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Resource Ecology & Ecosystem Modeling Program

Food Web Model Results to Inform Stock Assessment Estimates of Mortality

figure 4, see caption
  Longnose skate Pacific halibut Walleye pollock Squids   Longnose skate Pacific halibut Walleye pollock Squids  

Figure 4.  Mortality of species groups as partitioned by the GOA food web model, with case study groups arranged from highest (left) to lowest (right) trophic level.  Left: fishing, predation, and unexplained mortality as a proportion of total mortality for each species (mortality sums to 1).  Right: fishing, predation, and unexplained mortality relative to the total annual production rate of each case study species group (mortality sums to annual P/B ratio; P/B = Accumulation + Unexplained mortality + Predation mortality + Fishing mortality).  Note that for pollock, the Accumulation (BA) term is negative in this sum.

In a recently accepted publication in the Canadian Journal of Aquatic and Fisheries Sciences, Drs. Sarah Gaichas and Kerim Aydin (REEM), and Robert Francis (University of Washington) examined food web relationships for commercially important species to identify sources of variability in mortality and production which are not included in standard single-species stock assessments.

They used a static mass balance model to evaluate relationships between species in a large marine ecosystem, the coastal Gulf of Alaska (GOA). Four case study species groups were included to represent broader food web relationships: Pacific halibut, longnose skate, walleye pollock, and squids. For each, they presented the species' position within the food web, evaluated fishing mortality relative to predation mortality, and evaluated diet compositions.

High trophic level species, whether commercially valuable (Pacific halibut) or incidentally caught (skates), were found to have mortality patterns consistent with single-species assessment assumptions, where fishing mortality dominates natural mortality. However, assessments for commercially valuable (pollock) or incidentally caught (squids) mid-trophic level species could be enhanced by including food web-derived predation information, because fishing mortality is small compared with high and variable predation mortality (Fig. 4 above). Finally, food web relationships were outlined that suggest how production of species may change with diet composition or prey availability.

By Sarah Gaichas

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