Although harvesting strategies are usually assessed for single species, each is part of a complex, ever-changing ecosystem. Seasonal effects may cause fluctuations in fish survival, distribution and migration, while global climate change may alter fish stocks and the functioning of marine ecosystems. Predator-prey relationships are also important in regulating population numbers. The result of these interactions among populations and between these populations and the environment are studied. Research is being conducted to evaluate the effectiveness of marine reserves.
Reef fish inhabit particular reef types.
Such information can be used to determine whether marine reserves can support populations of species requiring protection. Tag recapture data have shown that many fish species display nomadic behaviour, moving to areas outside the reserve where they can be caught by fishermen. In addition, the protection afforded by reserves allow fish to reach larger sizes than fish outside reserves. This is because the biggest and most aggressive fish are usually removed from the population in heavily fished areas. Some fish change sex during their life, so removal of the largest fish may cause a shortage of one sex if the population is overexploited.