Decoding the Mystery: How Do Fish Mate? Everything You Need to Know

Mating is incredibly important in the animal kingdom because it's how animals make babies and ensure their species keeps going. It's a way for animals to pass down their genes and create new generations. Reproduction helps animals adapt to their environment and maintain a diverse population. Without it, species would die out, disrupting the balance of nature. 

70% of the planet is covered in water (plenty of space for fishing!). Consequently, there is a diverse range of fish species. The total number of living fish species—about 32,000— is greater than the total of all other vertebrate species (amphibians, reptiles, birds, and mammals) combined, according to National Geographic. Fish are some of the oldest animals on the evolutionary scale of complex life forms (multi-celled organisms). Thus, many different reproductive anatomy types, mating behavior, and spawning rituals exist.

This blog aims to water down some of the scientific terms you may have come across to help better understand this fascinating process and offer some useful suggestions to ensure responsible fishing and the continuation of these superb species.

What Is The Reproductive Anatomy of Fish?

The reproductive anatomy of fish is like its vertebrate cousins on land. In the fish world, most species have separate sexes (dioecious), with male anatomy having testes and female ovaries. However, some fish are hermaphrodites with both male and female reproductive organs. Reproduction requires an egg (roe) and sperm (milt) to make a newborn, but there are several different ways in which the roe and milt can meet.

Laying Eggs in The Water

Oviparity is a reproducing method that many animals, including fish, reptiles, and insects, use. When an animal is oviparous, it lays eggs to give birth to its offspring. The eggs are usually laid outside the parent's body, often in a safe place like a nest or a burrow. Once the eggs are laid, the parent usually leaves them alone and doesn't provide further care. The eggs contain all the nutrients and protection needed for the developing embryos to grow and develop until they are ready to hatch and emerge as independent young animals. The eggs are usually fertilized externally by males. Research shows that an estimated 90% of bony fish species are oviparous, according to the National Oceanic and Atmospheric Administration (NOAA)

Oviparity is a common and efficient reproduction method, allowing many species to produce offspring at once, providing food for predators while maintaining sustainable numbers of babies.

What Happens to The Embryos?

As mentioned above, the embryos (fry) usually grow in nests or burrows made by the male. Studies demonstrate that females prefer to lay their eggs in larger nests but without male parental care, they also prefer nests without other eggs, according to Reynolds, J. D., & Jones, J. C. (1999). Female preference for preferred males is reversed under low oxygen conditions in the common goby (Pomatoschistus microps). Behavioral Ecology, 10, 149–154.

Quite unusually, some species gather the fertilized eggs in their mouth and keep them there, refusing to eat so they can protect their eggs until they hatch. These are called mouthbrooders. Female African cichlids hold eggs in their mouths for 21 to 36 days, according to PADI.

Some eggs are laid in one place, and others are buried in the substrate, where the males will dive down and fertilize them. Some fish, like Goldfish, provide an adhesive substance that makes the eggs stick to a surface, like plants. They are then fertilized by passing males.

Internal Fertilization

For the small percentage of fish that copulate with internal fertilization, the males have a pair of claspers located along the pelvic fin. Claspers function similarly to a penis, although they are not separate appendages. The sperm is transferred into the female via this cartilaginous extension. It is rare to see sharks mating, but observations show that they only use one clasper while copulating. You can identify male sharks up close by looking (from underneath) for a groove between the pelvic fins and where the tail begins.

Ovoviviparous Fish

In the case of ovoviviparous fish, they don't lay eggs like other species. Instead, they carry their eggs inside their bodies until they are ready to hatch. The developing eggs contain all the nutrients required for development. Unlike egg-laying species, ovoviviparous fish produce fewer eggs, but each egg has a higher chance of surviving and hatching successfully.

Research shows that the first Sand Tiger Shark to hatch inside its mother usually consumes the remaining eggs, this is called intrauterine cannibalism and explains the occasional low birthing rates within that species, according to a study on 'The behavioral and genetic mating system of the sand tiger shark, Carcharias taurus, an intrauterine cannibal.’

Viviparous Fish

Viviparous fish have a unique reproductive strategy characterized by live birth, where the young are born alive rather than hatched from eggs. Within the mother's body, the embryos undergo internal development, growing and maturing until they are ready to be born. During this process, the developing embryos receive nourishment and support from the mother (similar to us) through a specialized organ, such as a placenta. Viviparous fish generally produce fewer offspring than other reproductive methods but compensate for this with higher parental care. The parent fish invests more time and effort in caring for and protecting the developing embryos until birth.

Mating strategies

There is a diverse range of mating strategies on display under the surface: 

• Promiscuity in fish is the primary mating system where both sexes breed with multiple partners. Mate choice is minimal or absent, and spawning occurs with multiple partners, either simultaneously or sequentially.

• Polygamy involves one sex having multiple partners during breeding while the other sex has only one partner. It increases the chance of passing on advantageous genes through mate competition.

• Polygyny is a common form of polygamy in which a male has multiple female partners during the breeding season. Polygyny is apparent when a large male protects several females with which he, alone, can mate. 

• Polyandry is a less common form of polygamy where a female has multiple male partners during the breeding season. This has been observed in anemonefish and possibly deep-sea anglerfish, where males become physically attached to the female and share their circulatory system in what appears to be a parasitic relationship. While anemonefish are typically monogamous, polyandry has been documented in certain cases.

• Monogamy refers to a mating system where partners form exclusive pair bonds, lasting for a single breeding season, several years, or even a lifetime. Some fish species, such as American freshwater catfish, certain cichlids, and various butterflyfish, including the Four-Eyed Butterflyfish, exhibit rare lifelong monogamy.

• Hermaphrodites are when a fish simultaneously produces egg and sperm and thus does not need a partner. Although this strategy is possible and has been observed, it is considered rare, according to ‘Avise J (2011) Hermaphroditism: a primer on the biology, ecology, and evolution of dual sexuality.’ A lack of genetic diversity within breeding means that hermaphroditic reproduction is not considered favorable for species continuation.

When Do Fish Reach Sexual Maturity?

In the diverse world of fish, the age at which they reach sexual maturity varies across species. As a general guide, smaller adult fish tend to reach maturity earlier, allowing them to engage in mating sooner. This means that certain species can start their sexual duties within mere weeks of hatching from their eggs. On the other hand, some fish require a patient wait of several years before they are sexually mature enough to enter the breeding scene.

Understanding the sexual maturation age in fish is important for anglers as it can be the difference between abiding and breaking the law. For example, if you want to catch Bluefish in Florida, you can check the link to learn about any restrictions.

Spawning Frequency

The spawning frequency among different fish species is as diverse as everything else marine-wise.

Most fish are classified as capable of breeding several times during their life. Iteroparous fish have the distinct advantage of contributing more to their species when conditions favor reaching adulthood. They can usually reproduce multiple times over multiple years.

There are also fish species that reproduce only once in their lifetime (semelparous) and unfortunately perish shortly after spawning, like the Pacific salmon or certain species of octopus.

Learning the spawning frequencies as an angler is vital, as every time a semelparous fish is hooked, there is a guarantee that the snagged fish cannot help continue its species. Additionally, Keeping up-to-date on the breeding seasons for iteroparous fish will allow them to repopulate in peace. Abide by the laws and be conscious of your actions when fishing! Quite literally, there are potential lives on the line!

Offspring Survival and Parental Care

Offspring survival is a big issue in the marine world. Research carried out shows shockingly high mortality rates, up to 95% in this individual study conducted by Langangen, Ø., Stige, L. C., Yaragina, N. A., Vikebø, F. B., Bogstad, B., & Gusdal, Y. (2014). Egg mortality of northeast Arctic cod and haddock.

Eggs provide excellent nutrition for a wide range of opportunistic species, but for other species to continue, some must be able to mature. The most prevalent ways to combat the inevitable destiny of becoming dinner is with nests and burrows. An alternative to these is parental care.

While most fish exhibit minimal parental care, there is a fascinating subset of species where caregiving plays a prominent role. Astonishingly, between one-third and one-half of these species entrust the task of parenting solely to the fathers. One notable and widely recognized example is the seahorse, where the male assumes the extraordinary role of pregnancy. He carries the fertilized eggs, courtesy of his mate, within a specialized pouch on his belly until the momentous occasion of hatching. 

Courtship displays

High male endeavor is not limited to just parental care but courtship too. An equal range of courtship displays exists among the aquatic vibrancy of colors and shapes we can observe on our watery planet. There is stunning evidence of artistic creativity from the Japanese Puffer Fish, as shown in the BBC Earth documentary series. This industrious little fish uses its fins to create textured and layered dunes in the sand that are geometrically perfect. The male must work diligently for up to a week without sleep so the current does not destroy its masterpiece and hope a female is sufficiently impressed! 

It may be easy to think of fish as mindless creatures, but their instincts to do what is necessary so they can reproduce are as strong as any other animal!

Environmental Factors Affecting Fish Mating

Temperature plays a crucial role in the reproductive process. These fish are most likely to spawn successfully within an optimal temperature range. However, critical temperature limits exist, above and below which fish cannot reproduce. Warm temperatures are particularly influential in stimulating gonad maturation in many fish species. For example, major carps typically breed within a temperature range of 24-31°C.

Light is another significant factor that influences fish reproduction. The duration of daylight (photoperiod) plays a key role in determining the timing of maturation and spawning in various fish species. Manipulating the photoperiod can result in early maturation and spawning in fishes like Fundulus and Oryzias. Even at low temperatures, certain species like Cirrhinus reba can achieve early maturation when exposed to artificial day lengths longer than the natural day, according to 'Role Of Different Factors On Reproduction Of Fish Mule Akash.B., Sarve Neha.S.'

Water currents and rainfall also play important roles in fish reproduction. Fresh rainwater and flooded conditions are primary factors that trigger the spawning of Carp. The sudden drop in electrolyte levels caused by heavy monsoon rains induces hydration in fish, stimulating their reproductive organs and resulting in natural spawning. Successful spawning has been observed in fish on cloudy and rainy days, particularly after heavy showers.

These various environmental factors demonstrate the intricate interplay between temperature, light, water currents, and rainfall in influencing the reproductive behaviors of fish. Understanding these factors is crucial for successful fish breeding and cultivation.

The Human Factor

Fish provides more than 2.9 billion people with at least 15% of their average per-capita animal protein intake, according to the World Fisheries and Aquaculture 2008 report by the Food and Agriculture Organization of the United Nations. The overexploitation and excessive harvesting of the biggest fish have generally led to a decrease in the size of fish, as they cannot reach full maturity. Smaller fish produce fewer and smaller eggs resulting in smaller generational populations that follow.

Studies show that larger fish tend to have higher levels of pollutants. So, avoiding these will have a double benefit in allowing fish to increase their average size and avoiding the accidental consumption of higher quantities of pollutants. 

The more practical aspect of fishing to eat is just one of many concerns currently highlighted by the interaction between humans and fish populations. Lessons need to be learned, compromises must be made, and it needs to be pushed now. Why not learn how you can help our fish populations by reading about The Ocean, Environment and Climate Change.