Sardines, anchovies, haddock, smelt, Atlantic cod and other shoal-forming species are also eaten. M. bassanus takes huge quantities from the waters around Newfoundland for the energy requirements of their growing chicks. Given the economic importance of the prey taken, whether they are in competition with human fishing industry in the area is an important question which remains unassessed. (But given the small amount taken, NL's cod take is probably unaffected.) On the other hand, they profit off of discarded bycatch and processed parts and the reduction in competition if humans are taking predatory fish. M. bassanus inflicts significant wasteage on the northwest Atlantic Salmo salar fishery, but does not actually eat much of them. This makes it difficult to resolve this conflict with the fishing industry in the area, as the only option would be to exterminate the birds, which is ecologically unacceptable. Breeding The oldest birds are the first to return to the northern gannet's breeding colonies. Birds not of breeding age arrive a few weeks later. In general, birds first return to a colony (not uncommonly the one in which they were hatched) when they are two or three years old. Once an individual has successfully bred in a colony it will not normally change to another. Nesting starts in March or April. Immature birds stay on the edges of the colony. They may even make a nest but they do not breed until they are 4 or 5 years old. Some birds of this age occupy empty nests that they will aggressively defend if they have sat on them for two or three days. If an apparently empty nest has an owner, the immature bird will leave without a struggle when the owner arrives to take possession. The preferred nesting sites are on coastal hillsides or cliffs. If these are not available northern gannets will nest in groups on islands or flat surfaces. As they find it more difficult to take off from such locations they will often cross the area occupied by an adjacent nest causing an aggressive reaction from the sitting pair; this means that the stress levels are higher in this type of colony than in those on steeper surfaces. Notwithstanding this, nests are always built close together and otherwise ideal nesting sites will not be used if they are some distance from a colony. On average there are 2.3 nests per square metre