In early March 2019, a humpback whale (Megaptera novaeangliae) was sighted in the Firth of Forth close to Edinburgh, Scotland (UK), entangled in static fishing gear (ropes and buoys associated with shellfish pots or creels). Despite the efforts of a large whale disentanglement team, releasing the whale from the gear was unsuccessful and the location of the whale became unknown. Several weeks later, on 23 April, a dead humpback whale washed ashore on Tyninghame Bay, close to the Ocean Science Consulting office in Dunbar, Scotland. Photographs of the deceased whale’s tail were compared to those of the individual seen entangled previously, and it was confirmed that it was the same animal spotted alive a few weeks back. The necropsy (post-mortem of an animal) conducted by Scottish Marine Animal Stranding Scheme revealed signs of drowning in the lungs, which was deemed to be the cause of death. It is believed that the drowning occurred, because the animal either became too exhausted, from dragging the fishing gear to the surface or that entanglement had become so severe, it impeded the animal’s movements. Since then, two minke whales (Balaenoptera acutorostrata) and another humpback whale have washed ashore along the Scottish coast after suffering fatal fishing gear entanglements.

Date and location of baleen whale strandings off the coast of Scotland, in which death was attributed to fishing gear entanglement from April to May 2019. Source: Scottish Entanglement Alliance (2019).

Ecological effects

Entanglement in fixed fishing gear is a major threat to baleen whales, not only in Scotland, but worldwide. Studies investigating scarring on humpback whales have estimated that more than a quarter of individuals in populations examined had been entangled at least once in their lifetime (van der Hoop et al., 2017), indicating that many animals are often able to free themselves. At individual level, entanglement can lead to sudden death (acute entanglement) through drowning or eventual death (chronic entanglement) attributed to inhibited feeding, exhaustion from increased drag, increased stress, weakened immune system, inability to avoid an attack by a predator, and trauma or injury such as lesions which could lead to infection (Basran et al., 2019). At population level, entanglement can have  potentially devastating long-term effects, because as mortality increases, the population declines (Volgenau et al., 1995). The decline is compounded if there is a high frequency of entanglements in juveniles, as it results in decreased recruitment (Knowlton et al., 2012).

Threats and solutions

Any rope, line or cable may pose an entanglement risk to a whale; however, inspection of gear recovered from entanglements revealed that the end line as well as the ground line from fixed fishing gear, as illustrated above, may contribute significantly to entanglements (Johnson et al., 2005). An entangled whale will often damage or even completely drag gear away (van der Hoop et al., 2016), leading to a loss to fishermen. Using a ropeless fishing system instead would decrease the number of whale entanglements; however, removing the end line attached to a buoy disables both purposes of simplifying trap retrieval and indicating trap location to mariners and enforcement agencies. A system using acoustic buoys would address the first and is being trialled at present; however, a satisfactory alternative for the second is still under development (Baumgartner et al., 2019).

References

Basran, C.J., Bertulli, C.G., Cecchetti, A., Rasmussen, M.H., Whittaker, M., and Robbins, J. (2019): First estimates of entanglement rate of humpback whales Megaptera novaeangliae observed in coastal Icelandic waters. Endangered Species Research 38, 67-77.

Baumgartner, M., Werner, T., and Moore, M. (2019): Urgent need for ropeless fishing: Removing end lines to protect right whales. Sea Technology March, 23-27.

Johnson, A., Salvador, G., Kenney, J., Robbins, J., Kraus, S., Landry, S., and Clapham, P. (2005): Fishing gear involved in entanglements of right and humpback whales. Marine Mammal Science 21, 635-645.

Knowlton, A.R., Hamilton, P.K., Marx, M.K., Pettis, H.M., and Kraus, S.D. (2012): Monitoring North Atlantic right whale Eubalaena glacialis entanglement rates: a 30 yr retrospective. Marine Ecology Progress Series 466, 293-302.

van der Hoop, J., Corkeron, P., and Moore, M. (2017): Entanglement is a costly life-history stage in large whales. Ecology and Evolution 7, 92-106.

van der Hoop, J.M., Corkeron, P., Kenney, J., Landry, S., Morin, D., Smith, J., and Moore, M.J. (2016): Drag from fishing gear entangling North Atlantic right whales. Marine Mammal Science 32, 619-642.

Volgenau, L., Kraus, S.D., and Lien, J. (1995): The impact of entanglements on two substocks of the western North Atlantic humpback whale, Megaptera novaeangliae. Canadian Journal of Zoology 73, 1689-1698.