Mortality and serious injury of northern right whales (Eubalaena glacialis) in the western North Atlantic Ocean

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Amy R. Knowlton
Scott D. Kraus

Abstract

Northern right whales in the western North Atlantic number about 300 animals and have shown little sign of recovery in recent decades.
Mortality and serious injury due to human activities, particularly commercial fishing and shipping, are thought to be significant factors
limiting their recovery. From 1970-1999, 45 right whale deaths were reliably documented. Sixteen of these fatalities (35.5%) were due to
ship collisions, and three (6.7%) were due to entanglement in fishing gear. The remainder were neonates (13; 28.9%) and ‘unknown cause’
mortalities (13; 28.9%). Criteria for defining serious injuries and mortalities from entanglement or ship strikes were developed and include
any animal carrying fishing gear, cuts from entanglement or ship strike deeper than 8cm, swelling or necrosis, evidence of poor health from
such interactions, and, in carcasses, evidence of haematoma, haemorrhaging or broken bones. A total of 56 animals fitting the defined
criteria were documented from 1970-1999: 31 (55.4%) from entanglement and 25 (44.6%) from ship strikes. Nineteen were fatal (16 ship
strikes, 3 entanglements), 10 were possibly fatal (2 ship strikes, 8 entanglements) and 27 were non-fatal (7 ship strikes, 20 entanglements).
The breakdown of potentially serious injuries by age and sex reveals no difference in levels between sexes but shows a 3.3:1 higher level
of interaction in juveniles and calves versus adults. The data show that ship strikes are more immediately lethal, but entanglements can result
in long term deterioration of an animal and may be responsible for higher levels of mortality than previously thought. Considering that some
animals become entangled, drown and never return to the surface, even these levels may be underestimated. Between 1986 and 1999, 84
animals were presumed dead based on a lack of resightings for six years. There were 32 confirmed deaths during this time period suggesting
that at least as many unreported deaths occurred as carcasses were reported. Definitive actions need to be taken to reduce the level and
severity of anthropogenic injuries and deaths. Actions could include continued disentanglement efforts, gear modifications, seasonal
closures for fisheries, mandatory ship reporting, ships’ routing measures and speed restrictions for commercial shipping.

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Knowlton AR, Kraus SD. Mortality and serious injury of northern right whales (Eubalaena glacialis) in the western North Atlantic Ocean. JCRM [Internet]. 2020Oct.30 [cited 2021Jun.24];:193-208. Available from: https://journal.iwc.int/index.php/jcrm/article/view/288
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