We used Raymarine's fishfinder to chase marlin. Here's what we learned

It’s a good sign, with the previous day washed out for many after a storm front swept through the NSW Hunter coast the previous day. The pontoons along which the boats are tied up are still shiny black with the moonlight reflecting off what’s left of the previous day’s constant drizzle.

We’re standing before Freedom, a 36-foot Black Watch, one of two boats belonging to Haven Sportfishing Charters and skippered by professional captain Scott 'Scotty' Thorrington, a seasoned marlin fisherman with a long track record in the industry. And that’s what today is all about – we’re hunting marlin.

And we have a bit of help, some of the best tools the industry can provide. Before us, on Freedom's bridge deck, is some of Raymarine’s latest chartplotting and fishfinding gear. Today, it will help us find bait, locate schools of fish under stress – a sure sign a predator is feeding upon them – and actually "mark" marlin. Well, that's the theory ...

Marlin is the main prize of the competition, worth double points for each one hooked up, brought to the boat and tagged, with bonus points added on for their estimated weight based on their length.

We take on a big slurp of fuel at the marina before Thorrington points Freedom’s bow out of Nelson’s Bay, heading for a spot he’s marked, a stop to jig for bait the only planned interruption to our progress.

Thorrington’s electronics setup is fairly typical for a game fisher. Before him is a spread of three flush-mounted Raymarine screens consisting of a pair of 19.0-inch Axiom Pro 2 units framing a smaller 16.0-inch Axiom 2 RVM screen. Above the console, on its own binnacle, is a 16.0-inch Axiom 2 XL screen.

Each screen has a different use. To the left, the sounder is reading through a distinct layer of thermocline, the algae-filled barrier between the warmer water above and the colder water below. To the right is the chartplotter, showing our progress out of the bay. In the middle, the screen is showing the Axiom’s side scan view, peering through the water profile on both sides of the boat. Above, the touchscreen-based Axiom is showing engine data as the twin 330hp Cummins 6BTA turbo-diesels hum away below us.

It’s still quite dark, although the cloud has now cleared away from the near-full moon and we can see the outline of Tomaree Head that marks the end of the no-wake zone. Thorrington switches the smaller central screen across to the FLIR M232 thermal imaging camera mounted on the canopy top and zooms in on one of the boats ahead of us, a Grady-White centre console running twin 425hp Yamaha XTO Offshore outboard engines. Its outboards glow hot white against the black that fills most of the screen, the ghostly outlines of people moving around the cockpit clearly visible.

“I mainly use this when you’re going in or out and it’s dark,” Thorrington says. “But it’s also great when you hit that man overboard button (a small icon on the top left of the screen) because the thermal camera will automatically turn and focus on the spot in the water where you were when you pressed it.

“The boat will turn around and the camera will stay focussed on that spot. It’s a great safety tool and one the rescue services use extensively on their boats.”

As we idle along, it gives Thorrington a chance to explain more of the electronics glowing in an array in front of the helm.

He flicks one of the larger Axiom units over to a screen fed by the spinning Cyclone Pro open-array radar that’s also mounted above us. We’re coming up on a slow-moving boat, and the icon representing Freedom in the centre of the screen glows red as the system detects we’re on a potential collision course. 

A slight turn to starboard via a small adjustment on the autopilot's dial sets everything back to normal.

Also visible on this screen are the automatic identification system (AIS) icons of the boats around us. Press on the boat’s icon and a raft of information about it pops up in a window, showing details such as its name, course and speed.

Thorrington says boats in fishing competitions will generally turn their AIS off so that rivals can’t dive into the chartplotter and see where they are catching fish. Occasionally, he laughs, they’ll forget and he’ll let them know.

The South Pacific is rough this morning, with a confused swell bouncing off the shoreline and washing back on itself, whipped up by the 30-knot winds that hung around until after midnight. The wind has died down and it’s still short-sleeve warm.

Once outside the no-wake zone, Thorrington sets the autopilot on a course for his spot about 15 nautical miles (about 30 kilometres) offshore in 80 fathoms (about 150 metres) of water, running slower than usual to stop the Black Watch slamming off the bigger sets of sharp swell.

An hour in and deckies Benny and Dylan have rigged up the overhead reels ready for action. We’re coming up to an icon on the chartplotter screen. It’s a mark that Thorrington has set on a previous visit, and it’s where we are going to jig for bait.

“When you see a big bait ball you can mark it, and even if you come back a few years later the bait will still be there,” he says. “If it’s not you don’t have to move very far, only 50 metres or so, but it will be there.”

He can’t explain why. There’s nothing physical on the plotter, such as a depression or reef, to suggest there’s anything different about this spot, but they’re always nearby.

The baitfish, a big school of slimy mackerel, shows up on the plotter as a dense cloud that swarms anywhere from just below the thermocline to the bottom almost 150 metres below us. Thorrington waits for the bait to rise to about 50 metres before he idles the engines and tells Dylan to drop the heavily weighted jig line fitted to a baitcaster rod but using a small overhead reel.

As Dylan reels in the macs and adds them to the live bait well, Thorrington explains what we’re seeing on the sounder screen. The transducer’s gain is turned down low and the background is white to maximise clarity.

The sounder can tell Thorrington a lot about what’s going on below the boat. He points to the feathered edge of the image on the screen. “That’s a happy bait ball,” he says. “The fish are all comfortable so they’re not being chased by a bigger fish.

“You’ll know when you see an unhappy bait ball because the fish are all tightly together, and you know something is feeding on them.”

A shout goes up from the cockpit as Dylan pulls up a small bonito in the mix of mackerel. “You’re a long way from home,” Thorrington quips.

Decky Benny prepares the live bait, which is then attached to overhead reels and set out the back of the boat, staggered about 20 metres on one side and 30 metres on the other. Attractors, umbrella-like structures with ribbons of flashy silver images of fish, are hung off the transom.

All of a sudden, something on the sonar attracts Thorrington’s attention. “50 metres,” he shouts to the deckies, calling out the depth as a short solid line, a sure sign of a marlin, appears on the sounder.

We circle the area for a bit but don't hook up. The marlin has gone. 

About 20 minutes later, one of the reels starts zipping – we’re on. Thorrington spots it from the bridge; it’s a mahi mahi, a wonderful golden green flashing in the sunlight as it leaps out of the water. It is reeled in beside the boat, tagged with a NSW Fisheries sportsfish tag and released. 

Minutes later, the rod on the other side of the boat starts screaming as a second mahi mahi hooks up. Again, it’s brought to the boat but flicks off the hook before it can be tagged.

Reports start to trickle in as other boats radio their catch back to the game fishing club: “One-one-one, mahi mahi, 16kg line, angler 1234.”  Points are higher for fish caught using lighter line.

The “one-one-one” is code for one fish hooked up, brought to the boat and tagged. 

All boats in the competition also must adhere to a strictly scheduled call-in every two hours to report their grid position, whether they’re trolling, anchored, drifting or jigging for bait, and how many fish they’ve tagged. Any report that starts with a one but ends with a zero is met with the reply: “Bad luck.”

Where we are fishing is right in the middle of a huge proposed floating wind farm. If the project goes ahead, the region’s reputation as a sports fishing mecca will die, as the proposal is for turbines set 100 metres apart with a 50-mere exclusion zone around each one.

Thorrington says sports fishing brings millions of dollars into the Port Stephens economy, all lost if the wind farm goes ahead.

Things slow right down as we continue to trawl in the sight of a host of other boats entered in the competition.

It’s a slow day, as reports trickle in announcing boats sitting about 15 miles south of us are starting to hook up to striped and black marlin. The radios around us stay largely silent. 

Thorington has chased marlin around Australia and over in the US. As far as marlin seasons go, it has not been a good start. This, he says, is the worst he’s seen it. The water has been a bit hotter than usual, and the East Australian Current is running hard.

The day is dominated by trawling and resetting the live bait.

A pair of lines, close to the bottom, appears on the sonar. Instantly everything switches over to action stations as everyone readies for a potential hook-up.

False alarm. The lines on the sonar are not crisscrossing as a pair of marlin would. Thorrington looks at the chart and realises we’re in an area where there are "acoustic" lobster traps laid, and what we’re looking at are the rows of release buoys that hang off one of them. 

The day drags on with a good chicken wrap for lunch but few signs of marlin below the boat. At least the seas have calmed down, although you still need to keep a firm hand on the boat to walk around.

Thorrington shows me the difference between how he tunes the fishfinder, and the automatic setting the unit uses by default. The difference is huge. On automatic, the thermocline disappears and instead of being a solid line, the bottom is a jagged, broken edge. The bait balls, too, lose definition on their edges and become less clear. He turns the gain down to get things back to the way they were and resets the background to white to get the best contrast.

“I never use automatic mode,” he says.

Nearby, someone has hooked a bronze shark on light line and is struggling to bring it back to the boat. About two hours into the fight it would report that it had lost it.

Talk soon turns to what Freedom will do if someone hooks a marlin and needs to fight it back to the boat before the hour-long cut-off between when everyone has to stop fishing and when they need to be back in Port Stephens. For us, it’s an hour-and-a-half trip, so the decision is made late in the afternoon to keep trawling on the way back in, just in case.

It yields nothing. Thorrington eventually makes the call to haul in all the gear and we set the engines for the run home.

As we run, Thorrington keeps an eye on the fishfinder. A strong arch appears. “That’s a marlin,” he says. We’re in around 50 metres of water, so it won’t be a big one.

Faster, sometimes larger boats blast past us, pushing big wakes as they race to beat the 6pm cut-off to have their tournament cards and fishing tackle verified, and sending Freedom rocking.

Thorrington says he remains hopeful that the slow start to the marlin season is about to turn around. He has another six weeks before he bumps Freedom out of Port Stephens to join his smaller charter boat based at Broken Bay, putting a pin in his north-central fishing season.  

“The next six weeks will be the best,” he says. “It has to get better. You’ll see. And tomorrow we’ll be one of the only boats out there.”

The NSW Game Fishing Association Interclub State Championships are held each year in February. The heaviest marlin caught in the three-day tournament weighed 146.0kg.