HOW TURBIDITY AFFECTS FISH BEHAVIOUR

For many years now I’ve noticed that catch rates are affected by the optical clarity of the water. Generally I have greater success if the water is optically clear than when it is murky or cloudy and full of suspended sediment. It is still possible to catch fish when the water is murky but it requires a change in tactics. More about that later.

Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates. The more total suspended solids in the water, the murkier it seems and the higher the turbidity.

There are various parameters influencing the murkiness of the water. Some of these are:

  • Tidal movement
  • Wind action
  • Phytoplankton
  • Sediments from erosion and urban runoff
  • Re-suspended sediments from the bottom caused by bottom feeders like carp
  • Waste discharge
  • Algae growth

Turbidity affects fish behaviour in several ways. Firstly, it makes it difficult for predators that rely on sight to catch prey. They can get close to targets using impulses received by their lateral lines (THE LATERAL LINE – A FISH’S SIXTH SENSE) but need good visibility to actually capture them by sight. This results in lower feeding efficiency and often a change in feeding strategy, especially for the predators who mainly eat smaller fish. As the turbidity increases predators move from a feeding strategy of chasing fast moving prey with a high chance of escape to a slow moving organisms with a low escape probability.

In the Manukau harbour this means that predators which normally feed on fish start to look elsewhere for food and become less selective. They switch to hunting worms, crabs and shrimps (known as macrobenthic prey) as they are easier to find. Kahawai and trevally both do this. It is especially noticeable over the winter as the flats are pock marked with holes at low tide and there are no snapper in the harbour to have caused the digging. If the water is murky, and you notice that the flat is covered in holes when you arrive, then attach a whirl tail soft plastic or crab imitation and retrieve it very slowly along the bottom. Fly fisherman should do likewise and dredge the bottom with a crab, worm or shrimp imitation.

Fish that mainly feed on worms and other crustaceans suddenly have more competition for food from the fish eaters and become slightly less selective in what they will eat. Fish, such as mullet, that feed on detritus are not as affected by turbidity which is why they are often visible feeding in close, even when the water is very murky.

Secondly, the suspended particles absorb heat from the sunlight, making turbid waters become warmer, and so reducing the concentration of oxygen in the water. As the water warms it slows the rate at which fish can digest food and this affects when they will next feed. What this means on the shallow saltwater flats is that fish which are susceptible to water temperature will shift to feed at high tide when the water is deeper and slightly cooler or, during summer, switch to feeding on the bottom at night. In freshwater, predators will seek out cooler thermoclines (often right on the bottom) or cooler, less turbid inflows into a lake. For more information on how water temperature affects feeding click on:- FISH DIGESTION AND HOW IT DRIVES FEEDING BEHAVIOUR

Thirdly, the suspended particles scatter the light which decreases the photosynthetic activity of plants and algae. This contributes to a lowering of the oxygen concentration even more. When this happens fish will leave an area and move to a cooler zone with higher levels of dissolved oxygen. If you want to read more on how changes in dissolved oxygen levels affect fish then click on:- HOW DISSOLVED OXYGEN AFFECTS FISH BEHAVIOUR

Finally as a consequence of the suspended particles settling to the bottom in shallow bodies of water fish eggs and larvae will be covered and suffocated and gill structures clogged or damaged. In freshwater rivers, especially when they are in flood, predators will actively seek out the margins or bottom, side branches off the main flow or clearer inflows where the water is marginally less turbid.

The key issue for predators feeding in turbid water is detecting a difference in contrast between their prey and the background murk. Brightness contrast is the determining factor for the visibility of objects underwater and objects must be lighter or darker than the background (reflect more or less light) to be seen clearly. This need for contrast is why black or dark lures are often recommended for use in turbid water. See the chart below:- (https://www.fix.com/blog/view-from-below-lures-underwater/)

However, this only works if the light levels are medium – high. At low light levels the detection of prey is largely governed by absolute light levels rather than contrast. So if you are fishing on a very dull day try to find seams or pockets of water where the clarity is better and target these. Often predators will sit just inside the murky water so they are invisible to any concentrated schools of prey that pass. The other thing to do in this situation is to go deep with a high contrast paddle or whirl tail soft plastic and retrieve it along the bottom. The soft plastic needs to have a distinct “sonic fingerprint” so that it can be located by predators. For more information:- CHOOSING A LURE COLOUR TO SUIT THE CONDITIONS

THE “VACUUM CLEANER” EFFECT

One phenomenon which contributes significantly to turbidity in shallow estuaries is spring tides. Spring tides, specially king tides where the high tide levels are significantly higher than normal, lead to a phenomenon which I’ve named the “Vacuum cleaner” effect. Put simply, the volumetric flowrate of water off the flat as a spring tide starts to ebb is greater than normal and this “sucks” sediment off the bottom and into suspension. The effect is greatest in between the second and fourth hours of the outgoing tide as this is when 50% of the water in the estuary moves out. If you are fishing the estuary as it ebbs from high tide then you can often see it getting murkier as time goes on. When this starts to happen try to target the clearer pockets or seams of water as the fish will be close to these. The best time to fish shallow estuaries during king tides is in the run up to high tide and the first hour of outgoing as this is when the water clarity is at its optimum. If you want to learn more about tides click on:- TIDES AND SAFETY – THE RULE OF TWELFTHS

Turbid water is one of the few occasions where scented soft plastics may be useful but only if you are fishing in weak to moderate current flows and are retrieving slowly enough for the dissipated scent to be picked up by any nearby predators.

In summary, turbidity has a massive effect on the behaviour of predatory fish. If you can understand how their behaviour changes as the turbidity increases then you can target areas where they will likely be concentrated with lures that will give you the greatest chance of success. Being observant and identifying the signs that the fish are feeding on the bottom because the water is murky is the key.

References:

1. Turbidity induced changes in feeding strategies of fish in estuaries. T. Hecht & C.D. van der Lingen. South African Journal of Zoology. 27:3, 95-107, DOI: 10.1080/02541858.1992.11448269

2. Visual feeding of fish in a turbid environment: physical and behavioural aspects. A.C. Utne-Palm. Marine Freshwater Behavioural Physiology. Volume 35, No. 1-2, pp. 111-128. 2001.

3. https://www.lenntech.com/turbidity.htm#ixzz6cOPXrbuc

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