Fish intelligence

Fish intelligence is "the resultant 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 Teleost fish 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, 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. This suggests that fish use their memory of negative experiences to associate capture with stress and therefore become less easy to catch. 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.

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. Some common rudd and European chub could remember the person who trained them to feed from the hand, even after a 6-month break. 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. Rainbow trout can be trained to press a bar to get food, and they remember this three months after last seeing the bar. Red Sea clownfish can recognize their mate 30 days after it was experimentally removed from the home anemone.

Several fish species are capable of learning complex spatial relationships and forming cognitive maps. They can orient themselves using multiple landmarks or symbols and they are able to integrate experiences which enable them to generate appropriate avoidance responses. 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.

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. 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. They can even learn to shoot at moving targets.

Whitetail damselfish clean the rock face where they intend to lay eggs by sucking up and blowing sand grains onto the surface. Triggerfish blow water at sea urchins to turn them over, thereby exposing their more vulnerable underside. River stingrays create water currents with their fins to suck food out of a PVC pipe. Banded acaras (''Bujurquina vittata'') lay their eggs on a loose leaf and carry the leaf away when a predator approaches.

In one laboratory study, Atlantic cod

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