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October 25, 2001

Population Fluctuations

Why do some seasons host greater numbers of gamefish than others?

Q: I have noticed that inshore fishing for big redfish and snook is better in the years when there are a lot of pinfish and mullet in the bay and bad when there isn't much bait around, but how can the size and number of fish fluctuate so much from year to year? - Tom LaMarque, Tampa, Florida

A: Predator-prey interactions are very complex. Eating is serious business - if a fish can't find enough to eat, it must either move to another area or starve to death. When there is plenty of food, game fish grow quickly, mature and spawn at an early age and produce lots of eggs for the next generation. If food is scarce, the game fish will be smaller and mature later.
Game fish are usually top predators, feeding on smaller fish. The smallest fish feed on zooplankton, which in turn feed on detritus, algae and other microscopic plants. Because the whole food chain depends on how much nutrition is available to produce the food the zooplankton eat - called primary production - the environment plays a large part in sustaining both bait and game fish. Coastal environments are rich in nutrients, while offshore environments are nutrient poor.
If the baitfish have enough food - and no other negative environmental event intervenes, such as a red tide or a cold-water fish kill - predators will have plenty to eat and may increase in numbers until they overwhelm the existing food supply. As the baitfish are reduced in numbers by the feeding predators, the predators may move on or switch to another food source. In this way, there is a natural balance between predators and prey in which both may flourish or decline together, in regular up and down cycles.
However, this balance is easily disrupted by humans overfishing either predators or prey, and by humans changing the environment itself by polluting or destroying habitat with industry or development. All of this can affect the food chain at its most basic level.