In commercial fishing, "by-catch" has come to mean any sealife that is unintentionally caught. Depending on the fishing or netting techniques used, this can include a wide variety of sealife - from non-commercial species to prohibited or endangered species to commercial species that do not have a lucrative enough sales value (known as "highgrading").
By-catch is typically discarded and that discarded percentage of the boat's total catch can range from 4% to a staggering 80%, depending on the prevailing regulations for a particular country's territorial waters. As an example, Norway has regulations in place that favor the lower percentages and generally induces fisherman to better utilize their catch commercially or use more effective netting techniques to reduce the overall level of by-catch.
However, the European Union (EU) has less restrictive regulations, allowing fisherman to dump enormous percentages of their catch - as much as 80% has been recorded (see video) - with very little chance of survival for the discarded animals. In fact, due to co-operative fishing rights, UE fisherman can fish in Norwegian waters, then sail back into UE waters and dump their by-catch!
Organizations like Oceana.org are pressuring EU governments to adopt regulations like Norway's but are getting resistance from the EU commercial fishing industry. The fact that Norway's regulations are effective and help support the conservation of commercially viable species, thereby protecting the longevity of the Norwegian fishing industry, seems to escape the EU's abilities of comprehension. According to Ricardo Aguilar, Oceana Europe's Research Director, "Wasting fishing resources is a global problem. However, the magnitude it reaches in European waters is unacceptable, especially when 88% of EU stocks are overfished."
Read Oceana.org press release.