NZ Scallops Part 3: Lifecycle

Ben Carey finishes his series on scallops with a quick overview of their lifecycle.

Many factors determine the survival of scallop spat, including the sea conditions and the predators that eat them, such as snapper, starfish, crabs and even octopus.

One of the main predators of small scallop spat is starfish – they love eating them. The next time you find starfish on your scallop bed or in your dredge, definitely go and have a nosy and check it out! They actually digest the spat whole by latching onto the scallop and smothering it internally, eating it from the inside out – muscle, roe, guts, skirt and all. They don’t seem to attach themselves to the really big ones, just the small spat (a 50-cent sized piece if you would call it that). Interestingly, if there are lots of scallops in an area, there will be very little in the way of starfish.

Starfish are the scallops 'public enemy #1'.

Where the spat land is another big determining factor in their survival rate. Spat grow best when they settle on clean surfaces free of mud and silt, and we all know that spat love a clear and sandy bottom. If the currents push the scallop larvae well out to sea, they may well be lost forever.

Scallop growth rates

Whether people realise it or not, scallops are actually very fast-growing and short-lived – so much so that a small 50-cent sized spat can grow to a 90mm+ scallop in just one year.

On average, a single 90mm scallop has taken one year to grow to this size, before slowing down and growing at a rate of about 5mm per year. This puts a 100mm scallop at about three years old – but this isn’t a hard and fast rule as some grow even faster. As a matter of fact, in places such as Little Barrier Island in the outer Hauraki Gulf, small spat can grow into a 100mm+ scallop in one year! 

Coromandel scallops take around three years to grow to 100mm.

Scallops become sexually mature and start spawning when they are around 70mm in length, at around eight months of age. When a scallop reaches about 150mm (typically 9-10 years old), they stop growing and start dying.

The 2012 deepwater bed that was found in the Hauraki Gulf saw the in-season commercial catch limit increase from 22 tonnes to 325 tonnes. Interestingly, this bed was actually found by a commercial snapper longline fisherman of all people. He had scallops coming up on his backbone and hooks, so let the commercial scallop fleet come in to check it out. It was full of big scallops; however, the bed itself was at the end of its days and dying off quite quickly as the spat landed in an area of mud and silt. Over the following two years, the bed became full of dead shells.

Whether it was actually one big mother-bed which provided the larvae and recruitment for the Hauraki Gulf and/or the Coromandel fishery is yet to be seen, but what we do know is that these fisheries are in some degree of trouble.

A tasty treat, but one we must be careful to preserve for future generations.

Scallop spat reseed other beds. What we have to remember is that not all scallop larvae will settle on another bed directly next to it. Larvae travel many miles to scallop beds in other regions, which just goes to show how much we still have to learn about this species.

So there you have it – a little information about scallops. Hopefully, this series has provided you with a bit of insight into these tasty little morsels.