Rounding out this series on fishes’ sense of smell, Sam Mossman looks into freshwater fish, the use of masking scents, and what may be the most potent attractor of all for predatory fish – prey rinse.

Saltwater fish are not the only types that have an acute sense of smell. Freshwater fish, including eels, salmon and trout, also have well developed nostrils, and these can be exploited to the angler’s benefit, too.

Natural scents can be added to artificial lures in other ways, apart from commercial scent additives. I fished for the giant salmon of Alaska’s Kenai River some years ago. Recreational and commercial fishing pressure was high and the guides would try anything to get an edge. A favoured lure was a ‘Quickfish’   a giant version of the Flatfish type we use for trout in NZ. Our guide used very fine nylon to lash a herring fillet to the belly of the lure and give it scent, occasionally dosing it with herring oil to keep it potent.

Salmon certainly have a great sense of smell – after all, they famously find their way back to their ‘home’ spawning rivers by using it. They also use smell to find food. A few years ago, the practice of fishing dead pilchard baits under floats in South Island salmon rivers was found to be so lethal that it was banned from all but one small area.

I am not sure if this same sensitivity is found in trout, but it may well be. Some years ago a bloke confessed to me that he had once experimented with running a pilchard-based berley trail in Lake Taupo and stray-lining with cut pilchard baits. He reckoned it was highly effective, but of course it is also highly illegal.

The northern hemisphere salmon from which our NZ stock were sourced, feed heavily on ‘herring’ a member of the same Clupeidae family as our pilchards, and the same is probably true of steelhead (sea-run rainbows) from which, it is argued, some of our rainbow trout stock may have been derived. This could explain their predilection towards pilchard scent.

Using scents on flies and lures fished for trout is only allowed in areas open to bait fishing. This includes lures such as various softplastics, where the scent is already built into the tail. When I was a boy, aniseed was rumoured to be the secret scent that would make trout jump onto your fly like a starving doberman on a porterhouse steak. One of my mates actually got hold of a small bottle of aniseed and tried it on his flies, but without noticeable results.

Who was that masked man?

Besides attracting fish and stimulating the bite, another function of additives can be blocking ‘bad’ scents that would otherwise repel fish. Experiments have been conducted in North American rivers where the scents of humans, bears and even dogs in a river had salmon ducking for cover for quite some distance downstream. Other ‘bad’ scents thought to be off-putting to a wide range of fish include: sunblock, insect repellent, soap, detergents, tobacco, the scent of human amino acids, along with petrol and diesel. Popular ‘masking’ scents often include powerful garlic or banana additives. You would think that all petrochemical products would be on the ‘naughty’ scent list, but this is not necessarily the case. My old man used to pretty much drown the hooks in his tackle box with CRC 5.56 to prevent rusting, and he caught as many fish – or more – as I did. Sometimes it seemed that the CRC actually attracted fish rather than repelled them. And then there is the old-time bait of cow liver soaked in kerosene. I thought this was a myth or some kind of joke, until I fished with an old bloke who actually used it. The kero toughened up the liver so it stayed on the hook better, and the snapper seemed to quite like it.

Prey rinse

As mentioned in previous parts of this series, fish in the water give off amino acids and pheromones at molecular level, and many predatory fish have the ability to sense and identify them, most especially when derived from the prey items they subsist on. It appears that when baitfish are under stress, such as when being pressured by or under attack from predators, they give out what may be termed ‘fear pheromones’. 
The combination of specific prey amino acids and fear pheromones is called ‘prey rinse’. It comes from live fish (and is supplemented when baitfish are being smashed up by predators, causing further amino acids to be released). The right kind of prey rinse can drive predators into an absolute frenzy.

There have been attempts to manufacture prey rinse, but they have not been particularly successful. Probably the best source is the overflow from your live-bait tank when it is filled with baitfish under stress from being in captivity, in addition to a berley trail.

As they are an important commercial species, a lot of testing of prey rinse has been done with yellowfin tuna. It has been found that their attention span to a scent trail is relatively short. It is thought that, due to their high metabolic rate, tuna cannot afford to spend a long time (and a great deal of energy) rapidly swimming in hunting or searching patterns unless the prey is nearby. If the baitfish are nearby, the scent will soon be reinforced by the tuna actually seeing the baitfish or feeling their vibrations. This is why cubing (or chunking) for tuna can draw fish from a distance: the rewards (something to eat) are there all the way.

Apparently the scent from a specific prey will focus ‘food search behaviour’ for that specific prey, as this will optimise the predator’s feeding efficiency. However, this behaviour is modified by prey abundance: if the predator encounters a new prey type regularly, this causes a gradual shift to maximum responses to the new prey’s odour. It makes sense for the predators to take advantage of whatever food is mostly on offer.

I well remember soaking a perfect blue (slimy) mackerel live bait in an area crawling with yellowfin off Whakatane. After quarter of an hour, when everyone was hooking tuna on jack mackerel live baits all around me while I couldn’t buy a bite, I switched to a jack mackerel and was instantly nailed! In trout-fishing terms this is called ‘selective feeding’ and the angler needs to ‘match the hatch’ to be successful.

To sum up:

• Each species selectively responds to a scent that contains a specific mixture of compounds
• Mixtures of natural amino acids are more effective than extracts or single compounds
• Even better results are achieved when the ‘fear pheromones’ put out by stressed prey are added.

But it must be remembered that scent, although a powerful tool when it comes to attracting fish, holding them in an area and turning them on to biting, is only one aspect of attraction. Fish also respond to visual and acoustic keys, and this is why a lure or bait’s presentation, action, colour and sound/vibration (some manufacturers add rattles to increase this) are also very important. In the mind of a predator, if it looks like a baitfish, acts like a baitfish and smells like a baitfish, it must be time for dinner, right?