ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on October 3, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(9):1791-1799; doi:10.1093/icesjms/fsm148
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Determining summer residence status and vertical habitat use of sailfish (Istiophorus platypterus) in the Arabian Gulf
1 NOAA Fisheries, Southeast Fisheries Science Center, 75 Virginia Beach Drive, Miami, FL 33149, USA
2 Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA
Correspondence to J. P. Hoolihan: tel: +1 305 365 4116; fax: +1 305 361 4562; e-mail: john.hoolihan{at}noaa.gov
Hoolihan, J. P. and Luo, J. 2007. Determining summer residence status and vertical habitat use of sailfish (Istiophorus platypterus) in the Arabian Gulf. – ICES Journal of Marine Science, 64.Pop-up satellite archival tags (PSATs) were deployed on 18 sailfish in the Arabian Gulf between 2001 and 2005 to determine summer geoposition and habitat preference. Programmed releases following periods ranging from 110 to 156 d provided an aggregate total of 533 monitoring days of data. Three PSATs failed to report and nine released prematurely after periods ranging from 3 to 93 d. Four were recovered in gillnets after periods ranging from 39 to 90 d, and two transmitted after programmed releases of 127 and 128 d. Pooled archival data from recovered PSATs showed a cumulative mean distribution of 83.9% for total time spent in the upper 10 m, with no significant difference between day and night (
24 = 0.84, p = 0.93). Depth ranged from 0 to 61 m, and ambient water temperature from 19.7°C to 30.1°C. Linear displacements ranged from 11 to 543 km and were all located inside the Gulf. Satellite- and light-level-derived geopositioning suggested that all fish remained in the Gulf. The two PSATs releasing on schedule validated summer residence inside the Gulf, providing further evidence in support of genetic analyses and conventional mark-recapture studies, which suggested that this billfish population confines itself year-round within a shallow marginal sea area. Preference for near-surface depths suggests a great susceptibility to capture by gillnets and other surface gears, raising concern for the effectiveness of regional management and conservation of the species.
Keywords: Arabian Gulf, movement, pop-up satellite tags, sailfish, vertical behaviour
Received 27 June 2007; accepted 27 August 2007; advance access publication 3 October 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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An increasing body of evidence indicates that overexploitation threatens the sustainable use of many of the world’s large apex predator fish (Hilborn et al., 2003). Billfish (Istiophoridae) are a particular concern, because they often face heavy regional harvesting in both targeted and non-targeted fisheries. A notable example is the population of sailfish (Istiophorus platypterus) in the Arabian Gulf (Hoolihan, 2004a).
Sailfish range throughout global tropical and subtropical waters, and are generally associated with continental shelf areas (Nakamura, 1985). In the Arabian Gulf (also known as the Persian Gulf, but hereafter referred to simply as the Gulf), the sailfish is the lone istiophorid in commercial and recreational fisheries. The Gulf is a shallow marginal sea with a maximum width of 338 km, a nominal length of 1000 km, and 
36 m average depth (Reynolds, 1993). It has a single, narrow (56 km) saltwater opening at the Strait of Hormuz, which connects to the Gulf of Oman (Figure 1).
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Previous studies have suggested that sailfish may be year-round residents in the Gulf. These include conventional mark-recapture studies that report an absence of any recoveries outside the Gulf, and mitochondrial DNA analysis which indicated phylogeographic isolation of the Gulf population (Hoolihan, 2003; Hoolihan et al., 2004). Conventional tag recoveries also revealed an annual April migration towards the northwest, further into the Gulf territorial waters of Iran (Hoolihan, 2003). Although in these northern waters, sailfish are subject to incidental bycatch in gillnet fisheries targeting Scomberomorus and Euthynnus. Although those tag recoveries were distributed over most months of the year, none were reported for summer (August–September; Hoolihan, 2003), a possible reason being the cessation of gillnet fishing then (Hoolihan, 2004a). Additionally, no landing data were available to support, or refute, the summer presence of sailfish in the Gulf. Determining summer presence is one of the preconditions required to validate year-round residence.
Gathering information on movement and behaviour of large vagile predators presents challenging obstacles. For billfish, the time and the expense required to locate and monitor these "rare-event" species over large geographic areas is often prohibitive. Further, it is often difficult for researchers to gain access or to move about regions experiencing political tension, as is the recent situation for the Gulf. However, because of recent advancements in electronic tagging technology, many inherent difficulties have been resolved. For example, pop-up satellite archival tags (PSATs) have proven instrumental for defining horizontal and vertical distribution, migration and post-release survival of several billfish species, including blue marlin (Makaira nigricans), black marlin (M. indica), striped marlin (Tetrapturus audax), white marlin (T. albidus), and sailfish (Graves et al., 2002; Domeier et al., 2003; Gunn et al., 2003; Kerstetter et al., 2003; Horodysky and Graves, 2005; Prince and Goodyear, 2006).
Developed as a fisheries-independent method to monitor movement and behaviour, a PSAT is capable of recording ambient water temperature and pressure (depth), as well as ambient light levels used for location tracking (Arnold and Dewar, 2001). On a user-programmed date, a corrosive link separates to allow the PSAT to detach and ascend (pop-up) to the surface. Contact is then made with an orbiting ARGOSTM system satellite, to which location and stored data are uploaded and forwarded to the researcher.
For Gulf sailfish, PSATs offer many benefits that allow researchers to study movement and behaviour. The objectives of our study were to monitor sailfish with PSATs to determine summer presence inside the Gulf, and to evaluate vertical behaviour in terms of time spent at depth and temperature.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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In all, 18 PSATs were deployed on sailfish in the Gulf coastal waters of the United Arab Emirates (UAE) during March and April of 2001, 2002, and 2005 (Table 1). All fish were captured using standard recreational fishing gears and techniques (Prince et al., 2002), followed by manual restraint (grasping the bill) until calm. The fish were then lifted aboard through a transom door and double-tagged with PSAT and conventional tags. Specimens were returned to the water in <30 s.
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Three PSAT models deployed in our study included PAT2 (n = 9) and PAT4 (n = 7) units manufactured by Wildlife Computers (Redmond, Washington). Sensors on those models can measure ambient water temperature from –40°C to +60°C (±0.05°C) and depth from 0 to 1000 m (±0.5 m) every minute. Additionally, two PTT-100 tags manufactured by Microwave Telemetry (Columbia, MD, USA) were deployed. Sensors on that model can measure ambient water temperature from 0°C to +35°C (±0.09°C) and depth from 0 to
670 m (±0.27 m) every hour. All models were positively buoyant in water. PSATs were attached using a monofilament tether and stainless steel "H" dart anchor, as described previously (Hoolihan, 2004b). For the Micowave Telemetry tags, depth and temperature data were transmitted as hourly readings. However, limited battery capacity and synchronization contact time with orbiting satellites prevented the transmission of minute-by-minute depth and temperature data archived in Wildlife Computer’s tags. To circumvent these problems, data were summarized into larger temporal blocks for transmission. For example, the data transmitted by the PAT2 and the PAT4 were condensed into percentages of time spent at depth and temperature across 12 user-programmed bins. These bin ranges were set at the factory default for the 2002 deployments, because it was uncertain whether the fish would migrate to deeper water outside the Gulf (Table 2). After surmising that movements were probably confined to the Gulf, the 2005 PSAT bins were programmed to a finer scale more characteristic of Gulf conditions (Table 2). To determine the summer geoposition of sailfish monitored during this study, PSATs were programmed to release and transmit data following deployments ranging from 110 to 156 d (corresponding to the period 1 July through 5 September). These somewhat lengthy periods were necessary because sailfish leave UAE recreational fishing areas towards the northern Gulf in April (Hoolihan, 2003), and are thereafter unavailable.
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Several PSATs were recovered intact, allowing access to the entire archived dataset stored in the non-volatile memory. For those data, time at depth was analysed by distributing day and night readings separately across 5-m bins, then testing for statistical differences between tags with a
2 goodness-of-fit procedure using an alpha level of 0.05. Day and night periods were separated between local sunrise and sunset at estimated longitudes. Time at temperature was distributed across 1°C bins separately for day and night. PSATs are programmable to detect tag-shedding or death of the animal, through the pressure (depth) sensor. When depth remains stable over a defined period, the PSAT initiates release and data transmission. For our study, PSATs were programmed to release if depth remained constant (±4 m) for 48 h. All distances discussed here refer to direct linear displacements between tagging and recovery or pop-up locations.
Light-level geolocation data were initially processed using the global positioning software WC-AMP (Wildlife Computers, Redmond, WA, USA), and then applying a sea-surface-temperature-corrected Kalman filter (Nielsen et al., 2006) to the light-level-derived locations. Finally, we developed a custom bathymetry filter to relocate the points that were on land or in shallow water, based on 2x2 minute grid ETOP02 bathymetry data (Anon., 2006) and the daily maximum depth from the tag. For each point where maximum daily depth was greater than the bathymetric depth, we selected all grid cells along the longitude where bathymetric depth was greater than the daily maximum depth within ±1° of the previous day’s latitude, then assigned a final location to a single cell randomly selected from that group.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The 18 PSATs were deployed in UAE coastal waters of Abu Dhabi and Dubai between 9 April 2001 and 12 April 2005 on sailfish with estimated weights ranging from 18.1 to 45.5 kg (Table 1). Three of the 18 PSATs failed to communicate entirely. Two of the 18 PSATs (N and P) transmitted successfully on their scheduled release dates (8 and 10 August) following 127 and 128 d at liberty, so confirming summer sailfish presence inside the Gulf. Nine PSATs detached prematurely after periods ranging from 4 to 93 d and transmitted summary datasets of various size via the Argos system. Four PSATs were recovered in Iranian gillnets and returned intact, allowing access to extensive stored archive data ranging from 39 to 71 d. All pop-up and recovery locations were inside the Gulf, and the linear displacement between points of deployment and pop-up release, or recovery, ranged from 11 to 543 km (mean = 268 km; Figure 1, Table 1). Movement tracks estimated from Argos and light-level readings for the ten PSATs (A, B, D, E, H, J, N, O, P, and Q) at liberty for more than 5 d suggested that all fish remained inside the Gulf (Figure 2), further confirming summer sailfish residence inside the Gulf.
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Five PSATs provided high-resolution temperature and depth data recorded at intervals of 1 h (A) or 60 s (B, D, H, and O). In all, these PSATs provided 235 aggregate monitoring days of detailed data (Table 1), with temperature ranging from 19.7°C to 30.1°C and depth from 0 to 61 m (Table 3). Markedly similar profiles were evident on comparing diel distributions for time spent at depth and temperature (Figure 3). On average, the five sailfish spent 83.9% of the total monitoring time in the upper 10 m of the water column, 71.9% in the upper 5 m alone (Figure 3). Moreover, a
2 distribution revealed no significant difference (24 = 0.84, p = 0.93) between day and night for time at depth shallower than 10 m. The distribution of mean time at temperature for the same five sailfish showed a preference (45.7%) for waters ranging from 24°C to 25°C (Figure 3).
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For five other PSATs, from which we received Argos-transmitted summary data for more than 5 d, three (E, J, and Q) exhibited time at depth and temperature profiles (Figure 4) similar to the five high-resolution PSATs (Figure 3). However, the two PSATs (N and P) operating successfully to full term spent less time shallower than 10 m (Figure 4), 60% for N and 57% for P. Moreover, those sailfish spent 59% (N) and 67% (P) of their time in water temperatures of 29–31°C, noticeably warmer than that of the other PSATs in our study (Table 3). Both N and P exhibited increased depth and temperature over the duration of their respective periods of deployment (Figure 5).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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The spatial and temporal characteristics of sailfish PSAT movements outlined here were consistent with earlier reports suggesting a spring migration northwestwards from UAE waters up the Gulf (Hoolihan, 2003). Moreover, our estimated PSAT track plots indicated that all sailfish remained inside the Gulf (Figure 2). To exit the Gulf, sailfish in our study were required to travel eastwards at least to longitude 56°32'E, to reach the Strait of Hormuz (the single entry and exit opening to the Gulf, Figure 1). After allowing for an error margin of ±1° for light-level-based longitude estimates, there was no evidence of any fish moving eastwards far enough to suggest that they had exited the Gulf (Figure 2).
We recognize that PSAT failure and premature recaptures severely limited our sample number of summer pop-up locations. Nevertheless, PSATs N and P successfully confirmed sailfish presence inside the Gulf during summer. This, together with conventional tag recovery data (Hoolihan, 2003), confirms sailfish occurrence in the Gulf for all months of the year, a necessary precondition to validating year-round residence. Another precondition, spawning, was evident from the ripe/running gonad stage of landed sailfish during June in the northern Gulf (Hoolihan, 2004a).
The phylogeographic isolation of a highly vagile istiophorid billfish, such as the Gulf sailfish, is unusual (Hoolihan et al., 2004). If indeed this accurately describes Gulf sailfish behaviour, then maximum sustainable yield may be limited by a lack of recruitment from outside the Gulf. As a bycatch species, Gulf sailfish are compromised by target species that move (i.e. higher recruitment potential) in and out of the Gulf. For example, mitochondrial DNA analyses of S. commerson, a primary target species of the Iranian gillnet fishery, indicate a single genetic stock structure for fish inside and outside the Gulf (Hoolihan et al., 2006). The implication here is that the S. commerson inside the Gulf may recruit from a much larger geographic area, and so be able to withstand greater fishing pressure. This scenario is consistent with the Gulf gillnet fishery, for which catch rates for S. commerson appear relatively stable, whereas sailfish numbers continue to decline.
The mean time at depth (83.9%) shallower than 10 m for the high-resolution PSAT data (A, B, D, H, and O) was almost identical to that observed (84.3%) during short-duration ultrasonic tracking studies of Gulf sailfish (Hoolihan, 2004b). However, this was not the case for the two PSATs (N and P) that attained their full-term deployment periods, for which just 60% of overall time was spent in the upper 10 m. We believe that the difference in time spent shallower than 10 m and the higher mean temperatures (Table 3) exhibited by fish N and P are a reflection of seasonal temperature increase (Reynolds, 1993) and a change in bathymetry encountered during the lengthier periods at liberty. Our explanation is supported by the increasing daily maximum depths and temperatures recorded over time by PSATs N and P (Figure 5). As a greater proportion of time at liberty for PSATs N and P was spent in areas of deeper bathymetry (Figure 2), perhaps the increased size of the water column influenced the preferential swimming depth. Less time spent shallower than 10 m may have afforded those two individuals some relief from the higher surface temperatures, or offered other advantages (e.g. a proximity to prey).
An overall preference for near-surface water has been observed for other istiophorid billfish. For example, both PSAT and ultrasonic tracking studies indicated that black marlin favoured the near-surface mixed layer (Pepperell and Davis, 1999; Gunn et al., 2003). For Atlantic blue marlin, Graves et al. (2002) reported a cumulative mean of 79.9% of time spent in the upper 10 m for eight fish, and Kerstetter et al. (2003) reported that two blue marlin spent 64.4% and 81.5% of their time in the upper 5 m. Further, Pacific blue marlin ultrasonically tracked off Hawaii exceeded 50% of their time shallower than 10 m (Holland et al., 1990; Block et al., 1992), and striped marlin ultrasonically tracked off Hawaii and California clearly preferred near-surface waters (Holts and Bedford, 1990; Brill et al., 1993). Lastly, Prince and Goodyear (2006) reported that the cumulative proportion of time spent shallower than 50 m by PSAT-monitored blue marlin (n = 17) and sailfish (n = 14) in the Atlantic and Pacific exceeded 70%.
Despite the preference for warmer near-surface waters, billfish often undertake deep dives of short duration, generally presumed to be associated with foraging or predator avoidance. Physiologically, billfish are adapted to tolerate the lower temperatures in deeper water by the presence of brain and eye heater tissue (Block, 1986). Other considerations, such as oceanographic features and prey availability, also factor in depth preference. For instance, Prince and Goodyear (2006) described how vertical habitat distributions of Atlantic and Pacific sailfish and blue marlin were directly correlated with suitable quantities of dissolved oxygen, and that hypoxic layers formed barriers to vertical movement. This was probably not the dominant factor controlling vertical distribution of sailfish in the present study, because both the shallow bathymetry and wind-driven wave action in the Gulf contribute to thorough mixing of the water column (Reynolds, 1993). Hoolihan (2004b) reported consistent temperature, salinity, and dissolved oxygen throughout the water column during sailfish ultrasonic tracking studies undertaken in the same area (Abu Dhabi, UAE) as the PSAT deployments; nevertheless, a clear preference for near-surface water was apparent. Notably, this area was <30 m total depth; given that water sampling was confined locally, it would not necessarily reflect the conditions found at the PSAT recovery locations in more northern Gulf waters.
Other studies have shown that istiophorids prefer a restricted temperature range. In contrast to Gulf parameters, such locations were characteristically deeper areas of the open ocean, with a thermocline separating the mixed surface layer from the much cooler water below. For example, an Atlantic blue marlin monitored for 5 d with a PSAT spent 98.6% of its time within 28.6–30.6°C (Kerstetter et al., 2003). Further, Graves et al. (2002) reported that eight Atlantic blue marlin monitored with PSATs for 5 d spent a major proportion of time at 
26°C and never ventured below 22°C. Similar behaviour was reported for a single black marlin monitored by PSAT for 39 d in the Coral Sea (Gunn et al., 2003); although that fish ranged from 22°C to 30°C, most of its time was spent within the range 26–27°C.
It has been suggested previously that istiophorids seek the warmest water available, thus explaining their affinity for the near-surface waters of the upper mixed layer. Brill et al. (1993) reported that movements of Pacific striped marlin ultrasonically tracked off Hawaii were controlled by the change in water temperature (relative to the mixed layer) rather than by the absolute temperature, provided oxygen was not limiting. Moreover, movements extending >8°C below the mixed layer temperature were unlikely. In contrast, an ultrasonic tracking study of striped marlin off California (Holts and Bedford, 1990) reported that two fish spent 25.3% and 76.7% of their time below the thermocline. Because the monitoring periods were of relatively short duration (<48 h), abnormal swimming behaviour resulting from capture and post-release trauma must be considered. Regardless of temperature, suitable levels of available oxygen would still be necessary for these fish to remain for extended periods below the mixed layer (Prince and Goodyear, 2006).
Some PSATs in our study failed to report, release, or transmit properly. We do not understand precisely the reason for these failures, but suspect physical damage or internal malfunction. Anecdotal claims from an Iranian fisher suggested that two of our PSATS were recovered in drift gillnets during 2002 and subsequently discarded into the sea (Mohammed Hajiani, pers. comm., 5 July 2004). Such action offers a possible explanation for damage and premature release. Data transmissions received from the premature releases of PSATs C, E, G, and J indicated that those units may have been shed or removed while the fish were alive, or after capture near the surface, and not as a result of death followed by sinking.
The large proportion of time that Gulf sailfish spend near the surface suggests an increased susceptibility to entanglement in gillnets and other surface gears, which may be further exacerbated by the small geographic area of the Gulf. This may explain the very high, and unprecedented, 27% recovery rate for PSATs in this study, and the >6% recovery rate reported for conventional tagging studies of Gulf sailfish (Hoolihan, 2004a). In contrast, of 144 PSATs deployed on istiophorid billfish in the Atlantic Ocean, only two (1.39%) were recovered by fishing gear (E. D. Prince, pers. comm., 11 September 2006). Moreover, a maximum recapture rate of just 2.8% was reported for billfish in a review of the world’s constituent-based conventional tagging programmes (Ortiz et al., 2003).
Our findings offer further supportive evidence that Gulf sailfish are an isolated population, with further implications that total population size is strictly limited. Given the drastic reduction in sailfish landings (Hoolihan, 2004a) and recreational fishing encounters, actions to reverse this trend warrant immediate consideration. One particular concern is the potential loss of a unique component of Gulf genetic diversity (Hoolihan et al., 2004). However, any steps implemented to reduce fishing mortality and rebuild this stock will rely heavily on international cooperation between the regional stakeholders to achieve success.
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Acknowledgements |
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We thank X. Eichaker, A. DeMaré, F. Launay, R. Gerard, M. Fadhili, G. Heinricks, and T. Z. Al-Abdessalaam for field assistance, and Iranian Fisheries Research Agency staff members S. Rezvani Gilkolaei, N. Niamaimandi, and H. Alizadeh for assistance in returning recovered pop-up tags. We also thank A. Nielsen for providing help in the use of sea-surface-temperature-corrected Kalman filter. Comments and suggestions by E. D. Prince and two anonymous reviewers certainly helped improve the manuscript, and M. A. Al-Bowardi, WWF-UAE, Emirates Wildlife Society, and Environment Agency–Abu Dhabi provided project funding and management support. All experiments complied with current laws of the United Arab Emirates.
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References |
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