Shark theory #24: It's because of salmon and leatherjacket

THIS white shark business has been blamed on everything from trifling river dredging right through to a shift in the Earth's magnetic field.

But it's all part of the boom-and-bust cycle of the sea.

I think I can explain why they're here now in numbers when they once were uncommon, but it's not up to me to determine what happens next.

As background, I received Sydney surgeon Sir Victor Coppleson's book 'Shark Attack' for my 10th birthday and half a century later, a well-thumbed copy has a place on my bookshelf.

I have fished, swum and snorkelled these waters since that time and for most of the 1980s and 1990s I fished off Evans Head at least 50 times a year. These days it's probably only about 20 but I walk the local beaches twice a day, every day, with an eye on the water.

It wasn't until winter 1990 that I saw a great white here. It approached our anchored boat on the surface from some distance away, took 10 minutes to slowly inspect us a very scary one metre away from the boat, and then leisurely swam off into the distance the way it had come. It was 4.2-4.5 metres, measured out along our 5.3m boat.

Ten minutes later, I looked over the side and, suspended four metres below, there it was again.

Great whites were protected in Australian waters in 1999, listed as vulnerable by the International Union for the Conservation of Nature, the Australian Government and NSW.

In 2002 a White Shark Recovery Plan was introduced, finalised in 2008 and further amended in 2013. That encompasses Federal governments of both persuasions, so it's hardly party-political.

The 2008 version concluded it was not possible to determine if the Australian shark population had shown sign of recovery.

I started identifying smaller whites, 1.8-2.5m, on my daily beach walks from 2007, accompanying massive influxes of Australian salmon unprecedented here in my lifetime.

Major commercial fishing effort on salmon ceased with the closure in 1999 of the HJ Heinz cannery at Eden, which had processed 1000 tonnes a year.

Salmon proliferated after netting closures north of Barrenjoey (Sydney) in 2000 and massive numbers were reported on northern coasts, to the extent that some years later NSW Fisheries and the Federal Fisheries Research and Development Corporation commissioned a comprehensive study by Stewart et al.

Salmon numbers peaked in local waters during 2008-2010 and the net ban was repealed in 2011. Salmon are now netted commercially to bait fish and lobster traps in NSW and WA and local numbers have declined to merely occasional encounters with small pods of fish for a few weeks a year.

Coinciding with the inshore salmon boom were huge influxes to local offshore waters of oceanic 'chinaman' leatherjackets, meaning further prolific food for juvenile great whites, whether they were off the beach or on the continental shelf. Leatherjackets experience extreme boom-and-bust population cycles and their numbers appear to have peaked a few years ago.

CSIRO shark researchers Barry Bruce and Russ Radford have identified Stockton Bight, near Port Stephens, and Corner Inlet, near Wilsons Promontory, as the focus of shark nursery areas from the surf zone to mid-shelf depths, used repeatedly on a seasonal basis across different years.

I know little about the Victorian nursery but Stockton Bight was the epicentre of massive, semi-permanent schools of salmon during the latter half of last decade, and was the geographic centre of the leatherjacket boom - perfect conditions for nourishing great white shark pups.

Dr Bruce says in a YouTube UTS lecture that in the summer of 2010 the patrolled area in front of the Hawks Nest Surf Lifesaving Club was closed 44 times because white sharks had swum between the flags.

A 3m specimen mauled surfer Glen Folkard off Redhead Beach in 2012 and a 2m white was responsible for an attack on surfer Ben Morcom inside Port Stephens in 2007.

With similar salmon and leatherjacket numbers occurring in our waters during those years, it was only natural that the young sharks, 1.5-2m, followed the food.

White sharks seem to retain location memory; they keep coming back to the same areas over many years, according to tracking data.

However, there isn't the local food for them that there serendipitously was when they were younger, although schools of smaller baitfish this season have attracted some tailor, mackerel and tunas.

Could the Byron Coast be becoming a hangout for some of these 'adolescent' sharks? Fisheries officials say sharks tagged recently in these waters have been 2.5-3m long - 5-7 years old, consistent with growth of sharks that were whelped back in the local heyday of the salmon and jackets.

The salmon that migrated to these waters have been 'managed' back to lower numbers and the jacket population has declined naturally. So we have some age cohorts of sharks lately recruited to the fishery that until recently have been able to feast on medium-sized fish but are now transitioning their diet towards larger prey.

They are familiar with the area and it's now a waypoint on their oceanic roamings.

I think you could say the recovery plan seems to be working. Now the question is, what do we do to stop becoming their food?

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