What sashimi is in season?

Visiting Tokyo end of July - beginning of August, just love sushi / sashimi but want to sample only the fishes in season, what might those be ? Would uni be out of season ? Love uni but heard there is uni now from Korea, China and Philippines but if we want only Japanese uni and preferably from Hokkaido, what restaurant can we go to to make sure we get that ?

Uni is supplied whole year but Hokkaido Shakotan uni is only supplied between Jun and Aug, three months a year only. I think luxury sushi and sashimi restaurants serve shakotan uni. They are branded uni, therefore very expensive.
The other seasonal fish include, aji, kanpachi, awabi(shell), aori-ika(branded squid, luxury fish), karei, hoya and iwa-gaki(or iwa-kaki, rock oyster).
"Ayu" is not served as sushi or sashimi, it served grilled with salt but is a typical summer fish and not very expensive.

That's great, thanks.

If you like uni, try uni bowl(uni-don, uni on top, rice under neath).
http://image1-3.tabelog.k-img.com/restaurant/images/Rvw/34982/320x320_...
I believe majority of sushi restaurants in Tsukiji serve uni bowl around 3,300 - 4,500 Yen. The restaurant normally advertise on notice board which part of Japan the uni is from. I think this time of season uni is mostly from Hokkaido. Ask the staff "Hokkaido uni ?", they will understand which uni you are looking for.
Conveyor belt and cheap sushi restaurants serve uni from overseas.

Awesome, thanks.