Vertical Movement and Behavior of the Ocean Sunfish, Mola Mola, in The

JEMBE-49259; No of Pages 9 Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx

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Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic

Inga F. Potter ⁎, W. Huntting Howell

Large Pelagics Research Center, Department of Biological Sciences, Spaulding Life Sciences Bldg., University of New Hampshire, Durham, NH 03824, USA article info abstract

Article history: Pop-up satellite archival tags (PSATs) were attached to 31 ocean sunfish, Mola mola. in the Northwest Atlantic Received 30 May 2010 between 2005 and 2008, in order to examine their vertical movement and behavior. Tags remained attached Received in revised form 11 October 2010 from 7 to 242 days, with a mean attachment period of 107.2±80.6 (SD) days. Fish spent greater than 30% of Accepted 13 October 2010 their time in the top 10 m of the water column, and over 80% of time in the top 200 m. The maximum depth Available online xxxx recorded by any fish was 844 m. Temperatures experienced by tagged fish ranged from 6 to 30 °C. Vertical behavior of M. mola changed over short-term and seasonal scales. Ocean sunfish in northeastern US waters in Keywords: Mola mola the summer months inhabited shallower depths and spent more time at the surface than those that moved Northwest Atlantic south in the winter and spring. This shift from shallow to deeper depths was especially apparent when fish Ocean sunfish entered the Gulf Stream, where they spent little time at the surface and dove to depths of 400–800 m. A diel Pop-up satellite tag pattern was observed in vertical behavior. Tagged fish spent more time at depth during the day and inhabited Vertical movement shallower waters at night. There was no observed relationship between the amount of time per day that fish spent in cold water (b10 °C) and the amount of time fish spent near the surface (0–6 m), indicating a lack of evidence for M. mola basking at the surface as a mechanism for behavioral thermoregulation. © 2010 Elsevier B.V. All rights reserved.

1. Introduction diel pattern has been observed in the vertical movements of ocean sunfish with nocturnal movements limited to near-surface waters and The ocean sunfish, Mola mola, is a large pelagic species distributed diurnal vertical movements including repeated dives below the worldwide in both temperate and tropical oceans. A member of the thermocline (Cartamil and Lowe, 2004; Thys et al., 2007; Sims et al., family Molidae in the order Tetraodontiformes, M. mola is known for 2009). This diel pattern, observed in other large pelagic fishes (blue its unique body shape, large size (reaching 4.2 m and weighing up to sharks, swordfish and tunas) (Carey and Scharold, 1990; Block et al., 2300 kg), and atypical anatomy (Gregory and Raven, 1934; Norman 2001; Sims et al. 2003), is thought to be related to prey acquisition. In and Fraser, 1949; Fraser-Bruner, 1951; Bass et al., 2005). Sunfish are addition to prey acquisition and thermocline depth, other factors have valued food fish in Asia and comprise a large portion of bycatch in been suggested as physiological influences on M. mola's diving Pacific and Mediterranean commercial fisheries (Silvani et al., 1999; behavior, including the depth of the DSL or chlorophyll maximum, Macias and de la Serna, 2002; Cartamil and Lowe, 2004; Lovgren, changes in temperature, and a decreased oxygen concentration at 2004; Thys, 2005). M. mola is a common resident of the offshore depth (Thys, 2003; Cartamil and Lowe, 2004; Fulling et al., 2007). waters of the Northwest Atlantic during the spring and summer Ocean sunfish were so named for the common behavior of lying on months, with an estimated summer population of 18,000 (Kenney, their sides near the surface, appearing to “sunbathe”, which has been 1996). Little is known about M. mola's basic ecology, distribution, or suggested to be a mechanism of “thermal recharging” after deep dives population dynamics, and the global status of the species is unknown. in cold water (Thys, 2003; Cartamil and Lowe, 2004), or to solicit Few studies have been conducted on the movement or behavior of cleaning/parasite removal from above the surface by seabirds, or M. mola, and there is no such information on the species in the below by fishes (Pope et al., 2010). northwestern Atlantic. Results from previous tagging studies indicate A tagging study on M. mola in the North Pacific found that dive they spend the majority of their time in the top 50 m of the water behavior was dependent on both location of the fish and time of year. column, with occasional dives to deeper depths (400–600 m) (Thys, As fish moved south into the autumn and winter months, their time at 2003; Cartamil and Lowe, 2004; Thys et al., 2007; Sims et al., 2009). A greater depths increased. As the fish returned north, they resumed a pattern of shallow diving (Thys, 2003). Recent studies of ocean sunfish in both the North Pacific and eastern North Atlantic Oceans found a seasonal pattern of migration related to sea surface ⁎ Corresponding author. PO Box 4524, Portsmouth, NH 03802, USA. Tel.: +1 603 380 5707; fax: +1 603 862 3784. temperatures/thermal tolerance and decreasing chlorophyll levels E-mail address: [email protected] (I.F. Potter). (Thys et al., 2007; Sims et al., 2009).

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 2 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx

The present study is the first to examine the vertical movement geolocation data, were used to identify the location of tagged fish. and behavior of M. mola in the northwestern Atlantic. The study's Depth–temperature plots were created from the raw PSAT data in R objectives were to attach pop-up satellite archival tags (PSATs) to using methods of Galuardi (unpublished). For some analyses, the fish ocean sunfish during the summer and autumn off the coast of New were divided into two groups—north and south of 35°N latitude, to England in order to examine behavior and identify movement separate “northern/mid-Atlantic fish” and “southern fish,” because patterns of M. mola in the region. when they moved south of about 35°N latitude, their vertical behavior changed. Those fish defined as “northern/mid-Atlantic fish” or 2. Materials and methods “northern” fish never moved south of 35°N (n=12). Those defined as “southern fish” were tagged in New England and moved south of 2.1. Tagging 35°N at some point during the course of their tagging period (n=11). Vertical behavior of fish was examined relative to water temperature Between 2005 and 2008, 31 pop-up satellite archival tags (PSATs: by comparing depth data from individuals when in warm water model PTT-100, Microwave Telemetry, Columbia, MD, USA) were (N24 °C) to that of the same individuals in offshore, cooler water deployed on ocean sunfish, M. mola, in the Northwest Atlantic. (b24 °C). The basking behavior of M. mola was examined by analyzing, Twenty-nine were deployed during the summer and autumn for each fish, the amount of time per day spent in cold water (b10 °C) (August–October) of 2005, 2006, or 2007. Fish were tagged in the and the amount of time during that same day that fish spent at the Gulf of Maine (n=7) and in shelf waters to the south and east (within surface (0–6 m). 80 km) of Nantucket Island (n=22). Five fish were tagged in 2005 as a pilot study. Fourteen fish were tagged in 2006, and 10 fish were 3. Results tagged in 2007. Two additional fish were tagged in March 2008 in shelf waters off the coast of Georgia, USA. PSATs were prepro- Of the 31 PSATs deployed, 25 successfully reported data back to grammed to release either five or eight months after deployment. Argos. Fig. 1 shows a map of the deployment and endpoint locations Fish were tagged from commercial tuna vessels (F/V Peregrine, F/V for all 25 tags. A summary of the dates and locations of the start and Tenacious; n=10) and a charter fishing vessel (F/V Monomoy; n=19) end of each tag deployment, days at liberty, distance traveled, and in the waters off the northeastern United States, and from a research percent data reported is provided in Table 1. Tag attachment periods vessel in Georgia (R/V Margaurite, Georgia DNR). Fish basking at the ranged from 5 to 242 days. Eighteen tags were attached for 50 days or surface were approached by the vessel, and tags were attached via a more, 10 of those were attached for 150 days or more, and 4 of them modified wooden harpoon tagging pole, 2.5 m in length. Monofila- were attached for at least 240 days. Mean attachment period of all the ment tethers (25 cm long, 250-lb test) were used to attach the PSATs tags reporting data was 107.2±80.6 days (SD), and mean data to black nylon umbrella darts (darts designed and produced by reported was 69.7±33.1% (SD) (n=25). Two tags with data retrieval Michael Domeier, Marine Conservation Science Institute). Tags were of less than 20% were removed from the analysis. placed into the musculature approximately 15 cm below the base of Fish spent 31.7% of their time in the top 10 m of the water column, the dorsal fin, just anterior to the clavus. Because the fish were not and 59.8% of their time in the top 50 m (n=23) (Fig. 2A). A very large brought onboard or retained during the tagging procedure, measure- proportion of total time (84.1%) was spent in the top 200 m, and the ments of size and weight were not taken. Based on observation, maximum depth recorded by any fish was 844 m. Tagged M. mola individual M. mola tagged in the region were estimated to weigh spent 24.0% of their time between 18 and 20 °C and 68.0% of their time approximately 130 to 230 kg with a total length of 1.3 to 1.6 m between 14 and 22 °C (Fig. 2B). Temperatures experienced by tagged (weights estimated using equation from Watanabe and Sato (2008)). fish ranged from 6 to 30 °C. The PSATs recorded ambient light levels, temperature (resolution Examination of the behavior of southern fish (n=11) compared to 0.17 °C), and depth (resolution 5.38 m). Temperature and depth were the behavior of northern fish (n=12) showed southern fish spent sampled in most cases at 15-min intervals and stored as summary more time at depth and less time at the surface time than northern data. fish (Fig. 3A). Southern fish spent 24.1% of time at depths greater than 200 m, whereas fish that stayed north spent 10.6% of time at these 2.2. Horizontal movement depths, and twice as much time as southern fish in the top 10 m of the water column. Southern fish experienced warmer water temperatures Daily geolocation position estimates of the fish were established than those that remained north (Fig. 3B), with 17.4% of time spent at from recorded high-resolution light data stored in the PSAT using a temperatures N22 °C compared to 3.3% of time spent in these proprietary algorithm (Microwave Telemetry, Inc.), sea surface temperatures by northern fish. Southern fish encountered warm sea temperature, and bathymetry using the methods of Galuardi et al. surface temperatures for a longer period of time in shelf waters (14 °C (2009). The horizontal movement of these tagged M. mola in the to 19 °C during the months of November through May) and were in northwestern Atlantic has been reported in Potter et al. (in press). closer proximity to the warm waters of the Gulf Stream (N26 °C) than fish that remained in northern shelf and offshore waters (8 to 20 °C 2.3. Vertical movement during the months of August through May). There was a diel pattern in vertical behavior. Tagged M. mola spent Depths and temperatures were recorded by the PSAT at 15-min more time at depth during the day and inhabited shallower waters at intervals. To determine if a diel pattern in vertical behavior existed, night. Mean daytime depth of all fish was 107.8 (±21.2) m and mean day–night differencing (mean daily depth−mean nightly depth) was nightly depth was 72.0 (±12.4) m (n=23). Mean day–night utilized, and a t-test was applied to test for differences. The periods of difference of all fish was 51.8 (±12.0) m (t-test, p=0.004). Though “day” and “night” were defined for each individual and day, those fish that traveled south experienced a greater range of depth, depending on the location of the fish on each particular day and the the day–night difference was the same as for fish that remained north resulting day length. Depth and temperature data retrieved by the (Table 2). Ocean sunfish changed their dive behavior over the course tags were used to construct depth/temperature profiles that provided of the tagging period, from shallow dives with a high proportion of information on the surrounding water mass characteristics. The depth surface time (0–6 m), to deeper dives and a lower proportion of and temperature profiles based on PSAT data were compared to data surface time. Mean daily depth of each individual (mean depth of fish from stationary buoys in the Gulf of Maine (part of the GoMOOS during daylight hours) increased, as did the mean day–night system) to confirm accuracy, and in combination with the corrected difference (Fig. 4A–C).

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx 3

Fig. 1. Map of deployment locations (gray circles) and endpoints (black triangles) of 25 PSATs attached to Mola mola between 2005 and 2008. Tags were attached in New England waters between the months of August and October in 2005–2007 (n=23) and in Georgia shelf waters in March 2008 (n=2). Tags were attached from 5 to 242 days.

Vertical behavior of M. mola changed as the fish traveled south. relationship between the amount of time per day fish spent in cold When in New England shelf waters, tagged fish exhibited shallow, water (b10 °C) and the amount of time fish spent in warm water frequent diving to depths of 50–100 m with a high percentage of (N20 °C). Because depth data were limited to 15-minute intervals, surface time. Aside from the initial few days after tagging fish in individual ascent and descent rates were not able to be determined, shallow inshore waters, bottom depth did not limit dive depth. As fish however a previous study of juvenile (small) ocean sunfish using moved off the shelf to the south and east they changed both the depth acoustic tags found the typical rate of descent to be 8.4±5.6 m/min and frequency of diving. Plots of the depth data of individual fish with (Cartamil and Lowe, 2004). Based on this information, it is unlikely that concurrent temperature data show M. mola making regular descents tagged ocean sunfish were surfacing from depths N200 m in between below the thermocline into cooler waters. As fish moved into a water the 15-minute intervals when depth was recorded by the tag. mass with a deeper mixed layer they adjusted their vertical behavior Two of the 25 sunfish tagged entered the Gulf of Mexico. One fish to dive below it (Fig. 5A–B). This was especially apparent when fish tagged in September 2005 in New England reached the Gulf of Mexico entered the Gulf Stream, as they spent little time at the surface and in December before the tag detached. The second fish was tagged in dove to depths of 400–600 m. In some cases, fish spent a period of Georgia shelf waters in March 2008, traveled south to the Gulf of days at depth with no surface interval. The mean hourly depth of four Mexico in June, and remained there until early July. Though data fish in warm, Gulf Stream water (sea surface temperature N24 °C) was return from these tags was limited (22% and 33%), analysis of data significantly deeper (150–400 m) than the mean hourly depth of the from the second fish shows that, like other individuals tagged in the same fish when in cooler, off-shelf water (15–35 m) (repeated study, the fish showed a diel pattern in its diving behavior, with a measures ANOVA, pb0.001; Fig. 6A–B). mean day–night difference of 23.2 m. Once in the Gulf of Mexico, the There was no observed relationship between the amount of time per fish altered its diving behavior to greater depths and shorter surface day fish spent in cold water (b10 °C) and the amount of time fish spent times when compared to its behavior in Southeast US shelf waters at the surface (0–6 m) in the next time interval (Fig. 7). At the beginning (Fig. 8). Prior to entering the Gulf of Mexico, in the southeast US, the of the tagging period, tagged ocean sunfish spent little to no time in cold fish spent 69% of its time in the top 25 m of the water column, and 90% water, and spent a high proportion of their time at the surface. As the of its time in the top 200 m. Once in the Gulf of Mexico, the fish spent tagging period progressed, fish spent more time per day in cold water, less than 3% of time in the top 10 m of the water column, 90% of its and little to no time at the surface. There was also no observed time between 25 and 200 m, and had occasional dives to 400 m. While

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 4 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx

Table 1 Date of deployment, location of deployment and pop-off, number of days attached, straight line distance (SLD) traveled, and retention rate of 31 PSATs attached to Mola mola between 2005 and 2008.

PSAT Deployment Deployment Pop-off Days SLD Data ID date location location attached (km) reported

(°N) (°W) (°N) (°W)

14538 9/3/2005 41.37 69.31 23.21 85.73 130 2520 22% 8776 10/2/2005 42.04 70.02 40.19 68.93 24 222 100% 67322 8/17/2006 43.15 69.98 31.35 79.298 153 1565 55% 67325 8/18/2006 40.96 70.51 32.23 78.98 153 1206 46% 67321 8/23/2006 40.82 70.66 26.94 79.94 143 1685 64% 67331 9/1/2006 41.29 69.96 39.42 74.21 13 415 82% 67332 9/8/2006 42.24 70.03 27.18 78.33 74 1830 96% 67330 9/17/2006 41.26 69.28 38.86 71.19 242 312 40% 67565 9/17/2006 41.29 69.24 33.89 76.30 242 1022 18% 67327 9/19/2006 41.20 69.59 32.38 78.71 153 1250 31% 67333 9/19/2006 41.19 69.59 30.46 81.41 240 1590 4% 67323 9/23/2006 41.17 69.58 32.56 78.37 153 1235 51% 67564 9/25/2006 41.31 69.99 39.92 70.17 14 148 100% 67328 9/25/2006 41.30 69.97 28.28 79.44 70 1690 78% 77073 9/7/2007 41.39 70.05 41.49 70.12 5 30 100% 77079 9/7/2007 41.29 69.95 37.30 73.17 98 527 84% 77070 9/14/2007 41.29 69.95 33.52 77.75 68 1105 92% 77075 9/20/2007 41.27 69.98 39.63 74.10 31 419 100% 77077 9/24/2007 41.13 70.03 41.23 70.62 11 50 100% 77069 9/24/2007 41.21 70.14 38.64 67.76 153 338 73% 77071 10/3/2007 41.26 70.22 40.37 73.33 9 283 100% 77072 10/6/2007 42.63 70.37 36.55 75.64 50 793 96% 67326 10/6/2007 42.38 70.27 36.74 71.64 152 645 72% 77074 3/11/2008 30.79 80.76 31.74 80.17 57 110 64% 77078 3/11/2008 30.90 80.66 32.51 78.38 242 280 33%

Fig. 3. A–B. Histograms showing proportion of time at depth (A), and time at temperature (B), for ocean sunﬁsh that remained above 35°N and ﬁsh that traveled below 35°N. Fish were tagged in New England waters between the months of August and October in 2005–2007. Fish that remained above 35°N had attachment periods from 7 to 242 days (mean=67 days; n=12). Fish that traveled below 35°N had attachment periods from 68 to 153 days (mean=127 days; n=11).

in the Gulf of Mexico, the ﬁsh experienced temperatures ranging from 8.3 °C to 28 °C, with an average water temperature of 18.23 (±4.96)°C.

4. Discussion

PSAT-tagged sunfish exhibited a wide range of vertical behavior, which varied over both spatial and temporal scales. We found that ocean sunfish in northeastern US waters in the summer and early autumn months generally inhabited relatively shallow depths (often b10 m), and warm temperatures (17–20 °C). The water column in this region at this time is typically stratified, with a strong thermocline and shallow mixed layer (Lalli and Parsons, 2006; Mann and Lazier, 2006). Mola spent a high proportion of their time above the thermocline near

Table 2 Mean daytime depth, mean nightly depth, and mean day–night difference (mean daily depth−mean nightly depth), for 23 tagged ocean sunﬁsh, divided by location: northern (above 35°N) and southern (below 35°N). p-Values indicate results of a paired t-test.

N Total Mean Mean Mean p number daytime nightly day–night of days depth (m) depth (m) difference (m)

Northern 12 802 67.4 (±23.6) 26.9 (±7.01) 45.3 (±18.2) 0.055 fish Fig. 2. A–B. Histograms showing proportion of time at depth (A), and proportion of time Southern 11 1396 150 (±32.2) 101.8 (±15.4) 45.2 (±18.8) 0.047 at temperature (B), for all fish (n=23). Fish were tagged in New England waters fish between the months of August and October in 2005–2007 and in Georgia shelf waters All fish 23 2198 107.8 (±21.2) 72.0 (±12.4) 51.8 (±12.0) 0.004 in March 2008. Tags were attached from 7 to 242 days.

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx 5

high foraging success. This pattern matches that of zooplankton distribution in the region, suggesting a co-occurring abundance in gelatinous prey (Fosette et al., 2010). Ocean sunfish that moved south into the waters of the mid- Atlantic and southeastern United States in the winter and early spring spent less time at the surface and more time at depths N200 m. This shift in vertical behavior was associated with a move to waters that are typically weakly stratified and have a deep mixed layer (Werner et al., 1999). It is probable that the ocean sunfish adjusted their vertical range to follow the deeper thermal gradient. Because M. mola are often located with, and presumably feed on gelatinous zooplankton (Desjardin, 2005; Houghton et al., 2006), and because gelatinous zooplankton exhibit diel vertical migrations (DVM) that range from a few, to hundreds of meters (Graham et al., 2001; Madin et al., 2006; Hosia and Bamstedt, 2008), it is also probable that the vertical movements of Mola are tied to locating their vertically migrating gelatinous prey. A study on ocean sunfish in the Pacific, as well as studies of other large pelagic species such as bluefin tuna and basking sharks in both the Pacific and North Atlantic, has noted similar differences in vertical distribution associated with water column structure on a seasonal scale (Lutcavage et al., 1997; Kitagawa et al, 2000; Sims et al. 2003; Thys, 2003; Wilson et al., 2005). Like ocean sunfish in the present study, leatherback turtles in waters off the southeastern United States dive deeply, for long periods of time, to forage in the deep scattered layer or DSL (Eckert, 2006). A recent study comparing the vertical niche overlap of both species in waters off South Africa suggested that like leatherbacks, ocean sunfish exhibit plasticity in their vertical behavior in response to patchily distributed gelatinous prey with differing levels of diel vertical migration. Both species experience a shift from predation on seasonally abundant surface medusa during the summer to vertically migrating gelatinous species found at deeper depths during the winter months (Hays et al., 2009; Pope et al., 2010). There was a diel pattern in vertical behavior of M. mola. Mola visited deeper depths during the daylight hours and remained in shallower waters at night. The diel pattern has been observed in other studies of the species (Cartamil and Lowe, 2004; Thys et al., 2007; Sims et al., 2009), in other large pelagic fishes (basking sharks, blue sharks, swordfish, and tunas) (Carey and Scharold, 1990; Block et al., 2001; Sims et al. 2003), and in leatherback sea turtles (Eckert et al., 1989). In the present study, ocean sunfish maintained these distinct differences in vertical behavior even as their overall depth profiles shifted to deeper waters over time, which reflects the prey searching and foraging behavior of the species in different water masses. The shift in vertical behavior from shallow to deeper depths was Fig. 4. A–C. Mean daily depth (A), mean nightly depth (B), and mean day–night especially apparent when tagged sunfish entered the Gulf Stream. difference (C), of PSAT ID 67328. The fish was tagged on 9/25/06 off Nantucket Island. There fish spent little time at the surface and dove to depths of 400– The tag was attached for 70 days and popped off when the fish was in offshore waters 800 m, and in some cases, spent a period of days at depth with no off Florida on 12/7/06. surface interval. The Gulf Stream flow field extends deep into the water column where nutrient concentrations are maximized (Palter the surface, but dove frequently to depths b200 m. Interestingly, the and Lozier, 2008), and a migratory DSL has been detected in the Gulf observed vertical behavior of Mola (shallow, short dives) is similar to Stream with diurnal migration to depths of 450 m (Cole et al., 1971). the vertical feeding behavior of the leatherback turtle, which is also The deep diving exhibited by ocean sunfish in the Gulf Stream is likely found in this area during the same time of year (James et al., 2005; areflection of prey searching behavior. In addition to the warm sea Eckert, 2006). Like leatherbacks, ocean sunfish are believed to feed surface temperatures and a deeply mixed water column, swift primarily on gelatinous zooplankton, and the distribution of M. mola northward moving surface currents may have motivated southward has been associated with areas where jellyfish are abundant moving fish to remain at depth and avoid the surface currents. (Desjardin, 2005; Houghton et al., 2006). Similarities in vertical In addition to a shift to increased depth, ocean sunfish also altered behavior and diets of the two species suggest that M. mola is foraging the frequency of vertical behavior, shifting from high-frequency in northeastern US shelf and slope waters, taking advantage of the diving and surfacing to low frequency diving—often with several high concentrations of gelatinous prey found in the region (Shoop and consecutive days at depth with no surface intervals. The high- Kenney, 1992; Madin et al., 2006). A recent study on foraging patterns frequency diving with extensive movement in the water column of leatherbacks relative to prey distribution noted similar diving likely reflects prey searching behavior, and has been observed in behavior in turtles in the northwestern Atlantic during the summer another study of M. mola in the eastern Atlantic (Sims et al., 2009). A months. Mean dive depths of turtles in the region were shallower study examining the vertical movement of M. mola, among other than when in southern waters, which the authors associated with marine predators, found evidence that the species adapts its vertical

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 6 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx

Fig. 5. A–B. Depth and corresponding temperature data from: A) fish 67321; and B) fish 67332. Fish 67321 was tagged on 8/23/06 in waters off New England. The tag was attached for 143 days and popped off when the fish was around the Bahamas on 1/23/07. Fish 67332 was tagged on 9/8/2006 in the Gulf of Maine. The tag was attached for 74 days and came off when the fish was around the Bahamas on 11/25/06.

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx 7 A 0

-100

-200

-300 Depth (m)

-400

-500 061218 23 Hour Fig. 8. Histogram showing proportion of time at depth for ocean sunﬁsh ID 77078 when it was in the Gulf of Mexico (n=28 days) and in southeast US waters (n=92 days). The B ﬁ 0 sh was tagged from 3/11/2008 to 11/11/2008.

-20 no statistical relationship between the amount of time per day fish spent in cold water (b10 °C) and the amount of time fish spent at the -40 surface (0–6 m). Tagged ocean sunfish spent a large proportion of time at the surface in Northeast US waters in the late summer and early autumn where the water was highly stratified with a shallow -60 Depth (m) mixed layer and the warmest temperatures at the surface. Basking at the surface in the shelf waters off the northeastern United States has -80 also been observed in swordfish, Xiphias gladius, and leatherback sea turtles, and is believed to be part of a recovery process to aid in digestion and warm body temperatures (Carey and Robinson, 1981; -100 061218 23 Eckert et al., 1989). If M. mola is also feeding on the large quantities of Hour gelatinous zooplankton in the region, it may equally benefit from recovery time at the surface. As sunfish moved south and encountered Fig. 6. A–B. Mean hourly depths (±SD) of four tagged ocean sunfish during a one-week warmer, mixed water masses (particularly the Gulf Stream) they dove period when they were in warm offshore water (N24 °C) (A), and a one-week period in cooler offshore water (b24 °C) (B). Time is in GMT. deeper and, if seeking a recovery period after time at depth, may have gained thermal benefits from the warm temperatures found at mid- water depths. This behavior has been observed in swordfish in the behavior over a broad range of depths to optimize its search for Southeast Pacific after feeding on organisms in the DSL (Abascal et al., patchily distributed prey in different environments (Humphries et al., 2009). Basking behavior in ocean sunfish has also been suggested to 2010). The retinal topography of immature M. mola was examined in a facilitate recovery after diving into the oxygen minimum layer, as has recent study, and results suggested that ocean sunfish detect their been observed in other large pelagic fishes (Carey and Robinson, prey during their descent through the water column, as well as on the 1981; Cartamil and Lowe, 2004; Thys et al. 2007). Given that the ocean floor (Kino et al., 2009). The frequent descents through the northwestern Atlantic is well oxygenated at depth, basking behavior water column observed in ocean sunfish in the present study are may be observed with more frequency in populations inhabiting presumably prey searching behavior. oceans where oxygen minimum zones occur. The large size of M. mola It has been suggested that M. mola basks at the surface as a in the present study may also play a role in basking behavior if related mechanism of “thermal recharging” after deep dives in cold water to thermal tolerance, as larger fish may be buffered against (Thys, 2003; Cartamil and Lowe, 2004). In the present study there was temperature differences more so than smaller fish. The importance of large body mass in maintaining elevated body temperatures relative to the environment has been suggested for sea turtles and bluefin tuna (Sato et al., 1994; Kubo et al., 2008). Basking behavior may also benefit M. mola as a mechanism of parasite removal. M. mola is known to harbor up to 40 different genera of parasites, and basking at the surface is likely to attract cleaning fish from below as well as seabirds from above, both of which have been reportedly associated with the species (Cartamil and Lowe, 2004; Pope et al., 2010). Two fish in the study entered the Gulf of Mexico during different times of year: winter (December–January) and summer (June–July). M. mola is a year-round resident of the northern Gulf of Mexico with the greatest densities reported in the winter and spring (Desjardin, 2005; Fulling et al., 2007). The fish that entered the Gulf of Mexico during summer encountered sea surface temperatures approaching 30 °C, warmer than previous studies suggest is the thermal preference of the species (Thys et al. 2007; Fulling et al., 2007; Sims et al., 2009). Fig. 7. Daily surface time (proportion of time at 0–6 m per 24 h) vs. time in cold water fi (proportion of time in temperatures b10 °C per 24 h) for four ocean sunfish (IDs: While in the Gulf of Mexico, sh spent little time at the surface, more 67321, 67332, 67328, and 67322). time at depth, and remained in close proximity to the shelf and Loop

Please cite this article as: Potter, I.F., Howell, W.H., Vertical movement and behavior of the ocean sunﬁsh, Mola mola, in the northwest Atlantic, J. Exp. Mar. Biol. Ecol. (2010), doi:10.1016/j.jembe.2010.10.014 8 I.F. Potter, W.H. Howell / Journal of Experimental Marine Biology and Ecology xxx (2010) xxx–xxx

Current. Similar behavior was noted in a sharptail mola, Masturus Block, B.A., Dewar, H., Blackwell, S., Williams, T., Prince, E., Farwell, C., Boustany, A., Teo, S., Seitz, A., Walli, A., Fudge, D., 2001. Migratory movements, depth preferences, and lanceolatus, tagged with a PSAT in the northern Gulf of Mexico (Seitz thermal biology of Atlantic bluefin tuna. Science 293 (5533), 1310–1314. et al., 2002). The deep thermocline in the Gulf of Mexico was a likely Carey, F.G., Robinson, B.H., 1981. Daily patterns in the activities of swordfish, Xiphias factor in the shift in vertical behavior of M. mola to greater depths gladius, observed by acoustic telemetry. Fishery Bulletin 79, 277–292. Carey, F., Scharold, J., 1990. Movements of blue sharks (Prionace glauca) in depth and when it entered the region. course. Marine Biology 106 (3), 329–342. Of the 31 PSATs deployed on M. mola between 2005 and 2008, 11 Cartamil, D.P., Lowe, C.G., 2004. Diel movement patterns of ocean sunfish Mola mola off remained attached until the programmed pop-up date. Six never southern California. Marine Ecology Progress Series 266, 245–253. reported data, and 14 detached prematurely. One reason for Cole, H.P., Bryan, G.M., Gordon, A.L., 1971. The deep scattering layer: patterns across the Gulf Stream and the Sargasso Sea. In: Farquhar, G.B. (Ed.), Proceedings of an premature detachment was the activation of the tag's fail-safe International Symposium on Biological Sound Scattering in the Ocean. Maury mechanism that caused its release due to what was perceived by Center for Ocean Science, Washington, D.C, pp. 281–293. the tag as constant pressure (i.e., the change in the fish's depth was Desjardin, N.A. 2005. Spatial, temporal, and dietary overlap of leatherback sea turtles (Dermochelys coriacea) and ocean sunfishes (Family Molidae). Master's thesis, less than the resolution of the tag (5.38 m) over a four-day period) Florida Atlantic University. which occurred in six tags, and in all of these cases the fish showed Eckert, S., 2006. High use oceanic areas for Atlantic leatherback sea turtles (Dermochelys steady horizontal movement in shelf waters. This often occurred coriacea) as identified using satellite telemetered location and dive information. – fi b Marine Biology 149, 1257 1267. when sh were in shallow waters ( 10 m) around Nantucket Island Eckert, S., Eckert, K., Ponganis, P., Kooyman, L., 1989. Diving and foraging behavior of where they presumably were feeding on large patches of gelatinous leatherback sea turtles (Dermochelys coriacea). Canadian Journal of Zoology 67 prey (salps and jellyfish) observed in the area. Lack of any horizontal (11), 2834–2840. fi Fosette, S., Hobson, V., Girard, C., Calmettes, B., Gaspar, P., Georges, J., Hays, G., 2010. displacement from one prematurely released tag indicates the sh Spatio-temporal foraging patterns of a giant zooplanktivore, the leatherback turtle. died, presumably as a result of being tagged. Other possible causes of Journal of Marine Systems 81, 225–234. premature detachment include tag shedding/tissue rejection due to Fraser-Bruner, A., 1951. The ocean sunfishes (Family Molidae). Bulletin of the British Museum of Natural History 1, 89–121. improper placement of the tag, corrosion or fouling of the tag, and Fulling, G., Fertl, D., Knight, K., Hoggard, W., 2007. Distribution of Molidae in the mechanical failure. A recent paper addressing the issue of signal loss northern Gulf of Mexico. Gulf and Caribbean 19 (2), 53–67. in satellite tracking studies identified antennae breakage, exhaustion Galuardi, B., Royer, F., Golet, W., Neilsen, J., Lutcavage, M., 2009. Complex migration fi of batteries, tag malfunction and animal mortality among the causes tracks from Atlantic blue n tuna tagged off Nova Scotia, Canada. Abstract in In: Nielson, J.L., Arrizabalaga, H., Fragoso, N., Hobday, A., Lutcavage, M., Sibert, J. (Eds.), of signal loss, and suggested more diagnostic data be incorporated Tagging and Tracking of Marine Animals with Electronic Devices: In Series: Review: into tag transmissions in order to better understand the factors Methods and Technologies in Fish Biology and Fisheries, Volume 9. 400 pp. contributing to this problem (Hays et al., 2007). Graham, W., Pages, F., Hamner, W., 2001. A physical context for gelatinous zooplankton aggregations: a review. Hydrobiologia 451 (1–3), 199–212. 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