Jack Galsworthy, from Cornwall, and Freddie Wright, from Gloucestershire, both 21, are one week into their 3,000 mile challenge and their "morale is high" after initial seasickness.
The pair are aiming to raise Â£150,000 for the Brain Research Trust.
They caught and ate tuna which they had as sushi for Christmas dinner, their team said.
They will be rowing two hours on and two hours off for three months in the race from La Gomera to Antigua, as part of the Talisker Whiskey Atlantic Challenge.
Two days from the nearest support team, they risk facing 40ft waves, sharks, howling winds, blisters, salt-rashes, sunstroke and sleep deprivation.
The Atlantic Castaways team spokeswoman Alexandra Durnford, said they were suffering badly from sea sickness at first but have now recovered and "morale is high".
"They tried their luck at a bit of fishing and managed to catch a very small tuna and had sushi I would imagine as there are no cooking supplies on board, so they did have a very special Christmas lunch, just an unusual one," she said.
The duo are expected to lose at least 15% of their body weight during the challenge, which they hope to complete in 90 days time.

It started happening when Apple's operating system, iOS 8.3 came out.
By tapping and holding on a chosen emoji, iPhone users of iOS 8.3 or above are now given the choice between five different skin tones and new flags.
Apple users who are still using iOS 8.2 and below are not able to see the new emojis.
Instead they are replaced with a picture of an alien face inside a black square, next to the original emoji.
Apple wouldn't give us a comment on this, but on their website it says: "If you get a message with unexpected characters instead of emoji, try updating your iPhone, iPad, iPod touch or Mac."
If you're viewing the new iOS emojis on devices other than Apple they can appear as lots of other things, like blank spaces, boxes, equal signs, question marks, or even line breaks.
Two university students have begun their bid to become the youngest pair ever to row across the Atlantic Ocean.
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Have you noticed any of these little guys creeping into your texts and social media, when you know they weren't intended to be there.