Kaisen don is the rare donburi where the topping is the light part. Ninety grams of mixed sashimi is 135 calories and 25g of protein. Compare that with the same bowl topped with beef for gyudon (633) or a fried cutlet for katsudon (866). The seafood is doing nothing wrong. The rice is the same 250g of sweetened, vinegared rice that goes under everything else, and that is 400 calories.
The mix matters a little. Salmon is the fattiest common fish at 200 calories per 100g, tuna akami is 125, squid is 85, and prawn is 90. A bowl that is all salmon and toro is 100 calories more than one weighted towards tuna, squid and whitefish. Ikura is 270 per 100g but you only get 10 to 15g, so 30 to 40 calories. Uni is similar. Neither is worth worrying about.
Where it goes wrong is the market 'deluxe' bowl. Tourist market kaisen don at 3,000 yen come piled with 200g of fish, a crab leg and double ikura, and the extra fish alone is another 250 calories. Still not terrible at 850 for that much protein, but it is not the 580 you had in mind. The miso soup on the side is 40 and worth having.