Sperm whales’ daily routine revealed by underwater robots
Image credit: Dreamstime
Researchers in the UK have used underwater gliders to monitor and document the daily habits of the endangered Mediterranean sperm whale.
The research, led by the University of East Anglia (UEA), used unmanned underwater gliders equipped with acoustic monitors to record sperm whales’ sounds - or ‘clicks’ - over several months and across 1,000km of ocean.
Sperm whales are highly vocal, producing distinct types of clicks for both echolocation and social interaction purposes. The study focused on the extremely powerful and highly directional ‘usual clicks’ produced while foraging – the act of searching for food.
The recordings obtained by the gliders confirmed the whales’ widespread presence in the north-western Mediterranean Sea and identified a possible hotspot for sperm whale habitat in the Gulf of Lion, where a higher rate of clicks was found. This could indicate a higher number of whales, but could also be for behavioural reasons, the team said.
In addition, continuous day and night monitoring during winter months suggest different foraging strategies between different areas. In the Ligurian Sea, mobile and scattered individual whales foraged at all times of the day. In the Sea of Sardinia, usual clicks were also detected at all times of the day.
However, the researchers found that in the Gulf of Lion, larger groups targeted intense oceanographic features in the open ocean, such as fronts and mixing events, with acoustic activity showing a clear 24-hour pattern and decreased foraging effort at dawn. This finding, according to the UEA team, could suggest the whales may have modified their usual foraging pattern of eating at any time to adapt to local prey availability. This also provides a clue to the sperm whales’ diet in this area.
According to conservation experts, there are fewer than 2,500 mature individual Mediterranean sperm whales. Threats to them include being caught as bycatch in fishing nets and, as was recently the case off the Italian coast, entanglement in illegal fishing gear. Other dangers range from ship strikes and ingestion of marine debris to disturbance by human-made noise and whale-watching activities.
Pierre Cauchy, a postgraduate researcher at UEA’s Centre for Ocean and Atmospheric Sciences (COAS) and CEFAS, said their findings would help tackle the threats encountered by the species while increasing conservation efforts.
“Information on the ecology of the Mediterranean sperm whale subpopulation remains sparse and does not meet the needs of conservation managers and policymakers,” he explained. “Increasing observation efforts, particularly in winter months, will help us better understand habitat use and identify key seasonal habitats to allow appropriate management of shipping and fishing activities.”
Cauchy added that the clear daily pattern identified in their results appear to suggest that the sperm whales are adapting their foraging strategy to local prey behaviour. “The findings also indicate a geographical pattern to their daily behaviour in the winter season.”
The study found that whales spend a substantial amount of their time foraging. When in a foraging cycle, they produce usual clicks 60 per cent of the time. As such, the detection of their clicks provides a reliable indicator of sperm whale presence and foraging activity and their specific features allow them to be identified and detected up to a distance of 20km.
The study involved analysing sounds recorded by passive acoustic monitoring (PAM) sensors, previously successfully used for weather observation, on gliders deployed by the team to collect oceanographic data during winter 2012-2013 and June 2014, covering 3,200km of the ocean.
Professor Karen Heywood, also of COAS, said the study demonstrated the possibilities of using existing glider missions to monitor the Mediterranean sperm whale over the winter months, for which there is a lack of crucial data for conservation.
“Our ability to successfully observe sperm whale distribution in different geographic areas of the north-western Mediterranean Sea, across the slopes and the open ocean, highlighted the complexity of sperm whale behaviour, foraging strategy and habitat use,” she explained.
“This study shows that the addition of PAM sensors to existing oceanographic glider missions offers the opportunity for sustained long-term observation, which would significantly improve sperm whale population monitoring and behaviour description, as well as identification of key habitat and potentially harmful interaction with human activities.”
Furthermore, Dr Denise Risch, a marine mammal ecologist at the Scottish Association for Marine Science (SAMS) said that experts need to understand the Mediterranean sperm whale population better in order to work towards their conservation by eliminating threats.
“This is also true for other marine mammal species globally,” she said, “and gliders allow us to go into new areas, which we wouldn't have any observations from otherwise, and also at times of the year when we are not usually monitoring.”
The study involved researchers from UEA and the Centre for Environment, Fisheries & Aquaculture Science (CEFAS), the Scottish Association for Marine Science (SAMS), the University of Gothenburg in Sweden and Sorbonne University in France.
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