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Shetland Sea Mammal Group

 

 

UPDATE:  Further work by Paul Fisher and Nick Tregenza using hydrophones to research activities of Harbour Porpoises in Yell Sound is available here as a pdf download (811Kb - Adobe Acrobat Reader required)

A day in the life of a Porpoise

Paul Fisher

With the assistance of Setterness salmon farm, a POD hydrophone was deployed in the mouth of Boatsroom Voe, Lunna Ness on Saturday 11th March 2000 to “monitor” the feeding activity of porpoises over a two-week period. The POD hydrophone, previously reported in The Shetland Times, is an acoustic detection device, which logs the time and counts the number of high frequency clicks porpoises produce when echolocating for food.  The advantage of this system over visual observations is that the hydrophone detects monitors twenty-four hours a day for up to three weeks, until the battery power supply is drained. However, from the acoustic data it is not possible to determine the number of porpoises producing the clicks. 

Porpoises have been studied in Shetland over the last decade, combined with anecdotal information from the general public, has provided a great deal of information. In particular, identified at which sites porpoises frequently feed during summer months, such as Mousa, Quendale, Noss and Out Skerries. There is less information regarding “how” and “why” porpoises use particular sites to feed (often with calves), and how they feed and move round Shetland in the wider scheme of things. The primary aim of using the POD hydrophone was therefore to (i) find out the daily activity patterns of porpoises feeding at particular sites, and (ii) whether coastal feeding sites are used all year round. This information can then be linked with other factors such as tidal patterns, sea state conditions, habitat characteristics and available prey species, to explain and understand the behaviour and habitat requirements of porpoises. 

An astounding 82,000+  echolocation clicks, produced by porpoises, were recorded by the hydrophone over a complete ten day period! The time and duration of the porpoise feeding activity, summarised from the log of the POD hydrophone, is shown in the Figure. The time of day is along the horizontal axis. Each solid vertical bar indicates that a porpoise was detected echolocating / feeding during the hour of day shown. The start of each day is marked by a vertical dotted line, the point mid-way between the dotted lines is mid-day (12 noon). The main results from the POD acoustic data were: 

  • Porpoises were detected every day at Boatsroom Voe, with echolocation clicks in 79% of the logs, summarised for each hour. 

  • Porpoises appeared to feed mainly during daylight hours, shown by the frequency of peaks, which is most obvious on Monday 20th (less activity close to the vertical dotted line). This feeding activity may be related to the tidal cycle and behaviour of prey. Spring tides were running, and slack water was generally around mid-day for the last week of the study period . 

  • When porpoises were detected feeding, periods of echolocating (feeding activity) were relatively short. Few peaks reach the top of the graph, which would indicate that porpoises were feeding for the complete hour represented. There could be a number of reasons for this pattern of behaviour. More information is required, in particular what is the prey porpoises are chasing and how does the distribution of prey change throughout the day?  

  • There was some evidence (from the frequency of peaks) to suggest that porpoises echolocated less, or were not present in the Voe, when the wind increased above force 7 (northwesterly) on Monday 13th and Sunday 20th. During such weather, the shallow Voe is exposed to northerly winds and heavy seas may disperse shoals of fish (sandeels / sprats?). The periods of porpoise activity Wednesday 15th-16th correspond with days of westerly winds, and Monday 20th-21st the winds were northwesterly force 1-3.  The sensitivity of the hydrophone has been tested under various conditions and considered reliable. 

Time and duration of Porpoise feeding activity between Sunday 12th-Wednesday 21st March 2000, Boatsroom Voe

The initial results of this study, using a POD hydrophone, are interesting and very encouraging in terms of the amount of data collected. The hydrophone was obtained with the support of a Transco Grassroots Environmental Award / SeaWatch Foundation. Thanks also to the Setterness salmon farm workers for their interest, co-operation and good environmental practice by maintaining the intrinsic nature of the site. 

Systematic land-based watches for porpoises are planned for August this year. 30 sites were watched last August, involving members of the public and individuals from local groups (RSPB and Shetland Sea Mammal Group).  For more information about  participating with this project please contact Paul Fisher on 01595 890371.

 

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