Disentangling evolutionary and plastic responses to over-fishing of Malawi Cichlid fish in a common garden experiment

Over-fishing is a consequential anthropogenic driver of demographic and phenotypic changes in aquatic fish populations. In Lake Malombe, Malawi, over the past 40 years, intense fishing pressure has led to local extinctions, dramatic population declines, and rapid life history shifts in several commercially important benthic cichlid species. In some of these species, size at maturation has decreased by approximately 50% within only 20 generations, suggesting a strong effect of either fisheries-induced evolution or phenotypic plasticity in response to human induced harvest pressure. To disentangle the genetic and plastic components of this life history shift, we conducted a common garden experiment using offspring from populations experiencing contrasting fishing pressures. Juveniles from heavily fished and weakly fished populations were reared under standardised laboratory conditions to control for environmental effects and isolate genetic differences. We observed an accelerated growth, earlier maturation, but reduced reproductive investment in offspring from heavily fished populations, consistent with a rapid evolutionary responses to fisheries pressure. Importantly, because of the non-size selective fisheries in Lake Malombe these shifts in life history traits are driven by biotic interactions such as competition for mates and reproductive opportunities, with smaller, faster-maturing individuals potentially gaining an advantage in heavily fished populations. This work and evolutionary genomic investigations we have carried out highlight how human-induced pressures reshape the genetic determination of life history strategies, with cascading consequences for intra- and interspecific interactions, population stability, and the long-term resilience of biodiversity hotspots like Lake Malawi.