Fleeing from Danger
Fish are extremely popular as prey among a diverse range of hunters. One reason is that they are an extremely rich source of food -- fishes bodies are packages of extremely lean swimming muscle that accounts for up to 80% of the fish's overall weight. But although this makes them an excellent menu item, it also means they can be tough to catch, so long as they become aware of the danger in good time.
What are the two main kinds of fish swimming muscles?
In simple terms, there are two main kinds of muscle, distinguishable by their color. White muscle is used to provide explosive power of the kind used by human sprinters and weightlifters. White muscle is comparatively poorly supplied by the animal's circulating blood, hence its color. It provides a great deal of power, but only for a limited amount of time. This is because it operates anaerobically -- without oxygen -- and quickly becomes fatigued. Red muscle, by contrast is very well supplied by the blood, as its color suggests, and is rich in oxygen. It provides steady, long-term power and is used by the animal as it cruises along.
Can fish swim out of danger?
Quick reactions and a turn of speed are essential weapons for a fish when a predator makes an attack. Both of these are provided by white muscle and in most fish, this white muscle accounts for a much higher proportion of the muscle mass than steady red muscle. This muscle bulk means that the fish can accelerate explosively from a slow or standing start -- an essential defense against approaching danger.
What's the best option for a fish that has been targeted by a predator?
Fish generally wait until the last moment to dart away.This is because the white muscle that powers this movement tires rapidly and if the fish sets off too soon it may merely alert the predator to its presence. By waiting until the threat is almost upon it, the fish can surprise its hunter and has a good chance of darting out of the predator's line of sight.
Darting out of a home territory into uncharted waters can be dangerous -- it could mean escaping from one predator only to blunder into another -- so the sudden dash is something of a last resort. Many prey fish curl their body into a kind of "J" shape as danger approaches, ready to spring away with a powerful flick of the tail. Not only does this serve as preparation for a dash while the prey waits for the predator to make the first move, it also signals to a stalking predator that it has been seen and that the game is up -- "come any closer and I'm going."
Netting a fish out of an aquarium can often try the patience, but by concentrating on one fish at a time, the aquarist can exploit the tendency of white muscle to fatigue quickly -- few domestic fish can maintain escape speeds for over a minute.