Fish demonstrate lateralisation as has been demonstrated in laboratory experiments and field study observations. What does this mean for fishermen and how can we use these results?
I won’t be using any of my own observations but just give a brief summary of experiments and findings that scientists have found. I will then try and give some examples of where I think this information might be used to put more or bigger fish on the bank. I fully expect everyone else to have their own ideas as to how to use this information (if at all) for their own benefit.
First I should explain what lateralisation is, it is the differential use of each hemisphere of the brain (Sloman et al., 2006). In lay mans terms “handedness”. In fish most of the experiments and observations have been done regarding which eye the fish prefers to use under certain conditions. When the fish is using its left eye the right side of its brain is being used and vice versa.
A theory as to why optical lateralisation came about in evolutionary terms is that it initially arose when the eyes were placed on the sides of the head and so each eye was observing a different scene. It is thought that lateralisation made it more efficient to perform to tasks simultaneously. These could include looking for food with one eye while keeping a look out for predators with the other (Rogers et al., 2006).
Professor Angelo Bisazza et al has been doing work in this area. How these experiments were worked was that mosquito fish were put in a round tank with a predator in the centre. The majority swam in a clockwise direction allowing them to keep the danger in their right eye. These results were published in 1997. In 1999 he published more results that showed that when these fish had a shoal mate on its left then it was more likely to investigate the predator. This was done by using mirrors so that the fish saw what it thought was a buddy either on its left or right depending on what side the mirror was placed.
Also published in 1999 was work done by Facchin et al. I believe Facchin and Bisazza were working in collaboration at the time but I haven’t been able to access the original paper. What I think was done during these experiments is that the fish swam down a channel and were then confronted with a variety of scenes. If what the fish encountered was a predator or a shoal of the opposite sex then 90% of the fish turned to the left so that they could observe with their right eye.
I won’t give anymore examples to prove that fish demonstrate lateralisation, I’ll just say that I believe they do and that in most cases they favour the right eye for observing events that require immediate reaction.
Of note in terms of lateralisation is the fact that fish that suffer predation seem to show a higher degree of lateralisation than those that don’t have to worry about being eaten (Brown et al., 2004). Panamanian bishop fish that had never been preyed upon did not show a preference for one side or another whereas fish that had been collected from areas where they were preyed upon did show lateralisation.
Also on lateralisation in fish it is not only whether the fish is right or left eyed, it has been shown in to be prevalent in the lateral line organ (Burt de perera et al., 2005), lateralisation has been observed in catfish strumming their fins (Fine et al.,1996) and it has also been seen to be a driver in natural selection in scale-eating cichlids and their prey fish due to which (lefties or righties) are more common in the population at any given time (Hori.,1992).
Enough science, there are plenty more examples that could be used but this gives an overview. It appears that in general fish use their right eye for viewing things that demand an immediate or emotional response and the left for more mundane tasks such as keeping a watch on other shoal members.
This article started as a result of a question on the forum by Barry (Baz) Flemming. To paraphrase, why do fish in still waters tend to swim in an anticlockwise direction? The truth is that I still don’t know. However, I think it might have something to do with this lateralisation, if not, I am sure that there is still something in it to be of relevance to us anglers.
Danger!
My hypothesis would be that on a lot of still waters there would be more of immediate interest or danger near the bank and this is why the fish, especially shoal fish would like to keep their right eye in that direction and that is why they swim in an anticlockwise direction. I realise that there thousands of other factors to take into consideration but this could be a piece of the puzzle.
Why do sometimes fish only take a bait or fly presented down one side rather than the other? Does this mean that fish would naturally prefer one side of the river, or different sides at different places depending on where and what was the likely predator or food source at particular locations? Which side of a snag would it be best to present a bait?
There are lots of questions on the bank that if we don’t know the answer we give it our best bet and hope for luck. It might be possible to ever so slightly narrow that guess down. Something that if everything else is equal might be a clue as to what maybe is the best option.
I know it is difficult when just one aspect of fish behaviour is being discussed to equate how it fits into real life instances. This is why people do experiments in controlled conditions as they can then control just one difference in the environment. In the river or lake it is so difficult to know even when fish do the same thing on two or more occasions whether it is for the same reasons. We can only hope that the more we know the more we can read the signs.
Matthew Barter.
