Diving in marine estuaries - fish nurseries

Have you ever thought what it would be like to live in the ocean from a fish’s perspective? Can you imagine how perilous and terrifying it would be growing up in an environment where everything wants to eat you?

The vast majority of fish never make it to adulthood because something slightly larger thinks they look really tasty. Let’s just say it’s a dog-eat-dog world in the ocean and only the fittest, luckiest or cleverest survive.

I guess as a fish you’d want to have some strategies so that at least some of your kids make it to adulthood and the family line carries on. You’d probably also need some favourable conditions and safe places to grow up in.

One of New Zealand’s best-loved fish, the snapper, uses a strategy of mass spawning, where huge aggregations of fish come together, with each female releasing millions of eggs. (Snapper may spawn a number of times between October and March.) It’s pretty clever: having so many eggs ensures that a small percentage (but still good numbers) of snapper hatchlings make it through to adulthood.

Against impossible odds – it must be like running the gauntlet – where the chances are stacked seriously against them, the first 28 days of a snapper’s life are spent drifting at the mercy of ocean currents, unable to swim, while fish, birds and other plankton feeders, including whales, pick off their fair share.

Once they reach about 12mm in length, they’re finally strong enough to swim towards shore and find somewhere to hunker down and hopefully grow as quickly as they can.

Harbours and estuaries provide a vital home for this next stage – but not just any estuary or harbour; snapper seem to be pretty snobbish about their accommodation! For example, I find it mind blowing to think that with so many west coast harbours to choose from, over 95% of west coast snapper grew up in just one harbour – the Kaipara!

We all know that where there are little fish there will be bigger predators not too far away hoping to eat them – john dory, kahawai, kingfish, trevally, to name just a few... Heck, I bet even bigger snapper hoover up the odd little one down!

Two really important survival strategies for juvenile snapper are: to have somewhere to hide so they don’t get eaten, and ensuring they have plenty to eat. The Kaipara has ticked these boxes historically, with its extensive sea-grass beds enabling these tiny snapper to hide from marauding predators – and there’s also plenty of food to aid their rapid growth. Once they get to about 65mm they start spreading around the open coastline away from the harbours.

However, some of the Kaipara’s seagrass beds are disappearing due to increased sedimentation, which is cause for concern.

Invasive Japanese mantis shrimp, which grow to over 350mm, have taken up residence in the harbour in recent years and may prey on juvenile snapper. Some people are excited about having these critters in the harbour as bigger fish love eating them, but they could potentially be one of the most lethal predators of the lot when it comes to eating our baby native fish!

I’d never go snorkelling in the Kaipara, as it’s a bit of a mud bath, but my local estuary is an epic place to see a nursery first hand, as it offers some of the clearest water on the coast. It’s the most bizarre thing: while shooting for an episode of Young Ocean Explorers, we started filming just outside the harbour, where the visibility was about 6m; then, as we moved inside the harbour, the visibility became an awesome 18m! What the…? This was exactly the same body of water as outside with 6m vis!

Here we were, witnessing firsthand a very graphic illustration of Nature’s pool pump in action: the ‘dirtier’ water coming into the harbour passes over the hundreds of thousands of filterfeeding pipis and cockles, which suck it in and spit out a cleaned-up version. My local estuary is the last harbour near Auckland with healthy beds of cockles and pipis and which enjoys this phenomenon. Amazingly, each pipi can filter about one litre of water every hour!

The small fish obviously enjoy this clean environment, with perfectly camouflaged schools of tiny snapper, barely visible amongst the ripples of the white sand, hovering near any obstacle they can hide behind. Baby parore, feeding on tiny particles of food in the water, quickly dart to the cover provided by Neptune’s necklace seaweed whenever danger is sensed. There are also schools of little trevally which race about in perfect synchronicity, each doing its best to survive the gauntlet, hoping to win the lottery and make it to adult life.

Healthy harbours are vital for the survival of our favourite fish, so looking after the places where baby fish grow will help ensure our kids have fish to catch in the future.

Estuaries are fantastic places to take kids snorkelling. The best times to snorkel are:

• After a calm period with little rain
• Just before high tide and during the slack tide, so there is minimal current. It’s also the time the water is normally the clearest, as the filter feeders have been cleaning the water as it comes in and the dirty water from further up the estuary hasn’t come out yet.

Use a dive flag, but still be aware of what boats are doing around you – it’s unbelievable how many people disregard dive flags and the safety of others – and ask a local dive shop for the best and safest estuary locations to try.