Plumbing the depths

Somerset Dam is one of the largest freshwater impoundments in south east Queensland, and is located in a valley above Wivenhoe Dam. Steep, rocky escarpments along its eastern side form sheltered bays, presenting good casting opportunities for anglers. A shallow delta runs into farmland extending to the hills on the opposite side of the valley.

According to echo sounder readings, the majority of the bottom of the catchment is void of structure like trees, indicating it may have been cleared grazing land before being submerged. The main riverbed is deep and the sounder shows an average depth of 7m on the delta dropping to just over 20m along the old riverbed.

This depth reading was taken when the dam was at a low level, and try as I might I couldn't find out what capacity it was when those readings were taken. Apparently I was deemed to be attempting to uncover a national secret without waiting the mandatory 30 years to utilise the Freedom of Information Act!

Suffice to say Brisbane Water did relinquish information on an average capacity for Somerset, Wivenhoe and another dams in its control as being 68% at the time. Sometimes I just wonder!

Brisbane Water Authority makes no secret of the fact that they do not want anglers here and is fighting with different bodies to make that attitude become law. This is a prime example of what happens when a government-owned facility is sold off to private enterprise. We can only hope that its plans never eventuate.

BOMBS AWAY
Somerset Dam is a perfect place to fish with downriggers with deep sections and steep submerged river banks providing the features required to attract and hold the bony herring bait schools. It's a vast, open and often windswept waterway that can make drift fishing and jigging lures a brief but frustrating affair.

Downriggers are nothing new to southern trout trollers, but only receive a small amount of attention in the mid to northern parts of the country. They are a brilliant way of getting small, shallow-diving lures in the sizes and shapes bass prefer down deep in the strike zone. That zone may well be on the bottom of the riverbed 20m below, and I don't know of any lures for this species that will troll that deep on their own. The downrigger bomb will do the job for you.

If you're looking to get into this aspect of the sport, here are a few pointers.

HEAVIER THE BETTER
Even though you are in waters with little or no current, get a downrigger that can handle a bomb with a minimum weight of 3.5kg. The heavier the bomb, the more instantly the hooks set when the fish strikes, pulling the line out of the clip being towed behind the bomb. Reduced weight in the bomb will allow fish to pull it towards them, which affects the hook penetration on strike.

With your downrigger mounted on the gunwale somewhere in the vicinity of the transducer, a heavier bomb will remain more perpendicular and will be easily read by your sounder. This will allow you to accurately read the depth of the bomb without having to rely on the counter, a feature installed on some models. These counters tend to be inaccurate after some of the wire is removed from the wheel from loss or breakage.

RELEASE CLIPS
There are a few models to choose from, but I have been using the clamp-action Scotty Snapper clip which has a pair of tapered pads fitted within the jaw. Greater release pressure can be achieved by placing the line further back in the jaw, or less pressure when the line is installed near the mouth of the jaw.

For heavier trolling with big-bibbed lures I use an Aftco Roller Troller with the outrigger line connector bar removed, fitted to a lanyard and onto the clip that fits on the bomb.

LINES AND LURES
Low stretch lines are perfect for downrigging. Fish will be more securely hooked with lines that have less 'give'. Gelspun lines are ideal for this sort of work. Braid line used in conjunction with a well set-up release clip and a heavy bomb can be a deadly combination.

With a bomb performing much of the diving work for you, care should be taken when selecting trolling lures. I prefer small lures because they are a small mouthful. In my experience, little lures will outfish large lures more consistently when targeting natives. Having said that, when you see a 16cm bony herring regurgitated by 50cm bass, one wonders why we don't fish bigger lures more often!

Pay attention to the manufacturer's lure diving capability claims, in particular how far back they have to be to get to the claimed depth. This should be taken into account when viewing your bomb on the sounder and factoring in the diving depth of the lure to the overall equation.

Using the graph on the side of the sounder screen will assist with putting the lure at the exact depth where you're seeing fish arches or baitballs.

CURIOUS BASS
When bass are in an inquisitive mood, they'll often come up and look at the bomb as it's trolled through their territory. This is an ideal time to have a lure close by, using the bomb as a teaser. A lure way back and down deeper than the bomb is not going to do the job. Try shortening right up, even to a few metres behind the bomb. You could also try towing a drone such as a small fly or lures that don't dive at all. Spinnerbaits, wobblers and small spinners make ideal drone lures. They will produce some surprising results when sent to the depths and flatlined. This is an accurate way to put lures into the exact position for a strike.