Most of the fish in the Bothnian Bay are terrified of divers. As soon as they either hear or feel the diver’s air bubbles, they swim away as fast as possible. To most fish, a diver represents either a potential predator or at least a completely new and unknown danger and is worth a quick escape.
Some fish, however, can sometimes be seen by divers or people wading in the water with a water binocular. These species are either too small to flee quickly (three-spined stickleback Gasterosteus aculeatus, ninespined stickleback Pungitius pungitius, which in Finnish is curiously named as a ten-spined stickleback, and fish larvae, which means fish babies and youngsters), ro they rely on their camouflage (European bullhead Cottus gobio, pike Esox lucius) or they are ill equipped for fast swimming (European eelpout Zoarces viviparus). Some species are really curious (perch Perca fluviatilis, Eurasian ruffe Gymnocephalus cernua) and they might even come and see, what a fellow diver looks like.
One time I was taking a grain size sample of the sand. I was in the middle of shoveling sand into a bucket when I realized that a school of Eurasian ruffes were trying to get into the bucket! There were maybe five or six of them and they were dashing in and out of the bucket. It took me a while to realize that they were trying to catch invertebrate animals that might have been hiding at the top layer of the sand which I was shoveling. I had to shoo them away to get back to my work.
The ruffe’s cousin perch is also a very curious fish. Quite often they come to check out the diver or a drop video camera and try to look directly into the “eyes” of the diver or the camera – either to see, what’s inside the mask or the camera lens or because they see their own reflection there.
Pike is a successful ambush predator which waits for its prey to come within a striking distance before making a dash for it. Quite often it can be found waiting in ambush between the reeds in the shallow water. A pike also relies on its camouflage to confuse both their prey and potential predators. You can quite often approach a pike if you just move slowly enough. Just be careful – when you’re close enough, or too close for the pike’s liking, the fish will very quickly make a run for it and you’re left with a nasty adrenaline rush.
Some of the smaller or benthic (bottom dwelling) fish are poor swimmers. Sticklebacks and European eelpout, as well as the tiny European bullhead, are not great swimmers. If encountered by a diver, the sticklebacks try to swim away, except when they are mating or guarding their nest. Then a courageous beast (The Daddy) will take over the small and normally so shy fish and they will come chasing even a diver off their property. The benthic fish, which usually have a flatter belly and larger pectoral fins compared to free swimming (pelagic) fish, are well equipped for resting on the bottom but not for swimming. The eelpout and the bullhead don’t want to waste extra energy by swimming clumsily away if they are not sure that the threat is for real.
The fish larvae obviously try to swim away as fast as possible because anything bigger than them is a threat to them but compared to a diver or even a wading human being in a survival suit, they are slow swimmers.
Some fish, such as shoals with small Cyprinidae, enjoy human company and like to eat dead skin and resuspended material that the marine team leaves behind when we snorkel. This is like one of those “fish massage” tourist attractions in Southeast Asia 🙂
If you want to see fish underwater, snorkeling very quietly along the reed belt or in an underwater meadow will do the trick. You just have to move very quietly and not splash around and scare the fish away.