As you get more involved with fishing, and interested in fish, you will come to the realisation that fish come in many shapes and sizes - and their fins do too. But why do fish have different fin shapes? Well, let’s start by looking at their caudal fin (the fish’s tail) as this will tell us a lot about how the fish moves and a lot about how it lives its life. Depending on the species, different fish will use different fins for movement, however most fish use their tail to be able to move forward and the shape of their tail has been perfected over millions of years of evolution to suit that fish’s life history.
Let’s look at a few examples. Fishes with rounded tails, like some gropers (or groupers), Murray cod, some gudgeons etc. are generally pretty stationary for most of the time. They might be classed as ‘ambush predators’. Their tails are designed to accelerate them from a standing start with a lot of power for short, rapid bursts of speed to eat something swimming by, for example, before the prey even knows what is going on. This tail shape is perfect for this type of activity, but not so great for swimming long distances at a fast speed.
Now let’s look at pelagic fishes like tuna and mackerel. These fish are the long-distance speedsters of the fish world. Tuna can travel across oceans quickly in search of food and need to be fast to outrun predators also. Their long “lunate” (so named because it’s shaped like a crescent moon) tail helps them do this as efficiently as possible. This tail shaped would not be good for stopping and starting in, say, a river environment or a pond.
But what about fish that need to travel long distances and also need to stop and start, and manoeuvre also? Well, there’s a tail shape for that too! Let’s look at salmon and steelhead. These are great examples of fish that need to travel long distances in oceans to feed, but also need to navigate the tight areas of rivers and need short, powerful bursts of speed to get up waterfalls and rapids to their spawning grounds. They have ‘truncate’ (squared-off) tails– not long and skinny like a tuna, and not short and round like a cod or grouper, but a kind of a compromise between the two. No tail does every job perfectly, but salmon and steelhead have evolved to have a tail shape that fits their life needs as efficiently as it can.
Fishes with forked tails are usually constant swimmers. The deeper the fork, the more they swim. Compare the tails of a tarpon and a largemouth bass. Which one do you think is a more efficient long-distance swimmer?
