1. High Mortality Rates:
* Predation: Many fish eggs and young are eaten by other fish, birds, and invertebrates.
* Environmental Factors: Eggs and young are vulnerable to changes in water temperature, oxygen levels, and currents.
* Disease: Fish eggs and young are susceptible to diseases and parasites.
2. Limited Parental Care:
* Most fish species provide little to no parental care for their eggs or young. This means that a large number of offspring are needed to ensure that at least some survive to adulthood.
3. External Fertilization:
* Many fish species release their eggs and sperm into the water for fertilization. This process is less efficient than internal fertilization, meaning that many eggs are unfertilized or destroyed.
4. Short Lifespans:
* Some fish species have relatively short lifespans, meaning that they need to produce large numbers of young to ensure the continuation of their species.
5. Exploitative Life History:
* Many fish species have an "exploitative" life history strategy, where they invest a lot of energy into producing large numbers of offspring, but provide little parental care. This strategy is successful in environments with high mortality rates.
6. Compensation for Overfishing:
* Overfishing has led to a decrease in the population sizes of many fish species. In response, these species may produce more offspring to compensate for the losses.
In summary: The high mortality rates, lack of parental care, external fertilization, short lifespans, and exploitative life history strategies of many fish species all contribute to their production of large numbers of young.