Trophodynamic modeling of walleye pollock (Theragra chalcogramma) in the Doto area, northern Japan: model description and baseline simulations

Abstract

An age-structured trophodynamic model was constructed to quantitatively analyze factors affecting postsettlement mortality and growth of walleye pollock (Theragra chalcogramma) in the Doto area, the main nursery ground of the Japan Pacific population. The model included (i) multiple age classes of pollock, (ii) a generic predator, (iii) fisheries, and (iv) major prey of pollock. Major processes considered were (i) recruitment, (ii) bottom-up control of somatic growth, (iii) mortality because of predation, cannibalism and fishing, (iv) size-selective prey selection, (v) temperature-dependent bioenergetics such as conversion efficiency and daily consumption rate, and (vi) production and advective supply of prey. By assuming that pollock select prey based upon both relative abundance and predator-prey size relationships, the model accurately simulated seasonal and ontogenetic variations in the diet. However considering ontogenetic segregation, the model showed that, due to cannibalism, newly recruited fish would be totally consumed within 6 months after settlement. By considering segregation (10% overlap during spring and 0.1% during other seasons), an agreement of diet between the simulation and empirical data averaged 82.7% for the different seasons and fish sizes. Euphausiids, the most important prey of pollock, suffered the highest predation impact (22.2 ± 5.3 WWg m )2 yr )1 ) exceeding annual production in the model domain (17.2 ± 0.1 WWg m )2 yr )1 ), indicating that an advective supply of prey is necessary to support the pollock population. The daily ration of pollock during spring and summer averaged at 1.2 and 0.6% BW day )1 for small (£200 mm) and large (>200 mm) pollock, respectively; this daily ration was reduced by half during autumn and winter.

Interannual variation in diets of walleye pollock in the Doto area, in relation to climate variation

“…cannibalism), appendicularians and anchovy, and a decrease in the consumption of copepods and micronekton. Like other gadid species, pollock are opportunistic feeders, whose diet is primarily determined by the relative abundance of ambient prey and the predator:prey size relationship (Yamamura 2004). Thus, the interannual variation in pollock diets revealed in this study may be attributable to changes in the availability of different prey items in the Doto area.…”

“…Such variability may impact higher trophic levels via bottom-up controls. In particular, the population dynamics of pollock could be severely affected because of their prominent cannibalistic behavior (Yamamura 2004, Mueter et al 2006, making juveniles more vulnerable to predation during years of reduced prey supply. On the Eastern Bering Sea shelf, the abundance of large zooplankton (Cala nus marshallae and euphausiids) declined abruptly during a period of warm temperatures (2002)(2003)(2004)(2005).…”

Life-history traits of walleye pollock, Theragra chalcogramma, in the northeastern Japan Sea during early to mid 1990s

“…This fish plays an important role in many regional ecosystems, as it is generally zooplanktivorous and is preyed upon by bottom-dwelling fishes, marine mammals, and seabirds (Springer, 1992;Yamamura, 2004). The species also forms one of the most important fishery resources in the world (Anonymous, 2009).…”

An overview of the Oyashio ecosystem

“…In Oyashio waters, young pollock feed mainly on copepods and copepod eggs. In this region, adult pollock eat mysiids, euphausiids, silver smelt and capelin (Yamamura, 2005). The pollock catch in the Oyashio region has decreased gradually from about 300,000 t in the early 1970s to 200,000 t in the mid-1990s (Fig.…”