Population structure of the Chilean jack mackerel ( Trachurus murphyi ) in the South Pacific Ocean

The Chilean Jack Mackerel (Trachurus murphyi Nichols), is one of the most important among the exploited stocks 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 strong variation in its biomass, and the world catches varied from less than 1 million tons to 5 million tons. Its management requires a correct understanding of the population structure. Although several hypotheses have been submitted, suggesting up to 4 independent populations, the genetic analysis of the fish shows a single population in the whole Pacific Ocean. We consider the two most probable structures for this population: a “superpopulation” 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 a long period, depending on the environmental conditions. The conditions for these hypotheses to represent the real case are studied. It is difficult to adapt the definitions of metapopulations to this large pelagic stock, and we suggest some adaptation of the concept to pelagic gregarious species. Using the knowledge obtained from a considerable amount of data from fisheries and acoustic survey on spatial distribution, demographic structure and dynamics, spawning areas, etc., we analyze the two hypotheses. None can be fully rejected. We present a model for metapopulation structure and conclude that, although no final choice can be done between the two hypotheses, the metapopulation is likely to better describe the stock; moreover it represents a safer definition as far as stock assessment is concerned.