Predators of Freshwater Fish Eggs and Fry: It's A Tough Life
It's a tough life being a fish, especially a young fish. Everything, but everything, can and will eat you. The prospects for a newly fertilized egg are grim. Survivorship for these eggs to adulthood can be as low as a fraction of 1%. Most of the mortality occurs in the first few days of life. Each week that a fish fry survives represents a huge step towards its ultimate survival. Danger lurks at every corner and much of it comes from a highly unexpected direction.
Do fish ever turn cannibal?
Although the image of an out-and-out predator such as a wolf fish (Hoplias malabaricus) may spring to mind when we imagine the risks that fish face, very often their greatest enemies are larger individuals of their own species. From the outset when fish spawn, the eggs are greedily gobbled up by any fish in the area, and for good reason: they offer excellent nourishment. Newly hatched fry hardly fare any better; they are eaten by nearly everything, including their own parents, in some cases.
Researchers in Nicaragua found that the largest single predator of convict cichlid fry were subadult convict cichlids themselves. Again, this makes sense. When fish eat, their bodies break down the food and, put simply, convert the food into the building blocks that they need to grow or maintain their own bodies. Eating conspecifics means that every necessary nutrient is present and in the correct proportions. For fishkeepers, seeing a livebearer such as a black molly (Poecilia sphenops) giving birth in the home aquarium can be quite harrowing, as the newly born offspring are rapidly chased down and eaten by the other tank inhabitants and sometimes, even their own mother. Although hard for us to understand, fish seem to eat their own fry when they assess that their chances of survival are slim -- "if the kids are going to be eaten anyway, I might as well eat them myself." This recycling allows the parents to recover some of the huge energetic expenditure of parenthood, preparing them for their next effort, when perhaps conditions will be better.
How do fish sense that eggs may be on the menu?
Just as the smell of our favorite food being prepared can cause us to hang around the kitchen, so the smell of hormones in the water during a breeding season can produce a similar effect in fishes.The ability of fishes to detect minute concentrations of chemicals in the water means that the smell of spawning draws eager predators from all around. The only defense that fish can muster is to produce a huge number of eggs -- more than the predators can eat at once -- and to scatter them widely or to guard them. Even so, the smell of eggs is a powerful stimulant for fish and the promise of rich, defenseless pickings is often just too good to miss.