Tracking the Fate and Habitat Preferences of White Marlin Released from Commercial Fishing Gear with Archival Pop-up Tags
John E. Graves, Andrij Z. Horodysky, and David W. Kerstetter
Virginia Institute of Marine Science, College of William and Mary, P.O. Box
1346, Gloucester Point, VA 23062
Of all the billfish species in the Atlantic, white marlin are the most depleted - current population estimates are less than 10 percent of the virgin or unfished stock. The majority of white marlin mortality occurs as incidental catch on pelagic longline gear set for tunas and swordfish, although they are also the target of a directed recreational fishery. More than 90 percent of the white marlin caught by recreational anglers are released alive, and international management now requires longline vessels to release all live white marlin (about one-half of them are alive when the gear is retrieved). We have been employing Archival Pop-up Tags to determine the fate of white marlin after capture, and to better understand the vertical behavior of white marlin in order to adjust historical catch rates for changes over time in the depth of longline gear deployments.
Fig. 1a. White marlin carrying 10-day MT HR
Archival Pop-up Tag offshore of La Guaira
Bank, Venezuela.
Fig. 1b. Positions of archival and conventional tags on
a white marlin tagged in the Mid-Atlantic region.
To study post-release survival and habitat preferences, we required high resolution information over a relatively short duration period. We selected the HR Archival Pop-up Tag programmed to record light, temperature, and pressure (depth) every one to four minutes and to release after five or ten days (mortalities noted in previous acoustic tracking studies of other billfish species typically occurred within the first 24 or 48 hours). This tag provides serial (as opposed to summarized) data allowing us to reconstruct the actual diving behavior of each fish. To date, we have deployed a total of 24 tags, 22 on fish taken on recreational gear, and two on fish released from the longline fishery (Figure 1a, 1b). Tagging locations include the Dominican Republic (5), U.S. mid-Atlantic region (11), Georges Bank (2), and Venezuela (6).
Overall, 23 of 24 tags responded and approximately 80 percent of the light, temperature, and pressure (depth) measurements from each reporting tag were received. Data were consistent with survival for one of two white marlin released from the longline fishery. The fish that died sank to the bottom within an hour after release. The tag was apparently consumed on the bottom by a shark about ten hours later, and regurgitated a few days after the expected release date, allowing us to retrieve the unusual data set (see the Microwave Telemetry Winter 2002 Newsletter for details).
Five of 21 white marlin released from the recreational fishery died. Three of the mortalities occurred within one hour of release, one at fifteen hours, and the longest time at liberty before death was 64 hours. A higher post-release mortality was noted for white marlin caught on standard straight shank (J) hooks (5 of 15) relative to those caught on circle hooks (0 of 6), presumably due to the greater incidence of deep-hooking and tissue trauma associated with the use of straight shank hooks (four of the five fish caught on straight shank hooks that died were deeply hooked, while all six fish caught on circle hooks were hooked in the jaw). If this trend persists, it suggests that a minor change in hook types could have a profound effect on post-release survival.
Fig. 2. Overlay of the ten-day depth track and summarized
time-at-depth
histogram of a white marlin tagged offshore of the Mid Atlantic in August
2002. This figure reveals both the overall depth preferences of the animal
and the specific, high-resolution signature of individual dives. Note that
many of the deeper dives are not readily apparent in the histogram.
Fig. 3a. Strong day/night differences in the diving behavior
of a white
marlin tagged offshore of the Dominican Republic in May 2002 with a
5day HR Archival Pop-up Tag. Black bars along the x-axis denote periods
of darkness, derived from time-of-day and light level data archived by the
tag.
Fig. 3b. Change in the dive behavior of a white marlin
tagged in the
waters of the Mid-Atlantic in September 2002 with a 10-day HR Archival
Pop-up Tag. Black bars along the x-axis denote periods of darkness,
derived from time-of-day and light level data archived by the tag.
The increase in dive depths concomitant with the increase in temperature
readings and the general path of this animal (not presented) suggest that
the change in diving behavior of this fish occurred as it entered the
Gulf Stream on 29 September.
Top Chart: Temperature (C) Bottom Chart: Depth (m)
The archival tag data demonstrate that white marlin spend the majority of their time in the upper ten meters of the water column, although dives to depths of 200 m were noted for some individuals (Figure 2). Because we received the data back in serial format, it was possible to reconstruct specific dive behaviors. Each fish typically made frequent, short-duration (20 - 40 min) dives in excess of 60 m, although there was considerable variation in the nature of these dives between tagging locations, among individuals at a tagging location, and even for the same individual over time. Typically, deeper dives of long duration were followed by longer times in surface waters. In addition, strong day/night differences in diving behavior were noted for some white marlin (Figure 3a), and the behavior of some individuals was observed to change as they entered different water masses (Figure 3b).
The frequency, duration, and patterns of white marlin dives suggest that these movements are probably associated with feeding. Although white marlin have evolved a special organ to heat the eye, the extended periods in surface waters may help to warm core temperatures after forays into cooler waters. If white marlin are undertaking dives into cooler waters to forage, this behavior pattern may account for the surprisingly high catch rates of a supposed surface feeder on deep-set pelagic longline deployments.
While we plan to continue Archival Pop-up tag studies of white marlin survival and habitat utilization over the next few years, our preliminary data indicate that live release of the animals can significantly decrease fishing mortality. Furthermore, it would appear that a change in hook types in the recreational fishery can have a dramatic increase in post-release survival.
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