The size of an individual organism is a key trait to characterize its physiology and feeding ecology. Size-based scaling laws may have a limited size range of validity or undergo a transition from one scaling exponent to another at some characteristic size. We collate and review data on size-based scaling laws for resource acquisition, mobility, sensory range, and progeny size for all pelagic marine life, from bacteria to whales. Further, we review and develop simple theoretical arguments for observed scaling laws and the characteristic sizes of a change or breakdown of power laws. We divide life in the ocean into seven major realms based on trophic strategy, physiology, and life history strategy. Such a categorization represents a move away from a taxonomically oriented description toward a trait-based description of life in the oceans. Finally, we discuss life forms that transgress the simple size-based rules and identify unanswered questions.

10 Size spectrum models have emerged from 40 years of basic research on how body size 11 determines individual physiology and structures marine communities. They are based 12 on commonly accepted assumptions and have a low parameter set, which make them 13 easy to deploy for strategic ecosystem oriented impact assessment of fisheries. We 14 describe the fundamental concepts in size-based models about food encounter and the 15 bioenergetics budget of individuals. Within the general framework three model types 16 have emerged that differs in their degree of complexity: the food-web, the trait-based 17 and the community model. We demonstrate the differences between the models 18 through examples of their response to fishing and their dynamic behavior. We review 19 implementations of size spectrum models and describe important variations concerning 20 the functional response, whether growth is food-dependent or fixed, and the density-21 dependence imposed on the system. Finally we discuss challenges and promising 22 directions. 23 24

Balanced harvesting, where species or individuals are exploited in accordance with their productivity, has been proposed as a way to minimize the effects of fishing on marine fish communities and ecosystems. This calls for a thorough examination of the consequences balanced harvesting has on fish community structure and yield. We use a size- and trait-based model that resolves individual interactions through competition and predation to compare balanced harvesting with traditional selective harvesting, which protects juvenile fish from fishing. Four different exploitation patterns, generated by combining selective or unselective harvesting with balanced or unbalanced fishing, are compared. We find that unselective balanced fishing, where individuals are exploited in proportion to their productivity, produces a slightly larger total maximum sustainable yield than the other exploitation patterns and, for a given yield, the least change in the relative biomass composition of the fish community. Because fishing reduces competition, predation and cannibalism within the community, the total maximum sustainable yield is achieved at high exploitation rates. The yield from unselective balanced fishing is dominated by small individuals, whereas selective fishing produces a much higher proportion of large individuals in the yield. Although unselective balanced fishing is predicted to produce the highest total maximum sustainable yield and the lowest impact on trophic structure, it is effectively a fishery predominantly targeting small forage fish.

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