SATELLITE-MONITORED MOVEMENTS AND DIVE BEHAVIOR OF A BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) IN TAMPA BAY, FLORIDA

MARINE MAMMAL SCIENCE, 11(4):452463 (October 1995) 0 1995 by the Society for Marine Mammalogy

SATELLITE-MONITORED MOVEMENTS AND DIVE BEHAVIOR OF A BOTTLENOSE DOLPHIN (TURSIOPS trUNCATUS) IN

TAMPA BAY, FLORIDA

BRUCE R. MATE, KELLY A. ROSSBACH, SHARON L. NIEUKIRK

Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, U.S.A.

RANDALL S. WELLS Chicago Zoological Society, % Mote Marine Laboratory,

Sarasota, Florida 34236, U.S.A.

A. BLAIR IRVINE

Dolphin Biology Research Institute, Sarasota, Florida 34242, U.S.A.

MICHAEL D. SCOTT

Inter-American Tropical Tuna Commission, % Scripps Institution of Oceanography, La Jolla, California 92037, U.S.A.

ANDREW J. READ Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, U.S.A.

Abstract

An adult, female bottlenose dolphin (Tursiops trucncatus) was radio tagged and monitored via satellite-based Argos receivers for 25 d from 28 June to 23 July 1990, in Tampa Bay, Florida. A total of 794 transmissions were obtained during 106 satellite passes. A mean of 3.9 (SE = 0.24) locations/day were determined by Service Argos and showed the animal remained in the bay, usually close to the southeastern shore. The dolphin moved at least 58 1 km at a minimum mean speed of 1.2 (SE = 0.1) km/h. Data from 63,922 dives were recorded. The animal spent an average of 87.1 (SE = 0.6)% of the time submerged, with a mean dive duration of 2 5.8 (SE = 0.5) sec. Mean dive duration differed significantly between four periods of the day, as did the mean percent of time spent submerged. During the early morning the animal spent more time at the surface, averaged shorter dives, and was submerged less than other times of da:y. This is the first study to demonstrate die1 dive cycles in a bottlenose dolphin. Four months after tag loss, the dolphin was photographed with no evidence of necrosis or disfigurement of the dorsal fin. Satellite telemetry was demonstrated

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MATE ET AL.: SATELLITE-MONITORED MOVEMENTS 453

as an effective means of documenting the movements and dive behavior of a small inshore cetacean.

Key words: satellite telemetry, radio-tracking, bottlenose dolphin, Tursiops truncatus, movements, dive behavior, dive patterns.

Dolphins are difficult to study in the wild because they spend most of their time underwater. Most observations are limited to daylight periods, during good sea and weather conditions. Many methods have been used to study dolphin movements: photoidentification (Wursig and Wiirsig 1977, Wiirsig 1978, Wells et al. 1987), aerial surveys (Leatherwood et al. 1978), mark and release (Irvine and Wells 1972, Irvine et al. 1981, Wells et al. 1987), theodolite tracking (Wursig and Wiirsig 1979), conventional radio telemetry (Evans 1971, Evans et al. 1972, Evans 1974, Leatherwood and Evans 1979, Leatherwood and Ljungblad 1979, Perrin et al. 1979, Irvine et al. 1981) and satellite-monitored radio telemetry (Tanaka 1987, Mate 1989, Mate et al. 1994). However, many questions regarding the movements of dolphins still remain unanswered.

Satellite telemetry offers the opportunity to study many aspects of movements and dive behavior (Mate 1989). It can provide data periodically throughout the day and night, even during adverse weather conditions. No field work is necessary after tagging to collect dive and location data, although close-range observations are still necessary to document data on tag attachment and behavior.

We used a satellite-monitored radio tag to monitor the movements and dive cycles of a bottlenose dolphin (Tursiops truncatus) in Tampa Bay, Florida, as a pilot test for small inshore cetaceans. The research objectives were to: (1) de- termine the utility of the attachment method, (2) examine the dolphin’s movements, and (3) characterize the dolphin’s dive behavior.

METHODS

The central west coast of Florida was chosen as the study area because it is the site of an ongoing, 24-yr study on bottlenose dolphins. The communities are well described, and identifiable individuals are resighted regularly (Wells 1978, 1986, 1991; Irvine et al. 1981; Wells et al. 1987; Scott et al. 1990a).

Tampa Bay (Fig. l), on the central west coast of Florida, is about 56 km long by 16 km wide. It is characterized by shallow sand flats and sea grass beds with an average water depth of l-3 m, dropping off to a bay bottom of 4-6 m depth. The Intracoastal Waterway traverses the bay and is dredged to a channel depth of 3 m. In midbay, a ship channel is maintained at 15 m depth.

On 28 June 1990 an adult female and a juvenile male bottlenose dolphin were caught at the north end of Anna Maria Sound in Tampa Bay, using an encircling net (Asper 1975). The tag consisted of a Telonics (Telonics, Inc., Mesa, AZ) ST-6 satellite-monitored radio transmitter in a stainless-steel cylinder which measured 12.5 cm long, 5.6 cm in diameter, and weighed 0.57 kg (Fig. 2). The tag was mounted to a molded, plastic “saddle” to fit the dorsal fin and was attached with five Delrin plastic pins (Scott et al. 19906). These inert


454 MARINE MAMMAL SCIENCE, VOL. 11, NO. 4, 1995

C, Tampa

Figure 1. Satellite-monitored movements of a bottlenose dolphin in Tampa. Bay, Florida. 0 = Argos-acquired locations; * = tagging location. The line connects locations for the first two days of travel showing that the dolphin only left the southeastern bay area immediately after tagging.

pins were designed to break and detach the saddle if the saddle or tag became entangled. After standard measurements were collected (Wells 1991) and a freeze-brand (Scott et al. 19906) was applied, the dolphin was tagged. The saddle attachment process took 14 min. The dolphin was released immediately after tagging in the company of the juvenile.

The tag summarized dive data every 6 h as follows: night [201 l-02 10 Eastern Daylight Time (EDT)}; early morning (02 1 l-0810 EDT); day (0131 l- 1410 EDT); afternoon/evening (14 1 l-20 10 EDT). Sunrise occurred at 064 1



Figure 2. Cylindrical satellite-monitored radio tag attached to molded plastic saddleon the dorsal fin.

EDT and sunset was at 2022 EDT on 28 June. These times drifted +11 minand –5 min, respectively, during the tracking period. The tag initiation time,which dictated the beginning of all summary periods, was established to max-imize the number of passes received during the tag’s transmission schedule. Ithad nothing to do with the animal’s biology or the times of sunrise and sunset.

Data were transmitted to Argos (Service Argos, Inc., Landover, MD) receiverson two NOAA TIROS-N weather satellites (NOAA 10 and 11) (Fancy et al.1988). The data were subsequently downloaded to ground stations and availablevia computer and modem from Service Argos.

Data from each 6-h summary period were transmitted during the first 2 hof the following period. During each 2-h transmission period, at least one satellitetypically passed overhead for 6–15 min. During transmission periods, a saltwaterswitch initiated transmissions only when the tag was above the water surface.Each transmission lasted 0.44 sec.

Transmitted summary data included mean dive duration (MDUR) and totalnumber of dives (DNUM) during each 6-h period. A dive was considered tobe any submergence > 6 sec. The tag also transmitted the duration of the mostrecent dive (DDUR). A reported DDUR of zero (0DDUR) represented a periodof 40 sec during which the animal did not submerge for more than 6 sec atone time.

On 7, 8, and 10 July, the tagged dolphin was visually relocated and pho-tographed during routine surveys by Scott and Wells. Dive data were recordedwhich we subsequently compared with tag-recorded data.

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Methods of data analysis-Argos locations are determined from the doppler- frequency shift produced by the rapidly moving satellite receiver (Argos 1990). A maximum of one location per satellite pass is provided by Service Argos. A location is only possible when multiple transmissions are received during a single satellite pass.

Location accuracy increases with the number of transmissions and the period of time between the first and last transmission in a satellite pass. Service Argos provides an estimate of location accuracy by class (LC) for each location: LC3 = f 150 m; LC2 = + 350 m; LCl = + 1 km; LCO = unrated (Argos 1990). Distances between Argos locations were calculated. Because animals mlay not have moved in a straight line between adjacent locations, reported speeds represent a minimum value for the time between determined locations. Speeds were used to screen LCO locations for errors.

From our experience, Argos sensor data typically have a 16%-20% error rate (Mate et al. 1992). All data were screened for “logical consistency”, and transmissions with identifiably erroneous data were removed before analyses. For example, it was impossible for a sampled dive to be 70 set long if a transmission occurred 60 set earlier.

Overall mean dive duration was determined by averaging the individual means from each period. The amount of time spent submerged was determined by MDUR x DNUM and was expressed as a percentage (%TSUB).

After testing for normality (Kolmogorov-Smirnoff test) and homogeneity of variance (Bartlett’s test), comparisons among periods were accomplished using ANOVA procedures (Sokal and Rohlf 1981). Multiple comparisons of means were done using Fisher’s Protected Least Significant Difference test (Peterson 1985). Where assumptions for parametric testing were not met, the Kruskal- Wallis test was used to compare means (Sokal and Rohlf 198 1). For all analyses the STATGRAPHICS@ (Manugistics, Inc. and Statgraphics Graphics Corp., Version 7.0 1993) and SAS@ (SAS Institute, Inc., Release 6.03 1985) packages were used and the level of statistical significance was set at 0.05. All means are reported with standard errors.

RESULTS

The tagged dolphin measured 2 57 cm and weighed 20 1 kg. She ‘was ac- companied by her five-year-old calf, as determined by evidence of lactation, genetic analysis of blood samples (Duffield and Wells 199 l), and age estimates from a tooth (Hohn et al. 1989) from the calf.

The tagged dolphin was tracked via satellite for 25 d (28 June to 23 July 1990). A total of 794 transmissions was recorded during IO6 satellite passes, with a range of 14-56 transmissions per day (R = 32 + 2 transmissions per day), and 2-6 satellite passes per day (Z = 4.1 -C 0.2 passes). Errors in sensor data were detected in 150 of the transmissions (19%).

Data were available for 91 of 99 (90%) possible summary periods in 25 d. Of these 91 periods, 5 (6%) contained insufficient data and were not included in the analyses.



Figure 3. Dorsal fin of tagged dolphin, photographed approximately four monthsafter tag loss.

Ninety-three useful locations were obtained via satellite, averaging 3.9 ±0.24 locations per d. Of 93 locations, 8% were rated by Argos as LC3 (themost accurate), 56% were LC2, 16% were LC1, and 19% were LC0 (unratedaccuracy). During the first two days after tagging the dolphin swam throughOld Tampa Bay, before moving to the southeast area of the bay where shesettled in a localized area for the next 23 d (Fig. 1).

The dolphin was visually located in the same general area on three occasionsbetween 7 and 10 July 1990. The dolphin was not seen again until 20 November1990, 114 d after the last received transmission, when we presume the transmitterwas shed. When photographed on 20 November 1990, the dorsal fin appearednormal with no signs of necrosis and little scarring (Fig. 3). From 20 November1990 to 17 September 1992 the animal was seen 36 times, primarily insouthwestern Tampa Bay. Further repigmentation of the minor scarring on thedorsal fin made it difficult to detect these marks.

The overall distance traveled during the 25 d the tag transmitted was at least581 km, with a minimum mean speed of 1.2 ± 0.1 km/h. The highestminimum mean speed recorded was 4.9 km/h during 1.6 h. The dolphinmoved fastest during the first 6 d (minimum average: 1.6 km/h). The longestdistance traveled in a day was 50.2 km.

A total of 63,922 dives was recorded via satellite during 86 six-hour summaryperiods. The overall mean DNUM was 743 ± 10 dives per period (124 dives/h), and ranged from 508 to 921 dives per period. The number of dives differedsignificantly among the four periods (ANOVA, P ± 0.001) (Table 1).

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Table 1, Dive durations, dive number, and percent of time submerged based on time of day, expressed as means and standard errors, for a satellite-monitored bottlenose dolphin.

Time of day

Mean dive Number Percent of duration of dives time submerged

n 2% SE z SE R SE

Night 25 24~6~ 0.5 762bc 16 85.9cd 0.8

Early morn 22 22.1c 0.6 806b 15 81.7= Day 22 26.6b 0.6 734c 89.7bd A’: Aft/Eve 17 31.5a 1.1 647”

:; 92.6b 0:5

* Significantly different periods are indicated with different superscript letters.

The overall mean dive duration was 25.8 + 0.5 sec. As expected from DNUM data, MDUR also differed significantly among the four summary periods (ANOVA, P -K 0.001) (Table l), and MDUR was negatively correlated with DNUM (r= -0.94, P < 0.001, n = 86). MDUR was longest in the afternoon/ evening (3 1.5 & 1.1 set) and shortest in the early morning (22.1 + 0.6 set). Consistent with the remote data, visual observations of dives on 7, 8, and 10 July taken between 1130 and 1545 EDT revealed a mean dive duration of 29.7 + 0.92 sec.

Of 644 DDURs recorded, only 14 (2%) were > 100 set, and 9 of the 14 (64%) occurred during the afternoon/evening. The longest DDUR (160 set) occurred during afternoon/evening. Forty-nine DDURs were measured as ODDURs (8% of all DDURs). Forty-six (94%) ODDURs occurred during early morning (20% of aI1 early morning DDURs), and 3 (6%) occurred during the day. No ODDURs were recorded during the afternoon/evening or night:. The DDURs, though a sample, reflected the same die1 patterns as the summary dive duration data.

The dolphin was submerged for 87.1 f 0.6% of the total time the tag recorded data. On average, more time was spent submerged during the afternoon/evening (92.6 1- 0.5%), and the least amount occurred during early morning (8 1.7 + 1.4%). The maximum mean percent time submerged for any one summary period was 95% (afternoon/evening), while the lowest mean percent time submerged for any one summary period occurred in the early morning (69%). Multiple comparisons showed significant differences in %T.SUB between periods (Kruskal Wallace test; Table 1). %TSUB was negatively correlated with DNUM (r = -0.5 1, P < 0.001, n = 86), and positively correlated with MDUR (Y = 0.75, P K 0.001, n = 86).

DISCUSSION

Effects of tagging-Irvine et al. (1982) reported two out of ten dolphins with conventional radio tags developed abnormal swimming behavior. Some of the radio-tagged dolphins experienced dorsal fin damage including, in one case, loss of the tip of the fin. This damage may have been due to pressure necrosis


MATE ET At.: SATELLITE-MONITORED MOVEMENTS 459

from hydrodynamic drag, tissue rejection, or attempts by dolphins to remove tags (Irvine et al. 1982).

During follow-up observations in the present study, the tagged dolphin appeared to exhibit typical swimming behavior and was in the presence of other dolphins and her male offspring (Bassos, personal communication). On 20 November 1990, 114 d after presumed tag loss, the satellite-monitored dolphin was photographed and positively identified by freeze brand and dorsal fin shape. The dorsal fin showed no signs of necrosis, and only light discoloration where three of the pins had been attached. These encouraging results may be due to the use of inert plastic pins, which were sufficiently large and numerous enough to reduce pressure necrosis from the hydrodynamic drag of the tag.

Movements--The dolphin in this study moved at least 58 1 km in 25 d and averaged a minimum of 23.7 + 2.4 km/d. Similarly, in Sarasota Bay, Florida, north-south movements of up to 30 km in a day have been occasionally observed in resident bottlenose dolphins (Irvine et al. 1981). Tanaka (1987) reported that a satellite-monitored bottlenose dolphin off Japan moved about 604 km in 18 d (33.6 km/d). VHF d - ra io monitored dusky dolphins (Lagenorhynchus obscurus) moved a minimum of 19.2 km/d in Golfo San Jose, Argentina (Wursig 1982).

The extended movements of the satellite-monitored dolphin during the first week after tagging may represent a period of adjustment. The first two days she moved extensively through Tampa Bay before settling in a specific area where she was visually identified on 39 occasions through 17 September 1992. Irvine et al. (1982) also saw “long movements” immediately after release. However, at the time of tagging the dolphin in our study was outside the range in which she subsequently settled. Photo I.D. surveys in the area during the past 24 years also indicate that she was not a resident of the region where she was caught for tagging (Wells 199 1). In addition, the animal showed a slightly elevated (2.5 ng/ml) progesterone level, indicative of estrus. Female bottlenose dolphins show baseline progesterone levels of < 1 ng/ml, whereas ovulating females show levels of 3-20 ng/ml (Kirby 1984). Some female bottlenose dolphins in Sarasota have been observed to range more widely while exhibiting elevated progesterone levels. Therefore, the extensive movements of the dolphin may reflect natural patterns.

Average swim speed has been reported from several bottlenose dolphin populations by various methods. Dolphins at Sanibel Island, Florida (Shane 1990), Argentina (Wursig and Wursig 1979) and Sarasota Bay, Florida (Irvine et al. 1981) usually travel at speeds less than 6 km/h in water < 10 m deep. A juvenile male bottlenose dolphin off the coast of Wales moved at an average speed of lo- 15 km/h (Lockyer and Morris 1987).

Off the coast of Japan, Tanaka (1987) found that minimum speeds of 11 satellite-monitored bottlenose dolphins varied from 0.1 to 12 3.7 m/min (O- 7.4 km/h) and noted that animals moved faster after release than other times. These speeds and the immediate post-release high speeds are similar to those calculated in our study (4.9 km/h). I n our study, speed was calculated from the straight line distance between location points and does not reflect movements


460 MARINE MAMMAL SCIENCE. VOL. 11. NO. 4. 1995

of the dolphin. The movements of the first two days after tagging may have been more linear than the next 23 d and thus been a better estimate of true speed.

Dive duration-Literature reports suggest that the average dive duration of bottlenose dolphins ranges from 20 to 40 sec. Coastal bottlenose dolphins in shallow Argentine waters (< 10 m) had a mean dive duration of 2 1.8 set (Wiirsig 1978), and bottlenose dolphins off Sanibel Island, Florida, averaged 20-25 set dives in waters less than 4.5 m (Shane 1990). Dolphins tagged with conventional radio tags in Sarasota Bay, Florida, had a mean dive duration of 30-40 set in water l-4 m (Irvine et al. 1981). Two bottlenose dolphins (Echo and Misha) reintroduced into Tampa Bay had mean dive durations of 20.5 set (recalculated SE = 1.6, n = 120) and 25.9 set (recalculated SE = 1.2, n = 197) respectively, during daylight hours (Bassos 1993). The dolphin in the present study is one of the top ten social associates of Misha (Bassos 1993). The daytime MDUR of the satellite-monitored dolphin (26.6 set & 0.6) was virtually identical to Misha’s (25.9 set + recalculated SE = 1.2, n = 197).

Behavior cycles-Diurnal behavior cycles have been recorded in several populations of bottlenose dolphins. Off the coast of Argentina, bottlenose dolphins rest in the mornings and are more active in the late afternoon (Wiirsig and Wiirsig 1979). In South Africa (Saayman et al. 1973), Sanibel Island, Florida (Shane 1990), and Mobile Point, Alabama (Goodwin 1985), bottlenose dolphin feeding is reported to peak during the morning and late afternoon. According to Shane (1990) bottlenose dolphins at Sanibel Island, Florida, socialize most during the evening. This is similar to those off South Africa where mating (synonymous with “socializing” in this case) behavior occurs mostly in late morning and late afternoon (Saayman et al. 1973). Diurnal behavior cycles have not been observed in Sarasota Bay dolphins (Irvine et al. 1981, Scott et al. 1990a). However, at night some radio-tracked dolphins there spent periods of time at the surface, with little movement (Scott et al. 1990a; unpublished data). In our study, the large number of ODDURs between approximately 0200 and 0400 EDT, together with a shorter MDUR, and lower %TSUB during the early morning, suggest the satellite-monitored dolphin may have been surface resting.

Two dolphins reintroduced into Tampa Bay fed most often during the morning, traveling peaked during the early morning and late afternoon, and socializing increased through the day (Bassos 1993). Further, the two dolphins had their longest dives while traveling and their shortest dives during social activity. Resting behavior was rarely seen during observations limited to the daytime (Bassos 1993). If the satellite-monitored dolphin had similar behavior and dive habits to the reintroduced dolphins, the longer MDUR and higher %TSUB in the late afternoon/evening may reflect traveling during these periods.

This study presents the longest track and the most detailed description of dive behavior for a bottlenose dolphin to date. It demonstrates the effectiveness of satellite-monitored tracking and a tag attachment technique that caused no discernible problems to the dolphin’s health or subsequent behavior. This is the first demonstration of die1 dive cycles in a free-ranging bottlenose dolphin.



Satellite-monitored radio tags may be useful in future studies to answer questions of synchronous school behavior, seasonal home range, migration, sightability, and stock identification of both inshore and offshore bottlenose dolphins.

ACKNOWLEDGMENTS

This project was funded by the Oregon State University Endowed Marine Mammal Program. We thank Oregon State University volunteers Will Emery, Jeannie Gorlick, Lisa Lammers, Rob Mate, Judy Mello, Gail Whitman, and Sunny Valley and Earthwatch for field assistance; Rod Mesecar for engineering and tag construction; Kim Bassos for sharing field observations; Toby Martin for helping identify data errors; Martha Winsor for assistance with statistics; Very1 Barry for clerical assistance; and Kate Stafford for review of manuscript drafts. We also thank two anonymous reviewers for improving the manuscript.

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Received: 18 August 1994 Accepted: 6 February 1995


SATELLITE-MONITORED MOVEMENTS AND DIVE BEHAVIOR OF A BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) IN TAMPA BAY, FLORIDA

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