Why do assessments of demersal stocks largely ignore habitat?

The divergence between most stock assessments and studies in marine ecology is characterized by the low priority usually given to assessing the holding capacity of marine habitats. Habitats of high structural complexity are relatively uncommon in shelf waters, are contagiously distributed, and are damaged incidentally by bottom-towed gear. Structurally complex habitats are used by many demersal fish and crustaceans for predation abatement and as a site for feeding forays. Successive life-history stages typically migrate through several structurally complex habitats which recent studies show often to have fractal properties. One consequence of fractal structures as cover is a rapid reduction of protection from predators with growth in size: migration is the only response possible when further growth of the recruiting age class renders individuals in that habitat vulnerable to predation. A common feature of structurally complex habitats with high vulnerability at size is the occurrence of population bottlenecks. It is suggested that identifying and rectifying shortages of structured habitat, and eliminating habitat bottlenecks, will be effective in stock enhancement. This will require placing strict spatial constraints on the operation of bottom gear. This paper reviews new methods of estimating juvenile predation mortality, including mortality-at-age and mortality-at-life-history stage, which depend on the fractal characteristics of structurally complex habitats.