Virtual Population Analysis of the critically endangered Scalloped Hammerhead (Sphyrna lewini) in the Eastern Tropical Pacific

Miguel de Jesus Gomez Garcia , Alex Hearn, Mario Espinoza, Easton R White 

The scalloped hammerhead (Sphyrna lewini) is one of the most critically endangered shark species in the Eastern Tropical Pacific (ETP). Despite its wide distribution and relatively high fecundity, overfishing and bycatch have led to sustained population declines across the region. We developed a spatially implicit, stage-structured, two-sex virtual population model for the ETP population of S. lewini using a Bayesian framework. Our model outputs were aligned with observed population trends using underwater visual census data collected at the Galápagos Archipelago, Ecuador, and Cocos Island, Costa Rica, from 1993 to 2024. Missing life-history parameters for the scalloped hammerheads population were estimated using model outputs. We conducted elasticity analyses to identify key life stages driving population growth and evaluated ten management scenarios aimed at species recovery. Our results provide the first stage-specific estimates of demographic parameters for both sexes in the ETP, including natural and fishing mortality. Adult and sub-adult stages were the main contributors to variation in population growth for both sexes. Growth to reproductive stages and in-stage survival were the most critical life history parameters for females and males, respectively. Under current conditions, the probability of scalloped hammerhead population recovery is < 1%. Most scenarios allowing continued fishing yielded recovery probabilities below 50%. A 75% reduction in adult mortality produced the most favorable outcome, with a 63% probability of positive population growth. Our results provide researchers and decision makers with a baseline for increasing the accuracy of demographic studies and improving management of scalloped hammerhead populations across the ETP.