The knowledge of fish fecundity is needed in establishing its production potential and consequently its exploitation and management rationale. It is imperative to understand and appreciate the diverse shades of meaning of this term.
Fecundity, derived from the word ‘fecund’ generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology the definition is more equivalent to fertility or the actual reproductive rate of an organism or population, measured by the number of gametes (eggs), seed set or asexual propagules.
This difference is due to the fact that demography considers human fecundity which is often intentionally limited, while biology assumes that organisms do not limit fertility. Fecundity is under both genetic and environmental control, and is the major measure of fitness.
Fecundation is another term for fertilization. Super fecundity refers to an organism’s ability to store another organism’s sperm (after copulation) and fertilize its own eggs from that store after a period of time, essentially making it appear as though fertilization occurred without sperm (i.e. parthenogenesis).
Meaning of Fish Fecundity Defined
The number of eggs produced per female has been variously defined in fisheries literature and different fecundity terms have been used in relation to the reproductive strategy of the fish and oocyte recruitment or stages.
In most studies, fecundity usually refers to potential annual fecundity, which represents the total number of advanced yolked oocytes matured per female and year, uncorrected for atretic losses. Nikolsky viewed fecundity of fish from two aspects; individual or absolute fecundity, that is, the numbers of eggs from the generation of that year and relative fecundity which refer to the number of eggs per unit body weight.
Bagenal defined fecundity as the number of ripening eggs in female prior to the next spawning season. Hansson defined fecundity as an ability of an organism to produce viable reproductive units.
In general, fecundity is one of the important aspects of life history of a species. It has been used in fish stock assessment studies in egg and larval survival studies, estimate of size of a stock and for stock discrimination.
The advantages of high fecundity cannot be overemphasized since the survival and continued existence of the species depend on numbers of eggs hatched.
High fecundity is also a desirable quality in aquaculture as it ensures regular (when reproduction can be obtained all-year round) and adequate supply of fingerlings for stocking culture enclosures.
Knowledge of the fecundity of a fish is necessary in establishing its production potential and consequently its sexploitation and management rationale
Fish species with large eggs comparatively have a low egg production than fishes with small eggs. As a rule, fecundity of a fish is inversely related to the degree of parental care it exhibits.
Low fecundity is associated with parental care during the developing stages of the embryo and fry. Generally, mouth breeders like tilapias have been associated with low fecundities.
Bagenal reported that the fecundity of a fish species approximate to the cube of the body length but noted there were several exceptions. In some fishes, fecundity approximates the square of the body length; while in others fecundity may approximate more than the cube of body length.
However, in some fish species, there is no relationship between fecundity and size of fish. Several workers have reported fecundity to be directly related to fish size or weight.
Methods of Determination
The estimation of fecundity usually refers to the determination of the number of vitellogenic oocytes (i.e.,potential fecundity). Different methods exist but their use will depend on the species under investigation, resources and laboratory facilities available.
Traditionally, potential fecundity is determined by a gravimetric or volumetric method. Although these methods are simple, inexpensive and give reliable results, the work is time-consuming and tedious
The gravimetric method usually involves sub-sampling by weight is often used for fecundity estimation: preserved eggs are washed in water to drain excess preservative usually Gilson’s fluid and exposed on filter paper for about 5 – 10 minutes after removal of ovarian tissue.
The eggs are weighed; five sub-samples of the eggs are weigh and number of eggs in the sub-samples counted. The total number of eggs in the ovary is calculated by proportion. This method of estimating fecundity is accurate to about 1%. The volumetric method is similar to this but volume measurement is used in place of weight.
However, new methods are developed to reduce the time and labor involved in measuring fecundity. For example, Thorsen and Kjesbu (2001) developed a method to measure oocyte density (number of eggs/g) using an image analysis system.
An image analyser was used to automatically determine mean oocyte diameter of a gonad sample and the oocyte density was determined with a calibration curve. In general, it took 5 minutes to prepare the sample, measure oocyte diameter and process the data.
Similarly, procedures based on image analysis have also been developed to measure efficiently and at low cost potential fecundity (Friedland etal., 2005; Klibansky and Juanes, 2008). Although some validation process is necessary before using these methods, they represent a major advancement in the efficient measurement of potential.
Biological Constraints of Fecundity
Potential fecundity is strongly influenced by female size, trade-off between egg size and egg number, reproductive strategy and spawning pattern of the species.
While size of oocytes will have a direct influence on maximum number of hydrated oocytes that can be produced in one time, reproductive strategy and spawning pattern will determine the possible number of eggs that can be produced in the spawning season.
Species with indeterminate fecundity where potential annual fecundity is not fixed before the onset of spawning will have the capacity to produce more eggs than species with determinate fecundity where new vitellogenic oocytes are not produced during the spawning season.
In the same way, batch spawners which released hydrated eggs in batches over a protracted spawning period will have the capacity to produce more eggs than total spawners where all oocytes are hydrated and shed in a single episode.
Because female fish retain their oocytes internally during their development, maximum reproductive output will be subjected to morphological constraints. The volume of the body cavity will limit the reproductive allocation at each spawning event.
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This limitation will result in correlations between reproductive investment and fish size. As the volume of body cavity is related to size, the potential fecundity in many fish species is strongly related to body size.
The constraint associated with the volume of the body cavity can also result in a trade-off between egg size and egg number; larger egg volume decreasing the maximum number of eggs that can be produced.
The inverse relation between egg size and egg number in spring and autumn spawning herring, a total spawner with determinate fecundity is a known example of this possible trade-off. Female attributes other than size have also been shown to influence potential fecundity.
Variations in the nutritional status estimated from different indices of condition like the condition factor or liver index significantly influence potential fecundity of Atlantic cod, plaice.
Ecological Constraints of Fecundity
Besides the determinant influence of female size, potential fecundity is also modulated by environmental conditions.
Numerous studies showed relations between potential fecundity and many environmental factors such as food abundance/ availability/consumption, temperature, fish density, and biomass index.
Variability in environmental conditions will directly or indirectly through their influence on growth and nutritional status result in a wide range of variation in potential fecundity.
This variability is clearly demonstrated by the different potential fecundity-size relationships (length is commonly used as the measure of size) observed for many species in the literature.
Annual variations in these relationships can be seen as the response of individual females to varying combination of biological and environmental influential factors detected between populations, geographic areas, and years
In summary, one of the most fascinating ideas to become established in recent decades is the concept that fish (and other organisms) have a life-history strategy, in which trait related to development and reproduction play a central role.
The life–history strategy of a species is a complex of evolved traits related to the allocation of energy; it defines a species, along with morphology, physiology and behaviour. Life cycle of a species is fine-tuned to the environment.
For example, among pelagic piscivores of the oceans, shark use a low-fecundity strategy of viviparity, tunas use a high-fecundity strategy of oviparity, and salmon use a strategy involving moderate fecundity and some parental care.
The evolutionary diversity of fishes has meant the development of unusual life-history strategies involving such things as multiple sexes, sex change and hermaphrodatism. Fecundity is important and well-studied in the field of population ecology.
Fecundity can increase or decrease in a population according to current conditions and certain regulating factors. For instance, in times of hardship for a population such as a lack of food, juvenile and eventually adult fecundity has been shown to decrease.
In addition to sex-ratio and proportion of mature individuals, fecundity is one of the most important determinants of a stock’s reproductive potential. A number of methods have been described that could be used to estimate fecundity for a variety of fishes.
However, no universal method could be recommended, Potential fecundity could be a biases method of estimating realized fecundity, if oocyte are continuously recruited into the pool of developing oocytes or if the number of this is reduced by atresia
Fecundity has been used in relation to the reproductive strategy of the fish and oocyte recruitment or stages. The advantages of high fecundity cannot be overemphasized since the survival and continued existence of the species depend on numbers of eggs hatched.
High fecundity is also a desirable quality in aquaculture as it ensures regular (when reproduction can be obtained all-year round) and adequate supply of fingerlings for stocking culture enclosures.
Knowledge of the fecundity of a fish is necessary in establishing its production potential and consequently its sexploitation and management rationale.).
Different methods exist to determine fecundity but their use will depend on the species under investigation, resources and laboratory facilities available. Traditionally, potential fecundity is determined by a gravimetric or volumetric method.
Although these methods are simple, inexpensive and give reliable results, the work is time-consuming and tedious.
Fecundity has been used in relation to the reproductive strategy of the fish and oocyte recruitment or stages. The advantages of high fecundity cannot be overemphasized since the survival and continued existence of the species depend on numbers of eggs hatched.
High fecundity is also a desirable quality in aquaculture as it ensures regular (when reproduction can be obtained all-year round) and adequate supply of fingerlings for stocking culture enclosures.
Fecundity is strongly influenced by biological factors such as female size, trade-off between egg size and egg number, reproductive strategy and spawning pattern of the species.
Numerous studies have showed the relations between fecundity and many environmental factors such as food abundance/ availability/ consumption, temperature, fish density, and biomass index.
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