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How Smart Are Fish?


Dec 28, 2013
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Ch4rlie said:
Humans have developed a means of communicating using words, and our species has developed in such a way that it is reliant on verbal language. Furthermore, an individual's intelligence is often inferred by their ability to communicate effectively with language.
How about deaf people?
We lack verbal language and therefore rely on body language, facial expressions, lip reading and sign language.
malfunction said:
So, a by-product of this is that non-human animals, who clearly lack the ability to form language, are deemed as less intelligent. Thus, when they do anything that remotely resembles human conceptions of intelligence, we're shocked because we have mistakenly linked their lack of language skills to a lack of intelligence.
In actual fact, i know of some dog trainers who have managed to train their dogs to obey commands using just sign language alone.
The same can be said of horses too, again, I know of a few trainers who can make their horses do things without uttering a single spoken word.
Dolphins and whales are reliant on hand signals when trainers want them to do certain commands too.
Chimps have been trained to use sign language too, in fact this is the closest we'll probably come to actual communication with an animal.
So its not a total unknown that animals can communicate or understand humans in one form or another, usually for rewards/treats though!
All very accurate arguments, but I think you misunderstand the point I'm making. My point is not that animals or humans that can't speak are stupid and unable to communicate at all. But rather that humans have a tendency to assess the intelligence of other creatures based on that creature's ability to communicate its thoughts using language. I'm arguing that people make assessments on intelligence based on the ability to engage in verbal language exchanges. I'm NOT arguing that these assessments are necessarily accurate.


Mar 2, 2009
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Question of intelligence/knowing.
A few weeks ago I noticed one of my angel fish had a white mark inside the iris of its eye. Having recently added assasin snails to the tank I came to the assumption the fish had eye flukes and treated the whole tank. After treatment there was little improvement in the eye and it appeared to be swelling (the eye not around it). I could not add any other medication at this point as I was clearing the fluke medication.
The following morning I find the angel being attacked by the other fish including angels. Here is the strange part, I have witnessed full on aggression with cichlids and it is horrific, fins get shredded, bites are taken out of the body etc, but in this case the fish were only attacking the bad eye, there was no other damage to the fish at all, not a single fin was ripped or a scale out of place only the eye was removed.
In a panic I removed the angel with the now missing eye to a hospital tank where I treated the fish and the wound began to heal. However just as a new layer of skin had grown over the missing eye a tiny white dot appeared in the middle of the eye. This dot grew and grew quite rapidly, dispite further treatment a substantial lump appeared where the eye once was, a tumour. Sadly there was nothing further I could do for the fish and I euthanised him.
Initially I was angry and upset at the other fish attacking the angel, but did and still do wonder why they did no damage other than remove the eye.  Were they aware before I was of a tumour? 


Jan 26, 2008
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Perth, WA
An old thread but an interesting one. And since nobody is needing help now I am going to tell you of some of my fish.

I had a number of 4 foot long fish tanks in the lounge room and they had different species of Australian and New Guinea rainbowfishes. They did the usual and swim up and down the front glass whenever I walked past. They were begging for food. They even knew which bedroom I slept in and whenever I opened my bedroom door, the fish raced down to the end of the tank closest to my room. People in the lounge room knew when I was opening my door even tho they couldn't see the door. They saw the fish move to the end of their tanks and said here comes Colin. When I came out of the room they went nuts begging for food. I use to feed them on different sides of the tank each time to vary it and make them think and look for food.
If another person came out of my room the fish would go back to doing whatever they were doing before. They could identify me from other people in the house, and associated me with food.

Some years ago we got a group of wild caught Melanotaenia boesemani (rainbowfish from West Papua New Guinea). The boesemani were only about 2 inches long and still young so we put them in a 6 foot tank with a group of about 50 adult cardinal tetras. The tetras were pretty good size and we didn't think there would be any problems because they were nearly the same size as the boesemani.

All went well for the first day or so then one day we noticed the cardinals hiding in one corner of the tank. We tested the water and it was fine so we watched and waited and nothing. We moved out of the room and watched from around a corner and the boesemani were hunting them.

The boesemani split up into 2 groups. One group went along the front of the glass towards the cardinals, while the other group created a wall with their bodies and basically pushed the cardinals into the corner. The group that went along the glass raced in and attacked the cardinals while the group that created the wall held position to prevent any cardinals escaping. After a few minutes the groups swapped position and repeated the scenario. After that we removed the boesemani and put them in a separate tank.

I have seen the same hunting/ feeding behaviour in Geraldton WA, when a group of tailor trapped a heap of bait fish in a man made bay that was about 20meters wide and had limestone walls on 3 sides. The tailor split into 2 groups and one group made a wall preventing the smaller fish from getting back out to see, while the second group went in and fed. Again after several minutes of feeding the groups swapped and feeding continued.

Interestingly none of the tailor were interested in fishing lures that we were cast in during the feeding frenzy. They were only going after the bait fish. We had a number of different lures being cast in and the fish did swim past the lures but completely rejected them. This feeding pattern happened pretty regularly in this same bay.

The boesemani are lake dwelling fish that occur in freshwater and have had no contact with oceanic species for thousands of years, and therefore thousands of generations. Yet they showed the same hunting behaviour as wild oceanic fish. Is this a learned behaviour or inherited?

Domesticated boesemani (fish that have been bred in captivity for 10 generations or more) never showed any behaviour like this. However, when the domestic boesemani were housed with the wild ones, they quickly joined in feeding frenzies and made walls to heard smaller fish into corners. We used Gambusia not cardinal tetras during these experiments. Once the domestic and wild boesemani were separated the domestic fish never behaved like that again, even tho the wild ones continued to do it.

Another time I went away for a couple of months and asked one of my sisters to feed my fish a couple of times a week. She fed them a few times and then stopped. When I got back most of the fish were fine, some had disappeared but they were small young fish anyway. When I got back, and as soon as I went into the fish room, all the fish swam up to the front glass looking for food.

I was talking to my sister after and she said she forgot about it and didn't feed them much. But she also said when she was feeding them they wouldn't eat while she was in the room. So she thought bugger em, they can starve. This would suggest fish have a pretty good long term memory because they could recognise me after I had been away for a couple of months. It also seems to suggest that fish have preferences for people and prefer the people that normally feed them compared to strangers.

Having said all this, when I have fed other people's fish, their fish ate quite happily. Does this mean some fish prefer food from their owners and others will eat anything from anyone? The only thing I can think of that may explain this, is most of the fish I kept were either wild caught or had only been kept in captivity for a few generations. Unlike other people's fishes that were common domestic fish like guppies and goldfish.

Do wild fish have a preference to who feeds them, and that disappears with domestication?

Fish also have to navigate waterways and defend territories, as discussed previously in relation to Plecos finding their holes in the mud. And they have to watch out for predators. I have seen fish (in aquariums & the wild) with only one eye swim along walls so they can keep their eye on the open water for potential predators. It takes a bit of common sense to do that.
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