Assessment of stock, spatial distribution, and recruitment of walleye pollock in the northern and eastern Bering Sea

Abstract

Abundance and biomass of walleye pollock Theragra chalcogramma in the Bering Sea exceeded the mean values in 2015 owing to the high-abundant year-classes of 2008 and 2012 and several medium-abundant year-classes (of 2006, 2009-2011, 2013, and 2014). In contrast to rather stable distribution on its spawning grounds, distribution of feeding pollock in the northern Bering Sea is very variable and depends on both biomass of the population and long-term and short-term variations of environments, as water temperature and zooplankton abundance. In some years including recent times (2005-2007 and 2010-2014), the pollock migrated earlier and faster into the northwestern Bering Sea and returned back to the eastern shelf also early: in late summer - early autumn. This scenario is supposedly conditioned by low zooplankton abundance over the Bering Sea that forces pollock to active feeding migrations, particularly a deficit of preferable food (large-sized zooplankton) on the north-western feeding grounds could be a reason for its leaving this area in late summer. Strength of pollock year-classes is highly variable, too, and depends mostly on young-of-the-year fish survival in winter. As usual, favorable conditions for reproduction and progeny survival, when strong year-classes appear, are observed in the years or short periods of transition from one state of marine environments to another, whereas long periods of either «warm» or «cold» regime are not favorable for forming of high-abundant year-classes of the Bering Sea pollock.