Chelonia Limited

  Cetacean Monitoring Systems

 

Towing PODs

Towed acoustic surveys have proved powerful in measuring sperm whale densities and for other species where the detection range far exceeds the range of responsive movement. For species such as porpoises, where that is not the case, large biases are present in the data and are difficult, or impossible, to remove accurately.

Surveys of finless porpoises in Hong Kong using the T-POD gave high detection rates with good correspondence to visual detections. By contrast, surveys in the Celtic and Irish Seas showed much lower rates of acoustic detection of sighted animals. The most plausible explanation appeared to be that Hong Kong is very noisy with intense shipping activity and so the porpoises are used to this environment and respond only to vessels very close to them, whereas in the much quieter Celtic and Irish Seas, avoidance behaviour was clearly seen. Porpoises here turned away from the approaching vessel and became acoustically undetectable until they re-orientated towards the towed sensor, which mostly did not happen while they were still within detection range. This kind of bias may show strong spatial variation depending on local exposure levels, habituation speeds and animal movement rates and the spatial resolution required to remove such biases may not be available from the relatively low spatial detection densities of line survey methods.

There are other factors to consider:

Population density estimates are made using DISTANCE line transect surveys. This method works by getting a distance from the trackine of all detected animals and fitting a curve - the detection function - to the graph of detection rates against distance from the trackline. g(0) is the term used to represent the proportion of animals on the trackline. If g(0) can be assumed or shown to be 1 (that is 100% detection on the trackline) then the detection function allows all detections within its scope to be used in estimating the density. This can work well if the detection function resembles the bell shaped curve (half of it!) of a normal distribution. If the animals respond strongly to the boat, errors can be large.

If a population consisted of a combination of both 'urban' animals in ship-noisy areas that were relatively detectable, because they did not turn away from boats, and 'rural' animals that were so strongly boat averse that they were rarely detected, then serious underestimation would result as the detection function would be dominated by the urban animals. These behavioural differences might arise within hours or days of being in either type of environment and persist for a similar length of time after moving away, so they may exist even where animals are not actually residents.

Differently noisy boats, changes in speed, in the use of sonars and in the length of the towing rig could all have large effects on the detection function.

Dolphins can be interested in the boat or the towed gear and stay with them for long periods, re-appearing from time to time as acoustic detections.

Porpoises mostly seem to move away from the boat and their directional sonar is then undetectable. Later, they re-orientate towards the boat and become detectable if the sensors are far enough astern. That distance is commonly greater than 100 metres.

Despite these problems, it has to be said that if the boat, its speed, its sonar use and the towing distance are all kept the same over a series of re-surveys of the same area, then significant changes may be demonstrable and useful data may be gathered from sea states that reduce visual detections to an insignificant level.

In that light, the following ideas are presented about how to tow the POD.

Towing rig

The ideal characteristics for a POD tow rig are that it must be:

  • towable at the same depth irrespective of speed
  • able to tow at up to 14km/h
  • buoyant so that nothing is lost if it breaks
  • seen easily at the surface if lost
  • light and have low drag
  • deployed without slowing the boat

All the above characteristics can be achieved using a skimmer rig, if a surface tow line is acceptable.

The rig consists of a diamond shaped skimmer board about 60 cm long, 35 cm wide, 4 cm thick is used. It is towed along the surface from one of the sharper corners and the two edges either side of this corner are 'sharp', while the two trailing edges are squared off. The POD housing is attached by a thin nylon line about 6 metres long to the back corner. The downward pull of the POD keeps the skimmer board stern-low so that it skims over the waves. It can be made of fibreglass, covering polyurethane foam, with attachment eyes at the front and back.

The POD is housed in a 110 mm plastic ground pipe with a hemispherical insert at the front with a central hole for the tow line, which goes through and is formed into a loop that can be attached to the POD to secure it. The 110 mm pipe is long enough to reach beyond the end of the POD by about 8 cm and the sides of the tube are cut away from the stern end far enough to give the transducer a clear view sideways.

A wooden bar about 2 cm square and 1 m long is clipped to the top of the stern of the tube using two screws in the bar and 'keyholes' in the tube. A groove in this bar holds an O-ring that goes around the POD transducer and pulls the POD forwards and the bar back, so that both are secured.

The hemispherical nose carries a hydroplane of stainless steel. This is about 30 cm long and bent at right angles into a strong dihedral. It is attached by two screws and is given a small angle of attack by a 3 mm nut under the leading screw.

In use, the POD has batteries only in the inner battery pack. The switch angle is set to allow it to be on at most angles.

The POD housing and skimmer board can be thrown over the side of the boat at the same time and brought into movement with some hand control of the tow line, which is let out to 100 or 200 metres as required. A light buoyant line is adequate. If the POD hits an obstruction underwater it floats to the surface with the wooden bar standing up vertically, which makes it easy to pick up.

The POD stays below the skimmer board by using the principle of a kite in reverse. Because of the low angle of attack of the hydroplane, it runs with an angle of around 20 degrees down, so that the POD does not feel the skimmer board tracking the surface too strongly.

An alternative would be to dispense with both the skimmer board and the hydroplane on the tow housing and force the towing line down near the boat. If the POD was just buoyant, it would tend to follow at the same level as the tow point behind and below the boat.