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22001177 SOUTHEASTERN NATURALIST 1V6o(2l.) :1269,7 N–3o1. 62
Movement Patterns of Sphyrna tiburo (Bonnethead Shark) in
a Shallow Tidal Creek System
Dontrece T. Smith1,* and Mary Carla Curran1
Abstract - The purpose of this study was to use acoustic telemetry to elucidate the
small-scale habitat-utilization patterns of Sphyrna tiburo (Bonnethead Shark, hereafter,
Bonnethead) in relation to diel and tidal cycles in a shallow tidal creek system along the coast
of Georgia. We found that Bonnetheads utilized the main channel of a tidal creek at night,
dawn, and during ebb and low tides. In addition, we found the first evidence of Bonnetheads
utilizing smaller 3rd-order creeks. Bonnetheads used these tributaries at night and dawn, during
flood and high tides when the water level facilitated access. The movement patterns of
these Bonnetheads could be representative of those in other areas with semidiurnal tides.
Introduction
Estuaries and nearshore habitats are dynamic environments with variable conditions
(Morin et al. 1992). These areas are highly productive and support the
development of many low trophic-level marine organisms (e.g., Callinectes sapidus
Rathbun [Blue Crab], penaeid shrimp, and teleost fishes) (Blaber and Blaber 1980,
Blaber et al. 1989, Zimmerman et al. 2000). These organisms serve as prey resources
for many upper trophic level organisms (e.g., sharks) (Cortés et al. 1996, Ellis and
Musick 2007). Sharks play an important role in the estuarine and nearshore foodwebs
causing a top-down effect in ecosystems (Cortés 1999). Many coastal shark
species utilize these ecosystems at all life stages, such as Rhizprionodon terranovae
(Richardson) (Atlantic Sharpnose Shark), Carcharhinus leucas (Müller and Henle)
(Bull Shark), and Sphyrna tiburo (L.) (Bonnethead Shark, hereafter, Bonnethead)
(Froeschke et al. 2010, McCandless et al. 2007, White and Potter 2004).
Bonnetheads are small sharks in the order Carcharhiniformes (Compagno 1984).
They are found in coastal and estuarine waters along the Atlantic and Pacific coasts
of both North and South America (Compagno 1984). Bonnetheads are migratory
and tend to travel in aggregations of 3–15 individuals (Compagno et al. 2005).
Some aspects of their life history have been well studied, including age and growth
(Carlson and Parsons 1997, Frazier et al. 2014, Lombardi–Carlson et al. 2003,
Parsons 1993a), feeding ecology (Bethea et al. 2007, Cortés et al. 1996, Lessa and
Almeida 1998), nursery areas (Gurshin 2007), and reproductive biology (Cortés
and Parsons 1996, Gelsleichter et al. 2002, Gonzalez De Acevedo 2014, Lessa and
Silva 1992, Manire 2001, Manire et al. 1995, Parsons 1993b).
The overall geographic distribution of Bonnetheads has been well documented,
but the pattern across regions is not consistent (Driggers et al. 2014, Heupel et
1Marine Sciences Program, Box 20467, Savannah State University, Savannah, GA 31404.
*Corresponding author - dtsmith@uga.edu.
Manuscript Editor: Lance Williams
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al. 2006, Ubeda et al. 2009). In South Carolina, Driggers et al. (2014) found that
Bonnetheads exhibited high degrees of intra- and inter-annual site fidelity. Those
authors marked a total of 2300 Bonnetheads with traditional external tags; 177
sharks were recaptured after 3 d to 8.9 y at liberty (noncontinuously), and 171 of
those individuals were recaptured in the same estuary in which they were tagged.
Furthermore, groups of 2–5 individuals that were tagged together were subsequently
recaptured together. Driggers et al. (2014) speculated that the site fidelity of
Bonnetheads in South Carolina waters may be attributed to the seasonal occurrence
of energetically valuable prey (e.g., ovigerous Blue Crabs), and the social transmission
of productive feeding area locations among individuals.
Heupel et al. (2006) studied Bonnetheads residing in a coastal Florida estuary in
the Gulf of Mexico and found that their movement was not related to time of day
or tidal cycle. Bonnetheads utilized small areas of the study site on a daily basis,
but did not exhibit site fidelity to any particular areas. Like Driggers et al. (2014),
Heupel et al. (2006) believed these highly variable movement patterns were associated
with the abundance of Blue Crabs, which are distributed throughout the area.
The widespread utilization of the area by Bonnetheads could be an example of “patrolling”
behavior in search of prey, which was observed by Myrberg and Gruber
(1974). In another study conducted in the Gulf of Mexico, the movement patterns of
Bonnetheads were affected by changes in salinity (Ubeda et al. 2009). Bonnetheads
were located in northern, central, and southern zones of Pine Island Sound, FL,
where salinity was ≥20.0–25.5 psu (Ubeda et al. 2009). Similarly, Belcher (2008)
found that salinity and turbidity influenced the presence of Bonnetheads in coastal
Georgia, where individuals were found in areas with low turbidity (mean = 16.3
NTU) and high salinity (mean = 32.0 ppt).
Although several ecological studies on Bonnetheads have been conducted
in the Gulf of Mexico and South Carolina, little is known about their behavior in
relation to tidal and diel cycles in the coastal waters of Georgia, where there are
high tidal ranges of ~2.5 m that provide access to a wide range of geographical
areas and creek orders that are inaccessible during low tide. These areas consist of
Crassostrea virginica Gmelin (Eastern Oyster) beds and Spartina alterniflora (Loisel)
(Smooth Cordgrass) salt marshes that support many marine organisms (e.g.,
callinectid crabs) that serve as a prey for Bonnetheads and other elasmobranchs
(Hettler 1989, Rozas and Reed 1993, Wells 1961). Heupel et al. (2006) determined
that the movement patterns of Bonnetheads were not correlated to tidal height or
time of day in a coastal Florida estuary that experienced a mixed semidiurnal tidal
cycle with a lower tidal range of ~1.0 m. These smaller tidal ranges allow Bonnetheads
and other shark species to frequent all areas throughout the entire tidal
cycle. However, Bonnetheads can only access certain areas of the estuaries during
flooding and high tides in coastal Georgia. Understanding the behavior of Bonnetheads
across a variety of tidal regimes in the southeastern US could elucidate
the importance of tidal-creek systems and estuarine waterways to the life history
of elasmobranchs. Therefore, the purpose of the present study was to provide the
first information on the small-scale movement patterns of Bonnetheads in relation
to diel and tidal cycles within a Georgia coastal system.
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Field–site Description
The Romerly Marsh Creek system (31°55'46.45''N, 80°59'4.66''W) is located
near Savannah, GA (Fig. 1). This estuarine area has a semidiurnal tidal cycle, with
a tidal range of 0.25–2.84 m. For the purposes of this study, we divided the creek
system into 2 components: the main channel of Romerly Marsh Creek and its
smaller tributaries. We followed the methods of Horton (1945) and classified the
main channel, which was the widest waterway, as a 4th-order creek and the tributaries
as 1st–3rd-order creeks. The 1st-order creeks in this area are exposed during low
tide (Brinton 2015). The main channel is ~1.5 km long and varies in depth from 0.6
m to 11.9 m at mean lower low-water level. Several creek tributaries that vary in
width from 5 m to 120 m and in depth from 0.5 m to 6 m flow into the main channel.
Smooth Cordgrass is the dominant vegetation, and there are numerous live and
dead oyster beds along the creek banks.
Methods
Shark sampling
Shark collection and tagging protocols were approved by the Savannah State
University Institutional Animal Care and Use Committee (IACUC). We collected 13
Bonnetheads using a hook and line baited with Loligo spp. (squid) during August–
September 2011 and April–June 2012. We determined the sex, took measurements,
and assessed maturity based on size (Compagno et al. 2005) of all Bonnetheads
caught. We predetermined the minimum size for tagging as 70 cm total length (TL)
and/or 0.6 kg so that the tag was no more than 2% of the body weight. We removed
each captured shark from the water and placed it in tonic immobility by dorsoventrally
inverting the individual, a technique described by Watsky et al. (1990). We
outfitted each shark with a VEMCO coded V16–4H acoustic transmitter tag (16 mm
x 67 mm, 158-dB power output, 12 g in water, 69 kHz, 60-s nominal delay; VEMCO,
Bedford, NS, Canada) and Dalton rototag (Dalton, Fort Atkinson, WI). Transmitters
were externally attached to the anterior margin of the first dorsal fin using cable ties
and marine epoxy by drilling 2 holes through the base of the first dorsal fin (Simpfendorfer
et al. 2010). We attached a rototag near the posterior margin of the first dorsal
fin. We completed the attachment procedure within 5 min of removing the shark from
the water. After tagging, we held the shark by the caudal peduncle and placed it back
Figure 1 (following page). Map of Romerly Marsh Creek, Wassaw Sound, GA, with locations
of 7–8 VEMCO VR2W acoustic receivers deployed from (a) 18 August–22 November
2011 and( b) 2 February–31 July 2012. Each black dot represents a VR2W acoustic receiver.
All sharks were caught and released in the main channel or tributaries within the study area
between both gates. The percentage of Bonnethead Shark detections per receiver during
each deployment is shown in parentheses. The creek orders are listed as 2nd, 3rd, or 4th. Receivers
8, 9, and 11 were placed in tributaries in 2012. Maps were created in ArcGIS® 9.3
(ESRI, Inc., CA, USA), using the National Hydrography Dataset (NHD) from the United
States Geological Survey (USGS; http://nhd.usgs.gov/). Maps were created using the Universal
Transverse Mercator (UTM) projection.
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Figure 1. [Caption on previous page.]
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in the water. Each individual was released once it was able to swim away on its own, a
period of 1–3 min. Methods are further described in Smith (2012).
Receiver array and deployment
We employed acoustic receivers to passively monitor the movement patterns
of tagged Bonnetheads. In 2011, the receiver array consisted of 7 VEMCO VR2W
single channel acoustic receivers (Fig. 1a). We deployed 7 receivers in the main
channel on 17 and 18 August 2011. Four of the 7 receivers (receivers 1, 2, 6, and
7) were configured as “gates” in the study area (Fig. 1a). We defined a gate as 2
receivers with overlapping detection ranges spanning the entire creek width. In
2011, the receiver array was designed to determine the utilization of this creek system
in relation to time and tidal cycle. We removed all receivers on 22 November
2011 and redeployed them on 1 and 2 February 2012, at which time we deployed
an additional receiver and reconfigured the receiver array to accommodate the new
receiver; receivers were still used to form gates. We redesigned the array in order to
monitor Bonnethead activity in smaller tributaries as well as in the main channel in
relation to time and tidal cycle. This array consisted of 5 receivers (#1, 2, 4, 5, 10)
in the main channel and 3 receivers (#8, 9, 11) in 3 of 6 adjoining tributaries (Fig.
1b). We caught and released all sharks in the main channel or tributaries within the
study area between both gates.
Statistical analyses
We included for analysis only sharks detected for ≥7 days, as recommended by
Heupel et al. (2006), who also studied Bonnetheads that were using the study area
on a consistent basis. Eight out of 13 sharks met this criterion. We downloaded and
sorted detection data into hourly bins. We used these data to analyze the presence
and movement of Bonnetheads in relation to tide and time of day. We determined
presence and movement based on whether a shark was detected on a receiver in a
given hour instead of on the total number of detections because several environmental
factors such as seasonal temperature changes daily tides, and episodic weather
events (Mathies et al. 2014), and biotic noises, such as Alpheidae (snapping shrimp)
(Cotton 2010), could potentially interfere with the detection range of acoustic receivers
and thereby make the absolute number of detections uninformative. We
considered a Bonnethead as present during a given hour only if ≥2 detections were
recorded per hour on that same day by ≥2 receivers and we classified any Bonnethead
as moving during a given hour if ≥2 detections were recorded by ≥2 receivers
in the same hour for that day. This approach helped eliminate any possibility of
counting false detections in the statistical analyses. We analyzed the main channel
for both Bonnethead presence and movement in relation to time and tide. Only one
receiver was placed in each tributary; thus, only presence was analyzed in these areas.
We employed a Rao’s spacing test to determine if the presence and movements
of Bonnetheads were evenly distributed over an entire day. To perform the Rao’s
spacing tests, we used the Naval Oceanography Portal (2011) sunrise/sunset data
to define hourly bins as dawn (0500–0559), day (0600–1859), dusk (1900–1959),
or night (2000–0459). Therefore, dawn and dusk were each only 1 h, while day and
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night were each 11 h. We displayed the number of detections or movements during
each hour as rose diagrams with 24 sections, each representing 1 h. We also classified
hourly bins as high, low, ebb, or flood tide; a complete tidal cycle was ~12 h
25 min. We converted the classified bins converted to angles between 0° and 360°
so that they could be used to create rose diagrams with 24 sections representing 1
of the 4 tidal stages. We used NOAA Ocean Service Education data (NOAA 2011)
to assign each tidal classification to a portion of the rose diagram (0–360º): low
(357–002º; 45 min), flood (003–176º; 327.5 min), high (177–182º; 45 min), and
ebb (183–356º; 327.5 min). We employed Rayleigh’s uniformity test to determine if
the presence and movements of Bonnetheads were uniformly distributed or showed
some directionality/uniform direction over time and tidal cycle. We created all rose
diagrams in Oriana (v.3) software (Kovach Computing Services, Pentraeth, UK).
We report significance of results where P < 0.05.
Results
Range test
We performed range tests in 2011 and 2012 to determine the reliable detection
range of receivers and transmitters. The range test transmitter had a 180-s nominal
delay in contrast to the 60-s delay of the transmitters that were attached to Bonnetheads.
We conducted repeated range tests 10–100 m from a VR2W receiver in the
main channel of Romerly Marsh Creek. We submerged the transmitter 1 m above the
benthos for 10 min at 10 m from the receiver and repeated this step every 10 m to a
maximum distance of 100 m. We employed ANOVA to analyze whether diel period or
tidal stage affected the detection frequency of the transmitter as distance increased.
The Pearson correlation coefficient and a Cross–Fourier analysis performed using
MATLAB® (MathWorks, Inc., Natick, MA) were used to assess the relationship
between current velocity (m·s–1) and the detection frequency of the transmitter. We
obtained current velocity and tidal-height data from the Nobeltec Tides and Currents
software (Nobeltec, Beaverton, OR). Time of day and tidal cycle did not have a
statistically significant effect on the detection frequency of the control tag (Table 1).
Current velocity and detection frequency were not significantly correlated in either
2011 (P > 0.05, r2 < 0.004) or 2012 (P > 0.05, r2 < 0.004) (Table 1). We defined as reliable
a detection range where at least 50% of detections were recorded by a receiver
(Bertelsen and Hornbeck 2009, Bessudo et al. 2011, Kessel et al. 2014, Ramsden et
Table 1. Range-test results for analysis of variance (ANOVA) on the number of detections h–1 based
on time of day and tidal cycle and for Pearson correlation coefficient on the number of detections h–1
based on current velocity for the control transmitter tag in 20 11 and 2012.
Statistical analyses Variables P R2
ANOVA (2011) Time of day 0.701 N/A
Tidal cycle 0.681 N/A
ANOVA (2012) Time of day 0.433 N/A
Tidal cycle 0.554 N/A
Pearson Correlation Coefficient (2011) Current velocity 0.945 0.003
Pearson Correlation Coefficient (2012) Current velocity 0.119 0.002
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al. 2016). Our maximum reliable detection range was determined to be 70 m because
fewer than 50% of the detections were recorded at and beyond 80 m.
Detections and receivers
We placed acoustic tags on 13 Bonnetheads in Romerly Marsh Creek (9 in 2011
and 4 in 2012; Table 2); 10 were females and 3 were males. We collected mature,
juvenile, and neonate Bonnetheads, but did not tag neonates because of the high
transmitter-to-body-weight ratio. We detected all tagged sharks after the date of
release except 2 individuals (Table 2). In 2011, we recorded over 29,000 detections
on the receivers; detections per shark varied from 158 to 8023 (mean = 4184.9 ±
3033.9) (Table 3). We recorded individual sharks for periods of 1–29 d (mean =
15.9 d ± 10.9; Table 2) over 2–58 tidal cycles (mean = 31.7 ± 21.7; Table 3). We
recorded >14,000 detections in 2012 (variation = 436–7979 detections per shark,
mean = 3659.2 ± 3165.3; Table 3). We recorded individual sharks for periods of
3–49 d (mean = 19.2 d ± 20.4; Table 2) over 3–98 tidal cycles (mean = 37.7 ± 41.7;
Table 3). In both years, the number of mean detections per day was greater than
200 (Table 3). We detected females for a greater number of days and for a longer
period than males in 2011 and 2012 (Table 2). Receiver 4 was located in the main
channel and recorded the highest percentage of detections in 2011 (31%; Fig. 1a)
and 2012 (33%; Fig. 1b). Receiver 11 was located in a 3rd-order creek and recorded
the highest percentage of detections in the tributaries in 2012 (9%; Fig. 1b). No
Bonnetheads tagged in 2011 returned to the study area in 2012. We detected 2 Bonnetheads
(a female of 105 cm TL in 2011 and a male of 86 cm TL in 2012) a few
days after they left the study area in Ossabaw Sound, GA (8 km south of Romerly
Marsh Creek) for periods of 3 and 13 d, respectively (C. Kalinowsky, GA Department
of Natural Resources, Richmond Hill, GA, pers. comm.). The majority of
Bonnetheads (8 of 13) left the range of the receiver array during the day (Table 3)
and 8 of the 13 Bonnetheads left the array during ebb or low tide (Table 3). All
Bonnetheads were last recorded near the gate receivers (Table 3). Of the 13 Bonnetheads,
only 8 individuals were detected for > 7d in the study area, and we included
those Bonnetheads in the subsequent statistical analyses (Heupel et al. 2006).
Diel period
We detected 8 Bonnetheads in the main channel throughout the 24-h period.
Rao’s test indicated that Bonnethead presence and movement were not uniformly
distributed over 24 h in the main channel (Table 4). Although Bonnetheads were
present in the main channel at all times of the day, they were primarily there at
night and dawn (Fig. 2a). Results of the Rayleigh’s uniformity test also indicated
that time of day had a significant effect on the presence of Bonnetheads in the main
channel; individuals were present in this section primarily at dawn (mean vector
= 05:48; Table 4). Time of day also had a significant effect on the movements of
Bonnetheads in the main channel; individuals moved within the main channel most
often at night (mean vector = 03:51; Table 4, Fig. 2b).
Bonnethead presence in the tributaries was not uniformly distributed over a
24-h period (Table 4). Similar to the main channel, Bonnetheads were present
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Table 2. Bonnethead Sharks tagged within the Romerly Marsh Creek system near Savannah, GA. The mean number of days detected and mean period of
detection ± 1 SD are presented for the 13 individuals collected. Number of days detected = the total number of days a shark was recorded during the period
of detection. Period of detection = the range of days a shark was recorded in the receiver array. TL = total length. FL = fork length. N/A means information
was not recorded. Bonnethead numbers marked with an asterisk (*) represent individuals that were included in the statistical ana lysis.
Bonnethead Transmitter TL FL Weight Date of # of days Period of
number ID Sex (cm) (cm) (kg) Date tagged last detection detected detection
2011
1 32233 F 96.0 81.0 3.8 19 August 2011 19 Aug 2011 1 1
2 32234 F 122.0 103.0 N/A 19 August 2011 N/A 0 0
3 32236 M 76.0 58.5 1.8 01 September 2011 N/A 0 0
4* 31925 F 87.0 68.0 3.5 09 September 2011 18 October 2011 29 40
5* 31927 F 105.0 85.6 5.7 09 September 2011 08 October 2011 28 30
6* 31928 F 72.0 56.0 2.1 09 September 2011 29 September 2011 17 21
7* 32235 M 80.0 61.6 2.3 13 September 2011 29 September 2011 16 17
8* 32237 F 103.0 89.0 6.3 15 September 2011 06 October 2011 17 25
9 32238 F 73.0 58.0 1.9 15 September 2011 17 September 2011 3 3
Mean ±SD 15.9 ± 10.9 19.6 ± 14.0
2012
10 31926 M 86.0 69.0 2.5 10 May 2012 12 May 2012 3 3
11* 30467 F 90.0 70.0 3.0 29 June 2012 28 Aug 2012 49 61
12* 30468 F 106.6 94.8 5.5 29 June 2012 17 July 2012 15 19
13* 30470 F 106.6 96.0 5.5 29 June 2012 19 July 2012 10 21
Mean ±SD 19.2 ± 20.4 26.0 ± 24.7
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most often during night and dawn (Fig. 2c). Bonnetheads were generally less
present during the day. The Rayleigh’s uniformity test indicated that time of day
had a significant effect on the presence of Bonnetheads in the tributaries, with
individuals present most often at night (mean vector = 00:30; Table 4, Fig. 2c).
Overall, Bonnetheads were nocturnal and crepuscular in the main channel and
tributaries. We did not assess movement in the tributaries because we placed only
1 receiver in each tributary.
Tidal stage
Neither the presence nor movement of Bonnetheads was uniformly distributed
over the tidal cycle in the main channel (Table 4). Bonnetheads were present
in the main channel primarily during ebb and low tide. Results from the Rayleigh’s
uniformity test also indicated that tidal cycle had a significant effect on
the presence of Bonnetheads; individuals were present within the main channel
during ebb tide (mean vector = 348.41º) (Table 4, Fig. 3a). Tidal cycle had a significant
effect on the movements of Bonnetheads—individuals moved within the
main channel most often during ebb and low tide (mean vector = 347.39º) (Table
4, Fig. 3b).
Table 3. Bonnethead Sharks monitored within the Romerly Marsh Creek system near Savannah, GA.
The mean number of detections, detections per day, and mean number of tidal cycles over which individuals
were detected ± 1 SD are presented for the 13 individuals. Number of detections = the total
number of detections for each shark on all receivers regardless of presence or movement for the entire
study period. N/A means information was not recorded. Rows marked with an asterisk (*) represent
individuals that were included in the statistical analysis.
# of Last Last Last
Bonnethead TL # of Detections tidal cycles time of day tidal cycle receiver
number (cm) detections per day detected detected detected detected
2011
1 96.0 183 183 2 Night Low Gate
2 122.0 N/A N/A N/A N/A N/A N/A
3 76.0 N/A N/A N/A N/A N/A N/A
4* 87.0 5080 175 58 Day Ebb Gate
5* 105.0 5297 189 56 Day Ebb Gate
6* 72.0 3890 229 34 Day Low Gate
7* 80.0 6663 392 32 Day Ebb Gate
8* 103.0 8023 472 34 Night High Gate
9 73.0 158 52 6 Day Flood Gate
Mean 4184.9 241.7 31.7
± SD ± 3033.9 ± 142.8 ± 21.7
2012
10 86.0 436 145 3 Day Ebb Gate
11* 90.0 7979 162 98 Day Low Gate
12* 106.6 3562 237 30 Day Low Gate
13* 106.0 2660 266 20 Day Ebb Gate
Mean 3659.2 202.5 37.7
± SD ± 3165.3 ± 58.2 ± 41.7
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Table 4. Rao’s spacing test and Rayleigh’s uniformity test for the presence and movement of Bonnethead Sharks in relation to time of day and tidal cycle
in the main channel and tributaries of the Romerly Marsh Creek system near Savannah, GA. U = the test statistic; μ = the mean vector for time (time of
day) and angle (tidal cycle); and r = the length of mean vector, which can range from 0–1; this value indicates whether the data were dispersed (close to
0) or clumped (close to 1). * indicates that detections were not uniformly distributed.
Time of day Tidal cycle
Rao’s Spacing Test Rayleigh’s Uniformity Test Rao’s Spacing Test Rayleigh’s Uniformity Test
Location Variable U P μ r P U P μ r P
Main channel Presence 352.96 less than 0.05* 05:48 (dawn) 0.100 less than 0.05* 354.28 less than 0.05* 348.41º (ebb) 0.304 less than 0.05*
Main channel Movement 348.98 less than 0.05* 03:51 (night) 0.179 less than 0.05* 343.59 less than 0.05* 347.39º (ebb) 0.341 less than 0.05*
Tributaries Presence 276.92 less than 0.05* 00:30 (night) 0.185 less than 0.05* 220.97 less than 0.05* 133.36º (flood) 0.118 less than 0.05*
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Figure 2. Rose diagrams representing
the (a) presence and (b)
movement of Bonnethead Sharks in
relation to time of day in the main
channel in 2011 (7 receivers, 5 individuals)
and 2012 (5 receivers, 3
individuals) combined, and (c) presence
in relation to time of day in the
tributaries in 2012 (3 receivers; 3
individuals). Movement in the tributaries
was not assessed because only
1 receiver was placed in each. The
concentric circles on the diagrams
represent the number of observations
per hour. The black line with
the arc represents the mean vector.
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Figure 3. Rose diagrams representing
the (a) presence and (b) movement
of Bonnethead Sharks in relation
to tidal cycle in the main channel
in 2011 (7 receivers; 5 individuals)
and 2012 (5 receivers; 3 individuals)
combined, and (c) presence in relation
to tidal cycle in the tributaries
in 2012 (3 receivers; 3 individuals).
Movement in the tributaries was not
assessed because only 1 receiver was
placed in each. Each tidal classification
was represented by a portion of
the 360º rose diagram: high (177–
182º), ebb (183–356º), low (357–2º),
and flood (3–176º). The concentric
circles on the diagrams represent
the number of observations per tidal
stage. The black line with the arc
represents the mean vector.
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The presence of Bonnetheads in relation to tide within the tributaries was not
uniform (Table 4). Bonnetheads were present most often during flood and high
tide, but were rarely observed during ebb and low tide within the tributaries (Fig
3c). Tidal cycle had a significant effect on the presence of Bonnetheads; individuals
were present most often during flood tides (mean vector = 133.36º) once water
levels in the tributaries began to rise (Table 4, Fig. 3c). We recorded these differences
in presence between the main channel and tributaries on multiple occasions.
For example, 2 female Bonnetheads (11 and 12) were present in a small tributary
(Receiver 11) during flood and high tide, and in the main channel (Receivers 1, 2,
4, 5, and 10) during ebb and low tide. Movement in the tributaries was not assessed
because we placed only one receiver in each tributary .
Discussion
This study provides the first documentation of the habitat-utilization patterns of
Bonnetheads in relation to tidal and diel cycles within a shallow tidal-creek system
on the southeast US Atlantic coast. Bonnetheads utilized the main channel primarily
during ebb and low tide, and the tributaries most often during flood and high tide
in Romerly Marsh Creek. These results indicate that tidal stage influences the habitat
utilization of this species; Bonnetheads move to shallower areas with the rising
tide. Other species of elasmobranchs have exhibited similar tidally oriented movements.
Triakis semifasciata (Girard) (Leopard Shark) and Carcharhinus plumbeus
(Nardo) (Sandbar Shark) showed pronounced movement-patterns with the incoming
and outgoing tidal currents (Ackerman et al. 2000, Medved and Marshall 1983).
Wetherbee and Rechisky (2000) found that juvenile Sandbar Sharks traveled with
the incoming tide into Delaware Bay, individuals were located farthest inshore at
high tide and close to the mouth of the bay at low tide. Brinton and Curran (2017)
reported that Dasyatis sabina (Lesueur) (Atlantic Stingray) utilized shallow creeks
significantly more often during the flood tide. In Brazil, Wetherbee et al. (2007)
observed neonate Negaprion brevirostris (Poey) (Lemon Shark) moving from shallow
tidal creeks at high tide to small tide pools on reef flats at low tide.
In the present study, Bonnetheads moved into the tributaries (e.g., 3rd-order
creeks) during flood and high tides on multiple occasions. Bonnetheads may have
moved into these tributaries with the incoming tides to forage or follow prey moving
with the tidal currents. Blue Crabs are a primary prey species of Bonnetheads (Cortés
et al. 1996), and are commonly associated with Eastern Oyster beds and Smooth
Cordgrass (Rozas and Reed 1993, Wells 1961) that are often only submerged at
flood or high tide. In addition, Hettler (1989) determined that callinectid crabs (e.g.,
Blue Crabs) were more abundant in subtidal 3rd-order creeks than 1st-order creeks.
In the present study, it is possible that Bonnetheads were following mature Blue
Crabs and other prey species into these 3rd-order creeks with the incoming tidal
currents. The movement of other elasmobranchs in relation to tidal cycles and prey
availability has been addressed in several studies. Campos et al. (2009) found that
Mustelus henlei (Gill) (Brown Smoothhound) moved into mudflats in Tomales Bay,
CA, during high and incoming tides, which was consistent with when prey were
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available in these areas. Teaf (1978) speculated that Atlantic Stingrays moved into
salt marsh areas at high tide due to higher food abundance/availability. Ackerman et
al. (2000) reported that Leopard Sharks exhibited no consistent movement pattern
on the mudflats of the muddy littoral zones at high tide, which was indicative of
benthic foraging for worms and clam siphons. Medved and Marshall (1983) documented
that Sandbar Sharks exhibited movement patterns opposite of tidal current
direction in areas where large schools of Brevoortia spp. (Menhaden) were present.
No tagged individuals were observed feeding, but those authors observed untagged
sharks feeding on the schools (Medved and Marshall 1983).
In 2011, the receiver array was originally designed to observe the utilization of
a creek system by Bonnetheads in relation to time and tidal cycle. Most detections
were recorded on Receivers 3–7. After examining our 2011 data, we rearranged the
array and placed a few receivers in the adjoining tributaries to determine whether
Bonnetheads were also utilizing these areas. In 2012, Bonnetheads were most active
near Receivers 1, 2, 4, and 11. From the rose diagrams, we determined that
Bonnetheads were using the main channel during ebb/low tide, and the tributaries
at flood/high tide. Combining the detection percentage per receiver and the rose-diagram
data, we could firmly conclude that Bonnetheads primarily utilized the main
channel during ebb/low tides. Bonnetheads tended to enter one particular tributary
(Receiver 11) during flood/high tides, and rarely frequented the other 2 tributaries.
Results of the present study also indicated that Bonnetheads were nocturnal and
crepuscular. It is possible that food abundance and/or availability may not only influence
the tidal movement patterns of Bonnetheads, but also the diel movement
patterns, as seen with other elasmobranch species. Bessudo et al. (2011) speculated
that Sphyrna lewini (Griffith and Smith) (Scalloped Hammerhead Shark) congregated
around Mapelo Island, Colombia, during daytime and ventured away to forage at
nighttime. Myliobatis californica (Gill) (Bat Ray) were present in the warmer, shallower,
inner-bay waters of Tomales Bay between 02:50–14:50 h and then traveled
back to the deeper, cooler outer-bay waters. Matern et al. (2000) suggested that this
movement reflected their behavioral thermoregulation as well as their foraging patterns
between 12:00–20:00 h. In another study, Atlantic Stingrays were most active
at dusk, potentially due to the increased crepuscular activity of prey species (Brinton
and Curran 2017). Based on the behavior of other elasmobranchs, the increased activity
of Bonnetheads at dawn and night in the Romerly Marsh Creek system could
reflect their foraging patterns or food abundance/availability.
In the current study, Bonnetheads utilized Romerly Marsh Creek from August to
October 2011 and May to August 2012. In other studies, Bonnetheads used coastal
systems for similar periods of time. They were present in South Carolina waters
from spring to early fall; Bonnnetheads were caught from April to November (Driggers
et al. 2014, Ulrich et al. 2007), and they exhibited a high degree of intra–and
inter-annual site fidelity to specific estuaries (Driggers et al. 2014). In Florida
waters, Bonnetheads were long-term residents (≥1 year) and some individuals
overwintered and did not undergo coastal migration (Heupel et al. 2006). We believe
that our findings may reflect the migration of Bonnetheads into other creek
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systems in coastal Georgia. In 2011, we collected Bonnetheads in August and
September. They were recorded in the study area on multiple occasions until most
individuals left in September and October 2011. A similar pattern was observed
in 2012—Bonnetheads migrated into the study area in May/June, and left in July/
August. In each year, 1 of our tagged Bonnetheads (2011 female: 105 cm TL;
2012 male: 86 cm TL) migrated south into another system (Ossabaw Sound, GA)
after leaving Romerly Marsh Creek (C. Kalinowsky, pers. comm.). Based on these
studies, Bonnetheads may be seasonal/transient residents in cooler regions, such
as Georgia and South Carolina, and year-round residents farther south in Florida
waters. This is a pattern also seen in Atlantic Stingrays (Ramsden, 2015).
The occurrence of Bonnetheads in the coastal waters of Georgia may be related
to parturition/reproduction. In the present study, we captured both mature
and neonate Bonnetheads, though we classified most as mature females based on
documentation that individuals greater than ~84 cm TL are mature (Compagno et
al. 2005). In nearby South Carolina waters, pregnant female Bonnetheads caught
in April/May were carrying embryos at early developmental stages, and pregnant
females were carrying pups and leaving South Carolina waters in September
(Ulrich et al. 2007). In early fall, 4 neonates were collected in Georgia estuarine
waters (Ulrich et al. 2007, C. Belcher, Georgia Department of Natural Resources
Coastal Resources Division, Brunswick, GA, pers. comm.). In addition, Driggers et
al. (2014) only recaptured female Bonnetheads in South Carolina waters. Belcher
(2008) and Gurshin (2007) determined that juvenile Bonnetheads use the coastal
waters of Georgia as secondary nurseries. The arrival of pregnant Bonnetheads in
South Carolina waters in April/May coincides with the arrival of Bonnetheads
in the coastal waters of Georgia. Given that pregnant females were found in South
Carolina waters and neonates were collected in Georgia waters, pregnant females
may be pupping in the estuaries of Georgia, with their neonates subsequently utilizing
these areas as primary nurseries before leaving in October .
Our acoustic telemetry data on the movement patterns of Bonnetheads have very
important conservation management and planning implications, even though detections
were highly variable and our sample size was small. In both years, we detected
tagged Bonnetheads over a span of 3–6 months with certain individuals frequenting
the area regularly (≥7 days), but none was observed year-round. We recorded some
sharks (38%) in the area for less than 7 days with <500 total detections. Only 13
individuals were tagged and monitored between sample years with 8 recorded for
≥7 days. In 2012, we recorded only 3 sharks, but only 1 was detected for more than
2 weeks. This one shark could have represented the majority of the acoustic data for
the main channel and tributaries in 2012. It is the presence/absence of Bonnetheads
at certain times and tides in a given location and the proportion of the detection data
across these parameters that are most informative. However, the low number of individuals
tagged and the possible disproportionate influence of a single individual,
which may have been detected more than the others, are limitations of our study. In
the current study, we recorded 3 of 13 Bonnetheads for ≤1 day, which could have
reflected fishing/tagging mortality. Hueter et al. (2006) analyzed the mortality rate
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of Bonnetheads during a catch–tag–release event. Those authors classified the condition
of each released shark from 1–5 (1 = best, 5 = dead). From their 1992–2004
gill-net data, they estimated that 40% of Bonnetheads died post–release (Hueter et
al. 2006). In our study, most tagged Bonnetheads were released in condition 2 or
3, but there were some individuals in condition 4. We estimate that only 1 tagged
individual died or shed its transmitter during the study .
Conclusion
Our results provide some of the first information on the small-scale movement
patterns of Bonnetheads within and between 3rd- and 4th-order intertidal creeks in
relation to diel period and tidal cycle. In both study years, we detected individuals
south of the study area a few days after their last detections in our array in Ossabaw
Sound, GA, indicating utilization of multiple coastal systems along the coast within
a year and possible migration south towards warmer waters. Thus, the movement
patterns of Bonnetheads observed in this study could be representative of other
semidiurnal tidal creek and estuarine systems in temperate waters along the East
Coast, which is in contrast to Bonnetheads that are year-round residents in the
warmer coastal waters of Florida and experience diurnal and mixed semidiurnal
tides. In addition, our small-scale acoustic telemetry data have the potential to be
used for spatial analysis for management purposes. In our study, we determined that
Bonnetheads utilized the main channel and tributaries of Romerly Marsh Creek during
different times of day and tidal cycle, and we observed this pattern on multiple
occasions from May–October. In the future, a Bonnethead acoustic telemetry project
spanning multiple creek systems along the Georgia shore or elsewhere on the
East Coast could be used to further understand their large-scale spatial use of coastal
estuarine waters in relation to tidal cycles and other environmental variables.
Acknowledgments
Funding for this research was provided by NOAA Living Marine Resources Cooperative
Science Center (LMRCSC) Program (Award# 06OAR4810163) and the Department
of Education Title VII (Award# P382G090003). We appreciate Dr. Matthew B. Ogburn, at
the Smithsonian Environmental Research Center, for assisting in the data analysis, circular
statistics, and control-tag analyses. We are grateful for the additional data from Chris
Kalinowsky of the Georgia Department of Natural Resources, Coastal Resources Division.
We acknowledge the field support of Dr. Charles Cotton, Michael Partridge, C.J. Carroll
Schlick, Adam Sapp, Kate Doyle, Jeremy Mitchler, Vinay Arora, Courtney Pegus, Christy
Pavel, Michele B. Sherman, Captain Jay Rosenweig, Captain Karega Moyo, Bull River
Cruises (Captain Michael Neal and Captain Buddy Lee), and Michael Richter and Captain
Harry Carter, from the Skidaway Institute of Oceanography. Editorial assistance was
provided mostly by Michele B. Sherman, but also by Dr. Tara Cox, at Savannah State University;
Dr. Carolyn N. Belcher, at the Georgia Department of Natural Resources Coastal
Resources Division; Dr. Chip Cotton, at Florida State University; Carolyn Kovacs; Brigette
Brinton; and Kate Doyle. Thanks to Sarah Webb for her help in creating GIS figures. This
publication is also listed as Contribution Number 1851 of the Belle W. Baruch Institute for
Marine and Coastal Science.
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2017 Vol. 16, No. 2
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