New Zealand pea crabs are known to damage the green-lipped mussels they live in, but what impact do they have on mussel farming in New Zealand?

Oliver Trottier (a PhD student at Leigh Marine Laboratory) wanted to answer this question. He carried out a study of a green-lipped mussel farm and found that mussels that contained pea crabs were about a third smaller than their crab-free neighbours. Based on his results, he estimated that pea crab infestation costs the mussel industry over $2 million a year in lost product.

Finding a farm

Oliver was interested in how farmed mussels – rather than wild ones – are affected by pea crabs. Because farmed mussels grow rapidly throughout their lives, Oliver thought the effect of pea crabs on mussel growth might be more pronounced than in wild mussels. To test his ideas, he identified a small farm on Great Barrier Island to study.

Defining the questions

Long before collecting any mussels, Oliver thought carefully about the questions he wanted to answer in his study. He decided on three key questions:

  • What proportion of mussels on the farm contain a pea crab?
  • How does the weight of mussels with and without a pea crab differ?
  • What effects do the pea crabs have on mussel size and shape?

Careful planning and consultation

Next, Oliver started to plan exactly how to carry out his experiment. In particular, he considered:

  • how many mussels to take in total from the farm
  • the age at which to harvest the mussels
  • how many different sites on the farm to take mussels from
  • how many to take from each site
  • what information to take from the collected mussels.

It was crucial that he took enough mussels from enough different sites on the farm to allow him to answer his three key questions confidently. After consulting with statisticians and other biologists, he decided to collect a total of 7000 mussels from a total of 225 sites spread evenly across the farm, including at three different depths. He chose to collect the mussels when they were approximately 10 months old.

Oliver also had to plan the logistics of the experiment carefully. He worked out what boat would be used to access the farm, asked his friends (fellow postgraduate students at Leigh Marine Laboratory) to help him dive for mussels on the day and planned food, drink, equipment and dive times for everyone involved. He discussed all his plans in detail with Peter Vitasovich, the mussel farmer.

Pilot study: “heaps of zeros”

On the day before the 7000 mussels were collected, Oliver and his colleagues carried out a pilot study – a small-scale collection of mussels to double check that the experimental set-up would give useful data. This was particularly important because of the cost of running his experiment – hiring the boat alone cost $1200 a day.

From the pilot study, Oliver discovered that pea crab infection levels on the farm were far lower than he had expected – only about 1 in 20 mussels contained a pea crab (the infection rate on New Zealand farms is typically 1 in 3 or more). He had to think on his feet: he decided to collect more mussels from each site on the farm and to collect from fewer sites.

Collection and analysis

The next day, Oliver and his friends carried out the big mussel collection. They spent all day diving for mussels, and the 7000 mussels were frozen for later analysis

Oliver’s analysis of the mussels was a time-consuming task. He measured the height and width of the shell, the weight of the flesh and whether it contained a pea crab. He then recorded the sex, life cycle stage and number of eggs for every crab he found. He estimates that processing each mussel took upwards of 5 minutes. That’s well over a month of full-time work to collect all his data!

A decrease in mussel size – and industry profits

As the mussels were processed, Oliver began to notice a trend. The mussels with a pea crab were, on average, a third lighter than crab-free mussels. That’s a big difference – it’s proportionally the same as the size difference between an average 12-year-old boy and an average 17-year-old boy in New Zealand.

By using these results, Oliver was able to calculate how pea crab infestation might affect New Zealand’s mussel industry as a whole. Assuming an infection level of 5% across all mussel farms, he worked out that pea crabs are likely to be responsible for an annual loss of over $2 million in mussel production. Because infection rates on farms are often much higher than 5%, the true loss to the industry is likely to be considerably higher.

    Published 14 June 2013