Murky water alters mbuna behaviour

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What do mbuna do when the water gets murky? They eat, according to a paper to be published in a forthcoming issue of the journal Ecology of Freshwater Fish.

Carrying out their study in three near-shore sites in the Lake Malawi National Park, Cape Maclear, Malawi, Suzanne Gray, Shai Sabbah and Craig Hawryshyn investigated the immediate behavioural response of cichlids to a sudden influx of turbidity caused by sediments meant to mimic a rainy season run-off near a deforested area.

The authors released a plume of turbidity by releasing slurry consisting of lake water and soil (collected from the shore) from a collapsible container by a diver swimming just below the surface. An underwater video camera was used to record the behaviour of the fish for five minutes before the plume release, and for five minutes after the release.

The cichlids tended to move away from the substrate as the plume was released, with a shift from foraging on the substrate to foraging in the water column seen among fish that were feeding.  

The authors also observed a significant decrease in courting and territorial behaviour when the plume was released, with fish engaged in such behaviours switching to those associated with foraging. These results indicate that sediment run-off can disrupt the natural balance of behaviours in fishes.

A more persistent change in turbidity may alter male reproductive success or species recognition during mate choice.  

Such a scenario has been postulated in Lake Victoria cichlids, where permanent shifts from clear to turbid waters contributed to a breakdown in visually mediated premating reproductive isolation.

Long-term consequences of increased turbidity in fish diversity are difficult to predict, but the authors point out that other studies have shown a pattern of reduced biodiversity associated with increased turbidity.

For more information, see the paper: Gray, SM, S Sabbah and CW Hawryshyn (2011) Experimentally increased turbidity causes behavioural shifts in Lake Malawi cichlids. Ecology of Freshwater Fish 20, doi: 10.1111/j.1600-0633.2011.00501.x