This is due to legislation restricting the numbers of fish, such as cod, that can be caught. Unfortunately often fishermen through no fault of their own, end up catching certain types of fish even if they are targeting alternative species.
If the quota has been reached for a certain species then they have no choice but to throw these fish back overboard.
I recommend having a look at http://www.fishfight.net/ to join the campaign to change this ridiculous legislation of wasting thousands of tonnes of fish each year, which is perfectly edible.
Also take a look at http://www.channel4.com/4food/the-big-fish-fight to read information about sustainable fish species, recipes and to watch the Big Fish Fight TV Program.
