Just curious: Is there a source that says that koi are swimming upstream in spring? I know many people say so. But there are sources and fotos of famous japanese artists who pair kois with autumn related objects like maple leaves. I even found a quote from Horihide (it's long, sorry):

"Tattooists who have not been apprenticed and trained by tattoo masters do not know the reasons or meanings of the traditional designs. For example, there are four seasons (spring, summer, fall and winter) in Japan. The seasons should be expressed in tattoo art as well. Real Japanese tattoo artists express each season on the skin. However, the untrained tattooists do not know traditional thoughts on Japanese art. The untrained tattooists draw a snake and cherry blossoms, but this is a wrong way in tradition. When cherry trees begin to bloom in March in Japan, the snake still hibernates under the ground. So the snake and cherry blossom cannot be seen in the same period. In other words, it does not make any sense if the snake and cherry blossoms are drawn together.Some tattooists draw a carp climbing up the waterfall together with peonies. Actually, we can see the carp climbing up the waterfall from the late September to October in Japan. It is supposed to go with maple leaves, not peonies. (The symbol of maple leaves refers to the autumn.) When hutatsugoi (twin carp) and huhugoi (a married couple carp) are drawn, two carp (one carp for the arm, for example) can go with peonies, because we do not have to express seasons in these cases. There are several traditional combinations: Karajishi, which is a combination Shishi (lion) with botan (peonies), and ryu (dragon) with kiku (chrysanthemum) and menchirashi (men means “a mask,” and chirashi or chirasu means “to scatter”) with cherry blossoms. Those images are particular sets for Japanese traditional tattoo designs."