Fish intelligence is "the result of the process of acquiring, storing in memory, retrieving, combining, comparing, and using in new contexts information and conceptual skills" as it applies to fish. Due to a common perception amongst researchers that teleosts are "primitive" compared to mammals and birds, there has been much less research into fish cognition than into those types of animals, and much remains unknown about fish cognition, though evidence of complex navigational skills such as cognitive maps is increasing.
Compared to similarly sized fish, mammals and birds typically have brain sizes fifteen times larger, though some species of fish such as elephantnose fish have very large brain-to-body ratios. However, fish still display intelligence that cannot be explained through Pavlovian and operant conditioning, such as reversal learning, novel obstacle avoidance, and passing simultaneous two-choice tasks. Some fish also match mammals and birds in the executive functioning capability of inhibitory motor control. Australian biologist Culum Brown has argued that fish may give the appearance of being less intelligent than they are due to differences between aquatic and terrestrial environments.
Fish hold records for the relative brain weights of vertebrates. Most vertebrate species have similar brain-to-body mass ratios. The deep sea bathypelagic bony-eared assfish has the smallest ratio of all known vertebrates.Fine ML, Horn MH and Cox B (1987) "Acanthonus armatus, a Deep-Sea Teleost Fish with a Minute Brain and Large Ears" Proceedings of the Royal Society B, 230(1259)257-265. At the other extreme, as mentioned above, the electrogenic elephantnose fish, an African freshwater fish, has one of the largest brain-to-body weight ratios of all known vertebrates (slightly higher than humans) and the highest brain-to-body oxygen consumption ratio of all known vertebrates (three times that for humans).
Brain
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal. However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials.
The cerebellum of cartilaginous and bony fishes is large and complex. In at least one important respect, it differs in internal structure from the mammalian cerebellum: The fish cerebellum does not contain discrete deep cerebellar nuclei. Instead, the primary targets of Purkinje cells are a distinct type of cell distributed across the cerebellar cortex, a type not seen in mammals. The circuits in the cerebellum are similar across all classes of vertebrates, including fish, reptiles, birds, and mammals. There is also an analogous brain structure in cephalopods with well-developed brains, such as octopuses. This has been taken as evidence that the cerebellum performs functions important to all animal species with a brain.
In mormyrid fish (a family of weakly electrosensitive freshwater fish), the cerebellum is considerably larger than the rest of the brain put together. The largest part of it is a special structure called the valvula, which has an unusually regular architecture and receives much of its input from the electrosensory system.
Memory Individual carp captured by anglers have been shown to become less catchable thereafter.Beukema, J.J. (1970). Angling experiments with carp: decreased catchability through one trial learning. Netherlands Journal of Zoology, 20: 81–92.Beukema, J.J. & Vos, G.J. (1974). Experimental tests of a basic assumption of the capture-recapture method in pond populations of carp Cyprinus carpio L. Journal of Fish Biology, 6(3): 317. This suggests that fish use their memory of negative experiences to associate capture with stress and therefore become less easy to catch.Raat, A.J.P. (1985). Analysis of angling vulnerability of common carp, Cyprinus carp /0 L., in catch-and-release angling in ponds. Aquaculture Research, 16(2): 171-187. This type of associative learning has also been shown in paradise fish (Macropodus opercularis) which avoid places where they have experienced a single attack by a predator and continue to avoid for many months.Czanyi, V. & Doka, A. (1993). Learning interactions between prey and predator fish. Marine Behavior and Physiology, 23: 63–78.
A number of studies have shown that fish can retain information for months or years. Anecdotally, channel catfish (Ictalurus punctatus) can remember the human voice call announcing food five years after last hearing that call.Reebs, S.G. (2008) Long-term memory in fishes Retrieved 9 July 2014. Goldfish remember the colour of a tube dispensing food one year after the last tube presentation. Sockeye salmon still react to a light signal that precedes food arrival up to eight months since the last reinforcement.Tarrant, R.M. (1964) Rate of extinction of a conditional response in juvenile sockeye salmon. Transactions of the American Fisheries Society 93: 399-401. Some common rudd and European chub could remember the person who trained them to feed from the hand, even after a 6-month break.Klausewitz, W. (1960) Ein bemerkenswerter Zähmungsversuch an freilebenden Fischen, Natur und Volk 90: 91-96; cited in: Bshary, R., Wickler, W., and Fricke, H. (2002) Fish cognition: a primate's eye view, Animal Cognition 5, 1-13. Crimson-spotted rainbowfish can learn how to escape from a trawl by swimming through a small hole in the center and they remember this technique 11 months later.Brown, C. (2001) Familiarity with the test environment improves escape responses in the crimson spotted rainbowfish, Melanotaenia duboulayi, Animal Cognition 4: 109-113. Rainbow trout can be trained to press a bar to get food, and they remember this three months after last seeing the bar.Adron, J.W., Grant, P.T., and Cowey, C.B. (1973) A system for the quantitative study of the learning capacity of rainbow trout and its application to the study of food preferences and behaviour. Journal of Fish Biology 5: 625-636. Red Sea clownfish can recognize their mate 30 days after it was experimentally removed from the home anemone.Fricke, H. (1974) Öko-Ethologie des monogamen Anemonenfisches Amphiprion bicinctus, Zeitschrift für Tierpsychologie 36: 429-512; cited in: Bshary, R., Wickler, W., and Fricke, H. (2002) Fish cognition: a primate's eye view. Animal Cognition 5: 1-13.
Several fish species are capable of learning complex spatial relationships and forming cognitive maps.Odling-Smee, L. & Braithwaite, V. A. (2003). The role of learning in fish orientation. Fish and Fisheries, 4: 235–246. They can orient themselves using multiple landmarks or symbolsReebs, S.G. (2007) Orientation in fishes Retrieved 29 August 2014. and they are able to integrate experiences which enable them to generate appropriate avoidance responses.Portavella, M. Torres, B. & Salas, C. (2004). Avoidance response in goldfish: emotional and temporal involvement of medial and lateral telencephatic pallium. The Journal of Neuroscience, 24: 2342–2335.Yue, S. Moccia, R.D. & Duncan, I.J.H. (2004). Investigating fear in domestic rainbow trout, Oncorhynchus mykiss, using an avoidance learning task. Applied Animal Behaviour Science, 87: 343–354. In 2020, a neurological study using wireless neural recording systems measured the neurological activity of goldfish and found strong evidence supporting the idea that fish form navigational cognitive maps.
Tool-like behavior
Tool use is sometimes considered as an indication of intelligence in animals. There are few examples of tool use in fishes, perhaps because they have only their mouth in which to hold objects.Reebs, S.G. (2011) Tool use in fishes Retrieved 10 July 2014.
Several species of wrasse hold bivalves (scallops and clams) or sea urchins in their mouth and smash them against the surface of a rock (an "anvil") to break them up.Coyer, J.A. (1995) Use of a rock as an anvil for breaking scallops by the yellowhead wrasse, Halichoeres garnoti (Labridae). Bulletin of Marine Science 57: 548-549.Pasko, L. (2010) Tool-like behavior in the sixbar wrasse, Thalassoma hardwicke (Bennett, 1830). Zoo Biology 29: 767-773. This behaviour in an orange-dotted tuskfish (Choerodon anchorago) has been filmed; the fish fans sand to unearth the bivalve, takes it into its mouth, swims several metres to a rock which it uses as an anvil by smashing the mollusc apart with sideward thrashes of the head.
Archerfish (family Toxotidae) squirt jets of water at insects on plants above the surface to knock them into the water. They can adjust the size of the squirts to the size of the insect prey.Schlegel, T., Schmid, C.J., and Schuster, S. (2006) Archerfish shots are evolutionarily matched to prey adhesion. Current Biology 16: R836-R837. They can even learn to shoot at moving targets.Schuster, S., Wöhl, S., Griebsch, M., and Klostermeier, I. (2006) Animal cognition: how archer fish learn to down rapidly moving targets. Current Biology 16: 378-383Dill, L.M. (1977) Refraction and the spitting behavior of the archerfish (Toxotes chatareus), Behavioral Ecology and Sociobiology 2: 169-184.
Whitetail damselfish clean the rock face where they intend to lay eggs by sucking up and blowing sand grains onto the surface.Keenleyside, M.H.A. (1979) Diversity and Adaptation in Fish Behaviour, Springer-Verlag, Berlin. Triggerfish blow water at sea urchins to turn them over, thereby exposing their more vulnerable underside.Fricke, H. (1975) Lösen einfacher Probleme bei inem Fisch. Zeitschrift für Tierpsychologie 38: 18-33; cited in: Bshary, R., Wickler, W., and Fricke, H. (2002) Fish cognition: a primate's eye view. Animal Cognition 5: 1-13. River stingrays create water currents with their fins to suck food out of a PVC pipe.Kuba, M.J., Byrne, R.A., and Burghardt, G.M. (2010) A new method for studying problem solving and tool use in stingrays (Potamotrygon castexi). Animal Cognition 13: 507-513. Banded acaras (Bujurquina vittata) lay their eggs on a loose leaf and carry the leaf away when a predator approaches.Keenleyside, M.H.A., and Prince, C. (1976) Spawning-site selection in relation to parental care of eggs in Aequidens paraguayensis (Pisces: Cichlidae). Canadian Journal of Zoology 54: 2135-2139.
In one laboratory study, Atlantic cod (Gadus morhua) given access to an operant feeding machine learned to pull a string to get food. The researchers had also tagged the fish by threading a bead in front of their dorsal fin. Some fish snagged the string with their bead, resulting in food delivery. These fish eventually learned to swim in a particular way to repeatedly make the bead snag the string and get food. Because the fish used an object external to their body in a goal-oriented way, this satisfies some definitions of tool use - though in a more rudimentary fashion, a human blowing air wouldn't qualify as tool usage just like a fish using water in the same fashion, or even in the case of certain wrasse, the bivalve is not a tool but the prey itself. If the wrasse had used a rock to access the bivalve, then it would fall under that classification. There appear to be very few to no observed behaviors of fish using tools in a fashion similar to humans, but instead altering water flows in ways that benefits their survival.
Construction As for tool use, construction behaviour may be mostly innate. Yet it can be sophisticated, and the fact that fish can make judicious repairs to their creation suggests intelligence. Construction methods in fishes can be divided into three categories: excavations, pile-ups, and gluing.