754 



Fishery Bulletin 102(4) 



We cannot attribute these chang- 

 es to a resuscitation of the fish for 

 three reasons. 1) The measured light 

 levels indicated that the tag was in 

 complete darkness for a period often 

 days, even though it was at the sur- 

 face during daylight hours. A mal- 

 functioning light sensor cannot ex- 

 plain this observation because the tag 

 recorded day and night differences 

 in light levels at the surface during 

 the seven-day transmission period af- 

 ter it was released from the fish. 2) 

 After a rapid increase, the tempera- 

 ture remained relatively constant, 

 between 23° and 26°C, even when 

 the tag was at depths in excess of 

 300 m. Although dive behavior may 

 be affected by location-specific con- 

 ditions, previous PSAT observations 

 of more than 20 other white marlin 

 indicated that temperature ranges of 

 individual dive events rarely exceed 

 8°C when, it is assumed, animals 

 make foraging dives to depth (Horo- 

 dysky et al., in press). 3) The PSAT 

 recorded several dives in excess of 400 m, and previous 

 observations of white marlin have revealed no dives in 

 excess of 220 m (Horodysky et al., in press). Finally, 

 the PSAT was scheduled to be released from WM1 after 

 ten days on 10 September. Although archiving of light, 

 temperature, and pressure data ceased on that date, the 

 tag did not begin transmitting until 12 September. 



WM2 



The shallow dive patterns reported by this fish may 

 indicate that it survived for approximately 24 hours 

 following its release. Between 12:45 and 3:07 pm (local 

 time), the light level fell abruptly from the maximum 

 light level value to zero. At 3:08 pm, the temperature 

 was 19.8°C at 166 m depth; by 4:37 pm, the tempera- 

 ture was above 24°C and remained above this value for 

 the remainder of the deployment period. At 5:58 pm on 

 12 September, the light levels returned to maximum 

 strength from zero — an indication that the tag had 

 likely been egested. For the 19 hours remaining of the 

 programmed deployment period prior to pop-off, the 

 depth, light, and temperature data all indicated that 

 the tag was floating at the surface. 



Opah 



Based on recovered data, our conjecture is that the tag 

 was attached to the live opah for the first 23 days. Then, 

 sometime during the 12-hour period from 2:00 pm 13 

 December to 2:00 am 14 December the tag was ingested. 

 From our data, we cannot discern whether 1) the tag 

 was detached prematurely from the opah and was float- 

 ing on the surface when it was ingested, 2) an animal 



D 

 CD 



■o 



Temperature 

 Depth 



Figure 3 



Delayed temperature changes recorded by tag WM1 following deep dive 

 events on the morning of 2 September 2002. Arrows indicate the lowest 

 temperatures recorded in association with a movement of the animal to 

 depth; note that these temperatures were often recorded while the animal 

 was at or near the surface and therefore represent a delay between depth 

 and temperature. 



attacked the opah and ingested the tag incidentally, 

 or 3) an animal ingested the tag alone. However, it is 

 unlikely that the opah died, sank to the ocean floor, and 

 was scavenged because the ocean floor in the area where 

 the opah was tagged is below 2000 m. We have observed 

 from other tags on opahs what we believe are mortalities; 

 these occur shortly after tagging and show that the tag 

 reaches depths in excess of 1000 m before detaching when 

 the emergency pressure release in the tag is triggered. We 

 did not observe depths below 600 m at any time during 

 this record, and therefore the pressure-induced detach- 

 ment mechanism on the tag was not triggered. 



The ingestion hypothesis for the failure of these three 

 tags to transmit data is supported by several lines of 

 evidence. First, the light level readings were consistent 

 with a tag residing in the complete darkness of an 

 alimentary canal. Second, although temperature varia- 

 tions occurred during the deployment period, the delay 

 in temperature changes during dives to depths indicates 

 that the tags were not directly exposed to ambient wa- 

 ter (see Fig. 3 for an example from WM1, as well as the 

 comparisons in Table 1) and further may indicate that 

 the scavenger was either endothermic or of large enough 

 size to mitigate heat loss at depth. 



There are several organisms that could have eaten 

 these PSATs, whether by scavenging a carcass or at- 

 tacking a moving fish. Clearly, each of these organisms 

 was sufficiently large to ingest the tag without seri- 

 ously damaging it. It is unlikely that a cetacean was 

 responsible for any of these events because internal 

 temperatures for odontocete whales (including killer 

 whales, Orcinus orca) range between approximately 36° 

 and 38°C (Whittow et al., 1974)— well above the range 

 of temperatures recorded by the PSATs. 



