It also reignites the row over the UK’s ban on the fishing of sand eels – designed to protect Britain’s population of puffins which eat the small fish, but hated by some EU countries that catch them in huge quantities – by insisting that Britain takes into account the “socio-economic consequences” on European fishing communities of its policies on managing stocks.

investigates

Apparently it’s Denmark.

https://ukandeu.ac.uk/eel-or-no-eel-an-early-test-for-the-uk-eu-reset/

The fishing of sand eels in English North Sea and Scottish waters was recently banned in order to protect depleted stocks, which are a vital food source for local marine wildlife including puffins and porpoises. The ban has wide support from normally pro-EU environmental groups in the UK, even though the EU questions whether a blanket ban is ‘evidence-based, proportionate and non-discriminatory’. The Danish government argues that it amounts to de facto discrimination against Danish vessels, who take 99% of the sand eel catch in that area.

Hmm.

https://en.wikipedia.org/wiki/Sand_eel

Traditionally, they have been little exploited for human food, but are a major target of industrial fishing for animal feed and fertilizer.[4] Increasing fishing for them is thought to be causing problems for some of their natural predators, especially the auks, which take them in deeper water.

An instance of this was the RSPB report linking a population crash of seabirds in the North Sea to fishing for sand eels.[5][6] This led to political pressure for the closure of this fishery; the seabird populations subsequently improved.[7]

I’m not sure that there’s a viable way to protect puffin food in a particular location in a way that’s going to affect countries by different proportions, unless one is going to just ship in food for the puffins from elsewhere.