"If you build it they will come": bivalve aquaculture boosts local biodiversity

The next time you sit down to a steaming bowl of mussels cooked in a savoury broth of wine and garlic, just think — the mussel farm where they were grown could be increasing the productivity and biodiversity of its local environment.

The commercial farming of mussels, oysters, scallops and other bivalves on Canada's east and west coasts has increased in recent decades to meet rising consumer demand. As with other forms of aquaculture, this growth has led to concerns about its impact on the environment and vice versa. Until recently, research has largely focused on the effect of organic waste from mussel farms on small organisms that live in the seabed and on the production carrying capacity of sites for mussel culture. Recent research on bivalve aquaculture led by Dr. Christopher McKindsey of DFO's Maurice Lamontagne Institute is exploring the issue from a different angle that has led to some very interesting findings and a more holistic view of aquaculture-environment interactions.

Mussel farmers in Lameque, N.B., harvest seed collection lines. Research by Dr. Christopher McKindsey at DFO's Maurice Lamontagne Institute is exploring the impact of mussel farms on fish, lobsters, crabs and other large, mobile organisms that live on the surface of the ocean bottom. Photo credit: Suzanne Taylor, DFO.

The low-down on organic loading

Bivalves are filter-feeders, straining phytoplankton, zooplankton and other organic particles and nutrients from the water. No additional food or antibiotics are used to grow them in culture. What they don't require for growth becomes waste that sinks to the bottom, potentially increasing the accumulation of organic matter there.

"Previous studies on the impact of mussel farms looked at the 'black spot' under a farm where a lot of the smaller organisms that live in the sediment, such as small worms and soft-shell clams, die or are displaced due to increased organic deposition. But that is only part of what's happening in the environment," says Dr. McKindsey. "When we look at larger organisms, we clearly see the opposite effect with an increase in productivity and biodiversity around mussel farms." In fact, McKindsey's research revealed organic loading increases the size of small worms at short distances away from the farms (30 metres).

Biodiversity and blue mussel "farms"

McKindsey's research explored the impact of blue mussel (Mytilus edulis and Mytilus trossulus) farms on "epibenthic macrofauna" — fish, lobsters, crabs, sea stars and other large, mobile organisms that live on the surface of the ocean bottom. The study, which is funded in part by DFO's Aquaculture Collaborative Research and Development Program, focused on four mussel farms in Prince Edward Island and compared them to non-aquaculture sites with similar conditions located 50, 100, 500 and 2,000 metres away. The research was carried out in collaboration with Dr. Philippe Archambault of the Institut des sciences de la mer de Rimouski at the University of Quebec at Rimouski.

The findings revealed an increase in the abundance and diversity of large, bottom-dwelling organisms — particularly sea stars and rock crabs — in the immediate vicinity of mussel farms. "Any big organism that you would see if you're diving you'll see more of within mussel farms than outside of them," says McKindsey. In general, the composition of those communities likely depends on the available species in the area and the suitability of a mussel farm as habitat for each species.

There is no evidence that the increase in biodiversity within mussel farms depleted adjacent populations. Communities 50 metres from the mussel farms were the same as those even farther away, indicating that the attraction of organisms to mussel farms is likely localized to areas within or immediately adjacent to mussel farms (i.e. within 50 metres).

"It's clear that large invertebrates and benthic fishes, including ecologically and commercially important species, seem to respond positively to the presence of suspended mussel culture," says McKindsey. "And there were no clear trends to suggest that mussel aquaculture had any negative impact."

Research by Dr. McKindsey reveals that mussel farms attract an array of other organisms, including ecologically and commercially important species such as lobster. For every five kilograms of mussels on a sock there may be up to 10 kilograms of other organisms growing on them. Photo credit: Chris McKindsey , DFO.

More and different prey

Although the mussels are grown for human consumption, their presence creates something akin to a buffet for other species due to changes in prey availability. In suspended mussel culture, the system most commonly used on the east coast, mussels grow on or in socks or mesh sleeves that are attached to a rope or long-line held in suspension by buoys.

"The bivalves in culture are one source of prey, especially if they become dislodged and fall to the bottom," says McKindsey. "The accumulation of empty mussel shells on the bottom may also create a permanent habitat for species that can live on or within the shells."

A survey at one mussel farm revealed that sea stars (Asterias sp.) were almost 10 times more abundant within than outside of the farm just after the August harvest, during which large quantities of mussels and fouling organisms were undoubtedly detached from the long lines. Three months later, just after farmers had restocked mussel socks with juvenile mussels, there were more sea stars located 50 and 100 metres from the farm. This suggests that they had been moving away from the mussel farm during the three months since harvest.

Reef-like communities

Fouling organisms — species that grow directly on mussel "socks" or fallen bivalves — may also increase both prey abundance and diversity. "If there are five kilograms of mussels growing in one 'sock', there may be 10 kilograms or more of other organisms growing on them," says McKindsey. "These include starfish, sea squirts, hyrdozoans, sea cucumbers and fish such as rock gunnel, cunner, snails, clams and worms. Organisms that normally live on a hard bottom community are now living up in the water column, creating a reef community that attracts a lot of fish."

Predictive computer modelling

McKindsey is also refining computer models to predict the influence on the bottom. "To date, our models have only included the amount of waste produced by the mussels. But since there's a lot of other biomass living on them, we are trying to determine the amount of waste produced by organisms such as sea squirts. This will help us build a predictive model to determine how mussel aquaculture influences organisms that dwell on the bottom," he says.

The physical structure of mussel farms, including ropes, anchors, buoys and mussel 'socks' also attracts mobile species by providing structure that increases foraging and refuge opportunities. "In the sites we studied, lobsters were often closely associated with the cement blocks that serve as anchors for the long lines," say McKindsey.

It is, quite clearly, a case of "if you build it they will come," including commercial and sport fishers who set traps around aquaculture sites because they know that's where the fish are, says McKindsey.

Research in support of sustainable aquaculture

Understanding the impact of bivalve aquaculture on the environment is important toward the ongoing development of regulations and sustainable practices, for which DFO shares responsibility with other federal, provincial and territorial governments. "Bivalve aquaculture is increasing around the world. A more holistic understanding of the role of this practice in the environment includes considering the organisms that culture sites attract and promote, and these may be included in future monitoring programs," says McKindsey.