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Scar-Based Inference Into Gulf of Maine Humpback Whale Entanglement: 2003-2006
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2009
[PDF-9.11 MB]
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Description:Entanglement in fishing gear is a known source of injury and mortality to humpback whales, Megaptera novaeangliae. However, eye-witnessed events provide limited insight into entanglement frequency, risk factors and biological impacts. The caudal peduncle is commonly implicated in humpback whale entanglements and is consistently presented during the terminal dive. Since 1997, peduncle scarring has been studied annually as a relative index of entanglement frequency among Gulf of Maine humpback whales. Between 2003-2006, a total of 2,155 suitable quality images were obtained of the caudal peduncle and flukes of 615 catalogued individuals. Preferred photographs were obtained while parallel to the whale and slightly ahead of its flukes during the terminal dive. Images were examined for evidence of wrapping scars, notches and other injuries that were believed to be entanglement-related. Nearly all (97.2%, n=35) individuals involved in eye-witnessed entanglements were independently scored as having a high probability of prior entanglement. The majority (64.9%, n=222) of individuals entering the study for the first time exhibited evidence of a prior entanglement. Unhealed high probability injuries were detected in the overall sample at annual frequencies ranging from 6.8 ± 3.59% (n=13, 2006) to 18.8 ± 4.16% (n=64, 2003). When baseline coverage was available, 35.8% (n=39) of new injuries occurred within one year, and 80.6% (n=87) to within 3 years. Inter-annual increases in scarring ranged from 6.3% (n=7, 2004) to 25.7% (n=27, 2003). There was significant annual variation during the study period, mainly related to a peak in adult entanglement between 2002 and 2003. There were no significant differences in entanglement incidence between the sexes, but juveniles were more likely than adults to acquire new injuries. A total of 156 new events were inferred from entanglement injuries during the study period. When reconciled with eye-witnessed cases, there were a total of 203 events between 2003-2006. Only nine events were detected and well-documented in progress, resulting in a 5.7% reporting rate for the period. Annual humpback whale mortality from entanglement was estimated at approximately 3% (19-29 whales per year, depending on assumptions of population size). Mortality estimates were substantially greater than observed deaths, but not inconsistent with population vital rates. Entanglement mortality estimates also exceeded Potential Biological Removal (PBR) by a larger margin than observed deaths, reinforcing the importance of reducing the risk of entanglement.
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