Contents:1. Nothing as practical as a good theory -- Part I. Individuals. 2. Size spectrum theory ; 3. Individual growth and reproduction -- Part II. Populations. 4. Demography ; 5. Fishing ; 6. Fisheries-induced evolution ; 7. Population dynamics -- Part III. Traits. 8. Teleosts versus elasmobranchs ; 9. Trait-based approach to fish ecology -- Part IV. Communities. 10. Consumer-resource dynamics and emergent density dependence ; 11. Trait structure of the fish community ; 12. Community effects of fishing -- Part V. Epilogue. 13. The size- and trait-based approach -- Part VI. Appendixes. A. Single stock size spectrum model ; B. Consumer-resource model ; C. Community model.