It’s possible it could keep populations at a lower average size without being unsustainable. With standard simple fishery models (Gordon-Schaefer models, no predators besides humans), there are actually equilibria at every population size below the “natural” one, corresponding to each fixed harvest rate (share of population caught per period). Any (relative) harvest rate per period less than 100% can be sustainable long term if fixed, but not every absolute catch number below the natural population size can be sustainable long term if fixed.
It’s possible it could keep populations at a lower average size without being unsustainable. With standard simple fishery models (Gordon-Schaefer models, no predators besides humans), there are actually equilibria at every population size below the “natural” one, corresponding to each fixed harvest rate (share of population caught per period). Any (relative) harvest rate per period less than 100% can be sustainable long term if fixed, but not every absolute catch number below the natural population size can be sustainable long term if fixed.
The maximum sustainable yield for fisheries occurs with the population being at or under half of the natural population. I think well-managed fisheries (with quotas) are in fact at around half their natural populations, setting natural fluctuations aside. https://ourworldindata.org/fish-and-overfishing#what-does-sustainable-fishing-mean