Echolocation and navigational distortion hypothesis of the recent short finned pilot whale beaching in Southern Coast of Tamil Nadu

Cetacean stranding is a phenomenon wherein cetaceans strand themselves on land, usually on a beach. Several explanations of the stranding have been proposed. In an unusual phenomenon starting from 11th Jan 2016 afternoon, 45 dead, short-finned pilot whales were washed ashore between Kallamolli, Alanthalai and Manapad in Thoothukudi district, of these, 37 were adults and eight were sub-adults. As many as 36 whales, part of the group which survived the beaching, were rescued in a joint operation by several government agencies and pushed back to sea. Interestingly, a similar incident on January 14, 1973 when 147 whales were stranded on the beach between Kulasekarapattinam and Manapad, almost the same location has also been reported. Hypotheses that try to explain the phenomenon range from  biological - including climate and oceanographic variation, navigational errors arising from particular magnetic configurations or bathymetric and ocean current features, and anthropogenic noise and sonar interference to the less-supported, including sensory stimulation distraction, regression to instinctive behaviours, and even the suggestion that earthquakes are responsible. Based on bathymetry and topographical features of the area there is a possibility that echolocation and navigational distortion could have caused the beaching. I propose a hypothesis which is mainly on navigational disturbance due to low slope and shallow water up to about 3 km in the seas in the coastline between Alanthalai and Manapad. The main beaching zone was between Alanthalai and Kallamoli and the 16 km stretch till Manapad. Note this is the second beaching in this area, here, in this case analysing the data obtained from bathymetry of the ocean in and around this area we see that at Manapad there is a drop of just 5-17 metres in 3 kilometres from Manapad beach, which means the sea does not get deeper than 17 m even after 3 km into the sea, Manapad has a unique coastline that was formed by multiple layers of lava accumulated over a vast period of time. This gives Manapad the rare distinction of witnessing the best wave formations all through the year. The echolocation system of the whales can have difficulty picking up very gently sloping coastlines. The wind and the surf in the ocean shallows approaching the beaches can also affect echolocation navigation system of the whales.  

Short-finned pilot whales Globicephala macrorhynchus are larger members of the dolphin group reaching mean lengths of 3.7 m for females and 5.5 m for males with maximum male size of 7.3 m. Adult weight is 1000 to 3000 kg and their habitat is primarily warm tropical waters, but usually stays offshore in the deeper waters and in squid areas as squid is its food.  They are very sociable and are always in a pod of about 20 or more.  The possible sequence of event could have been that this pod of whales may be 70-90 is swimming towards the Manapad coast from east to west, unaware land is close. The Pilot whale which uses echolocation to sense objects - in echolocation, a high-pitched sound is sent out by the whale. The sound bounces off the object and some returns to the whale are called pilot whales as they guide the rest of the pod - pick up the shore on their sonar and so have to change course, either along the coast in deep sea or turn back east. Their echo-navigation and sonar gives them clear water on these sides. Due to the jagged coast line and small slope the pilot gets strong readings from may be two directions and get confused.

The water is shallow here and that adds to the confusion as repeated reflections between the surface and ocean bottom in gently sloping shallow water may attenuate sound so much that the echo is inaudible to the whales.

They start to panic and the waves of the surf take over and drive them further inshore. Distress signals are sent to the rest of the herd that they have grounded. This pod may be a nursery pod with a strong social cohesion. The behaviour is when the others hear the distress signal, they come close to be supportive as in all toothed mammals that show this strong social cohesion and structure. Support for the hypothesis from comes from other such beaching in low slope coastlines wherein mass beaching repeats in the same spots have been seen and sonar termination has been explained as possible cause in places such as Ocean Beach, Tasmania and Geographe Bay, Western Australia where the slope is about half a degree - 8 m deep 1 km out to sea.