Differential Risk of Bottlenose Dolphin (Tursiops truncatus) Bycatch in North Carolina, USA

Abstract: Common bottlenose dolphins (Tursiops truncatus) in North Carolina (NC) are vulnerable to fisheries bycatch (fisheries interactions [FI]), particularly in gillnets. Although observed bycatch is relatively rare, strandings with evidence of FI are common and can be used to evaluate relative levels of and influences on bycatch. Strandings from 1997 through 2012 that had evidence of FI (n = 191) or had no evidence of any type of human interaction (n = 170) were evaluated to assess effects of possible predictor variables (i.e., sex, age-class, season, area, and time period [TP]) on whether a stranding was FI (bycatch risk). Due to sample size constraints, contingency tables assessed variables singularly, and significant variables were used in a Generalized Linear Model. Bycatch risk varied among three sequential TPs with a significant decrease between TP1 and TP2 coincident with regulations that effectively closed the NC spiny dogfish (Squalus acanthias) gillnet fishery. Bycatch risk increased slightly during TP3 despite implementation of the Bottlenose Dolphin Take Reduction Plan, perhaps due to regulatory changes allowing for increased spiny dogfish effort. During TP2-3, more male than female strandings were recovered (p = 0.0224), but sex did not affect bycatch risk. Age-class (p < 0.0001) and season (p = 0.0308) were significant predictors of bycatch risk. Bycatch risk of older calves and subadults was 1.5 (summer) to 3.5 (spring) times greater than for an adult or young-of-year. Thus, young dolphins surviving past their first year of life, which typically experience low natural mortality, are experiencing fishery-related mortality at levels that appear to exceed natural mortality, at least as determined from stranding data. Bycatch risk was not affected by area despite spatial differences in the relative abundance of dolphins and gillnet effort. Additional studies are needed to determine the long-term effects of this anthropogenic pressure on population dynamics for stocks occurring in NC waters.
Key Words: bycatch, strandings, fisheries interactions, age-class, mortality, bottlenose dolphin, Tursiops truncatus
Document Type: Research Article
DOI: https://dor.org/10.1578/AM.43.5.2017.558
Page Numbers: 558-569

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