My method which took me from around the mid 60's to 100 (but it works for all levels) was just following the White river from Riverwood all the way to around Yngol's Barrow catching all of the little buggers jumping up the rapids (which can be difficult/frustrating) and Fus Fishing all of the ones that are just swimming around (which also yields Salmon Roe). The best part about this is while you're taking your midday swim down the river, you can acquire enough of the other reagents in the crazy overpriced waterbreathing potion such as Histcarp and Nordic Barnacles. By the time I was in the 80's with all of the Alchemist perks and up to Benefactor, my Potions had a BV of well over 4000 even without alchemy gear!
Almost every character I play, I like to max out my crafting skills (alchemy, enchanting, and smithing) as fast as I can so I can have some good gear to play on Master and now Legendary (sue me for loving my OP Iron armor and weapons). This is the first time alchemy has ever gotten to 100 before the other two!
tl;dr salmon roe is the Kajiit's pajamas due to it's awesome waterbreathing potion multiplier. It's basically giant's toe but better and easy to get in large quantities.