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Learn about fish

Fish rarely gets the acknowledgment they deserve. Upon discussing about farm animals, fish do not get the same attention as other species. However, they are sentient beings who deserve as much as other animals on this planet. Let's find out more about them who populate the oceans and rivers. 

Let's get scientific about fish!

a) General characteristics

There could be two categories of fishes based on their skeleton type, namely cartilaginous fishes or bony ones.  Examples of cartilaginous fishes can be sharks, rays, sawfishes, and ratfishes.  It is believed that the bony fishes (or teleosts) have evolved during the Mesozoic era, which was around 70-155 million years ago alongside birds and mammals (Wootton, 1998). While cartilaginous fishes have replaceable teeth, stiff fins and scales, bony fishes generally have non-replaceable teeth (if present at all) and flexible fins and scales, which grow throughout their lives.  There are more documented species of bony fishes (over 24,000) than cartilaginous ones (over 1000) (Godin, 1997). 

Fishes inhabit almost all kinds of water body, from the hot springs, the Antarctic areas, deep under the seas, in high elevation lakes, in salty or fresh water. Whereas most species inhabit in one type of of environment, those that migrate between the two (freshwater or saltwater) have developed bodily processes that permits quick adjustment to changes in salinity (Grubb, 2003).

Most fishes develop senses of vision, hearing, and smell whilst some are capable of detecting water movement, generating and interpreting electrical signals as well as navigating using the sun. For example, sharks and skates can detect a difference in voltage of 0.01 microvolt per centimeter, which is considered to be the greatest in the animal world  (Reebs, 2001).

Some fishes are known for their long migrations. Many species often travel shorter lengths between three habitat types: reproductive, feeding and refuge. Other causes of migration could be displacement from floods, drought or daytime exploratory activity (Lucas and Baras, 2001). 

Fishes eat a variety of foods, from zooplankton to large fish and mammals. Schooling behaviors seen in fish are similar to terrestial herding animals. Many species have vast home ranges whereas some inhabit very small areas (Nelson, 2006).  

In several species, hermophroditism occcurs to elevate their reproductive success in cases where the ratio of males and females changes constantly (Wootton, 1998). Lower than 1% of all fish species die after spawning, the rest reproduce over many seasons, even in cases where notable migrations are involved (Nelson, 2006).

Fishes use many methods to avoid detection by predators. Some may resemble non-edible items, some develop shading body surfaces or transparency to cause confusion (Smith, 1997). 




b) Senses

Most species of bony fishes have well-developed eyes whose retinas contain color-sensitive cones. Fishes who live in dark, deep waters possess more rod cells that enhance vision in low light, while shallow water fishes possess more cone cells, allowing them to see better in conditions with increased light. Fishes that live in the blue water of reefs and the open ocean have vision most sensitive to shades of blue and green. Research also suggests that fishes gain better visual accuracy with age (Wootton, 1998). 

Fish are most sensitive to low frequency sounds below two to three kHz (Wootton, 1998). As sound travels through water, it moves through the
fish’s body and makes the inner ear’s otolith organ vibrate. This vibration triggers movement in the sensory cell hairs embedded in the otolith, which signals the fish’s brain. Interestingly, the swim bladder, an apparatus dedicated to flotation and depth modification, is an integral component of a fish’s hearing as it transforms pressure waves from the surrounding water into pulsating movements that are received by the ear (Reebs, 2001). Fish are believed to be able to determine the direction and distance of a sound source as well as discriminate among calls with different pulse patterns, amplitudes and frequencies (Hawkins, 1993).

Olfaction, or chemosensory information, is thought to be important for fish in finding food and mates, avoiding predators, and navigation (Hara, 1993). However, olfactory signals may become diluted in water and move slowly without a strong water current (Wootton, 1998).  

The lateral line is a series of pores that line the flank of a fish’s body from the head to the tail. It was originally thought to be associated with the sense of touch or hearing (Bleckmann, 1993), but recent evidence suggests it is used for detecting subtle levels of water displacement in the immediate environment. This sensory system is best used over short distances though, as water disturbances tend to dissipate quickly. Not only can the lateral line assist fish in locating potential prey, it also helps them navigate within their surroundings by detecting water movement around nearby objects. 

Electroreception abilities are found most often in fish species who inhabit murky waters or feed at night. Catfish have the ability to detect electric stimuli from hidden prey as well as distortions in the electric field caused by other fish (Wootton, 1998). One species of catfish is even capable of producing electric discharges up to 600 volts (Lucas and Baras, 2001). 


c) Migration

Migrations are synchronized movements that are larger relative to the average home range for that species and which occur at specific stages of the life cycle (Lucas and Baras, 2001). Even though many fishes migrate short distances between spawning, feeding, and refuge habitats, salmon and trout are well-known for their strenuous spawning-driven journeys from the ocean to their natal stream.

A number of factors dictate whether an individual fish engages in migratory behavior: genetics, hunger, homing, predator avoidance, and other environmental conditions like temperature, water flow and quality. Cues like geographic landmarks, celestial information, water currents, electric and magnetic fields, olfaction, salinity, and memory may assist fish as they navigate during their migration (Lucas and Baras, 2001).

Some adults leave the ocean after only one winter while others may stay at sea for two or more years before embarking on their upstream migration. Prior to and during their return migration, they cease to eat, drawing on their fatty energy reserves to survive  (Grubb, 2003) (Lucas and Baras, 2001).




d) Spawning

Fish utilize a variety of mating systems. Species that require bi-parental care for their young, or who are part of very small populations, are often monogamous, but such pairings are not necessarily long-lasting. Polygamous mating is most common in fish and occurs in several forms: polygyny, polyandry, and promiscuity. Marine fish are often polygynous in that one male mates with several females, while females usually mate with one male. Polyandry, a rare type of mating system seen in some anemone fish, involves one female mating with several males and each male mating with only one female. Promiscuity, a common mating system in nesting fish, involves several partners per individual fish (Berglund, 1997).

Hermaphroditism occurs in several species of fish. Simultaneous hermaphroditism, when an individual possesses male and female anatomy
at the same time, is most common in very small populations in which an individual’s likelihood of finding several potential mates is low. Sequential hermaphroditism, however, is more common and causes a fish of one gender to transform into the other gender as a result of a change in the group’s gender ratio or the absence of a dominant individual (Wootton, 1998)

In species that require parental care, one or both genders often prefer large mates whose potential to either provide protection for the nest (males) or contribute more eggs (females) is increased (Reebs, 2001). According to Grubb (2003), as fish have indeterminate growth,  in that they continue to grow with age, large individuals are perceived by potential mates to be healthy, have a proven ability to find food and possess the knowledge necessary to survive (Reebs, 2001) For species in which paternal care is common, the female often chooses larger males based on their likely good health and ability to defend the nest (Dugatkin & Fitzgerald, 1997). Additionally, females have shown a preference for males with symmetrical markings and high quality territories when selecting a mate. Species that do not show mate choice are those that rarely encounter the opposite sex and must take advantage of whatever opportunities arise, as well as those that engage in “group spawning” where the eggs and sperm are released simultaneously by all members of a school (Reebs, 2001).

e) Territoriality

A “home range” as a well-defined region of space to which fish or other animals restrict their activities (Lucas and Baras, 2001).  While some experts use “home range” and “territory” interchangeably, others characterize territories as specific areas, much smaller than home ranges, that can be defended against intruders (Gibson, 1993).

It has been theorized that in the case of fish, if parental care is more pervasive, territoriality would also be more common.  Fish who live in coral reef habitats tend to display more defensive and territorial behaviors. Grant proposed that the high productivity of these reef environments allows individuals to occupy small home ranges, which are more easily protected than larger areas. Godin elaborated that intruder pressure, and resource density and dispersion, dictate whether fish defend territories. Due to their less prolific environment, freshwater fish do not show such defensive strategies as it relates to food, although they will defend a food resource in the laboratory when it is arranged in such a way that it can be protected. Researchers have found that mates, spawning sites, nests, and offspring elicit defensive and territorial behavior more often than food in many fish (Grant, 1997).

f) Foraging

Fish employ a variety of techniques to forage for food; some species use a “sit and wait” strategy while others join schools of fish. Researchers have established that many fish rely on sight to obtain food and navigate within their environment.

For fish that have established a feeding position or territory, they will likely employ a “sit and wait” strategy whereby they swiftly grab any prey items that pass by with the current (Metcalfe, Taylor & Thorpe, 1995). For others, foraging takes place while swimming and artfully dodging other fish’s territories (Puckett & Dill, 1985). 

g) Social and parental behaviors

Fishes exhibit a range of social tendencies; some live largely solitary lives while others prefer the protection and company of schools. Familial relationships vary in importance from species that recognize and reduce aggression toward those with whom they are related to those that do not. Communication also plays an important role in communicating social and reproductive status.


Schools provide fish with protection from predators, as each individual’s chance of being taken is decreased when part of a group (Pitcher & Parrish, 1993). These schools of fish, or shoals, may be more or less tightly grouped, with loosely knit groups common in low light conditions and tightly grouped schools likely when predators are nearby (Brännäs, Alanärä & Magnhagen). The environment often dictates what type of social arrangement the fish adopt. Large bodies of water, like lakes and oceans, more often require fish to accept the added protection of a school, while streams and shallow lakes allow individuals to remain solitary. Schools of fish not only decrease a fish’s predation risk but they also increase the likelihood of finding food as many individuals are foraging at once. However, this common desire to forage can result in considerable competition for food.

A connection between body size and dominance has been observed in many fish species. However, scientists increasingly question whether large body size is not so much the cause of a fish’s dominance as the result of superior access to prime feeding areas enjoyed by dominant individuals. Communication appears to be common in fish, particularly via sound, in the contexts of competition, territoriality, aggression, and mating Hawkins, 1993). Olfaction is another widespread form of communication that assists fish with predator avoidance through alarm signals and in attracting mates (Reebs, 2001).


Other species use a unique form of communication involving electric charges in which each species has distinct discharge patterns. In such species, both sexes are assisted in mate choice by deciphering the unique electric charge characteristic of his, or her, own species (Reebs, 2001).

Parental care is not as widespread in fish as it is in other animal species: 78% of today’s fish families are made up of species that do not practice parental care. Of the remaining 22%, species engaging in bi-parental care make up 32%. In fish families that dedicate only one parent to care of the young, males guard the nest in 50% while females are the sole protector in 18% (Sargent, 1997).

h) Intelligence

A variety of fish species have shown the ability to learn through classical conditioning. Imprinting, an important mechanism that facilitates survival in other young animals, is seen in fish as it relates to their natal stream.  

Instrumental conditioning, when the frequency of a behavior increases or decreases depending on whether it is rewarded, has also been observed in fish. A number of species have been trained to push levers to receive a food reward, but researchers realize that there are limitations to what behaviors fish will perform for rewards due to their innate behavioral repertoire.

Individual recognition is often associated with intelligence in animals, and a number of fish species appear capable of the task.

Let's get scientific about fish
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Fun facts about fish

Fun facts about fish


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