Capture and live storage of flatfish using species-selective Danish seine nets

Background

There are several species of flatfish living inside the fjord lines along the Norwegian coast, such as European plaice (Pleuronectes platessa) and lemon sole (Microstomus kitt), which are currently not used much. 

The industry has shown an interest in fishing for these species, but there are several challenges relating to their usage that need to be resolved first in order to develop sustainable fisheries, i.e. first and foremost the development of a good range of fishing equipment and ensuring that such fishing would become profitable. 

Using Danish seines for fishing flatfish, such as plaice and lemon sole, has long been a traditional activity in some areas of Northern Norway, especially Vesterålen/Lofoten and Finnmark. No large quantities are landed, but these fish could be very important for some fishermen. 

The main problem with such fishing activities is that the fish can sometimes become mixed up with significant quantities of round fish, particularly coastal cod. This is a well-known problem in multi-species fisheries where one wants to use some species, but avoid secondary catches of other species. 

In 2015 and 2017, the Institute of Marine Research conducted a number of experiments aimed at developing a seine net so that flounders could be caught without being mixed with too much cod. These experiments found that it is possible to almost exclusively catch flounders by using a seine net with a low opening height. 

Despite the positive results obtained in these experiments under the project entitled “Controlling Catches in Seine Nets: Main Project” (FHF-900865) and also in later trials, the industry has reported challenges associated with the efficiency of the flounder seine net that was developed. 

Specifically, the fishermen reported that they were unable to get the flounder net to “settle” properly on the seabed and that the equipment did not fish as it was supposed to. Further development is therefore required in order to make the equipment more efficient. 

The possibilities for landing flounders are limited. With sporadic deliveries of small catches the recipients of the fish do not obtain a profitable turnover. In some cases, fishermen are offered a grinding-up price for their catches. 

In order to make such fishing profitable, the live storage of flounders is considered to be an attractive possibility. Large quantities of flounders could then be stored in fish farming cages and be slaughtered when required in order to ensure stable deliveries to the markets. 

The industry has proposed storing the fish in the “trampoline cages” that are currently used for the live storage of cod. The main seasons for the live storage of cod are the spring and summer, while flounders are typically fished during the autumn. Therefore, with this concept it might be possible to use the same trampoline cages that are used in connection with the live storage of cod. 

Live storage for 4 weeks (without feeding) is expected to provide good quality (controlled slaughter) and at the same time help to ensure delivery capabilities throughout the autumn season. 

There is also a need to develop “best practices” for the live storage of flounders in Norway. Best practice procedures should ensure the highest possible survival rates and fish welfare from catch to delivery.

Objectives

The aim of the project is to increase the utilisation of flatfish inside the fjord lines by further developing methods for Danish seine fishing, live storage and processing. 

Subsidiary objectives:

• To develop and test Danish seine nets for catching plaice, with minimal subsidiary catches of other species (e.g. cod). 
• To store flatfish live, focussing on the best possible survival rates and fish welfare.
• To identify and test available technology for the mechanical gutting and filleting of flatfish.

This is what we are doing:

This project is divided into three professional work packages based on our subsidiary objectives, as well as a separate work package for project management and presentation. 

At least two trips will be undertaken, one during the autumn of 2022 and one during the autumn of 2023. 

Testing of Danish seine nets for catching flounders (The Norwegian Institute of Marine Research) 

To develop and test Danish seine nets for catching flounders and achieve efficient sorting of subsidiary catches, focussing mainly on cod and halibut. The aim is to first identify and improve any potential weaknesses in an experimental seine net with a low opening, and then compare it with a larger net with a higher vertical opening. This project will also investigate mesh selection of flounders and propose a bag design that effectively sorts out both sub-target flounders and cod. 

Live storage of flounders (Nofima) 

Fish density and welfare indicators will be key factors in respect of the fish caught, from live storage and until the fish are slaughtered. In connection with the first expedition in AP1, attempts will be made to keep the flounders alive on board the fishing vessel. The flounders will be kept alive in 800-litre live fish tanks with perforated double bases and circulation based on the upstream principle (0.5 litres of seawater per minute per kg of flounders). 

Enough plaice should be collected in order to conduct experiments on live storage in commercial trampoline cages which are normally used for live cod, for up to 4 weeks without feed. All the flounders shall be slaughtered within 4 weeks of being caught. 

Evaluating mechanical gutting/filleting of flounders (Nofima) 

Testing a relevant gutting/filleting machine for yield and quality. Due to our limited budget, we only have the option to test one machine. If mechanical testing cannot be carried out, monitoring the yield produced by manual filleting after 0, 2 and 4 weeks of live storage would be an alternative.

These experiments can be done on individually marked fish. The water, protein and fat content of the fillets will then be measured. Fillet quality will be recorded by undertaking sensory measurements of colour, softness, gaping and smell.