Photo: DTU Fødevareinstituttet

Catching crabs can benefit the Isefjord’s fish stocks

Wednesday 08 Jul 15
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Bo Munk Jørgensen
Associate Professor
National Food Institute

The catch of fish for human consumption in the Isefjord can be significantly boosted if a considerable part of the fjord’s abundance of shore crabs is caught. The crabs – which make fishing in the fjord more difficult and often damage the catch in the fishermen’s fish traps – can fx be used in chicken feed. These are some of the findings from a research project at the National Food Institute, Technical University of Denmark. 

The stock of shore crabs in the inner Danish waters has risen steeply during the last 20-30 years. Shore crabs represent a large but untapped resource in the inner Danish waters. In the Isefjord alone stock is estimated to be between 15-20,000 tonnes. The crabs are a problem, partly because they compete with the fjord’s valuable fish species for food and partly because they damage the catch that is caught in the fishermen's fish traps.

Modified fish traps

The National Food Institute has since 2002 conducted a number of research and development projects to identify how shore crabs can be used for consumption or in animal feed. In a recently completed project researchers from the institute have studied the best ways to catch and process the Isefjord’s shore crabs into chicken feed.

The researchers have cooperated with three local fishermen to develop a fish trap which catches the crabs but avoids bycatch of eel or other fish. The fish traps ensure that any fishing for crabs does not contravene the action plan on eel and that there are no problems with fish being discarded because they are either undersize or the fishermen do not have a quota to catch them.

Subsidized chicken feed production

Shore crabs from the project have been processed into crab meal and used in feed for biodynamic layer hens. The research project has neither shown a positive nor a negative effect on the hens – except that the yokes of their eggs were more yellow in colour. If crab meal was mixed in with the feed, it would therefore no longer be necessary to add a colouring agent such as paprika to the feed.

However the costs involved with catching, transporting and processing the crabs are greater than the value of the crab meal. As such fishing for shore crabs solely for chicken feed would only be profitable if the fishery was subsidized as part of attempts to reestablish the fjord’s stock of eel, sole and flounder. Another possibility would be to develop other and more profitable products from the shore crabs or to find other markets.

Work for 30 fishermen

Although 26 tonnes of shore crabs were caught from the fjord and removed during the 18-month project, it had no impact on the catch rates of fish for human consumption in the fjord.

In order to reduce the crab stocks to a natural level, 15,000 tonnes of crabs would have to be caught over a three year period, according to the National Food Institute’s researchers. This would at the same time provide enough work for 30 fishermen. This could be done sustainably without severely damaging shore crab stocks, while at the same time having a positive effect on the rest of the fjord’s fauna, which is currently under a lot of pressure due to the large number of crabs.

Read more

The resultats of the project have been discribed further in the report: Pilotprojekt for udvikling af fiskeri af strandkrabber til foderproduktion (pdf – available in Danish only). The project has been financed by the Ministry of Food, Agriculture and Fisheries of Denmark and the European Fisheries Fund.