Mussel Farming – low flushing areas

This is an article regarding mussel farming in ‘low flushing areas’. Much of Port underwood may not be regarded as low flushing, although parts may be.

Marlborough Express

Future of Sounds a balancing act


November 29 2015

OPINION: Sounds mussel farming has been a topical matter of late.

The Kenepuru and Central Sounds Residents’ Association has serious concerns with the existing level of mussel farming in the low flush heavily farmed areas of the Central Pelorus (e.g. Beatrix, Clova and Crail Bays) and in the Kenepuru Sound.

There is very good cause for this concern, including present farming levels and the threat they pose to the natural ecosystem.

Mussels are very efficient and voracious filter feeders – indiscriminate consumers of what is in the water column going past them. They strip from the water column phytoplankton (microalgae), zooplankton (living organisms), fish eggs, and palatable detritus. In low flush densely farmed bays their ecological impact is potentially huge such that it is generally accepted that an ecologically safe threshold for mussel farming is the point beyond which there would be any significant change to the ecosystem structure.

Perhaps the most telling indicator that this point has been surpassed in the Central Pelorus is the recently released NIWA biophysical model. This model was commissioned by the Marlborough District Council to measure physical matters such as flush rates as well as to help understand what mussel farming impacts are in the Sounds on key biological indicators.  One of the model’s tasks was to predict what changes would occur to the system if all mussel farms that existed as at 2012 were to be removed.

The association has reviewed these results for the particularly low flush but heavily farmed bays in the Central Pelorus. They show that zooplankton levels in the likes of Clova Bay, Crail Bay, Beatrix Bay and North Side Kenepuru would be up to 10 times higher in the “without mussel farm” scenario whilst other palatable detritus would be up to four times higher. As well, ammonium and nitrate levels across the bays would be halved without the mussel farms.

Bearing in mind that mussel farming in the Pelorus has gone from 1000 hectares in 1995 to 2500 hectares now, there is little doubt that these base level food chain and water column changes are significant.  To illustrate, it has been shown that the average sized green-lipped mussel will filter 19 litres of seawater per 24 hours. It is calculated that the approximate 250,000,000 mussels growing in Beatrix Bay at any point will filter 4.75 billion litres of seawater per day. That is 7.2 times the entire volume of water in the bay per day. Adding to this is that the NIWA model shows that Beatrix Bay takes around 40 days to flush.

Exactly how these changes might manifest themselves further up the food chain was not predicted by the NIWA model.

However, there can be little doubt that an activity that is extracting up to 90 per cent of the zooplankton, up to 80 per cent of other palatable detritus and doubling the ammonium and nitrate levels throughout the bay is having a serious and unacceptable effect on the ecosystem structure, right up the chain to fish and natural shellfish populations.

Indeed mussel farming in Beatrix Bay is calculated to now be more than 10 times over the appropriate level suggested by the internationally recognised Aquaculture Stewardship Council. Clova Bay is even worse than that.

A lot of the increase in farmed area has been through new mussel farms and extensions in the Beatrix, Clova and Crail bay area. Accordingly, to locals in these areas, the significant demise of natural sea vegetation, shellfish and fish life over this period of rapid increase is no coincidence and is a serious concern. It is of little comfort to them that the scientists are only now starting to wake up to the downside of this industry.

There are of course economic benefits from mussel farming and these are important.

The Marine Farming Association suggests we may be worse off locally by up to $18 million of spending a year if 50 per cent of Sounds mussel farms are removed. No-one is suggesting this, although we welcome the MFA tabling targeted reductions in farms.

But whatever the economic effect, mussel farming cannot be at the expense of core environmental values. Economic benefits determine why we should have mussel farms. They do not determine how many mussel farms. And of course having ecologically bountiful waters brings its own economic benefits through the likes of tourism, recreation and holidaymaking services.

The Sounds has been a great nursery for the mussel industry and Havelock has greatly benefited. Nobody wants this to change. But if environmentally sacrificing vast areas of the Sounds is the only way we can economically sustain this industry then clearly we have a problem.

We do not believe the future of the industry lies in the very low flush waters of the Sounds. Fortunately things are evolving elsewhere. One example is the opening up of 2100 hectares of new high flush offshore mussel farm space in Golden and Tasman Bays.

The association accepts mussel farming has a part to play in the Sounds. We are comfortable to see mussel farming continue but not at the currently environmentally damaging and indeed dangerous level that we believe is occurring in the Central Sounds area right now.

Trevor Offen is a Pelorus Sounds resident and sits on the marine committee of the Kenepuru and Central Sounds Residents’ Association.