Mon, 18 Nov 2019

A humpback whale was recently spotted in the River Thames near London. This unusual sighting sparked national media interest, similar to "Benny" the beluga who also called the river home for several weeks last year. However, while Benny eventually left the Thames and headed home to the Arctic, the humpback whale was not so lucky. Ironically, despite the human-interest factor, the whale died as a result of human impact. In doing so, it had the dubious honour of being the first humpback whale known to have died in UK waters from being hit by a vessel.

Whale scientists like us call such incidents a "ship strike" or "whale strike", and they happen in all oceans around the world, both on the high seas and near the coasts. Injuries, which are not always fatal, can be sustained from blunt trauma from the boat's hull, or sharp propellers.

The risk is highest when lots of vessels overlap with areas where susceptible animals occur in large numbers. For example, sperm whales like to gather in the part of the Mediterranean between Corsica, southern France and the coast of northwest Italy. It's an area with lots of commercial and merchant vessel traffic, but of course the whales don't necessarily know that. Though the area has been turned into a large special protected area for marine mammals - Pelagos Sanctuary - research has estimated that 74 sperm whales could still be hit by ships in a single summer.

In the case of the Thames humpback, it was likely in the wrong place at the wrong time. A necropsy (an autopsy for animals), carried out by scientists from the UK Cetacean Stranding Investigation Programme, discovered the whale was in poor health, which may explain its journey into the Thames in the first place. However, a wound on its head clearly showed the cause of death was a collision with a ship. Such accidents are not particularly unusual, but the fact that it happened to an animal in the public eye makes it a fairly unique case.

One of the problems with ship strikes is the lack of knowledge. Dead whales sink more often than they float, so the evidence is usually quick to disappear beneath the waves. The few cases of dead whales washing ashore with clear ship-strike injuries are just the tip of the iceberg, compared to all the collisions that likely go unnoticed. As a result, there is still a lot that we don't know about ship strikes, including key information such as how often they happen, and to what degree they impact whale conservation.

We do know that ship strikes have led to population declines in the now critically endangered North Atlantic right whale. So named because it was considered the "right" whale to hunt due to its slow speed and tendency to float when dead, the North Atlantic right whale is a species with a particularly unfortunate history. Before a 1986 moratorium, they were decimated by commercial whaling. Since then, the species has been unable to recover as the animals' location in busy Atlantic shipping lanes and fishing grounds puts them at risk of ship strikes and entanglement in fishing gear.

The threat of ship strike is truly a global issue, also affecting seals, dolphins, sharks and turtles in coastal and offshore areas around the world. The level of risk varies from species to species. For example, some whales predominantly feed in surface waters, within the reach of commercial ships' draughts. Almost a quarter of fin whales that have washed up on UK shores since 1990 were killed by ships, and ship strike is the leading cause of death for large whales in nearby France.

Understanding the current situation is only part of the problem, however, as the oceans are changing at an unprecedented rate. Shipping grew fourfold between 1992 and 2012, making collisions with whales more likely, and increasing noise pollution which can further alter natural cycles and behaviours.

Climate change is changing the distribution of plankton, one of the preferred items on a whale's menu. This could result in whales also moving, potentially to new areas that overlap with shipping lanes. Many whale species are only just beginning to recover from historic whaling - the worry is they will suffer the same fate as the northern right.

Whales dying from ship strike is also not great for shipping companies and crews. Pulling into port with a dead whale draped across the bow is never a great look, but another worry is human health and safety. There have been cases where whale-vessel collisions have resulted in sinking vessels, human injuries, and even deaths.

We still don't know enough about whale strikes though. That is why we have teamed up with ORCA, a conservation charity, to investigate exactly how busy shipping lanes in European waters overlap with whale habitats. We want to reveal the true scale of the whale / ship strike problem, and suggest ways ships can minimise the risks.

Ultimately, we need to manage our marine spaces to minimise mortality, so that the death of the Thames humpback remains an oddity rather than the norm.

Authors: James Robbins - PhD Researcher, Whale-Ship Collisions, University of Portsmouth | Sarah Marley - Lecturer in Marine Vertebrate Zoology, University of Portsmouth The Conversation

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