138 



Fishery Bulletin 105(1) 



could never be introduced as an auxiliary factor in the 

 aging of POFs. In the present study, temperature had 

 a significant effect on the rate of POF resorption. This 

 effect appeared to be limited because an increase of 1°C 

 in ambient temperature was estimated to accelerate the 

 rate of POF resorption by almost 3%. Nevertheless, the 

 maximum range of 4-5°C that can be observed during 

 sardine DEPM surveys off the Iberian Peninsula cor- 

 responds to a 12-15% maximum difference in the rate 

 of resorption and thus to a maximum of 8 hours lag 

 in the degree of POF degeneration. The results of this 

 analysis imply that in each DEPM survey, temperature 

 differences between the subareas of the survey area 

 are not expected to introduce serious bias in the cor- 

 rect classification of POFs and to subsequently affect 

 estimates of their ages. Finally, given that the late 

 daily classes of POFs are usually excluded from the 

 estimation of spawning fraction, the maximum effect 

 between the most extreme temperatures is even less 

 important (<10%). 



Tests, such as described in our study, should be per- 

 formed at least once for each species or population to as- 

 sess bias in the criteria used to determine POF stages 

 and ages. Moreover, the test would provide compara- 

 tive information on POF resorption rates, the impact 

 of embedding material, and the effects of temperature 

 and other environmental parameters on the estimates. 

 In routine DEPM analyses, the measurement of POF 

 cross-sectional areas could increase technical work, 

 but not necessarily the precision in the estimates of the 

 spawning fraction because females are again broken 

 down into spawning nights as they were with the his- 

 tological staging method. However, in cases where such 

 relationships of POF age on POF size are already avail- 

 able, correspondence of POF sizes to spawning nights 

 would be much more realistic than simple histological 

 staging, which strongly depends on the experience of 

 the observer and the quality of the slides. 



Acknowledgments 



This work was supported by the program PELAGICOS 

 (PLE/13/00) funded by the Portuguese Ministry of Sci- 

 ence and the National Sampling Plan for DEPM sur- 

 veys funded by the European Union. The contribution 

 of K. Ganias was funded by a postdoctoral scholarship 

 in Portugal (FCT-BPD/17488/2004). We thank four 

 anonymous reviewers for helpful recommendations. In 

 particular, we thank all IPIMAR (Instituto de Inves- 

 tigafao das Pescas e do Mar) staff that contributed to 

 the collection of DEPM samples and the histological 

 preparations. 



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