Both biotic and abiotic factors affect the survival and reproductive success rate of fishes in an ecosystem.
The number of predators that a particular type of fish will have will influence the survival and reproductive rate of that fish. An increased number of predators will mean that the fishes will not be able to survive in that ecosystem and will be eaten up.
The number of preys will also affect the survival rate of fishes in an environment. The lesser the number of preys, the more the competition for food among species.
Abiotic factors like the concentration of salts in water will also influence the survival and reproduction of fishes. If a fish is not adapted to live in saline water conditions, then accumulation of salts in the ecosystem might destroy the whole fish species in that ecosystem.
Human activities, like the throwing of wastes into the aquatic ecosystem, drainage of fertilizers in the aquatic system can also destroy the survival and reproduction rate of a particular kind of aquatic animal species.
A geneticist would know if an organism is homozygous or heterozygous for a certain trait based on if the organism had two of the same alleles or 2 different alleles for that same trait.
