Estimating humpback whale (Megaptera novaeangliae) entanglement rates on the basis of scar evidence


Title: Estimating humpback whale (Megaptera novaeangliae) entanglement rates on the basis of scar evidence
Publication Date:
May 13, 2004
Document Number: Order Number 43EANF030121
Pages: 22
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Robbins, J.; Mattila, D. (2004). Estimating humpback whale (Megaptera novaeangliae) entanglement rates on the basis of scar evidence. Report by Provincetown Center for Coastal Studies. pp 22.

Entanglement in fishing gear is a serious source of humpback whale, Megaptera novaeangliae, injury and mortality in the Gulf of Maine. The flukes and caudal peduncle are commonly implicated in entanglements and consistently presented during the terminal dive. Between 2000 and 2002, photographs were systematically obtained of these features while parallel to the whale and slightly ahead of its flukes during the terminal dive. Images were coded to reflect the presence or absence of wrapping scars, notches and other injuries that were believed to be entanglement-related. Assumptions were successfully tested against documented caudal peduncle entanglements during the study period. Approximately half (48%-57%) of each annual sample exhibited scars that were likely to have resulted from a prior entanglement. However, as individuals acquire and lose scars over their lifetime, the inter-annual acquisition rate (8.0- 10.4%) was considered more likely representative of events during the study period. Composite (lifetime) rates of high probability scarring, inter-annual scar acquisition and the standardized minimum number of events were all comparable to the previous three-year period (1997-1999). However, neither recent documented entanglements, nor scar acquisition rates supported prior evidence of a higher male risk of entanglement. Based on the weight of the current evidence, it does not appear that breeding ground injuries alone can explain the higher frequency of healed caudal peduncle injuries among Gulf of Maine males. Continued efforts to refine scar analysis techniques and examination of historic caudal peduncle data may help to clarify this issue. With further development, the study of raw entanglement-related injuries may allow for annual estimates to be developed without a baseline sample. Nevertheless, caudal peduncle monitoring can only provide a relative index of entanglement, as it does not account for events that do not involve the caudal peduncle, nor those that result in the death of the animal prior to sampling. The present work confirmed that even animals that die with evidence of fisheries interactions do not necessarily exhibit extensive external injuries. Thus, tracking even apparently minor entanglements may be important in detecting and preventing increases in severe injuries or death. Given the low rate at which these events continue to be reported (10%), management initiatives should focus on prevention, in addition to human intervention (disentanglement).

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