Wetenschap - 23 maart 2017

Mussels need better dispersal

tekst:
Tessa Louwerens

Only five percent of the juvenile mussels that farmers spread around plots in the sea eventually end up as mussels on your plate. Researcher Jacob Capelle of Wageningen Marine Research discovered that this yield can be increased by getting a better dispersal of the ‘mussel seed’.

A mussel farmer flushes mussel seed through holes in the hold as he steers his boat in circles around his plot. Photo: Jacob Capelle

Every spring, millions of mussel larvae float in the waters of the Wadden Sea, looking for a place where they can attach themselves and grow. Juvenile mussels that are one to two centimetres in size are known as ‘mussel seed’. The mussel seed cluster together and form seed beds. In the past, the mussel farmers would fish out the seed and transport it to their plots where they would disperse the mussels.

But you can no longer cultivate mussels using the same methods as in the olden days, explains Capelle. ‘There is a lot of uncertainty about the effects of traditional mussel cultivation on wild mussel beds, which is why mussel seed now has to be collected using mussel seed catching equipment rather than taken from wild beds.’ It is more expensive and labour intensive to use this equipment, which consists of ropes and nets suspended in the water. That is why the yield has become more important for mussel farmers.

Underwater farming
Cultivating mussels is really underwater farming, explains Capelle. He investigated the effect of various factors on the growth and survival of mussels. ‘For a long time, it was not clear what exactly happens to the mussels after the seeds have been dispersed. If we have a better understanding of this, growers will be able to act accordingly.’

Dispersing the mussels turns out to be an important step in the cultivation process. The farmers drive a boat in circles around their plot and flush the mussel seed through holes in the hold. ‘If they drop the mussels close to one another, the shellfish lie there like a kind of mat and start fighting and competing for food,’ says Capelle. Three quarters of the mussels in areas with the highest seed density die within four weeks. ‘If the mussels are dispersed evenly across the plot, they organize themselves into groups and survive better.’ However, spreading them too thinly is not good either. ‘That is because they have to cling to one another to prevent themselves from being washed away.’

Crab banquet
Competition among one another is not the only problem for young mussels —predators are also lurking in the wings. Beach crabs adore them, for example. Capelle: ‘The process of dispersing the mussel seed attracts lots of crabs and the mussels are still easy prey because they have yet to attach themselves to something.’ The researcher discovered that this crab banquet is responsible for about one third of the losses in the first five weeks after the seed is dispersed. The seed density had no effect. ‘Crabs are territorial, so they claim a section of sea bed. It doesn’t matter whether you dump one kilo of mussels or ten.’

Mussel growers use this starfish-mop apparatus to clear their plots of starfish. Photo: Jacob Capelle


Starfish

Capelle also compared survival rates on mussel farm plots with those on wild mussel beds. ‘Wild beds have a better survival rate in fresh water, despite the poorer quality of the food in fresh water. That is because there are more starfish in saltier water and they eat the mussels.’ Mussel farm plots are located in places with higher salt concentrations as that is where the mussels grow better. To protect them against starfish, the mussel farmers regularly clean their plots with a special starfish mop. This underwater pest control method is effective, says Capelle.

He concludes that a better spread of the mussel seed and moving mussels to protected cultivation plots can help increase the mussel population in the Wadden Sea. This is important not just for mussel farmers, says the researcher; it can also benefit birds such as the common eider, which needs about five million kilos of mussels every year to get through the winter.


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