Does life history connectivity explain distributions of Chilean jack mackerel Trachurus murphyi caught in international waters prior to decline of the southeastern Pacific fishery?

Abstract

In severely depleted stocks, biological data taken prior to fishery decline are invaluable as a reference for recovery; and to understand the spatial population structure, especially when connectivity crosses management boundaries over the life history. Prior to a precipitous decline from 2010, Chilean jack mackerel (Trachurus murphyi) showed strong cyclic migrations hypothesized to encompass coastal areas regulated by national authorities along western South America and unregulated international waters offshore. In a preliminary analysis, we present data taken in international waters between 2000 and 2003 by a program sampling commercial catches from Chinese trawlers. Overall, catch length distributions occupied fork length classes from 240mm to 600mm, with a modal length class of 320mm for females and 340mm for males and the weight-length relationship was BW=0.000 006 52FL3.0708. Males showed increasing predominance with length. Nearly 80% of fish had prey items in their stomachs; only 1% had full stomachs, suggesting that few fish were feeding intensively. However, mean fork length declined from 327mm to 259mm between 2000 and 2003, and population juvenescence as a result of exploitation may help explain these data. Nevertheless, adult spatial distributions were also consistent with migrations between feeding and spawning grounds. Distinguishing the spatial structure of such life history connectivity, and how it interacts with fishing activity, is critical to refining the conservation measures necessary to rebuild large oceanic fisheries and restore marine ecosystems.