Cawthron Mussel Breeding

July 2009

Selective breeding of mussels produces superior yields

Selective breeding has been practiced for over a thousand years by land based farmers. Livestock, crop plants, and trees have been developed that are higher yielding, of better quality, and tailored to specific consumer markets. In contrast virtually all Greenshell™ mussel stock farmed in New Zealand are collected as spat from the wild. The Cawthron Institute in Nelson is running a breeding programme for the Greenshell™ mussel with the objective of giving the New Zealand mussel industry the same production and marketing benefits that have been delivered to land based farmers through selective breeding.

The programme is reaching the point where the first benefits will soon become available to mussel growers. The most likely route for passing on these benefits to the industry is via the production of superior stud mussels or broodstock. These broodstock will be available to hatcheries for the production of mussel spat that grow faster and produce a higher quality product at harvest. Mussel farmers will have the option of choosing the mix of characteristics they would most like to see in their mussel spat. For instance mussels that grow faster with a higher meat yield, and even have a greener shell.

One of the first steps in building a selective breeding programme is to decide on breeding objectives or which of these characteristics the programme should focus on. Many breeding programmes start out by breeding for fast growth and in aquaculture gains of 10 to 20% per generation are typical. A producer can make use of fast growth either by growing larger individuals if this attracts a premium or by growing to the standard size but in a shorter time. This can be particularly important in a seasonal crop like mussels where a small increase in growth rate can move the production cycle from 2 years back to 1 year.

A sometimes overlooked consideration is product quality. Even though breeding for faster growth usually results in more consistently sized individuals, it’s easy to be seduced by the economic gains of faster growth only to realize that product quality has been sacrificed along the way. While it’s important to ensure that product quality isn’t compromised, it’s also possible to breed for improved quality which can result in a more marketable and competitive product which may attract a premium. Cawthron is working with Crop and Food Research to identify post-harvest characteristics that may be improved via the breeding programme.

It’s important to remember that breeding objectives are specific to a particular species, particular industry, and that industry’s markets. And often the priorities of the farmer will be different to those of the processor who in turn has different priorities to the marketer. Currently the broad objectives are to increase production efficiency and to improve product quality and consistency. This means mussels that grow to harvest size faster and have an increased meat weight so more of the harvest goes into the higher paying grades.

The breeding programme uses what’s called a family approach. A mussel family is the offspring of one male and one female mussel. In other words two mussels are mated together to produce a whole lot (millions!) of brothers and sisters. This is done many times resulting in 50 or more mussel families, each composed of several hundred thousand siblings.

The families are hatchery reared until they’re about 20mm in length. Up to this point the families have been kept separate but there’s a risk that certain families might have an unfair advantage because some are getting more food than others for example. For this reason it’s better to grow all the families together to minimize the effects of environmental differences. This is possible once the mussels get to 20mm and are big enough to stick plastic ID tags on using super glue. The families can then be mixed and sent out to mussel farms for on-growing to harvest.

The mussels are ready to be harvested after one to two years and are brought back to the hatchery to be measured. The ID tag is used to find out which family each of the mussels came from. That way it’s possible to identify the families with the fastest growth, highest meat yield, or any other characteristics that meet the breeding objectives.

And then comes the selective part of selective breeding, because the families are chosen that best meet the objectives and are used to produce the next generation of families. They’re also used to produce mussel spat for commercial farming. Currently the focus is on selecting mussels that have the longest shell length and also the highest meat weight. Ultimately selection will be for things like meat appearance, shell colour, and what ever traits the industry considers valuable.

So far the programme has been very successful with its production of families. Initial results have shown significant differences in both shell length and meat weight between families, and these traits are relatively consistent within individual families. The implications of this are that the offspring of stud mussels from a fast growing family will grow to harvest in a shorter time and also have more even grading.

The differences between families and the similarities within families show that growth rate is heritable. This means that a fast growing family is likely to have fast growing offspring. Other traits are heritable as well. For instance shell colour shows strong differences between families. Selective breeding will only work for traits that are heritable.

The next steps in the breeding programme will be to carry on assessing the families, and to use the best mussels from the best family selections to create further sets of families. It’s also important to find out how the research results translate into commercial gains so another priority is to get spat from the best stud mussels out on marine farms at commercial scale. That way they can be grown and harvested as a commercial crop to assess the potential for increased revenue for the farmer.

Cawthron also has projects with industry partners looking at bridging the gap between the hatchery and the farm, where a large proportion of spat are currently lost. Further down the track there’s a large investment required in hatchery infrastructure to be able to produce the volumes of spat required by the NZ mussel industry.

Parallel to selective breeding programme Cawthron is developing a gene database to help understand the way in which genes influence mussel reproduction. Some of the goals for this research will be an ability to condition broodstock for spawning at a pre-determined date for hatchery spat production, to breed mussel lines that mature at different times of the year and mussel lines that hold their market condition for a long time and without spawning during harvest.

The development of this gene database has encountered a number of unexpected technical problems, highlighting, once again, the biological peculiarities of the mussel.