ABSTRACT
In the ocean, population requirements often imply the use of different habitats, either due to variable environmental conditions (e.g., temperature, dissolved oxygen), or a change in the necessities of the population itself (e.g., foraging habitat vs. spawning habitat) (Binder et al. 2011). In such cases, individuals benefi t from moving to an alternative habitat and as a result, many fi sh species develop a life history that involves migration (e.g., Block et al. 2001; Sims et al. 2003; Walli et al. 2009; Chiang et al. 2014). The term migration, although frequently used in biology, is diffi cult to defi ne satisfactorily; this is because in different fi elds, or more specifi cally, when studying different species, it can have different meanings (Baker 1978; Aidley 1981; Smith 1985). Therefore, some defi nitions fail to include movements that would otherwise be classifi ed as migrations in some specifi c cases (Smith 1985). For example, long-distance, directed movements such as wide-scale foraging trips are often not considered migratory movements. However, as long-term movements can have considerable biological importance for oceanodromous species (and the whole sequence is usually necessary for successful completion of the life cycle), such cases should be classifi ed as migrations (Smith 1985). Using a broader defi nition, Baker
1 CIBIO/InBIO - Universidade do Porto, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrário de Vairão, Rua Padre Armando Quintas, 4485-668 Vairão, Portugal.
