734 



Fishery Bulletin 101(4) 



(33.1 cm) was significantly different (Kolmogorov- 

 Smirnov test, /zs=0.6966, P<0.001) from the mean 

 size of loggerhead sea turtles caught in longline 

 fisheries in the Azores during an experiment con- 

 ducted in 2000 (49.8 cm; n=224; Bolten, in press). 

 The catch curves for both samples (Fig. 2) re- 

 veal a change in slope at age 6. The 7-year age 

 class includes loggerhead sea turtles with curved 

 carapace lengths >46 cm — the size at which they 

 begin to leave oceanic habitats and recruit to 

 neritic habitats (Bjorndal et al., 2000). Thus, the 

 sharper decline beginning with age 7 reflects the 

 migration of turtles out of the sampling area. 

 Estimates of total instantaneous mortality rate 

 (Z) and annual apparent survival probability 

 (4>) were generated for three age ranges for each 

 sample: from threshold age to final age, from 

 threshold age to pre-emigration age (6 years), 

 and from pre-emigration age to final age. For the 

 total sample, the age ranges were 2 to 12 years, 2 

 to 6 years, and 6 to 12 years, respectively; for the 

 tuna sample, the ages were 4 to 11, 4 to 6, and 6 

 to 11 years (Fig. 2, Table 2). 



Discussion 



Estimates of mortality and survival generated 

 from catch curves should be interpreted with 

 caution for several reasons. First, the analysis 

 assumes a stable age distribution, which we 

 cannot confirm for North Atlantic loggerhead sea 

 turtles. Second, the analysis assumes that mor- 

 tality rates were consistent over the years of the 

 study. The similarity of the mortality and survival 

 estimates between the total sample ( 1984-95) and 

 the tuna sample ( 1990-92) suggests that the esti- 

 mates for the total sample are not greatly affected 

 by heterogeneity among years. Third, converting 

 size distributions to age distributions based on a 

 size-at-age function introduces some level of error. 

 We believe the error from our size-at-age function 

 is small, as discussed in Bjorndal et al. (in press). 



Fourth, the analysis assumes no size or age effect on mor- 

 tality rates. The catch curves for both the total sample and 

 tuna sample reveal a size or age effect with a pivot point at 

 age 6. This size or age effect reflects the beginning of emi- 

 gration out of our study area. Loggerhead sea turtles begin 

 to leave oceanic habitats around the Azores and recruit to 

 neritic habitats at 7 years of age, at -46 cm curved carapace 

 length (Bjorndal et al., 2000, 2003). This change in slope 

 demonstrates the fifth difficulty in interpreting catch- 

 curve estimates: permanent emigration and mortality are 

 confounded in the estimates. That is, declines in numbers 

 with age, whether they are due to emigration or mortality, 

 are included in the estimate of mortality. The confounding 

 of emigration and mortality can introduce a major error in 

 estimates of mortality in populations — such as sea turtle 

 populations in oceanic and neritic habitats — that undergo 

 developmental migrations. 



— I 1 1 1 1 1 1 1 1 1 1 r- 



01 23456789 10 11 12 



- 6n 



5 

 4 



3 - 

 2 - 

 1 - 



^ 



B 



• •---^--_ 



1 



5 6 7 

 Age (years) 



10 



— r— 

 11 



Figure 2 



Catch curves for (A) total sample and (B) tuna sample. Dashed lines 

 are linear regressions for entire sample, threshold age to age 6, and 

 age 6 to final age (see Table 2 for regression statistics). 



Because little permanent emigration apparently occurs 

 before the age of 7, the survival estimates for the ages 

 prior to 7 years are our best estimates of true survival (S). 

 As can be seen in Table 2, the estimates of total instanta- 

 neous mortality and annual survival are similar for the 

 two samples. We believe that the estimate of S (0.911 and 

 0.894, respectively, for the total sample and tuna sample) 

 would apply to the entire life stage over the size range from 

 20 to 65 cm CCL for most sources of mortality other than 

 fisheries biased to large sizes, such as longline fisheries. 

 For predation, as sea turtles increase in size, they outgrow 

 the prey size of some fish predators, but they also grow into 

 the prey size of the largest predators, such as killer whales 

 and humans (although the latter source is now very low in 

 the Azores as a result of legislation and education [senior 

 author, personal obs.]). Death from ingestion of or entangle- 

 ment in marine debris would probably not vary substan- 

 tially over this size range. However, mortality from inciden- 



