Simple SummaryBycatch, a direct negative effect of the fishing industry, is one of the top research priorities in the field of seabird ecology and conservation, being the major cause of seabird mortality. The Mediterranean Shag (Gulosus aristotelis desmarestii) is a seabird endemic to the Mediterranean and Black Seas and is considered threatened. The most important site for reproduction for the Mediterranean shag is an islet near Corsica and Sardinia. To evaluate the impact of bycatch in the sex ratio population dynamic, we sampled, extracted DNA and sexed Mediterranean Shags found drowned in nets and newborn chicks in the colony of Corcelli islet, northeast Sardinia. A skewed sex ratio in the effective population was caused by the higher rate of male Mediterranean shags bycaught in nets. At the same time, in the Corcelli colony’s offspring, a skewed sex ratio toward the male sex was recorded. According to the Sex Allocation Theory, the skewed sex ratio of the offspring may represent an adaptive arrangement of the sex ratio by the mothers, to restore the sex ratio in the effective population.AbstractMediterranean Shag (Gulosus aristotelis desmarestii) is a seabird endemic to the Mediterranean and Black Seas, recently included in the IUCN list of threatened Species. Most of the reproductive colonies are hosted in Sardinia and surrounding islets. Bycatch in fishing nets is one of the most significant threats for this population. Our work aimed to assess alterations in the sex ratio caused by bycatch and to study the adaptive response of the population to a skewed adult sex ratio. The sex ratio of Mediterranean Shags found drowned in the gillnets near the colonies and that of the nestlings of the Corcelli (northeast Sardinia) colony was determined using the sex-linked polymorphism of the gene Chromobox-Helicase-DNA-binding 1. The data of the shags found drowned in gillnets evidenced a high mortality rate (83.3%;p< 0.001) and a larger size of males (35% heavier than females,p< 0.05) compared to females, supporting the theory that heavier individuals are able to forage at great depths. With 64.8% of the nestlings being male, the sex ratio of nestlings was statistically different from parity (p< 0.05). Furthermore, it was related to the brood size. In one- and two-chick broods, 73% and 70% of nestlings, respectively, were males, while in three-chick broods, only 33% were males. Our data identify the higher rate of male shags drowned in gillnets as a factor causing an alteration of the sex ratio in the Mediterranean Shag population. According to the Sex Allocation Theory, an adaptive adjustment of sex made by adult females restores the Mendelian sex ratio in the population.