The Buddhist Fish Symbol
In Japan, the fish means well-being, happiness and freedom. It is one of the Eight Auspicious Symbols used in Buddhism imported from China. The fish symbolises living in a state of fearlessness, without danger of drowning in the ocean of sufferings, and migrating from place to place freely and spontaneously.
Gargoyle-like carp, known as shachihoko, often appear on opposite ends of the ridge of a castle roof to symbolise diligence and ecstasy. There is also a fertility association with this pair since one fish is male and the other female. Another interpretation is that they are symbolic of the elongated eyebrows of the Buddha. As tiger-headed sea monsters, they also symbolise water and its defensive capabilities against fire.
The fish effigy often seen in Japanese temples is the mokugyo ('wooden fish'), fashioned in a roundish shape from a solid block of wood. It is carved with fish-scales and often with a lion/dragon head. Yet despite its peculiar appearance, its deep polish gives a warm consciousness. The mokugyo is hollowed out, so that when the priest strikes it with a leather-padded drumstick, the sound has a strange hypnotic effect on the hearer. This drum is often used to accompany a kyouten (sutra-reading).
Coincidentally, British 'sport' fishermen kill their catch by clubbing the salmon or trout with a drumstick called a 'priest stick'. But in the Buddhist temple, the priest is not ceremonially killing the mokugyo; just using it to generate sound.
The significance of this fish is its eyes. Because fish live in darker environments, their eyes tend to be relatively large. Also, because their eyes are always surrounded by water, they don't need to moisten them as we do when we blink our eyes, so most fish have no eyelids. These large, constantly open eyes of the mokugyo remind us that God is always watching what we do.
What do you call a fish with no eyes?
Other Japanese fish symbols
In early May, tubular koinobori ('climbing carp') streamers made of cloth, paper or plastic, are flown all over Japan to celebrate Boys' Day. They represent the fish's struggle to swim upstream and are flown high to show they succeeded. Japanese pray on that day that their sons will be similarly successful, healthy and strong.
Then of course, there's the Japanese namazu (catfish). These creatures are believed to be especially sensitive to an earthquake's precursors and behave in a peculiar way, warning people to take pre-emptive action. There is scant scientific basis for this.
There's also a legend that a giant catfish called Namazu lives in mud beneath the Japanese islands. This catfish likes to thrash about; something that could cause untold calamity for the people living above, since this catfish really is huge.
Fortunately, however, Namazu is kept under control by the demigod Kashima. He keeps a huge magical rock in position over the catfish, and as long as Kashima maintains this position, people above ground are safe. However, if Kashima relaxes, then people suffer an earthquake.
When East meets West
The Christian fish symbol is used in Japan but not as frequently as in the West. The Christian fish is associated with evangelism, and Christianity has always struggled to find a foothold in Japan. (See potted history of Christianity in Japan.) There are relatively few Christians in Japan anyway, which makes even the cross an uncommon symbol (with the exception of jewellery).
Japan has a justifiable history of fear from Western colonialists using religion to overtake their culture. Evangelical Christians in Japan are often regarded as cultists and rejected. Conversely, where Christians make a modest approach, their doctrine is at least tolerated if not welcomed. Part of this softly-softly approach is reflected in Christian symbolism. The cross is recognised in Japan as Christian, and the fish remains Buddhist. (And in sushi restaurants of course.)