The major part of the reproductive cycle is gametogenesis - sperm and egg production - ensuring individuals are capable of breeding. Various accessory activities are also essential to ensure that reproductively competent individuals congregate at an appropriate place and time, and that eggs and sperm release are synchronized to produce fertilized eggs.
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EFFORT |
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GAIN |
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fecundity etc. |
fertilized eggs etc. |
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ß |
ß |
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risks for future reproduction |
risks of survival through metamorphosis to reproduction |
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NET GAIN |
Effort should only be put into reproduction if it realizes a gain, as measured in terms of fitness, and fish should put out effort in such a way as to maximize lifetime fitness. The environment is also risky in that death to adults and/or loss of products of effort can occur in whole or part. In order to maximize gains, exposure to such risks must be minimized. This is achieved by distributing effort in space and/or time, as bet-hedging and/or to time reproduction to times of least environmental hazard and/or maximum productivity for growth and survival of offspring.
Effort is the total energy used in all aspects of reproduction, including migration, nesting building, parental care, defense of eggs and breeding territories, as well as egg and sperm production. However, most of these components have not been measured and are often very difficult to measure. Instead, effort is most commonly measured as the energy allotted to the production of eggs and sperm, or more simply for females as fecundity - number of eggs per female. Annual female reproductive effort varies from amount 1 to 11% of the food consumption. The amount of effort varies with food availability:
Effort typically initially decreases per year as longevity increases because fish expect multiple breedings. However, as fish age, effort should increase because the probability of future reproductions decreases as future life expectancy decreases. Thus there is a trade-off between current and future reproduction in terms of the effort expended.
Sperm production may be limiting to reproductive success in many species with external fertilization.
Fecundity (number of eggs per females) is the easiest measurement to make. This will overestimate true fitness, and basically assumes that fitness is simply a numerical gain.
Risk
The goal of reproduction is to maximize gain/effort. Gains depend on survival of YOY to reproduction in a variable and hazardous environment. Risks cannot be eliminated. Much of reproductive biology, therefore, actually is concerned with minimizing the effect of risks. Timing is a major factor, leading to a range of reproductive strategies with various trade-offs.
Reproductive cycles are presumed to be adaptive by placing young where growth is maximized and risks minimized.
Annual cycles are most common in temperate and polar climes where food availability also various on an annual seasonal basis.
There are often no obvious seasons in tropical climes. Factors affecting reproduction are more subtle, but include rainy versus dry seasons, flow rates due to seasonal flooding, availability of spawning sites, droughts, water chemistry that may change throughout the year, slight changes in temperature or day length.
The environmental cues are usually coarse, resulting in maturation of eggs and sperm, creating individuals capable of breeding. The final stages of reproduction must be fine-tuned to the environmental, and the short time-scale variations which could place the reproductive effort at risk. These often involve courtship.
The interplay of effort, gain, and risk have been codified into reproductive strategies derived from the logistic growth curve.
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r-selected |
K-selected |
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dN/dt ® rN, exponential growth. |
dN/dt ® 0, population at carrying capacity |
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unpredictable habitats with high density-independent mortality |
predictable environments with density dependent mortality rates |
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rapid individual growth (because resources are not limiting) |
slow individual growth rates |
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short life expectancy. |
long life expectancy |
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early reproduction |
delayed reproduction |
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high fecundity |
low fecundity |
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many young |
few large (competitive) young |
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high effort - even if it results in increased adult mortality; e.g. Pimephales promelas adults spawning at age I have less chance of surviving than adults spawning at age II. At age I they have not yet had a chance to grow as much and set aside extra energy for post-reproduction survival. At age II the fish are larger and have more extra energy to spare for survival. |
low reproductive effort - but often parental care |
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semelparity is common when fish should put as much energy as possible reproduction. E.g. Pacific salmon. |
iteroparity common K selected fishes have a choice whether or not to be semelparous or iteroparous. The fish that are semelparous are in trying to maximize compound interest on a single investment. Putting everything in one shot and then dying. Since the environment is relatively stable, the adult fish are also likely to survive until the next spawning season so iteroparity is an option as well. While k selected fish can be either iteroparous or semelparous and the r selected fish only have one option |
Responses to various factors affecting adult longevity.
Exploitation increases mortality rates, shifting habitats towards the r-end of the continuum. Exploitation of lake whitefish (Coregonus clupeaformis) and lake trout (Salvelinus namaycush) results in higher fecundity.
Predation, by reducing longevity, shifts fish towards r-selected patterns.
White suckers exposed to stress show increased growth, earlier maturation, increased fecundity, and decreased egg size.
Parasite loads typically reduce fecundity.
Cichlasoma nigrofasciatum (convict cichlid) shows increasing aggression with increasing density. This is associated with reduced reproductive success and reduced survival of young.
The r-K continuum may be useful when population mortality schedules are distributed across all year classes in a common habitat. Its usefulness decreases when different life history stages occupy different habitats with different risks and mortality schedules as with complex life cycles, typical of fishes.
Adults compete for spawning sites. Larvae compete for food, and sometimes physical resources such as oxygen.
Cannibalization by adults is common. Other juvenile and adult fish prey on larvae, ctenphores and medusae, chaetognaths, copepods, euphausids, annelids, etc. will also feed on larvae and cause reductions in populations.
Hjort first feeding hypothesis - critical period - postulated to determine larval mortality and eventually recruitment. Food particle sizes must be adequate for larvae to first feed in terms of food particle size and abundance. Many factors affect whether the requisite conditions are met and if they persist.