Workshop on technical solutions to reduce cod bycatches in the Baltic, 30 November 2020

Innovative modifi cations to existing gears

Cod is one of the most commercially important species in the Baltic Sea providing a livelihood to fi shers from countries across the region. Arresting the steady deterioration in the state of cod stocks calls for a slate of measures. Adapting fi shing gear is one of them.

Thuenen Institute

The Bacoma codend has a standard mesh with a square mesh panel on the top section of the codend and is a standard fi shing gear in the Baltic.

There are two cod stocks in the Baltic the eastern Baltic cod and the western Baltic stock. For several reasons, many of them related to the environment, the eastern Baltic stock is in particularly poor condition and ICES, the international body that assesses these stocks, has estimated that the stock is below safe biological limits and that it will remain so in the medium term even if it is not fished at all.

According to ICES, the stock has further declined since last year and the organisation therefore advised zero catches in 2021. Th is however would choke fi sheries where cod is caught as a bycatch. Based on an updated ICES assessment of levels of cod bycatches in other fi sheries, the Commission elected to limit the TAC for eastern Baltic cod to unavoidable bycatches. Th e TAC for bycatch which was reduced by 92 to 2,000 tonnes in 2020 has been further reduced to 600 tonnes in 2021.

Baltic Sea Advisory Council, Baltfi sh, and DTU Aqua collaborate on fi shing gear workshop

Th e state of the cod stock thus impacts cod fi shers as well as fi shers targeting species where cod is a bycatch. At the end of November, the Baltic Sea Advisory Council, Baltfi sh, and DTU Aqua jointly hosted a virtual workshop to discuss technical solutions to reduce unwanted catches of cod in the Baltic Sea fi sheries. Th e results of the discussion would feed into a Baltfi sh joint recommendation for more selective gear that would target fl atfi sh and minimise bycatches of cod, said Estonia’s Kaire Märtin, the chairperson of Baltfi sh. She also expressed the hope that the meeting would result in suggestions to streamline the process of developing and implementing new or improved gear solutions.

Scientists studying the eastern Baltic cod stock have noted that today natural mortality is six times higher than fi shing mortality, a highly unusual condition as most cod stocks exhibit the reverse. Th e fi sh since about 1990 have become smaller, skinnier, and also mature when they are smaller, meaning that the volume of roe (the number of eggs) is also less compared with fi sh that mature when they are larger. Marie Storr-Paulsen, a researcher at DTU-Aqua attributes these developments to a lack of oxygen, less prey for the fi sh to feed on, and an increased seal population. Studies of the oxygen levels in the Baltic Sea show that the area with little or no oxygen has increased over the years. Part of this is due to the infl ow of the nutrients, nitrogen and phosphorus respectively from agriculture and from municipalities. Although this has declined signifi cantly since peaking in the 80s, it may not be refl ected in higher oxygen levels in the water as oxygen depletion is measured at the sea fl oor. Another complication is the climate change infl uenced rise in water temperature which increases the fi sh’s demand for oxygen. Studying cod prey revealed that the number of a certain crustacean (Saduria entomon) in stomachs of large cod had declined drastically since 1994 compared with before the 90s, probably because the crustacean could not tolerate the low oxygen conditions at the sea bottom. Th e distribution of sprat, a fi sh on which cod feed, has also changed so that there is now little overlap between the two species. Grey seal abundance in the Baltic Sea has increased steadily since 2003. Th e seals prey on the cod, but another more serious impact comes from playing host to a parasite that attacks cod livers which in turn infl uences the growth of the fi sh. Studies have shown that the number of parasites per fi sh in the eastern Baltic is considerably higher than in the western Baltic. As a result of these factors, conditions for cod have become less favourable over time contributing to the phenomenon we see today of undersized fi sh and unhealthy stocks. Various measures have been taken to reduce pressure on

Thuenen Institute

A T90 codend using a 125 mm mesh proved to be the most effi cient at allowing the cod to escape while retaining the fl atfi sh.

the stock including spatial and temporal restrictions, the use of selective gears, changes to the minimum landing size, and bans on targeted fi sheries, but as Ms Storr-Paulsen suggested, perhaps something else needs to be done.

Modifi cations to existing gears could reduce cod bycatches in fl atfi sh fi sheries

Various kinds of selective gear have been deployed in the Baltic fi sheries to reduce the capture of unwanted species or sizes. Among the two options currently available is the T90 (a diamond mesh rotated 90 degrees) codend which has been used for the last decade in the Baltic. Jordan Feekings, a gear expert from DTU-Aqua explained that the T90 mesh is designed to allow smaller cod to escape. Th e other option is the Bacoma panel which is a window of square mesh in the upper half of the codend through which the smaller cod can escape. However, as a targeted fi shery for cod in the eastern Baltic is no longer permitted, the emphasis has switched to preventing a bycatch of cod in the fl atfi sh fi shery. Th is can potentially be accomplished by changing the codend, the design of the trawl, the behaviour of the fl eet, or some combination of all three. Th e researchers modifi ed the codend by replacing the square mesh Bacoma window with a large diamond shaped opening. Th is allowed all the cod to escape (rather than just the undersized specimens) yet retained the fl atfi sh. Trials with the modifi ed gear showed that retention rates for cod were low (25) while those for fl atfi sh were high (75) and that the addition of a piece of netting attached along the top and halfway down the sides of the codend at the back of the opening slightly improved the retention rates, increasing them for fl atfi sh and decreasing them for cod. Th e low cost and simplicity of these modifi cations make them potentially very useful to reduce bycatches of cod in fl atfi sh fi sheries. Mr Feekings added that the data shown came from trials in the Nephrops fi sheries using a 90 mm diamond mesh and that selectivity is likely to increase with a T90 codend using a larger mesh. Trawls can also be modifi ed for greater selectivity by stagnating the headline so that it is behind the ground gear. Normally, the headline is ahead of the ground gear so that when the fi sh swim up to avoid the ground gear they are guided into the trawl by the headline. By stagnating the headline, upward moving fi sh will swim over the trawl rather than into it. An alternative to

Thuenen Institute

cutting away the top panel (to stagnate the headline) is to replace the front section of the top panel with a large mesh through which the fish can escape. This has the added benefit of maintaining the geometry of the trawl making it easier to use than if the headline is stagnated. A further possibility to potentially reduce bycatches of cod could be to target flatfish at night when cod are higher up in the water column and therefore less vulnerable to being caught.

Trials show a less than 4% retention of cod and an 87% retention of flatfish

The increasing seriousness of the state of the eastern Baltic cod stock can not only be gauged from the decline in the stock and the actions introduced to protect it, but also from a letter from the Commissioner to Member States warning of additional measures if solutions were not identified and implemented. Daniel Stepputtis with his colleagues from the Thünen Institute in Rostock, Germany has been working on the issue of avoiding cod catches in Baltic trawl fisheries by modifying the codend and/or changing the trawl design. To find the best way of avoiding cod yet retaining flatfish, different codends were studied: Bacoma, which has a standard mesh with a square mesh panel on the top section of the codend; a full square mesh codend; and several codends with a T90 mesh. A T90 codend using a 125 mm mesh and equipped with Lastridge ropes (which keep the mesh stable and open) proved to be the most efficient at allowing the cod to escape while retaining the flatfish. In general, modifying codends is simple, effective in terms of bycatch reduction, cost efficient, and easy for the authorities to control, however their efficiency is influenced by the population structure—large cod, for example, do not get sorted out.

Modifying the trawl design by changing the extension, the part between the codend and the front section of the trawl, showed that removing the roof altogether was the best way of allowing the cod to escape. The presence of even a very large-meshed net through which they could easily swim deterred the cod. One variation of the roofless extension in particular allowed about three quarters of the cod to escape while retaining a high (87) proportion of the flatfish. Another advantage was that unlike with the codend modification, cod escapes were independent of size. In addition, the roofless extension was cost efficient, easy to control by the authorities and effective at allowing cod to escape. In contrast, modifying the front section of the trawl, by cutting away the top was not found to be an optimal solution as it was expensive, complex, of varying efficiency, and hard for the authorities to control. Finally, combining the most effective codend (T90, 125 mm, Lastridge ropes) with a roofless extension resulted in the retention of less than 4 of the cod. The use of this combination, Dr Stepputtis pointed out, would reduce the bycatch of cod by 90 compared to the Bacoma trawl currently being used in the Baltic. And while the combination is more complex it is still cost efficient, easy to control, and most effective at reducing the bycatch of cod.

Twin codends also show promising results

Studies on reducing the bycatch of cod have also been conducted in the Skagerrak with a trawl featuring two codends, an upper for roundfish and a lower for flatfish. The lower codend was equipped with a grid and a square mesh while the upper had an open frame and a large diamond mesh. Hans Nilsson from the Department of Marine Resources, Swedish University of Agricultural Sciences (SLU), considered the escape behaviour of the fish and the shape. From tests conducted in 2016 and 2017 in Skagerrak and Kattegat he showed that compared to the control (120 mm diamond mesh codend) the catches of flatfish in the upper codend were very low compared with the control. Using a topless trawl also reduced catches of cod by half compared with the control and catches of whiting and saithe even more significantly without impacting catches of plaice, witch, or nephrops. A trawl that was divided by a horizontal panel running from the extension section to the codend showed haddock, whiting and saithe caught almost exclusively in the upper codend, Cod, on the other hand, was caught in both the lower and the upper codends. Both codends had an identical mesh, so the purpose of the experiment was to see where the fish was ending up. In trials in the Baltic Sea a flatfish trawl was used with a frame in the front section. The frame had horizontal bars at 50 mm intervals and the trawl had a 105 mm diamond mesh with a 130 mm square mesh at the codend. Cod bycatches were on average 5 of the catch which was mainly flounder. A standard T90 codend that was also used to fish at the same time and in the same area caught cod and flatfish in more or less equal proportions.

The results from the research presented at the meeting should contribute to the recovery of Baltic cod stocks and ultimately of the economies of the fleets that depend on this resource.

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