Starvation is a situation undergone and tolerated by many species of fish in their natural environment in response to several factors (e.g. temperature, spawning migration, reproduction, etc.). To survive these periods of unfavorable feeding conditions, fish reduce their energy expenditures, which in a high percentage are derived from protein synthesis, and mobilize their endogenous reserves to obtain the energy required to maintain the vital processes. This response entails physiological and metabolic changes that are species dependent. Moreover, intra-specific adjustments to these conditions will depend on different factors such as fish age, nutritional state, etc.
Like most animals when starved, fish will metabolize fat reserves before protein. When subjecting fillets of starved fish to proximate analysis, the percentage of protein will be elevated relative to other components, although the actual amount might be less than before starvation. Therefore, it is the percent that protein comprises the flesh that actually "grows."
One need to be very careful to interpret the relative amount of nutrients. If lipids are reduce according to protein, then the protein will increase relative to the lipids. This does not mean that protein increases itself. One could for instance try to manage net changes. If you have the initial weight from the animals with their proximate composition in absolute numbers, you can compare to those data after starvation. By analysing their reduction in weight in absolute number an related to the proximate composition.