Marine Biology (1999) 135: 369±380 Ó Springer-Verlag 1999

J. G. Pepperell á T. L. O. Davis Post-release behaviour of black marlin, Makaira indica, caught off the Great Barrier Reef with sport®shing gear

Received: 17 January 1997 / Accepted: 16 June 1999

Abstract The post-release behaviour of eight black (September to December). The sport ®shery, which is marlin (Makaira indica), caught by standard sport®shing timed to coincide with this aggregation, concentrates its techniques o€ the Great Barrier Reef, Australia, was activities at the outer edge of the Great Barrier Reef investigated using ultrasonic telemetry. Five marlin between Cairns and Lizard Island. Most of the black between 100 and 420 kg were successfully tracked for marlin are caught within 2 km of the outer edge of the periods of 8 to 27 h. Of the three others tagged, one was reef. killed by a shark and two shed their tags, probably as the Since 1972, over 20 000 black marlin have been tag- result of poor attachment. The black marlin spent most ged and released inside and outside the Barrier Reef by of their time within 10 m of the surface, both day and the sport ®shery, and 190 have been recaptured (Squire night. During the day, however, they also spent some and Nielsen 1983; Pepperell 1990; Matthews and time between 40 and 140 m depth. They rarely pene- Thompson 1996). Given the emphasis on tag-and-release trated the thermocline, and then only brie¯y, remaining strategy, it is important to gain some understanding of at temperatures no more than 8 C° below that of surface post-release behaviour and mortality. waters. The deepest dives were to 178 m. Four of the ®ve The black marlin is one of the least studied of the marlin tracked, initially moved o€shore before heading istiophorid bill®shes, and there is no information on its parallel to the shore, whereas the other marlin stayed vertical movements. Conventional tagging has shown close to the reef edge. The average mean swimming the species to be highly mobile, dispersing from eastern speeds over the ground for entire tracks ranged from 0.7 Australia throughout the western Paci®c. Black marlin to 1.02 m s)1. tagged o€ the Australian east coast have been recap- tured in the Philippines, Micronesia, the Solomon Islands, Kiribati, the Cook Islands, and as far away as the Marquesas, French Polynesia (Pepperell 1990; Deguara Introduction and Matthews 1994). Black marlin tagged o€ Baja, Cali- fornia, and Christmas Island in the central Paci®c have Sport ®shing for large marlin o€ the Great Barrier Reef been recaptured near eastern Australia (Pepperell 1990), near Cairns, Australia, targets black marlin (Makaira which also indicates probable mixing over a large part of indica). The sport ®shery o€ Cairns is based on the Paci®c. As well as undertaking long-distance move- pre-spawning ®sh, with the prized, larger ®sh (>180 kg) ments, homing may also be indicated ± ®sh have been being gravid females (Pepperell unpublished data). This recaptured near their release points o€ the Great Barrier aggregation of black marlin is predictable and highly Reef after 1, 2, 3, 4 and 5 yr at liberty (Pepperell 1994). seasonal, occurring during the southern spring While information on the vertical and horizontal movements of black marlin is of considerable interest to anglers, it is also useful to ®sheries' management. For Communicated by G.F. Humphrey, Sydney example, knowledge of vertical behaviour in relation to J.G. Pepperell temperature may be useful in interpreting longline catch- Pepperell Research & Consulting Pty Ltd., e€ort data, and possibly indicate ways of reducing the P.O. Box 818, Caringbah, New South Wales 2229, Australia bycatch of marlin in longline ®sheries (e.g. Boggs 1992; T.L.O. Davis (&) Hinton and Nakano 1996). CSIRO Division of Marine Research, Black marlin have not been ultrasonically tracked P.O. Box 1538, Hobart, Tasmania 7001, Australia Fax: 0061 (0)3 6232-5195, before, although other members of the family e-mail: [email protected] Istiophoridae have (e.g. Yuen et al. 1974; Jolley and Irby 370

1979; Holland et al. 1990; Holts and Bedford 1990; A ®ltering program was developed to process depth data for Block et al. 1992a; Brill et al. 1993). It is possible that analysis. The program compared each depth record with the seven previous points, and deleted spurious records based on maximum di€erent species of bill®shes may behave di€erently, and minimum depths and maximum vertical speed. A ®lter was also since they have somewhat di€erent distributions in used to delete spurious temperature values based on maximum and relation to surface temperature and land-forms minimum temperatures and maximum temperature-change rates. (Nakamura 1985). Finally, no very large ®sh of The program then interpolated missing data and generated data any species had been tracked before and, given the ®les at required time intervals. Depth pro®les of tracked ®sh were plotted at 20 s intervals. The position and sustained swimming availability of very large ®sh (300 to 500 kg) in the study speeds of tracked marlin were assumed to be the same as that of the area, there was a strong chance of tracking ®sh in this tracking vessel. These were determined at 5 min intervals by GPS. size range. Speed was calculated from the straight-line distances travelled be- This paper presents the results of tracking six black tween points. Vertical movements and horizontal movements rel- ative to the vessel were not considered. During tracking, an attempt marlin that were caught, tagged and released under was made to match both the heading and speed of the tracked normal sport-®shing conditions during the years 1992 to marlin rather than stop and catch up, in order that the vessel's 1995. We examine their vertical and horizontal move- movements best re¯ected the movements of the ®sh. ments and activity patterns, and diurnal e€ects on these Depth distributions of individual marlin were determined every 20 s by day and night. The counts of depths in each 5 m depth- behaviours. We compare the behaviour of black marlin stratum were expressed as a proportion of the total counts. Max- to that of other species of istiophorid bill®shes and, by imum rates of descents and ascents were determined from depth way of contrast, broadbill sword®sh. data at the highest time-resolution available. Rates (m s)1) were determined from the regression of depth (m) on time (s). The mean speeds in each 10 m stratum were calculated for each ®sh every 20 s. The short time intervals resulted in many zero-speed esti- Materials and methods mates, but ensured that GPS position changes were closely matched to depth strata. The vessel used for all tracking cruises was a 14 m sport-®shing, ¯y-bridge cruiser. The hydrophone was mounted amidships 2m below the water line on a central rotating shaft housed within a cylindrical mast. A free rotating aluminium fairing was ®tted to the Results mast to reduce drag and strumming from forward movement or when the vessel rolled. A remote-controlled shaft rotator mounted Description of tracks on the mast was used to control the direction of the hydrophone. The control and display showing its direction relative to the vessel were housed in the saloon of the vessel. The signal from the Details of all Makaira indica tagged and tracked are hydrophone was decoded and displayed on a Vemco VR60 shown in Table 1 and Fig. 1. The ®ve successful tracks ultrasonic receiver. While tracking, we attempted to maintain a are described ®rst, in sequence, followed by details of distance between the vessel and the ®sh of about 400 m, judged by the strength of the signal, and based on distance/signal strength three marlin that were tagged but not successfully calibrations made before tracking. tracked. Two types of transmitters (Vemco, Halifax, Novia Scotia, Canada) were used: V32 (32 mm diam) tags transmitting on 32.8 kHz, and V22 (22 mm diam) tags on 40 kHz. Both types of tag were equipped with pressure sensors producing pulsed signals Marlin No. 1 linearly proportional to depth. One dual-channel V22 tag was ®tted with both temperature and pressure sensors. Decoded depth and The ®rst marlin tracked was caught north of Linden temperature data were logged on a computer together with GPS latitude and longitude using Vemco VSCAN/GPS software. Under Bank. It was hooked on the outside of the operculum normal conditions, depth data from single-channel transmitters and the single-strand wire trace wrapped itself around were logged at 1 s intervals, while data from dual-channel the pectoral girdle. It was released in good condition at transmitters were logged at 3 s intervals. 18:00 hrs. All Makaira indica were caught by standard sport®shing tech- For the ®rst hour, the ®sh stayed within 20 m of the niques (Goadby 1991). The ®sh was held alongside the vessel, and the tag was inserted 5 cm into the dorsal musculature below the surface, moving relatively slowly (Fig. 2A). It then rose anterior half of the ®rst dorsal ®n by means of a stainless steel, to very near the surface (coinciding with sunset) and arrow-shaped anchor mounted on the pronged tagging pole. The tag swam slowly eastwards (Fig. 1). After 20:00 hrs, the was attached to the anchor by crimped, multi-strand, stainless steel ®sh maintained close contact with the surface for several wire. After the ®sh had been tagged, the hydrophone was swung into the water and tracking began as soon as the signal was detected. hours, making several dives, the deepest (near midnight) Failure of an electronic temperature±depth pro®ler on the ®rst to 77 m. After this dive, the ®sh returned to the surface cruise in November 1992 resulted in no temperature data being col- and sped at up to at least 8 knots (4.1 m s)1) ± faster lected during Tracks 1 and 2. However, a temperature±depth pro®le than the boat could track ± for some minutes before it was obtained at the end of the two tracks with a Niskin cast and reversing thermometers. Supplementary information on tempera- was again located. At times during the night, especially ture pro®les was hindcast from satellite data and oceanographic after surfacing at 03:00 hrs, this ®sh became quite in- models of the region (K. Ridgway personal communication 1996). active, apparently drifting with the prevailing current for On subsequent tracks, conductivity±temperature±depth pro®les up to 20 min. Periodically, the ®sh would become more were taken with a data logger (Digital Data Loggers International) at active and make short dives. the start and ®nish and at convenient times during each track (6h intervals). Bottom depths <200 m, and times of high and low tide At ®rst light, the ®sh dived deeply after being at the were also recorded during all tracks. surface for >2 h. At 07:20 hrs, it became active, 371

Table 1 Makaira indica. De- Date Fish Estimated Time on Duration of Av speed Distance tails of black marlin tagged o€ )1 Cairns between 1992 and 1995 tagged No. wt (kg) line (min) track (h) (m s ) moved (km) (Distance moved horizontal dis- tance travelled along each 1992 track) 21 Nov 1 120 5 27 1.02 112 25 Nov 2 180 5 18 1.26 81 1993 24 Nov 3 420 45 24 0.83 72 1994 21 Nov 4 100 5 0a ±± 22 Nov 5 350 80 0a ±± 23 Nov 6 400 30 15 0.74 39 1995 12 Dec 7 110 20 8 0.70 18 16 Dec 8 120 10 21 0.74 56 a Tag lost diving and changing horizontal direction several times. The track was terminated at 21:30 hrs because of At 09:40 hrs, it ascended rapidly, jumping clear of the worsening weather; the tracking vessel was then >20 water several times, and then swimming away faster nautical miles from the outer edge of the reef. The than the tracking vessel could follow. The signal was weather had otherwise been calm. The moon was in the lost for 42 min, but was found again to the west, at last quarter plus 3.5 d. which time the ®sh was at 50 m depth, and still swimming quite quickly (2 m s)1). Having swum mostly eastwards during the night, the ®sh changed direction Marlin No. 2 to swim virtually north all day, generally at a depth of 50 to 70 m. After 17:30 hrs, its activity changed. It This ®sh was caught south of Linden Bank at the reef swam faster, zigzagging with bursts of 3 to 4 m s)1 for edge. During capture it was bleeding from the mouth, nearly 2 h. but no blood was apparent when it was tagged.

Fig. 1 Makaira indica. Hori- zontal tracks of eight black marlins; No. 1 was released on 21 November 1992, No. 2 on 25 November 1992, No. 3 on 24 November 1993, Nos. 4 and 5 on 21 and 22 November 1994, respectively, No. 6 on 23 November 1994, No. 7 on 12 December 1995, and No. 8 on 16 December 1995 [Large arrows and tag numbers points of release (points of release for Nos. 4 and 5 are marked by + only); symbols on tracks hourly positions; ®lled symbols night; open symbols day; dashed line 200 m depth contour] 372

Fig. 2 Makaira indica. Vertical tracks of Black Marlins Nos. 1 (A) and 2 (B). Depths of tracks plotted at 20 s intervals as a function of time of day. Speed was determined by straight-line distance (GPS) travelled in 5 min (Shading sea bottom; dashed lines approximate 1 C° isotherms from Niskin cast taken after tracking; H, L high and low tides, respectively; black bars night-time)

Initially the ®sh dived to 60 m, but for only a few rise, it dived and stayed at depth for the next 2.5 h, minutes. It then swam gradually to near the surface swimming between 100 and 160 m depth and only before making the ®rst of many dives at 17:15 hrs making two brief excursions to the surface. We stopped (Fig. 2B). Initially it swam faster (up to 3.5 m s)1) than tracking at 10:45 hrs. The seas during this tracking were Marlin No. 1, and it stayed near the reef edge during the moderate throughout. The phase of the moon was new ®rst hour after release. At 17:30 hrs it moved away on the night of tracking. from the reef, heading close to due east. At dusk, it made one sharp, deep dive to 120 m, but came up again and settled into a fairly regular pattern of ascents and de- Marlin No. 3 scents throughout the night. This ®sh also spent pro- portionally more time near the surface during the night. This ®sh, which was caught near the eastern edge of The ®sh was quite active and, unlike Marlin No. 1, did Linden Bank, was the largest we tracked (estimated not undergo periods of inactivity during the night, but 420 kg). It was apparently exhausted when brought to continued swimming at an average of 1.5 m s)1,with the boat, and after tagging ¯oated belly-up, virtually some bursts of up to 3 m s)1 after midnight. The ®sh motionless on the surface. After release, it sank head continued heading east until dawn, when it turned ®rst and continued sinking to a depth of 105 m just at slowly and headed north, and later northwest. At sun- the edge of the reef (Fig. 3A). For the next 2 h its only 373

Fig. 3 Makaira indica. Vertical tracks of Black Marlins Nos. 3 (A) and 7 (B)(xxx period when small marlin in A jumped) Further details as in legend to Fig. 2

movement was vertically between 80 and 118 m. We At about 09:00 hrs, another black marlin of 100 kg thought the ®sh had possibly died, but it was holding jumped clear of the water directly ahead of the tracking station against a strong southward-¯owing current that boat, virtually in the position of the tracked ®sh. This was pushing the tracking vessel away from the ®sh. smaller ®sh jumped eleven times over the next 75 min, At about 18:30 hrs, at sunset, the ®sh made its ®rst each time where the tracked ®sh was estimated to be. strong ascent to <20 m from the surface and began The jumps were ``lazy'', with the ®sh only just clearing swimming away from the reef. It then dived and ascended the surface before ``¯opping'' back on its side. Because at a frequency similar to Fish No. 2 (30 min apart). of its size, it is highly likely that the smaller ®sh was a After midnight, the ®sh stayed closer to the surface for male, and that it was following the ®sh being tracked, 4 h, moving very little but drifting with the current, as which was a female. had Marlin No. 1. In the hour before dawn the ®sh made After midday, the tracked ®sh began to dive more two deep dives, and just after dawn made its deepest dive frequently to depths between 80 and 100 m. The pattern of the track, to 130 m. It then returned to near the after 13:00 hrs was similar to that of both previously surface and remained there until noon, making only four tracked ®sh, with infrequent, brief excursions to the short, shallow dives to a maximum of 50 m. surface. This ®sh had headed east during the night, and 374 continued in this general direction during the following night of the track. This tagging (16 December) took day. Tracking was intentionally terminated when the place after the close of the black marlin ®shing season. weather deteriorated. Moon phase was 2 d before full. The moon rose at 15:14 hrs and set at 02:27 hrs on the night of the track. Other ®sh tagged Two black marlin tagged in November 1994 lost their Marlin No. 7 tags within minutes (see Table 1). Both tags were relo- cated on the following mornings on the bottom, where This ®sh was caught o€ Norman Reef. It was hooked in they had fallen. the side of its mouth and was the smallest of the ®ve Marlin No. 4 was tagged on the eastern edge of tracked (estimated at 110 kg). The ®rst tag fell o€, but a Linden Bank. It maintained a depth of between 8 and second tag was placed successfully in the shoulder and 15 m for 2 min after release, after which the tag sank to the ®sh was released after longer than usual handling the bottom at 0.92 m s)1. Marlin No. 5 was tagged just near the boat. north of Linden Bank. This ®sh stayed within 20 m of After release, the ®sh descended slowly to 95 m, just the surface for 4 min after release, after which the tag above the bottom. It then ascended gradually over the sank to the bottom at 0.83 m s)1. A video tape of this next hour to 25 m, apparently following the bottom tagging indicated that the tag had been placed poorly. contour as it moved over the reef (Figs. 1 and 3B). The The positions of both tags on the bottom, as determined ®sh remained near the reef edge at a depth of 20 to 25 m by GPS, had not changed on the following mornings. over a bottom depth of 44 to 55 m. At about 14:30 hrs, Marlin No. 6 was tagged just north of Linden Bank in the ®sh left the reef edge, and immediately dived to November 1994 (Fig. 1). It was released in reasonable 69 m, after which it made a series of gradual, shallow condition, although its stomach was everted. After re- dives and ascents, moving back over the reef. It then lease, it stayed at a depth of about 50 m for 12 min, made moved along the reef edge, twice descending to near the one brief ascent to 26 m after 12 min, and then descended bottom (60 m). The signal abruptly ceased at 18:40 hrs to the ocean ¯oor (140 m: Fig. 4B). After remaining there when the hydrophone failed. for several minutes, it ascended again up the face of the The marlin was apparently trailing closely behind a reef drop-o€ and onto the reef plateau itself. After some school of surface-feeding skipjack tuna (2 to 3 kg ®sh) periods of inactivity, the ®sh moved slowly and remained during most of this track. The tuna were travelling along on or near the bottom for the next 7 h, after which it the edge of the reef, periodically breaking the surface in became more active and moved into shallow water (30 to bursts of feeding activity. This was the only marlin 50 m). The tag used was a VPT model, which recorded tracked that stayed near the reef, but it was also the only temperature as well as depth. Subsequent analysis of the marlin that was tagged before noon and not tracked at temperature data indicated that the temperature of the night. The weather during this track was relatively tag did not drop when the ®sh dived into cooler water, rough, with winds from the southeast persisting at 18 indicating that it was decoupled from the ambient water knots. The moon was full plus 4.7 d. temperature (Fig. 5) and strongly suggesting that it had been swallowed by a shark. Marlin No. 8 Diel behaviour This ®sh, caught 3.7 km east of Finn Reef (Fig. 1), was hooked in the corner of the mouth. It dived to 30 m and Four black marlin were tracked during both day and settled down very quickly (Fig. 4A). After 1 h, it left night. The time they spent at each 5 m stratum is shown the reef edge and started heading slowly eastwards. in Fig. 6. Fish Nos. 1, 2 and 3 showed a marked pref- During the night, the ®sh stayed near the surface for erence for shallower depths (5 to 10 m) at night than in long periods interspersed with short dives every 90 min the day. Fish Nos. 1 and 3 mostly remained at depth or so. After midnight, it made several shallow dives until during the day, in a relatively narrow depth range. Fish 90 min after dawn (07:00 hrs) when it again dived to No. 8, which showed a strong preference for the top 65 m. It returned to the surface and remained there for 10 m during both night and day, had a slight secondary most of the morning, travelling slowly southeast, par- preference for deeper water during the day. Throughout allel to the reef. After 11:30 hrs (at low tide), the ®sh the tracking, this ®sh stayed closer to the surface than became much more active, making far more frequent the other three marlin. dives and moving faster horizontally. Its deepest dive during this period was again 65 m. The marlin was quite active, making a number of dives to 50±60 m be- Swimming speed fore we terminated tracking at 16:44 hrs. The weather throughout was very calm, with a slight current (1.5 The horizontal swimming speeds of ®sh were measured knots) from the north. The moon phase was in the last by GPS on the tracking vessel, and are therefore quarter plus 0.6 d. Moonrise was at 01:09 hrs on the indicators of speed over the ground only. The speed of 375

Fig. 4 Makaira indica. Vertical tracks of Black Marlins Nos. 8 (A) and 6 (B). Further details as in legend to Fig. 2

each ®sh is indicated above the vertical tracks in Figs. 2 have been tracked. Data from these studies, including to 4. The average speeds ranged from 0.7 to 1.26 m s)1 the present study, are summarised in Table 2. Most of (Table 1). The fastest speed was at least 5 m s)1 (the the bill®shes tagged and tracked have been caught by tracking vessel could not travel faster because of the sport-®shing methods. They were tracked for between a hydrophone). On one occasion Marlin No. 1 swam away few hours and several days, and post-release survival for very rapidly after jumping clear of the water several these periods has been high (84 to 91%) (Table 2). times, and was lost for some 40 min. On other occasions, the ®sh would outpace the tracking vessel for up to '15 min before slowing to the usual swimming pace. Vertical movements Horizontal speeds examined at di€erent depth strata gave no evidence of a relationship between depth and A striking feature of all the istiophorid bill®shes studied swimming speed (Fig. 7). Again, individual ®sh had is that they show a preference for the mixed layer, consistently higher or lower speeds at all depths. seldom cross the thermocline, and do not stay in water 5 to 10 C° cooler than the mixed layer for long. Striped marlin (Tetrapturus audax) o€ California stayed mostly Discussion above the thermocline; any dives crossing the thermoc- line lasted for only several minutes (Holts and Bedford Although this is the ®rst study to track the black marlin 1990). In Hawaii where the water is much warmer than Makaira indica, most of the other istiophorid species o€ California, striped marlin spent more than 85% of 376

Fig. 5 Makaira indica. First hour of Black Marlin Track No. 6. Depth plotted at 3 s intervals as a function of time of day. Water temperature was calculated by pinger depth and temperature/depth pro®le taken just before tracking. Further details as in legend to Fig. 2 the time within the mixed layer (<90 m deep; Brill et al. 1993). Five blue marlin (Makaira nigricans) tracked o€ Hawaii spent, on average, 82% of daylight hours and 97% of night hours in the mixed layer or the upper 2 C° of the thermocline (Holland et al. 1990). Other blue marlin o€ Hawaii spent 50% of their time within 10 m of the surface between 26 and 27 °C (Block et al. 1992a). In Block et al.'s study, the deepest dive of a blue marlin was (brie¯y) to a depth of 209 m, where the water temperature was 17 °C. The four black marlin which moved o€shore in our study remained in the warm mixed layer (24 to 28 °C) most of the time, making some short dives into cooler water, as reported for other istiophorids studied. The deepest dives (to 178 m) were made by Marlin No. 2 (Fig. 2B) on two brief occasions. The temperature at that depth was estimated to be 21 to 22 °C (based on the temperature at 85 m and hindcast oceanographic infor- Fig. 6 Makaira indica. Proportion of time spent by four tracked mation) (K. Ridgeway personal communication 1996). marlin at various depths (5 m intervals) by day (unshaded bars)and The two marlin tracked in 1995 (Nos. 7 and 8) swam at night (shaded bars) the shallowest depths, remaining above 65 m (26 to 28 °C) most of the time [Fish No. 8 dived to near the thermocline acted as a virtual physical barrier, regard- bottom (98 m) immediately after release]. less of its absolute temperature in relation to the mixed The shallow swimming depths exhibited by tracked layer. They also noted that, in both regions, striped black marlin provide some scope for standardising marlin did not dive to waters >8 C° cooler than the catch-and-e€ort longline data according to depth of mixed layer, regardless of the depth of the dive. The ®shing. Hinton and Nakano (1996) used such data for same applied to the black marlin we tracked, the lowest blue marlin to re-interpret historic Japanese longline- temperature recorded for a dive was also 8 C° cooler catch data. Also, these data provide some scope for than the surface temperature (Fig. 3A). The black reducing the by-catch of black marlin by longlines marlin's general pattern of vertical movements is similar targeting deeper-dwelling tuna species such as bigeye to that of other istiophorids. Some of the tracked ®sh tuna. made fairly regular, brief dives of <10 min (Figs. 2B, Although marked thermoclines were seldom evident 3A and 5), and it is possible that their return to the during our study, when they were, black marlin crossed mixed layer was a behavioural response to cooling. them only brie¯y (Figs. 3A and 4A). Blue marlin The habitat of istiophorids is therefore largely con- (Holland et al. 1990; Block et al. 1992a) and striped ®ned to the surface layers. The deepest dive of any black marlin (Holts and Bedford 1990; Brill et al. 1993) marlin was 178 m, and none of the other marlins or display identical behaviour. Brill et al., comparing the sail®sh (Istiophorus platypterus) tracked dived deeper behaviour of striped marlin in surface waters of 27 °C than 210 m (a blue marlin tracked by Block et al. (Hawaii) and 20 °C (California), suggested that the 1992b). In contrast to the istiophorids, broadbill 377 Edwards (1995) Source death

Fig. 7 Makaira indica. Mean speed of ®ve tracked marlin in various depth strata (10 m intervals) sword®sh (Xiphias gladius ) dive much deeper (measured to >600 m) into colder water up to 20 C° less than surface temperatures, paying little heed to the the- rmocline (Carey and Robison 1981). This ability is

probably facilitated in sword®sh by their possession of a not applicable)

large brain-and-eye heater (an energy-producing, modi- na ®ed eye muscle; Carey 1982). Istiophorids have also evolved this type of organ, but to a lesser degree (Block

1990). longline; LL

Horizontal movements ????na

A second striking feature of istiophorid movements is that many, although not all, tracked ®sh moved away rod and reel;

from the coast after release. This tendency has been RR noted in blue marlin (Yuen 1974; Holland et al. 1990; Block et al. 1992a), striped marlin (Holts and Bedford 1990; Brill et al. 1993) and sail®sh (Jolley and Irby handline; Track duration (h) Estimated size (kg) Time on line (min) Cause of 1979). Range Mean Range Mean Range Mean In the present study, all black marlin were hooked HL and released on the inside of the drop-o€ of the Great Barrier Reef (Fig. 1). Four of the ®ve ®sh tracked moved o€shore after release, while the ®fth stayed over 25 5.5±22.5 14 15±1205 56.6 70±270 170 60±220 104 8±27 ? 20 15±45 22 ? 120±400 200 Shark ? 5±45 Block et al. (1992a) 17 Yuen et al. (1974) Shark Present study

or close to the edge of the reef for the 8 h of its track. Of No. surviving the four ®sh that moved away from the reef drop-o€, the two tracked in 1992 turned north to eventually swim parallel to the reef; the ®sh tracked in 1993 turned a a a

south, but by the end of the track was again heading No. tagged further to sea; the last ®sh tracked in 1995 moved away from the reef for only 3 h before swimming south, parallel to the reef.

The four black marlin that moved o€shore were all Fishing method tagged within 2 h of sunset. The ®sh that did not (No. 7) was tagged before noon, and tracking ended just before sunset. O€shore movement may therefore be a normal night-time activity. This is supported by the noticeable change in direction of Marlin Nos. 1 and 2 after dawn. Both these ®sh had been moving away from the reef edge Summary of tracking studies on istiophorid bill®shes ( during the night, but at ®rst light began making slow, CaliforniaHawaiiNE Australia RR LL RR 12 6 6/8 12 5 3±48 20.9 14±51 30.2 50±80 61 37±57 45 ? na ? na na ? Holts and Bedford (1990) Brill et al. (1993) Florida RR 8 7 3±28 8.2 11±28 14 8±20 12 Shark Jolley and Irby (1979) HawaiiHawaiiUS Virgin Islands RR RR RR 6 6 5/6 6 6 <6 7±42 <6 28 60±160 97 20±36 27 na Holland et al. (1990) Hawaii HL 4/5 Location sweeping turns that brought them parallel to the reef. Of Based on the tag sinking rate, it is fairly certain that tags fell o€ ®sh soon after release Striped marlin Black marlin Sail®sh Table 2 a Blue marlin Species, 378 course, recovery time would a€ect the ®sh's behaviour 5ms)1 for Fish No. 2 (bearing in mind that tracking after release: Marlin No. 3, for example, remained in- was limited to this speed). shore of the reef ledge for the ®rst 2 h after release, but The average swimming speeds of other istiophorids then moved seawards at dusk. are very close to those of black marlin: for blue marlin It has been suggested that the o€shore movement is a from 0.6 to 1.12 m s)1 (Holland et al. 1990); for striped response to tagging, while the change in direction to marlin 0.5 to 1 m s)1 (Brill et al. 1993). The speed of swim parallel to the coastline indicates the end of the swimming blue marlin measured directly over 160 h post-release recovery period (Holland et al. 1990). This ranged from 0.1 m s)1 to 2.2 m s)1 (Block et al. 1992b). may be the case, but other variables such as time of day, Thus, the sustained swimming speeds of bill®shes are no and perhaps current strength, may in¯uence the direc- faster than many ®sh species, even though they are ca- tion of swimming with respect to the coast or reef edge. pable of bursts of high speed (reviewed in Block et al. While tracking striped Marlin, Brill et al. (1993) found 1992b). As blue marlin tended to swim faster at depth evidence that horizontal movements were in¯uenced by than near the surface (Block et al. 1992b), we brie¯y currents. We did not measure the currents, although in considered the relationship between depth and speed of 1993 when Marlin No. 3 was tracked, there was a par- swimming in black marlin. However, the black marlin ticularly strong southerly surface current in the area. we tracked showed no marked change of speed with While it is likely that such currents would in¯uence depth (Fig. 7). movement, the question requires further study.

Tides and moon phase Diel behaviour The times of high and low tides in the study area are Four black marlin were tracked both day and night. given in Figs. 2 to 4. The tide had no obvious general Three of these showed marked preferences for the top 5 e€ect on the vertical behaviour of black marlin. Other to 10 m depths at night (Fig. 6), but during the day they studies on istiophorids have not taken possible tidal mostly stayed at the surface or at a deeper layer near the e€ects into account, but this too could be a useful thermocline. The exception was Marlin No. 8, which retrospective exercise. stayed near the surface during both day and night Black marlin were tracked during various moon (Fig. 4A). phases. For example, Marlin Nos. 1 and 2 were tracked These ®ndings are consistent with those of Holland close to the new moon, whereas Fish No. 3 was tracked et al. (1990) for blue marlin, which demonstrated a during a full moon. No obvious e€ects of moon phase marked preference for swimming deeper during daylight on activity or vertical behaviour were noted. Broadbill than night hours. As in our study, the tendency of sword®sh may swim deeper at night under a full moon Holland et al.'s and Block et al.'s (1992a) blue marlin than a new moon (Carey and Robison 1981). However, was not always clear-cut, but if there was a di€erence in they have much larger eyes than the istiophorids, so are swimming depth, it was a tendency to swim deeper probably much more a€ected by ambient light. during daylight than night. Holts and Bedford (1990) found that, where di€erences were discernible in striped marlin (4 of 7 ®sh), there was a preference for deeper Recovery and survival after release swimming during the night than in the day. No obvious day/night di€erences in swimming depth for striped In all other studies but one involving isiophorids, the ®sh marlin tracked o€ Hawaii were found (Brill et al. 1993). were caught by standard sport-®shing techniques. The Therefore, no generalisations can be made about diel exception was that of Brill et al. (1993) who used depth-preferences for istiophorids as a group. 120-hook longline gear. Whether di€erent capture methods a€ect subsequent behaviour is unknown. Brill et al.'s study was the only one on istiophorids that noted Speed the apparent mortality of a ®sh (other than by shark attack) during tracking. The horizontal speeds of tracked black marlin varied For large ®sh such as marlin, there is much specula- more between than within ®sh. One ®sh (No. 2) main- tion about whether exhaustion, caused by the strenuous tained consistently higher speeds than the other four, activity of the ®sh hooked on rod and reel, a€ects its averaging 1.26 m s)1 compared with averages ranging subsequent recovery and survival. We are necessarily from 0.70 to 1.02 m s)1 for the other four ®sh. Inter- con®ned here to discussing short-term survival, since our estingly, the average speed of the presumed shark that longest track was 27 h. Other authors have stated that was tracked after ingesting a tag was 0.74 m s)1 which is behaviour during the ®rst few hours after release is close to the lower average speeds recorded for black signi®cantly di€erent from that during the remaining marlin and also to speeds measured for other species of track. The main quanti®able di€erence noted has been shark (e.g. Weihs 1984; Carey and Scharold 1990). The speed of horizontal swimming, which was fastest after highest sustained speed attained by a black marlin was release for blue marlin (Block et al. 1992b) and striped 379 marlin (Holts and Bedford 1990). No such di€erence in striped marlin has been interpreted as sleeping (Holts was recorded for black marlin. and Bedford 1990), which may be an apt interpretation Released marlin also have been observed to dive deep for our observations. The attendance of a presumed after release, often to the greatest depth observed for the male marlin in close proximity to Marlin No. 3 (a large track (e.g. Holland et al. 1990). This was also the case female) for at least 90 min strongly suggests social in- for Black Marlin Nos. 3 (Fig. 3A) and 7 (Fig. 3B), but teraction, possibly courtship behaviour. The presumed while the other ®sh all dived immediately after release, male made many short jumps near the tracked ®sh, they did not dive to unusual depths, and they all which was close to the surface at the time, both seem- ascended again within 60 to 90 min. It is possible that ingly oblivious to the presence of the tracking vessel. the short ®ght times for the black marlin tagged during Marlin No. 1 was probably exhibiting ¯ight behaviour this study may explain their apparent comparative lack when it suddenly made several energetic jumps before of recovery problems. The one ®sh that was on the line swimming away faster than the tracking vessel could for a long time (Fish No. 4 for 45 min) initially dived (or follow. Just before this, an oceanic whitetip shark sank) to near the greatest depth of its track, and stayed (Carcharhinus longimanus) had been seen in the vicinity, relatively motionless at about that depth for a little more and possibly the marlin was reacting to its presence. than 2 h. It then rose quite rapidly, and thereafter Contact was re-established with the marlin 45 min later, behaved like the other marlin. This was the largest ®sh by which time it had settled back into the much slower tagged, and it was ¯oating motionless upon release. swimming speed typical of the rest of its track and of the Although it appeared to be motionless at depth after other marlin tracked. Lastly, throughout its relatively release, there was a strong surface current in which the brief track (8 h) Marlin No. 7 seemed to be trailing a tracking vessel needed to maintain station, so it is school of surface-feeding skipjack tuna (Katsuwonus possible that the ®sh may also have been recovering by pelamis). Both were following the edge of the barrier holding station in the prevailing current. reef, but when the skipjack tuna moved into deeper Of the eight marlin tagged during this study, one water, or back to the reef, the marlin did likewise. All de®nitely died, the cause of mortality being almost these observations, together with the repeatable, pre- certainly shark predation (as indicated by the tag not dictable general behaviour of tracked black marlin, recording temperature changes as the ®sh moved verti- suggest that post-release behaviour re¯ects normal be- cally: Fig. 5). Anecdotal reports of sharks attacking haviour. hooked black marlin in the study area are relatively common. During the course of this work, we observed many hook-ups of black marlin, but only one de®nite Possible e€ects of tracking vessel on ®sh behaviour shark attack, that of a ®sh of 300 kg. Shark attack is thought by the charter captains to be a local problem on On some occasions, the tracking vessel may have in¯u- some parts of the reef and, in those areas, the risk of enced short-term behaviour. When marlin were near the shark attack is thought to increase with proximity to the surface, and if the vessel overran the ®sh, it would often reef. make sudden, brief dives ± probably because of the The two tags that sank to the bottom shortly after presence of the vessel. Holts and Bedford (1990) also tagging may also indicate shark-induced mortalities. noted this behaviour during their tracking experiments However, it is more likely that the tags were not placed with striped marlin, but they were the only authors to correctly and became dislodged, sinking to the bottom at )1 do so. constant rates of 0.83 and 0.94 m s , respectively. Two During tracking of the last marlin (No. 8), we felt tags that apparently came loose from blue marlin sank )1 that the ®sh was using the vessel as a reference point to the bottom at rates of 0.46 and 0.69 m s (Block during the track. On several occasions, to test this hy- et al. 1992a), while a striped marlin (presumably dying) )1 pothesis, we stopped the tracking vessel and allowed the sank more slowly (0.17 m s ; Brill et al. 1993). There marlin to keep swimming away. On these occasions, it was no evidence in our study of black marlin dying returned to the vicinity of the vessel, perhaps treating us of exhaustion after release, and in other studies of as a natural ¯oating object. This ®sh was very surface- istiophorids mortality appears to have been low (Jolley oriented, and the seas were calm, facilitating visual and Irby 1979; Holland et al. 1990; Holts and Bedford contact with the vessel. 1990; Block et al. 1992a; Brill et al. 1993).

Acknowledgements Not everyone who has helped with this study can be mentioned here. However, we would especially like to thank Behavioural observations the following who contributed funds and assistance: The Austra- lian Fisheries Management Authority, Flamingo Bay Research, the Some observations made during tracking provide indi- Game Fishing Association of Australia, Shimano, Toyota, Caltex cators of the normal behaviour of black marlin. A ®sh's Australia, Quest Australia, Lizard Island Game Fish Club, Cairns Professional Game Fishing Association, NSW Game Fishing As- activity would often decrease between midnight and sociation and Cairns Game Fishing Club. Individuals who greatly dawn to the extent that it appeared to drift with the assisted include: D. Tomlinson, Flamingo Bay Research, who made current at the surface for up to an hour. This behaviour available the tracking vessel; J. Gunn, M. Sherlock, J. Cordell, 380

L. MacDonald, all of CSIRO, who gave technical assistance; ational Fisheries, May 8±10, 1995. Virginia Institute of Marine R. Barstow, B. Beath, W. Howell, J. Martin, R. Smith, D. Still, Science, Gloucester Point, Virginia (Internal publication) S. Mott, the crew of the tracking vessel; and W. Billson, R. Jones, Goadby P (1991) Saltwater game®shing: o€shire and onshore. J. Gill and C. Tilley, the captains of cooperating vessels. The ®l- Collins/Angus & Robertson, North Ryde, New South Wales tering program was originally developed by M. Ryba and subse- Hinton MG, Nakano H (1996) Standardizing catch and e€ort quently modi®ed for temperature-®ltering by A. Betlehem, both of statistics using physiological, ecological, or behavioral con- CSIRO Division of Fisheries. This manuscript bene®ted greatly straints and environmental data, with an application to blue from critical reviews by B. Bruce, K. Holland and two anonymous marlin (Makaira nigricans) catch and e€ort data from Japanese referees, and from editing by V. Mawson. longline ®sheries in the Paci®c. Bull inter-Am trop Tuna Co- mmn 21: 177±200 Holland K, Brill R, Chang RKC (1990) Horizontal and vertical movements of Paci®c blue marlin captured and released using sport®shing gear. Fish Bull US 88: 397±402 References Holts D, Bedford D (1990) Activity patterns of striped marlin in the southern California Bight. In: Stroud RH (ed) Planning the Block BA (1990) Phylogeny and ecology of brain and eye heaters in future of bill®shes. National Coalition Marine Conservation, bill®shes. In: Stroud RH (ed). Planning the future of bill®shes. Savannah, Georgia, pp 81±93 National Coaltion Marine Conservation, Savannah, Georgia, Jolley JW Jr, Irby EW Jr (1979) Survival of tagged and released pp 123±136 Atlantic sail®sh (Istiophorus platypterus: Istiophoridae) deter- Block BA, Booth DT, Carey FG (1992a) Depth and temperature of mined by acoustical telemetry. Bull mar Sci 29: 155±169 the blue marlin, Makaira nigricans, observed by acoustic te- Matthews J, Thompson K (1996) Game®sh tagging newsletter lemetry. Mar Biol 114: 175±183 1994±95. New South Wales Fisheries, Sydney Block BA, Booth DT, Carey FG (1992b) Direct measurement of Nakamura I (1985) Bill®shes of the world. FAO Fish Synopes 125 swimming speeds and depth of blue marlin. J exp Biol 166: 267± (5): 1±65 284 Pepperell JG (1990) Movements and variations in early year class Boggs CH (1992) Depth, capture time, and hooked longevity of strength of black marlin Makaira indica o€ eastern Australia. longline-caught pelagic ®sh: timing bites of ®sh with chips. Fish In: Stroud RH (ed) Planning the future of bill®shes. National Bull US 90: 642±658 Coalition Marine Conservation, Savannah, Georgia, pp 51±66 Brill RW, Holts DB, Chang RKC, Sullivan S, Dewar H, Carey FG Pepperell JG (1994) Dispersal and homing of black marlin in the (1993) Vertical and horizontal movements of striped marlin southwest Paci®c: 25 years of recreational tagging. In: Kleiber (Tetrapturus audax) near the Hawaiian Islands, determined by P, Rasmussen R (eds) Proceedings of the 45th Annual Tuna ultrasonic telemetry, with simultaneous measurement of oceanic Conference, May 23±26 1994. National Marine Fisheries Ser- currents. Mar Biol 117: 567±574 vice (SWFSC), La Jolla, p. 105 Carey FG (1982) A brain heater in sword®sh. Science, NY 216: Squire JL Jr, Nielsen DV (1983) Results of a tagging program to 1327±1329 determine migration rates and patterns for black marlin, Carey FG, Robison GH (1981) Daily patterns in the activities of Makaira indica, in the southwest Paci®c ocean. NOAA natn sword®sh, Xiphias gladius, observed by acoustic telemetry. Fish mar Fish Serv tech Rep US Dep Commerce SSRF-772: 1±19 Bull US 79: 277±292 Weihs D (1984) Bioenergetic considerations in ®sh migration. In: Carey FG, Scharold JV (1990) Movements of blue sharks (Prionace McCleave JD, Arnold GP, Dodson JJ, Neill WH (eds) Mech- glauca) in depth and course. Mar Biol 106: 329±342 anisms of migration in ®shes. Plenum Press New York, pp 487± Deguara K, Matthews J (1994) Game®sh tagging newsletter. New 508 South Wales Fisheries, Sydney Yuen HSH, Dizon AE, Uchiyama JH (1974) Notes on the tracking Edwards R (1995) Assessing, release mortality by ultrasonic of the Paci®c blue marlin, Makaira nigricans. In: Shomura R, tracking: blue marlin, tarpon, king mackerel, Spanish mackerel, Williams F (eds) Proceedings of the International Bill®sh red drum, snook and sharks. Abstract of paper presented at the Symposium 1972. Part 2. US Department of Commerce, Kai- National Conference on Release Mortality in Marine Recre- lua, Hawaii, pp 265±268 (NOAA tech Rep NMFS SSRF-675)