Horizontal and Vertical Movement, Residency and Mortality of Gray Triggerfish (Balistes capriscus) on Artifical Reefs in the Northern Gulf of Mexico

Abstract

The present telemetry study evaluated movements of gray triggerfish (Balistes capriscus) on artificial reefs in the northern Gulf of Mexico from 23 January 2013 through 5 September 2017. Five Vemco Positioning System (VPS) acoustic receiver arrays assessed the fine-scale (m) horizontal and vertical movement, volume use, residency, site fidelity and mortality (F and M) of transmitter tagged gray triggerfish (n = 49). Additional single, surrounding receivers (n = 21) recorded presence-absence data to validate emigrations, document large-scale movements (km) and confirm mortality events. All VPS arrays and single receivers were associated with small steel cage artificial reefs (2.5 x 1.3 x 2.4 m) each positioned 1.4 – 1.6 km apart at depths of 18 – 35 m, encompassing a 64 km2 area. Tracked gray triggerfish had a mean residency time of 7 weeks and an annual site fidelity of 18% year-1. Fish remained closely associated with artificial reef structures (mean ± SD = 18 ± 41 m ) and used much of the water column with a mean distance from the seafloor = 4.2 ± 4.2 m. Fish displayed seasonal and diel patterns of behavior with home range areas (95% kernel density estimates, KDE) smaller in the spring from March through May and larger in fall and winter from September through February. Gray triggerfish were closer to the seafloor from November through March and were higher up in the water column from April through October. Fish used smaller home range volumes (95% three-dimensional KDE) during winter and spring from December through May and the larger volumes during summer from June through August. Area use was greater at higher mean bottom water temperatures and dissolved oxygen (DO) concentrations. Fish were closer to the seafloor when temperatures were lower and shallower when DO was lower. Home range volumes increased with higher bottom water temperature and with lower DO. Gray triggerfish made fewer movements and remained significantly closer to the seafloor and reef habitat at night than during the day, transitioning during crepuscular periods. Most (n = 41, 84%) of the transmitter tagged gray triggerfish emigrated away from their VPS site during their tracking period. However, patterns differed among fish, where some fish emigrated and did not return (n = 25, 51%), some fish emigrated once and then returned (n = 3, 6%), while others emigrated and returned multiple times (n = 13, 27%). High emigration from VPS sites (97% of the fish present, n = 28) occurred during two tropical storms, after which 32% (n = 9) returned. Gray triggerfish movement patterns were most likely related to foraging and resting behaviors, spawning and associated competitive interactions and predator evasion. For all years combined, annual fishing mortality F = 0.23 (95% CI = 0.07 – 0.50), natural mortality M = 0.25 (0.07 – 0.57) and total mortality Z = 0.48 (0.18 – 0.85). Present instantaneous mortality estimates suggest that while past management efforts were likely successful, the gray triggerfish stock may still be experiencing overfishing and values support the Gulf of Mexico Fishery Management Council’s decision for increased fishery restrictions in 2018.