One stock or more?

Could a combined view of jack mackerel hide differences between parts of the population?

The answer affects how catches and regional observations are interpreted, and whether a management rule performs acceptably for all the population components represented in the analysis.

What the research has examined

The benchmark and management evaluation consider alternatives that represent jack mackerel as one stock or as two stocks. They examine how those assumptions affect the analysis. These alternatives test the consequences of different biological explanations; assessing their plausibility requires empirical evidence. Sources: JM02, JM03, JM06, JM07.

Why it matters for management

A favourable result for the population as a whole may conceal a less favourable result in one component. Where an evaluation represents more than one component, its results should be examined for each as well as in total. The questions raised for SC14 include this concern. Source: JM06.

What has been agreed?

The current record describes the stock-structure alternatives under consideration. A Committee determination and its implications for management would need to be documented in the agreed meeting report.

What remains uncertain?

Bringing together the earlier genetic, tagging, distribution and modelling studies is the next historical research priority. Managers should be able to see what evidence supports each explanation, and how much the advice changes between them.