Size-selective harvesting effects in laboratory fish populations

Recent theoretical advances on size-structured interactions predict several so far unanticipated emergent phenomena at both the level of single populations as well as whole communities when size-selective mortality increases, e.g. due to harvesting. Among those phenomena is size-specific overcompensation in biomass at the population level or Allee- and facilitation effects at the size-structured community level. These phenomena result from mortality-related changes in the individual life history via food-dependent growth and fecundity.
To test for these effects, especially overcompensation, we performed a harvesting experiment with negative and positive size-selective mortality at different rates on laboratory populations of a small poecilliid fish (Heterandria formosa).
Intermediate positive size-selective mortality rates led to biomass overcompensation not only in both ends of the size-spectrum (basics for emergent Allee-Effects and facilitation) but also in total biomass. We attribute these findings to a combination of higher food-dependent individual growth and harvesting-induced bias in adult sex-ratio towards females in this highly size-sex-dimorphic species.
Both negative and positive size-selective mortality stabilised the baseline cyclic population dynamics, which were seemingly also partially due to temporal changes in adult sex-ratio.