Beach users warned of harm to shellfish

File photo / Michael Craig

Boaties towing trailers onto the beach this summer might want to follow existing tyre tracks to reduce harm to shellfish.

New research shows driving vehicles and riding horses on the beach poses a significant threat to shellfish living and feeding in shallow water.

Canterbury University student Gareth Taylor, who received his doctorate this week, looked at the impacts of vehicles and horses on shellfish beds within Pegasus Bay in Canterbury.

He found tuatua were immediately disturbed by their activities, and continued disturbance could be highly detrimental to the population.

The results have been used to provide management options to Environment Canterbury to minimise the impacts of beach users on shellfish.

Dr Taylor said the intertidal zone, where most people drove cars on a beach, contained a wide range of shellfish including juvenile tuatua.

Most light activities like sunbathing and running had no impact on the environment, but heavier activities like driving vehicles could.

Dr Taylor had some advice on how drivers could minimise their impact.

"The easiest way is to not drive on the beach; however, if a vehicle is required there are other ways to mitigate impacts.

"Driving within the same tracks made by other vehicles will significantly reduce tuatua mortality caused by vehicles."

Doing so caused shellfish mortality of only 0.3 per cent - in stark contrast to 5 per cent mortality when creating new tracks.

Mortality could also be increased by heavy four-wheel drive vehicles and off-road tyres, which penetrated deeper into and displaced more sediment.

Tuatua are a food source and also filter the water, reducing its cloudiness and keeping it clean.

Adult tuatua are found in the sub-tidal area, but juveniles sit in the top 5-10cm of sediment about 30m below the high tide line - where most vehicles and horses are used.