Why We Need to Understand Population Structuring
Catch control measures designed to manage the abundance of fish stocks are, for practical reasons, often applied at geographic scales which can encompass the annual ranges of more than one fish population. The migration routes of some fish populations may straddle different management units. This means the number of reproductively distinct populations in a management unit may vary seasonally.
Many declines in fish stock abundance have been associated with a contraction in spatial distribution and loss of distinct reproductive populations. Failure to recognise population structuring has been linked with inflated estimates of particular year-classes and the over-estimation of total stock abundance. The presence of these populations with differeing growth and reproductive responses to environmental variability is now believed to be important in sustaining the productivity of fish stocks.
Maintaining Population Diversity
Maintaining this population diversity is not recognised as an objective of current fisheries management, although it forms part of the Rio Convention on Biodiversity, and has been identified in reviews of the precautionary approach to fisheries management.
Understanding the importance of population structuring within fish stocks, particularly marine stocks, is now a major research effort of fishery institutions throughout the world. Such work has only recently become possible through the development of improved genetic markers combined with advanced field tagging and modelling techniques for studying early fish life-history and movements.
Marine Scotland Science is focusing on a number of species of commercial importance to Scotland. These include herring (Clupea harengus) (see links to collaborative project websites HERGEN and WESTHER), monkfish (Lophius piscatorius), sandeel (Ammodytes marinus) and several of the gadoid species, such as cod (Gadus morhua) (see links to collaborative project websites METACOD and CODYSSEY) haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus).
These studies aim to ensure that future approaches for conserving or restoring population diversity can be developed. This is of great importance to future stock recovery plans.