Sunday, April 3, 2016

Diving Behavior of Freshwater Finless Porpoises


Often confused with dolphins, porpoises are another member of the order Cetacea. The main differences are in the physical structure of the organism. Porpoises have blunt noses, spade-shaped teeth, and a triangular dorsal fin whereas dolphins have elongated beaks, cone-shaped teeth, and a curved dorsal fin. Both are extremely intelligent animals, however dolphins are very vocal creatures while porpoises are not. It is thought that this could be due to structural differences in the blowholes of the two creatures. Dolphins are much more common having 32 known species across the world compared to the 7 known species, one of them being the finless porpoise.


A study conducted in Tian-e-zhou Baiji Nature Reserve in the Yangtze River in central China looked at the diving behavior of freshwater finless porpoises. Researchers equipped three of the nine captured porpoises, a juvenile female and two adult males, with velocity-time-depth data loggers on the animals' pectoral fins. Temperature and dive-depths were recorded on the data loggers once every second, and velocities were calculated using a rotating blade on the front of the device. 

Only the data loggers from porpoise no.1, the juvenile female, and no.3, one of the adult males, were able to be retrieved. Two different dive types were recorded: shallow-dive and deep-dive. Shallow dives were categorized as any dive with a maximum depth of less than 2.7m. The results for the number of dives of each dive type for porpoise no.3 are shown in the figure below. According to the data, the majority of the dives were shallow dives, probably to conserve more energy.


This study found that the daily horizontal travel distances for the finless porpoise, 94.4km and 90.3 km, are significantly higher than those previously seen in harbor porpoises, 32-53km. Another important observation in this study was the evidence of a speed drop to less than .25m/s when porpoises dove deep. This is considered a "turn-around" behavior the finless porpoise uses to forage for fish. The number of these drops in speed to less than .25m/s are shown in the figure below, and the majority of them are at the maximum dive-depths reached during deep dives.


Velocity is quickly recovered from the turn-around; however, this slow reaction time involved in their turn-around behavior results in these organisms becoming captured and entangled in fishing gears and colliding with commercial vessels, resulting in their death. Based on this study, a decrease in vessel speed could potentially decrease porpoise mortality in high density areas. Further research is needed in this area on porpoises to create effective regulations on vessel travel, but it is an important research area since porpoises are crucial predators at the top of the food-chain. 


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