The Chilean jack mackerel (Trachurus murphyi) is one of the most
important exploited fish populations in the world. Its area of distribution covers the
sub-tropical waters of the South Pacific Ocean, from South America up to New Zealand and
even Australia. It is subject to high variations in biomass, with annual world catches
varying from less than 0.5 to almost 5 million tonnes. Management of this species and its
fishery requires a correct understanding of the population structure. Although several
hypotheses have been proposed, suggesting that there are up to four independent
populations, genetic analysis indicates only a single population in the South Pacific
Ocean. We consider the two most probable structures for this population: a “patchy
population”, where all the fish belong to a single population that expands and contracts
depending on the biomass; and a metapopulation, where a source population creates several
subpopulations that can remain independent during prolonged periods, depending on the
environmental conditions. We studied the conditions for these hypotheses to represent
reality, especially the demographic structure and existence of different spawning areas.
Using knowledge obtained from fisheries and acoustic surveys, we show that if we apply
their definitions strictly, neither of the two hypotheses can be fully rejected.
Nevertheless, some particular spatial characteristics observed recently between the
Peruvian and Centre-South Chilean parts of the distribution lead us to conclude that the
metapopulation definition is likely to better describe the stock. The consequences of each
population structure for fisheries management are listed, showing that the metapopulation
hypothesis represents a safer definition for stock assessment. We conclude that T.
murphyi can be described as a metapopulation, but also that the definitions
proposed in the literature on metapopulations are not fully adapted to pelagic fish
populations with a basin-scale distribution range, and suggests specific research that
could be done in this field.