Humans influence shark behavior: Evidence from the COVID-19 lockdown

Abstract

Achieving the UN SD Goals requires the conservation of keystone marine species. As top predators, many sharks play a crucial role in balancing marine ecosystems; yet, they have been experiencing severe population declines. Overfishing is indisputably the main threat to shark populations, but little is known about the impact of non-extractive human pressure. This study tests the effect of varied human presence on shark behavior at an insular marine reserve. In 2020, humans were virtually absent from Fernando de Noronha (FEN), Brazil, during a 211-days lockdown period prompted by the COVID-19 pandemics. A local shark tracking program rendered nearly 280000 acoustic detections in coastal waters from 2016 through 2021. Lemon sharks, Negaprion brevirostris, showed a 43% increase in detection rate during the lockdown which was ascribed to the concomitant decrease in human presence. In contrast, the detection rate of tiger sharks, Galeocerdo cuvier, decreased by 67% during the lockdown, but this trend was likely related to seasonality in tiger shark behavior. After the lockdown, both species’ detection rates tended to return to previous levels. Further, lemon sharks increased diurnal coastal residency and made more use of areas less restrictive to human use during the lockdown, whilst tiger sharks increased nocturnal coastal residency during and after the lockdown period. Contrasting trends might relate with lemon sharks completing their life cycle in FEN and tiger sharks being migrants arriving to FEN at older ages and being less exposed to local anthropogenic stimuli across ontogeny. This study demonstrates that non-extractive human disturbance can induce significant changes in the way marine predators explore crucial habitats for accomplishing trophic, reproductive, and ontogenetic functions. Ascertaining the ecological impacts of marine-based human development should thus consider cryptic responses by keystone megafauna to the anthropogenic landscape produced by an ever-growing coastal population. The designation of human exclusion marine areas could emerge as the single solution to endow shark populations with suitable habitats for optimizing their resilience to human pressure.