), applying force of 125 N. The square-mesh codend measured on average 132.5 mm (SD = 1.5 mm) and the small-mesh codend 76.4 mm (SD = 1.7 mm).
The fishing operations were carried out as in commercial fishing. The seine was set on fish aggregations detected on the echo sounder and towed forward at a speed of 0.7–0.9 ms−1 for 25–35 min, until the ropes were approximately parallel. Then the ropes were hauled until the wing ends were in the tow block (additional 24–25 min). The wing ends were then taken through a Triplex block on the vessels starboard side and the seine spooled until the codend was in the Triplex block (10–12 min). A vacuum pump was then connected to the codend and the fish pumped onboard.
The connection between the square mesh and small-mesh codends was closed during bottom hauling. Half of the hauls (control) were taken using an automatic codend releaser (https://jatronic.no/wp-content/uploads/2018/08/lav_POA.pdf) that opened the connection between the two codends at 30 m depth during ascend. When the gear was hauled and reached 30 m depth, a pressure sensor activated the releaser and the fish entered the small-mesh codend which prevented fish from escaping at the surface, i.e. no surface selection (Figure 2). The other half of the hauls (test) were taken without using the releaser and the square-mesh codend was tied as in conventional fisheries. When at the vessel’s side, the codend was opened and the catch released into the small-mesh codend to avoid surface selection during onboard taking of fish.
The procedure for preventing surface selection by use of the automatic codend releaser.
Nine control (with releaser) and nine test hauls (without releaser) were taken. Three pairwise test and control hauls (i.e. with and without the codend releaser) were obtained using long extension and the remaining six sets were unpaired. We consider hauls being paired when time between deployments is less than 6 h, distanced within 1 nm. The intention was to compare codend lengths, but that plan had to be abandoned due to time constraints. Therefore, only two test hauls with the shorter extension were obtained.
The catch was pumped onboard from the codend in batches. The first ∼100 haddock were taken from several batches from each haul and their total length measured to the nearest centimetre below. Most of the catch comprised haddock which was the main subject species. Measurements of haddock were prioritized, therefore fewer cod were measured. Haddock weight was obtained from a grader. The cod that was gutted onboard was also weighted on the grader. Number of haddock and cod were calculated based on length–weight relationships. Some of the cod were kept for live storage, counted, and weight estimated when delivered. While number of measured cod from each haul was in general low, a pooled analysis of surface selection was achievable.
The full study the Norwegian demersal seine fisheries can be read here.