Fishing shapes cetacean population density patterns in the Mediterranean basin

Human impacts are forcing species towards marginal and suboptimal portions of their historical ranges. Cetaceans are now under protection, but are still threatened by fishing activities, which reduce fish stocks, alter their feeding behavior, and can cause mortality due to bycatch. Here, we investigated how different fishing activities affect cetacean population density patterns in the Mediterranean, one of the most impacted and fished seas. We collected 366 population density estimates for eight cetacean species. We then classified species into four trophic groups (planktivorous, piscivorous, teutophagous, generalist) and modelled their density as a function of both environmental and fishing variables (artisanal, demersal destructive, demersal non-destructive with low bycatch, demersal non-destructive with high bycatch, and pelagic fishing with low bycatch). Finally, to quantify human contribution to the observed geographic pattern of population density, we predicted and compared population density patterns under a baseline fishing and a minimum fishing scenario. The four groups of cetacean species exhibited diverse responses to environmental and fishing variables. Demersal destructive fishing consistently had a negative influence on species population density. In contrast, others, such as demersal non-destructive fishing, showed mixed effects, including a potential attraction effect on piscivorous species. Overall, we predicted a probable change in the geographic pattern of population density of cetaceans in response to fishing activities, especially along the coasts in planktivorous species and in offshore areas in generalist species. Our study provides evidence of the negative impact of fishing activities on cetacean population density, while highlighting functional group-specific responses to different fishing practices. These findings enhance our understanding of human-induced changes in marine ecosystems, suggesting probable alterations to the natural population density patterns of cetaceans in the Mediterranean Sea.