Sustainability, to me means keeping the original source going. This definition does some what line up with what Solow says about making appropriate policies regarding fish resources. I some what agree that substitutions need to made, regarding sustainability, but it also depends what those substitutes are and if they negatively impact human health or the health of the surrounding ecosystem. If we were to look at it in a general way than I suppose that aquaculture is sustaining wild fish, but at what cost to the environment and health.
You have a good point on questioning Solow’s opinion on sustainability and substituting resources. I agree with you that we need to watch when substituting resources for it can negatively impact us humans and the ecosystem.