Historical roots of the migration triangle

The current theory of fish migration specifies that philopatry and population structure are inextricably linked, requiring a circuit of migration to and from localized spawning habitats. The circuit, popularized by Harden Jones as the migration triangle, has been a first principle in fisheries science since Hjort’s early work demonstrating circuits of migration by Norwegian cod. The roots of the migration triangle theory are unearthed by examining a central dialectic on population attributes in fisheries science: Hjort and Dannevig’s debate on the effectiveness o f the cod hatching program. Resolution of this early debate led to an assumption of closed populations (philopatry) and related theory development during the 20th century (e.g., surplus yield and stock recruitment). Unresolved issues include: 1) the interaction between sea basin populations and localformen (e.g., tjord cod, estuarine herring, resident salmon); 2) the consequence of straying on population persistence; and 3) allopatric migration circuits within populations. While philopatry has been verified in countless studies using various "certificates of origin", recent investigations indicate that polymorphism in migration circuits and reduced philopatry can be important in the regulation of fish populations.