Sardines are pretty much top of the list when it comes to oily fish and are packed full of omega-3 goodness and our are flash frozen quickly after they have been landed so all of that goodness is locked in.
They are also gutted and headed so all you need to do is defrost and get cooking. i don't think you can beat covering them with salt and oil and then grilling on the BBQ. Gremolata goes very well with grilled sardines and takes seconds to make.
The Cornish sardine fishery is healthy and these sardines are part of the MSC certified Cornish sardine fishery. more information on this can be found here.
Most Cornish sardines are caught using ring nets. This involves targeting specific shoals of fish and surrounding them with a curtain like net. this has minimal environmental impact and is a very targeted method (i.e. low incidental by-catch).