Satellite tracking the world's largest jelly predator, the ocean sunfish, Mola mola, in the Western Pacific

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Abstract

Twelve ocean sunfish, Mola mola, were tagged with pop-up satellite archival tags off the coast of Kamogawa, Japan during the spring of 2001 and 2003–2006. Transmitted data were obtained from seven tags. An eighth tag was recovered and provided a highly detailed four-month dataset from which several recognizable diving patterns emerged. This M. mola spent considerable time at the surface, possibly warming itself. These apparent basking events were punctuated by regular deep dives below the thermocline to depths as great as 600 m and temperatures as low as 2 °C. The vast majority of dives occurred during the day, with relatively little vertical movement at night. Geolocation estimates were possible between tag and release for five individuals. No large, basin-scale movements were apparent with most molas remaining relatively close to their initial tagging location over a six to nine month period. Seasonal movements were apparent for some molas and corresponded to regional shifts in oceanography. Northward movements from the Kuroshio Current into the Kuroshio–Oyashio transition zone and the Oyashio Current during the summer months coincided with an increase in temperature and reduction in chlorophyll a concentrations in waters near central Japan. In the fall, most molas returned inshore to the coast of Japan. Molas are targeted and incidentally captured by fishermen in these waters, and the information presented here is vital for effective fisheries management and to forecast changes in mola behaviors associated with environmental variability.

Introduction

Ocean sunfish [Mola mola (Linnaeus, 1758)] are found in all tropical and temperate ocean basins and are the world's heaviest bony fish, reaching more than 2200 kg (Carwardine, 1995, Roach, 2003). According to fossil evidence, the family Molidae diverged from their puffer fish relatives approximately 40 million years ago, abandoned life on the reef, and took to the open sea (Tyler and Bannikov, 1992, Tyler and Santini, 2002). Currently, three species are recognized (Nelson, 1994), all of which lack a true tail (Bigelow and Welsh, 1924, Fraser-Brunner, 1951, McCann, 1961): M. mola (common mola), Masturus lanceolatus (Liénard 1840) (sharp tailed mola) and Ranzania laevis (Pennant 1776) (slender mola). The English common name of the group, ocean sunfish, stems from the fish's characteristic behavior of lying at the sea surface, apparently basking (Norman and Fraser, 1938).

Molas forage near the base of the food web like most of the largest whales, sharks, and rays. They may consume krill and other crustaceans (Aflalo, 1904), but their primary food source appears to be a mixed assemblage of gelatinous zooplankton, referred to here as jellies (Fraser-Brunner, 1951). One of the few large pelagic organisms that share this unique trophic niche is the leatherback sea turtle, the largest of the extant marine turtles. Jellies comprise one of the most dominant yet poorly understood assemblages of pelagic fauna (Mills, 1995, Mills, 2001), and their global abundance and distribution may be changing due to a number of factors including climate change, pollution, and overfishing, (Brodeur et al., 1999, Purcell et al., 2007, Richardson et al., 2009). In turn, the changes in the abundance and distribution of jellies may be impacting the organisms that rely on them, such as the mola.

While general distribution patterns are known, information on the movements of M. mola is based on relatively few studies. In 2004, Cartamil and Lowe (2004) reported on the horizontal and vertical movements of eight acoustically tracked molas off Southern California for periods ranging from 24 to 72 h. More recently, Hays et al. (2009) published a study comparing the geographical movements and vertical tracks of four M. mola to those of leatherback sea turtles (Dermochelys coriacea) off Capetown, South Africa, with results supporting the claim that molas are deep divers (Norman and Fraser, 1938, Wheeler, 1969). In a similar study, Sims et al. (2009) provided evidence for seasonal migration of three M. mola in the northeast Atlantic, and highly variable dive patterns. These studies suggest that molas likely alter their vertical behavior in response to environmental conditions and prey distribution (Hays et al., 2009, Sims et al., 2009). While research to date has provided important insights into their geographic and vertical habitat use, previous studies have been limited in time, space, and/or sample size and there is currently no information available for the western Pacific where molas are captured in fisheries.

Although caught and sold in only a few locations worldwide, such as Japan and Taiwan, M. mola are taken incidentally in a large number of fisheries. They are the most common bycatch of the broadbill swordfish drift gillnet fishery off California and Oregon. According to reports from the National Marine Fisheries Service (NMFS) Southwest Region, between 1990 and 1998, 26.1% of the drift net catch consisted of M. mola. This translates to a catch of more than 26,000 individuals (Rand Rasmussen, NMFS Southwest Fisheries Science Center, pers. comm.). In the Mediterranean, M. mola constitute up to 90% of the catch of the illegal Spanish driftnet swordfish fishery off the Gibraltar Straits (Silvani et al., 1999). Off the coast of South Africa, ocean sunfish bycatch rates from the tuna and swordfish longline fishery are estimated at 340,000 sunfish annually (Petersen, 2005, Sims et al., 2009). Unfortunately, without even crude estimates of the population structure, connectivity, or size, the impact of these fisheries is difficult to assess. Also, while molas are considered to be highly fecund (Fraser-Brunner, 1951), recruitment is unknown.

As mentioned above, one location where molas are captured for food is in the western Pacific off the east coast of Japan. Known as ‘manbou’, molas occupy a special place in Japanese culture. From being a tax payment to shoguns in the 17th and 18th centuries (Jingushicho, 1910) to becoming modern day town mascots (e.g. the town of Kamogawa) and the focus of ecotourism, molas have been revered and consumed in Japan for centuries. Currently, genetic and pop-up satellite telemetry data are being collected for M. mola to document movements, behaviors, population subdivision, and migratory rates and routes around the Pacific. Our objectives for this study were to examine the horizontal and vertical movements of molas in the western Pacific and relate changes in those patterns to changes in environmental parameters. Results are presented from satellite tagging efforts on M. mola off Japan in the spring of 2001 and 2003–2006.

Section snippets

Pop-up satellite archival tags

The pop-up satellite archival tag (PSAT, Model PAT2, PAT3, PAT4 and Mk10, Wildlife Computers, Redmond, Washington USA) (Block et al., 1998, Lutcavage et al., 1999, Sedberry and Loefer, 2001) was used in this study. The precision of temperature measurements was 0.1 °C between 12 and 26.95 °C and for values outside this range was 0.2 °C. The precision in depth measurements was depth dependent and measured within 1, 2, 4, 8, and 16 m over ranges from − 20 to 99.5, 100 to 199.5, 200 to 299.5, 300 to

Results

Data were collected from the eight of the 12 tags deployed (Table 1). Tag 795 was recovered by a set-net fisherman in the Shiranuka Aomori Prefecture in northern Japan. Prior to the battery failing, 110 days of temperature, depth and light level data at two minute intervals were obtained. Of the remaining seven tags, two released early and all transmitted limited amounts of data. Tag 758 released when the fish went below 980 m (the depth limit of the tag), presumably triggering the RD1500 and

Discussion

The data obtained from satellite tagging M. mola in Japanese waters provide new insight into this poorly understood fish. The only detailed information on behaviors of this globally distributed species comes from one acoustic telemetry study of eight individuals (Cartamil and Lowe, 2004) and two satellite-tagging studies totaling seven individuals (Hays et al., 2009, Sims et al., 2009). This is the largest satellite tagging study to date and provides the first data on geographic and vertical

Conclusion

The data collected on the movement, behaviors, and habitats encountered by M. mola off Japan provide important insights into their vertical and geographic movements. These molas do not appear to make basin-scale migrations. If movements are indeed localized, over-exploitation off the Japanese coast may result in a loss of genetic diversity and potentially the loss of a discrete population. However, it should be noted that the molas in this study are juvenile fish and mature fish may make

Acknowledgments

We would like to thank the generous staff at Kamogawa Seaworld including K. Arai, Y. Maeda, Y. Genta, K. Mori, A. Osawa, and Y. Saito. The fishermen of the Fisheries Cooperative Association of Kamogawa provided critical assistance in capturing and releasing the tagged mola particularly T. Sakamoto and Y. Watanabe. E. Freund provided field support during the Japan 2001 field season. We would like to express our appreciation to the captain and crew of the R/V Kaiyo, as well as to the operations

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