NOTE Brill and Holts Natural mortality of North Pacific Thunnus alalunga from drift nets 



801 



a much larger number of albacore ( 12,085 unmarked 

 and 381 net-marked individuals^; and from which the 

 fish used in this study are a subsample, show that the 

 frequency distributions of 3- and 4-year-old net-marked 

 and unmarked fish are more nearly identical (Fig. 3B); 

 3-year-old fish were found to be only slightly dominant 

 in both groups. 



Our objective in calculating condition factor, relative 

 girth, and relative condition factor (also referred to as 

 relative weight) was to determine if there was any indi- 

 cation of differences in fitness between unmarked and 

 net-marked fish. No differences were found (Table 2). 

 The use and misuse of condition factor and relative 

 condition factor have received much recent attention 

 (Bolger and Connolly, 1989; Cone, 1989; Murphy et al., 

 1991). The use of such indices in groups of fishes with 

 different mean lengths and size-frequency distributions 

 may not be justified because disparities may be due to 

 differences in body size, rather than to physiological 

 fitness (Cone, 1989, 1990). Because indices of fitness 

 were not different for unmarked and net-marked fish, 

 we considered the question of suitability moot. 



No differences were found either in measures of 

 short-term (RNA:DNA ratio) or long-term (relative 

 otolith weight) growth rates (Table 2). The RNA:DNA 

 ratio can be affected by the age of the fish (Haines. 

 1973); however, it was not corrected for body size (i.e.. 



'N. Bartoo. D. Holts, C. Brown, and L. Halko. La Jolla Laboratory, 

 Southwest Fish. Sci. Cent., unpubl. data. 



age) because we found no correlation between it and 

 body weight. 



The relative leukocyte counts also did not differ be- 

 tween unmarked and net-marked albacore (Table 2). 

 We assumed that net-marked albacore could show ei- 

 ther elevated or reduced leukocyte counts. Acute bac- 

 terial infection has been shown to increase leukocyte 

 counts in fish (Wedemeyer et al., 1990). Conversely, 

 reduced leukocyte counts can be caused by elevated 

 circulating corticosteroid levels (an acute stress re- 

 sponse) or by chronic bacterial infection causing 

 leukocytolysis (Shreck, 1981; Wedemeyer et al., 1983). 

 The lack of difference between unmarked and net- 

 marked albacore suggests that none of the processes 

 that can affect relative leukocyte counts was occurring 

 in the latter. 



We were hampered in interpreting our results be- 

 cause we could not determine precisely how long the 

 fish were at liberty following escapement from drift 

 nets. Based on observations of skipjack and yellowfin 

 tuna held in shoreside tanks at the Kewalo Research 

 Facility (described in Brill, 1992), skin damage result- 

 ing from capture generally heals completely within 

 weeks in feeding fish (R. W Brill, unpubl. observa- 

 tions). Fish with damage codes 1-3 (Table 1), there- 

 fore, probably encountered drift nets within one month 

 of recapture. It was impossible to estimate how long 

 fish with damage code 4 had been at liberty or how 

 long net damage remains visible, although Bartoo et 

 al. 2 speculated that fish with damage code 4 may have 

 been at liberty more than a year since encountering a 



