Population size estimates and migrations of the queen conch (Aliger gigas) in the nursery ground of the Xel-Há coastal inlet based on a mark-recapture and census surveys

This project aims to analyze the spatiotemporal variations of the queen conch (Aliger gigas) population in the sanctuary of Xel-Há Inlet (Mexico). During 2022 and 2023, visual censuses (on 42 fixed transects per sampling) and biometrics (siphonal length SL and lip thickness LT) were carried out to estimate average densities and size-frequency distribution within four zones: Bocana (BO), Centro (CN), Brazo Norte (BN), and Brazo Sur (BS). In addition, the average population size (${\hat{N}}_t$) and migrations were estimated following a mark-recapture methodology on 7 occasions, tagging 574 specimens (279 in 2022 and 295 in 2023). Regarding the ontogenetic phases (based on LT) of the organisms observed during censuses in 2022 (n = 279), 81 corresponded to adults and 198 juveniles; meanwhile, in 2023 (n = 531), 36 were adults and 495 juveniles, indicating that the inlet is mostly a nursery ground. The overall mean densities have increased since Feb-2022 (0.543 ind./100 m²) up to Nov-2023 (5.079 ind./100 m²). Benthic variables and density of A. gigas varied spatially, i.e., CN and BO had higher benthic coverage of sand and seagrasses related to elevated densities of adults; meanwhile, BN and BS were dominated by turf and macroalgae and related to higher densities for juveniles. A total of 284 (54.2 % of tags) specimens were recaptured at another time, and based on the Jolly-Seber model, ${\hat{N}}_t$ was estimated at 938 specimens (ranging from 538 to 1369). Regarding migrations, 79 tagged specimens that moved to other zones were identified, and 205 specimens were again observed in the same zone. Inter-annually, 108 tags in 2022 were later observed in 2023, and 36 specimens (earlier as juveniles) a year later were recaptured as adults; meanwhile, 72 adults in 2022 were again observed in 2023, thus indicating that queen conch used to return to breed within the inlet.