READ and GASKIN: RADIO TRACKING HARBOR PORPOISES 



The movements of three radio-tagged porpoises 

 (RT-2, RT-4, RT-6) were tracked through the ma- 

 jor passages around Deer Island on seven occasions. 

 In all cases, the direction of movement coincided 

 with the direction of tidal flow. The strong correla- 

 tion between porpoise movements and current direc- 

 tion in these areas was demonstrated on 30 August 

 1981, when RT-2 moved up Western Passage with 

 the flood tide, turned at slack high water and moved 

 out with the ebb. 



Two radio-tagged porpoises were resighted on 

 several occasions. RT-2 was observed resting at the 

 surface in the approaches to Head Harbour Passage 

 on 22 August 1981. Although the porpoise was 

 alone, several groups of resting animals were pres- 

 ent in the vicinity. RT-3 was resighted on six occa- 

 sions; during five of these sightings the radio-tagged 

 animal was accompanied by a single large porpoise. 

 These observations gave no indication that the 

 transmitter packages affected the behavior of tag- 

 ged porpoises. 



Attempts to relocate radio-tagged animals 

 demonstrated some of the inherent problems in- 

 volved in censusing harbor porpoise populations. 

 Even with the aid of directional receivers and bright- 

 ly painted transmitters, it was difficult to sight a 

 tagged porpoise or to follow its movements after it 

 had been located. It proved particularly difficult to 

 see radio-tagged porpoises while they lay motion- 

 less at the surface. 



distance from porpoises swimming just below the 

 surface (see also Frost et al.^). 



Radio-tagged porpoises exhibited two readily 

 discernible activity states (Fig. 5). Low activity (or 

 relative inactivity) was characterized by frequent sur- 

 face resting periods interspersed with rolls; resting 

 periods accounted for over 55% of all signals in this 

 activity state Porpoises were considered active (high 

 activity) when resting periods were absent or infre- 

 quently recorded. It is important to note that por- 

 poises did not rest at the surface when wave height 

 was >30 cm and winds speeds exceeded 13 km/h (see 

 also Dudok van Heel 1962; Andersen and Dziedzic 

 1964). 



^Frost, K. J., L. F. Lowry, and R. R. Nelson. 1983. Investi- 

 gations of belukha whales in coastal waters of western and north- 

 ern Alaska, 1982-1983; marking and tracking of whales in Bristol 

 Bay. Final Report, Contract NA 81 RAC 00049, 104 p. 



J__J LI- 



ACTIVITY LEVEL HIGH 



u. 



 11 



  



J I I I L 



JJ I l—L 



_■_■ I L 



Patterns of Activity 



In total, 39.2 h of ventilation sequences were 

 recorded from four radio-tagged porpoises (RT-2, 

 RT-3, RT-4, RT-7). These sequences comprised 4,680 

 individual dives, lasting from 2 to 195 s. 



Two types of signals were received from radio- 

 tagged animals. The most common signal was brief 

 (1-3 s) and indicated that the porpoise had surfaced 

 and submerged in a continuous motion. Such action 

 patterns are commonly referred to as rolls (Amun- 

 din 1974; Smith et al. 1976). Other signals were more 

 prolonged (4-100 s) and are referred to here as sur- 

 face periods. 



Prolonged signals received from radio-tagged har- 

 bor porpoises have previously been interpreted as 

 near-surface swimming (Gaskin et al. 1975). 

 However, visual observations of radio-tagged animals 

 RT-2 and RT-3 indicated that such signals originated 

 from porpoises resting motionless at the surface. The 

 strength of the transmitted signal attenuated rapidly 

 as the length of exposed antenna decreased, making 

 it unlikely that signals could be received at any 



    



-60 seconds- 



ACTIVITY LEVEL LOW 



-60 secoods- 



FlGURE 5.— Examples of signal patterns used to derive activity 

 states of radio-tagged harbor porpoises (each example represents 

 a continuous record). Activity level was considered high when 

 signals were dominated by rolls (signal duration 1-3 s). Activity level 

 was considered low when signals were dominated by surface resting 

 intervals (signal duration >3 s). The signal pattern used to 

 demonstrate the high activity level (top) is characteristic of Pat- 

 tern B respiration (Watson and Gaskin 1983). 



549 



