They are transparent and are called elvers or glass eel which acquires new teeth. The elvers, when become three years old, start ascending the rivers in huge numbers, and are able to cross all obstacles and finally on reaching their suitable resting place they feed and grow for some years and ultimately develop into yellow eels.
Finally on attaining maturity, they finally change into silver eels, and start for their own breeding migration to the sea, which ends in death. Another closely allied species is called the American eel, Anguilla rostrata , and is found in the rivers and along the coast of North America. The breeding area of both the species overlaps, and the larvae of both the species are found together. The larvae of the American eel migrate towards the rivers of North America.
According to Schmidt, the larvae of the American eel grow more rapidly and become metamorphosed into elvers in one year, while the larvae of the European eel take three years to become elvers. The distance to be covered by individual species is dependent on larval lifespan.
The long larval life of the European eel is an adaptation for covering a great distance to approach breeding grounds which is located far away from the coasts. However, a British Zoologist, Dr Tucker, has put forward another hypothesis.
He opined that the two species of eels are really one and the same. The small morphological differences between the two are due to environmental factors. According to Dr Tucker, the European eels are not able to survive the presumed miles journey from Europe to the Sargasso Sea, and they do not return to the spawning grounds as also mature European eels have not been observed in the said area. The American eels are able to reach the spawning area as they cover shorter distance.
Thus it appears that the European eels are also derived from the American eels. The actual spawning area determines whether the larvae will be carried towards America or Europe, along with the currents. The temperature and other ecological factors determine the larval age. Extensive field and experimental studies needs to carried out before drawing conclusion.
The development of an egg begins soon after it is fertilised by a sperm. The egg of bony fishes has a relatively large amount of yolk, which remain segregated from the active superficial cytoplasm. Cleavage is confirmed to the superficial layer of the cytoplasm and is incomplete underside meroblastic. In the earlier stages cleavage planes are all vertical so that all the blastomeres lie in one plane only. The blastomeres are separated from each other by furrows but lie over the yolk.
In the later stages, cleavages occur in the horizontal plane also, so that the blastomers become arranged in more than one row. The marginal cells are in contact with the yolk. The disc of cells thus formed on the animal pole of the egg, is called the blastoderm. The central cells of the blastoderm divide to form a number of "free" blastoderms which subsequently arranged on the top of the yolk so as to form a layer of cells called the periblast.
The space between the blastoderm and periblast is the blastocoel and the embryo now is in the blastula stage. The blastoderm gives rise to embryo proper while the cells of the periblast probably serve to digest the yolk, and supply it to the developing embryo. It is possible to identify various regions of the blastula wall that are destined to give rise to specific organs in the embryo.
Thus a fate map of the teleostean blastula can be constructed showing the presumptive ectoderm, endoderm, mesoderm, notochord, neural plate etc. In the fish blastula, the areas which are destined to give rise to the organs of the dorsal region of the animal, tends to be concentrated towards one side of the blastodisc.
This indicates the posterior end of the future embryo. At this end and along the margin of the blastoderm, lies the presumptive endoderm which gives rise to gut. Anterior to this lies the presumptive notochord, and the centre of the blastoderm, holds the area of the nervous system.
The presumptive mesoderm lies along the sides of the areas destined for endoderm, notochord and neural plate. The mesodermal area extends mainly along the margin of the blastoderm and in Fundulus , is not continuous at the anterior end due to the presence of presumptive epidermis. But in Salmo the presumptive mesodermal area is present all round the margin of the blastoderm.
Fish may lay eggs, ie be oviparous, or give birth to live young. Those species which give birth to live young can be further divided into the truly viviparous species, in which the developing embryos are supplied with nutrients by the mother, and ovoviviparous species, in which there is egg retention but no additional nutrient supply from the mother.
Within this broad framework of the three basic modes of reproduction- oviparity, ovoviviparity and viviparity- there are large interspecific differences with respect to both the numbers of offspring produced, and the degree of protection and care given to the developing eggs and young by the parents.
Within the oviparous species, for example, the variations may range from the production of large numbers of freely floating pelagic eggs at one extreme to nest building and extensive parental care at the other.
For example, many marine species produce large numbers of small pelagic eggs, whereas stickleback species Gasterosteidae build nests and engage in elaborate courtship and parental behaviours. By studying the relationships between fecundity the number of eggs produced per female , egg size and degree of parental care it is possible to make some generalisations about reproductive patterns in fish Tables 9. Thus, it can be said that the following general principles appear to apply. These generalisations refer to interspecific differences, but even within species there can be considerable variations in both fecundity and egg size Table 9.
There may, for example, be considerable intraspecific variability in egg numbers, both with respect to differences between females spawning within a given season, and with respect to the fecundity of a given female in consecutive years or breeding seasons. Variations in fecundity of up to 45 percent have been recorded for plaice, Pleuronectes platessa , and variations of about 25 percent, 34 percent and 56 percent have been found for pike, Esox lucius , hering, Clupea haregus , and haddock, Melanogrammus aeglefinus , respectively.
The intraspecific variations in egg size and fecundity can usually be traced either to differences in the sizes, or ages, of the females making up the spawning stock, or to year-to-year fluctuations in the food supply. For example, in many species of fish there is a clear relationship between fecundity and size of the female.
Larger females tending to produce more eggs than do smaller conspecifics. Intraspecific variations in fecundity and egg size may also be related to the time of spawning.
For example, it is frequently reported that fish spawning late in the season tend to produce smaller eggs than do early spawners. Nine aquafeed companies from across the globe have officially joined the F3 Challenge - Carnivore Edition — which aims t…. Researchers in Spain claim that growing Salicornia and grey mullet together in an aquaponics system has considerable pot…. Norcod has reported NOK Finnforel, which curently produces 1, tonnes of rainbow trout at an RAS facility in Finland, has announced plans to i….
Maturity stage in the female fish On the basis of shape, size, colour of the ovary and other histomorphological features, at least six maturity stages can be recognized. Resting phase immature : The ovaries are small, thin, thread like, translucent, pale or dirty white in colour with inconspicuous vascularisation.
The ovaries occupy only a small part of the body cavity and ova are not visible to the naked eye. Histologically, the ovary shows ovigerous lamellae, having nests of oogonia, and immature oocytes in the stage I and II are visible under a microscope.
Early maturing phase: Ovaries become slightly larger, thicker, opaque and are light yellowish in colour. Vascular supply increases and the blood capillaries become conspicuous.
Immature oocytes are reduced in number while stage IV and stage V oocytes are present in large number. A few stage VI oocytes may also be seen. Mature or prespawning phase: The ovaries are further enlarged occupying almost the entire body cavity. They are turgid, deep yellow in colour and a large number of spherical ova are visible to the naked eye through the thin ovarian wall. The blood supply increases considerably.
Both translucent and opaque ova are present and the ovaries attain their maximum weight. The fish becomes gravid due to ripe ova tucked inside, and the abdomen becomes round.
The ova are not discharged till the environmental conditions become favourable. Histologically, a large number of ova in stage VII, and ripe eggs are seen in the ovary. Spawning phase: Overies are very much enlarged, occupying the entire body cavity. They are turgid and yellow in colour with a large number of translucent eggs. Ovarian wall is very thin, almost transparent. Eggs are present in the oviduct also, and the fish spawns a number of times during this period.
The ovary is now said to be in running phase. At the beginning of this phase, ova are extruded by applying a gentle pressure on the abdomen. Spent phase: The ovaries are flaccid, shrunk and sac-like reduced in volume and have a dull colour. The vascular supply is reduced. Some unspawned large ova and a large number of small ova are present.
Histologically, the ovary shows atretic and discharged follicles, along with stage I and II oocytes. Hypertrophic or phagocytic atresia In the early stages, the nucleus of the oocyte disappears, and there is erosion of the vitelline membrane zona radiata , which become wavy, losses contact with the granulose cells, and may become broken at some points. Non-hypertrophic atresia The nucleus of the oocyte disappears and the vitelline membrane is eroded.
The discharged follicle post-ovulatory corpus luteum Besides the pre-ovulation corpus luteum corpora atretica described above, comparable structures develop from the follicula cells after the discharge of the mature ovum. Length weight relationship and condition factor Several studies have shown that the length of the fish maintains a steady relationship with the weight ie, an increase in length is related with the increase in weight of the fish, but the degree of this relationship varies with the species of the fish and there is no effect on season or sex.
Based on the data obtained as above, the condition factor is calculated by using the formula:. Ovulation and fertilisation The egg gradually enlarges and projects into the lumen of the ovary. Eggs The number of eggs produced by a single female differs considerably and depends upon several factors like her age, size, condition and species.
Fecundity Fecundity can be defined as "the number of ova that are likely to be laid by a fish during the spawning season". It can be estimated by any of the following methods: In the volumetric method, the total volume of the ovary is measured. Small pieces of the ovary are taken in random samples from the anterior middle and posterior parts of the ovary, and the number of ova in each sample is counted along with the volume of the sample. The total number of ova in the total volume of the ovary is then calculated.
In the gravimetric method, the formalin preserved ovaries are used. After determining the weight of the ovary, three small samples of mg each are taken at random from the anterior middle and posterior part. The number of ova in each sample is counted under a binocular microscope. Total number of ova are then calculated as below:.
Sexual dimorphism, courtship and parental care Some species of fishes exhibit well marked sexual dimorphism. Migration for spawning Many marine fishes migrate to the rivers for spawning. Eels are are of two types Yellow eels: Represent their feeding and growing phase. Silver eels: Represent their breeding phase Yellow eels are found in both salt and fresh water and vary in length from a few inches to several feet. Development Cleavage and the formation of blastula The development of an egg begins soon after it is fertilised by a sperm.
Fate map of blatula It is possible to identify various regions of the blastula wall that are destined to give rise to specific organs in the embryo. Offspring numbers and parental care Fish may lay eggs, ie be oviparous, or give birth to live young. Fecundity tends to be high where eggs and sperm are liberated freely into the water, is less in those species giving some form of protection to the eggs and is lowest in species that show parental care.
Fecundity and egg size are inversely related, that is fish produce many small eggs or a few large ones. The production of pelagic eggs is largely restricted to marine species. These cavities can be found between rocks, in hollow logs, or burrows in banks. After the female picks a nesting location, she lays her eggs. A male will then come and fertilize the eggs. After he fertilizes the eggs, the females leave and let the males protect them.
The male fish keeps predators away and fans the eggs so they do not get buried by sediments. After about a week the eggs will hatch. When bass are ready to spawn , it only happens in the spring. They wait for the waters to be anywhere between degrees.
Even though bass spawning season only take place in the spring, they have three phases that they go through. The pre-spawn is when bass begin eating much more.
They increase the amount of food they eat because they know they will not eat while they are actually spawning.
While bass are spawning they become very aggressive. Species that they used to feed on will now be the ones they attack just to keep their eggs safe. During the post-spawn the eggs have hatched. The males will guard the newly hatched fish while the females leave to rest in deeper water.
Salmon migrate upstream to lay their eggs. They swim back to the same area where they hatched to spawn, and many die during or after the process. Salmon are born in freshwater rivers and streams and return there to spawn, but they spend most of their life in the ocean. They can change habitat, and even change colors when spawning — many turn a bright red.
Most anglers catch salmon during their migration on the way to spawn. Salmon usually spawn in the Spring through the Summer, and there are resources that track salmon runs by specific area or even body of water. Spawning bettas can be very difficult. The breeding pair needs to carefully matched because the males tend to be aggressive.
The best time for bettas to spawn is when they are young. Since they are fish that are typically pets, the time of year does not matter. We aim to study the local ecological knowledge that Brazilian coastal fishers have about reproductive aspects season, places and migration of 13 coastal fish species of commercial importance. In the interviews, we used standardized questionnaires and showed photos of fish species. The main fish migratory patterns mentioned by interviewees were: long migrations along the coast, usually in the South to North direction, short migrations among reefs, fishes that do not migrate, migrations between the shore and open ocean and migrations between the sea and coastal rivers.
We compared such ethnoichthyological information with available scientific data, indicating promising insights about reproduction and migration of Brazilian coastal fishes. Data gathered from local fishermen may provide inexpensive and prompt information, potentially applicable to fisheries management. Our approach might be useful to several other small-scale fisheries, especially the tropical ones, where there is a high diversity of target species and a low biological and ecological knowledge about these species.
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