On the Benefits of Including Age-structure in Harvest Control Rules

This paper explores the benefits of including age-structure in the control rule (HCR) when decision makers regard their (age-structured) models as approximations. We find that introducing age structure into the HCR reduces both the volatility of the spawning biomass and the yield. Although at a fairly imprecise level the benefits are lower, there are still major advantages for actual assessment precision of the case study. Moreover, we find that when age-structure is included in the HCR the relative ranking of different policies in terms of variance in biomass and yield does not differ. These results are shown both theoretically and numerically by applying the model to the Southern Hake fishery.

Lake Kainji water hyacinth infestation and the control strategies

The paper documents the aquatic vascular plants situation of Lake Kainji, particularly with the invasion of water hyacinth (Eichhormia crassipes) hitherto alien to the lake system. The frequency of occurrence of some aquatic plants on Lake Kainji have increased whilst a few decreased over the period between 1984 and 1995. More than 53.6% of the lake surface has now been covered by aquatic vascular plants of which water hyacinth alone covered 30.6% (16.4% of the system). Apart from the characteristic rapid multiplication and biomass build-up of the plant, more than 9.1 clumps with a diameter range between 0.8-4cm enter the lake per minute. Thus more than 16.4ha per day or annually 6000ha of water hyacinth are added as drifts. Because of the peculiar hydrological cycle of the lake and the plant aggressiveness and invasiveness, NIFFR (with the support of the GTZ) had developed an approach of control which combines manual, biological, natural, boom construction and chemical measures. These are discussed