Last Night's Viewing: Hugh's Fish Fight: Save Our Seas, Channel 4 Bank of Dave: Fighting the Fat Cats, Channel 4

I hardly dare watch Hugh's Fish Fight: Save Our Seas anymore, and I certainly wouldn't do it while eating a fish supper. You just never know when he's going to knock something else off the ethical consumer's shopping list.

This week it was king prawns, a staple of the supermarket freezer cabinets, but also – if Hugh is to be believed – another nail in the coffin of the seas. And I do believe him, because he went all the way to Thailand to show how industrial farming of prawns is contributing to the overfishing of its seas.

That's terrible, I thought, as he showed us the contents of a recently hauled net, full of immature fish. That must smell terrible, I thought, as he visited the fish-meal plant where such "trash fish" is processed into feed. But Hugh wasn't satisfied with that. He wants me to fear that I might be terrible too: "If you eat prawns and they're not organically farmed then you're part of this problem," he told us sternly.

Fortunately, being part of the solution isn't just a matter of self-denial. Because Hugh's Fish Fight – perhaps emboldened by its success in pushing fish discards up the political agenda – is very big on the idea of armchair people power. "Let's get Fish Fight trending during this commercial break," said Hugh, after issuing the Twitter handles for the supermarkets that had stonewalled him when he asked to talk to them about their buying policies.

The preview I watched suggested they were going to do an immediate live update on just how big a Twittermob they'd galvanised, perhaps to shame into action viewers too torpid even to put their thumbs into action. We may need a hideous neologism to describe this new hybrid of documentary and lobby-group mobilisation – activainment perhaps, or televactivism. But that it can work seems undeniable. I'll be checking the label next time I pick up a pack of prawns.

Hugh's Fish Fight ended on a downbeat, with disappointing news about Marine Protected Areas. But Bank of Dave: Fighting the Fat Cats offered the perfect antidote, giving us a people's hero and an unexpected last-minute triumph. This follow-up to Channel 4's earlier series about a Burnley man's attempt to set up a simple local bank started with a knockback. The Financial Standards Authority, as useful as a chocolate fireguard when it came to the ethical hooliganism of the big banks, had ordered Dave to stop taking deposits because he was an "unregulated savings scheme". Without deposits he had nothing to lend and it looked as if the experiment was over.

A tiny bit more information about the default levels on the loans Dave has already made wouldn't have gone amiss. Or more of a sense of how he judges the reliability of new applicants. "We'll get paid... we'll definitely get paid because she's passionate about her business," Dave said confidently about one client, sidestepping the fact that there are a lot of passionate bankrupts out there. But his basic idea was so good, and his manner so infectiously can-do that you simply wanted to cheer when the FSA eventually abandoned its pettifogging and let him take savings again. It was like a Frank Capra movie with Burnley accents. And it was true.

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