Fishing patterns affect depredation interactions of longline fishing-odontocetes in Southern Ocean

Fishing methods have evolved and changed over the years. With the decline in landings and effects of climate change, fishing methods became more selective, especially toward longline fishing. However, this method is associated with increased depredation interactions with odontocetes (toothed whales). Depredation interactions occur when a wild species consumes a resource that is caught or grown/raised by humans. When interacting with fishing vessels, odontocetes remove the fish directly from the hook which affects stock assessment and thus conservation efforts for fish species, as well as conservation of odontocetes due to potential lesions or even death. In the Southern Ocean, longline fishing is used to catch Patagonian toothfish (Dissostichus eleginoides), the most important economic activity in the region.

A recent study looked at the effects of fishing patterns in depredatipn interactions between longline fishing vessels and two odontocete species, orcas (Orcinus orca) and sperm whales (Physeter microcephalus) in the Antarctic Ocean. Data was collected in longline fishing vessels in seven different study areas: southern Chile, and around the Falkland, South Georgia, Prince Edward Island and Marion (PEMI), Crozet, Kerguelen, Heard and MacDonald Islands (HIMI) islands.

The results obtained showed that the two species interact with the vessels differently, depending on extension and depredation opportunities. The increase in fishing effort during the winter, especially in HIMI and South Georgia, coincided with a decrease in depredation interaction with sperm whales, probably due to the low number of individuals in this region. For this species, a decrease in interactions was observed from 2003 to 2015 near Chile, Crozet and Kerguelen Islands, but an increase was observed near Falkland Islands. For orcas, there was a decrease in interactions near Chile, but an increase in South Georgia.

In relation to fishing areas, in larger areas depredation interactions were reduced of both orcas and sperm whales with vessels, likely due to reduced probability of vessel detection by the whales. Higher vessel mobility lead to a decrease in interactions with orcas. However, the opposite was observed for sperm whales likely due to highest density of individuals, except for HIMI. In this region, depredation interactions were only reported once in 2010, year at which the fishing season was extended until the Spring, when sperm whales migrate to the region.

Some vessels were equipped with “cachalotera”, a fishing system that prevents whales from catching the fish from the hooks. Yet, the results showed no effect of this system in depredation interactions, and these even increased in some cases. The whales were still able to get fish and the vessels ended up remaining in the region to fish for longer.

Lastly, the depth at which the vessels fish influenced the probability of encounters with orcas and sperm whales, that is, these species tend to be distributed in shallower areas used by vessels. This can also be related to oceanographic conditions, i.e. the abundance of prey may be higher in shallower waters for both fishing and depredation. It is important to improve fishing methods and control fishing vessels in these regions to mitigate this issue. Since sperm whales interact more with vessels, more attention should be payed to take conservation actions for this species.


Source: Tixier, P., Burch, P., Richard, G., Olsson, K., Welsford, D., Lea, M. A., Hindell, M. A., Guinet, C., Janc, A., Gasco, N., Duhamel, G., Villanueva, M. C., Suberg, L., Arangio, R., Söffker, M., & Arnould, J. P. Y. (2019). Commercial fishing patterns influence odontocete whale-longline interactions in the Southern Ocean. Scientific reports, 9(1), 1904. DOI: 10.1038/s41598-018-36389-x

Author: Inês Ribeiro


Leave a Reply