131 



Abstract — Inaccuracy in the aging of 

 postovulatory follicles (POFs) and in 

 estimating the effect of temperature 

 on the resorption rate of POFs may 

 introduce bias in the determination 

 of the daily spawning age classes 

 with the daily egg production method 

 (DEPM). To explore the above two 

 bias problems with field-collected 

 European pilchard iSardina pilchar- 

 dus, known regionally as the Iberian 

 sardine), a method was developed in 

 which the time elapsed from spawning 

 {POF age) was estimated from the 

 size of POFs (i.e., from the cross-sec- 

 tional area in histological sections). 

 The potential effect of the preserva- 

 tive type and embedding material on 

 POF size and the effect of ambient 

 water temperature on POF resorp- 

 tion rate are taken into account with 

 this method. A highly significant log- 

 linear relationship was found between 

 POF area and age; POF area shrank 

 by approximately 50% per day. POFs 

 were also shown to shrink faster at 

 higher temperatures (approximately 

 3'7c per degree), but this temperature 

 effect is unlikely to be an important 

 source of bias in the assignment of 

 females to daily spawning classes. 

 The embedding material was also 

 shown to influence the size of POFs, 

 the latter being significantly larger 

 in resin than in paraffin sections. In 

 conclusion, the size of POFs provides 

 an indirect, reliable estimation of the 

 time elapsed from spawning and may 

 thus be used to test both the validity 

 of POF staging criteria for identify- 

 ing daily classes of spawners and the 

 effect of other factors (such as tem- 

 perature and laboratory processing) 

 in applications of the DEPM to S. 

 pilchardus and other fish species. 



Degeneration of postovulatory follicles in the 



Iberian sardine Sardina pilchardus: 



structural changes and factors affecting resorption 



Konstantinos Ganias^-^ 

 Cristina Nunes^ 

 Yorgos Stratoudakis^ 



Email for K Ganias: kganias@bio.auth.gr 



' School of Biology 

 Laboratory ol Ichthyology 

 Aristotle University of Thessaloniki 

 54 124 Thessaloniki, Greece 



^ Institute Nacional de Investigacao Agraria e das Pescas 

 Institute de Investigacao das Pescas e do Mar 

 Avenida de Brasilia s/n, 1449-006 

 Lisbon, Portugal 



Manuscript submitted 12 May 2006 

 to the Scientific Editor. 



Manuscript approved for publication 

 5 July 2006 by the Scientific Editor 



Fish. Bull. 105:131-139 (2007). 



The postovulatory follicle (POF) 

 consists of the follicular layers that 

 remain in the ovary of fish after the 

 release of the ovum during spawning 

 (Saidapur, 1982). Initially, the POF 

 is a distinct structure, but it rapidly 

 deteriorates and becomes undetectable 

 within a few days (Hunter and Gold- 

 berg, 1980). The study of POF degen- 

 eration is important in fishery studies 

 because it permits the assignment of 

 spawning females to daily classes to 

 provide estimates of the daily frac- 

 tion of spawning fish in the population 

 (POF method, Hunter and Goldberg, 

 1980). The most common application 

 of the POF method is in the daily egg 

 production method (DEPM; Parker, 

 1980), where spawning fraction, 

 together with other adult parame- 

 ters, are used to estimate daily spe- 

 cific fecundity (Picquelle and Stauffer, 

 1985) for the fisheries-independent 

 estimation of spawning biomass. A 

 prerequisite for such applications is 

 the existence of an accurate aging key 

 that describes the time course of POF 

 degeneration (Hunter and Macewicz, 

 1985). 



In most DEPMs, the degeneration 

 of POFs is described by a small num- 

 ber of histomorphological stages (see 

 "Materials and methods" section) that 

 are usually assumed to correspond 

 to distinct daily classes (see review 

 by Stratoudakis et al., 2006). How- 



ever, because the process of POF de- 

 generation is continuous and DEPM 

 samples are usually obtained oppor- 

 tunistically throughout the day, the 

 direct assignment of POF stages to 

 daily classes of spawning fish can be 

 imprecise. Also, morphological stages 

 are often attributed to daily classes 

 without prior validation and thus 

 can lead to biased estimates of the 

 spawning fraction. Validation is best 

 performed in the laboratory by sacri- 

 ficing female spawning fish at known 

 time intervals after ovulation (e.g.. 

 Hunter and Goldberg, 1980; Perez et 

 al., 1992). Alternatively, in fish with 

 daily spawning synchronicity, such 

 as with the Iberian sardine Sardina 

 pilchardus (also known as the Euro- 

 pean pilchard, FAO, 1985) (Bernal 

 et al., 2001; Zwolinski et al., 2001; 

 Ganias et al., 2003), validation can 

 be performed indirectly through the 

 examination of field samples collected 

 at different hours of the day (Gold- 

 berg et al., 1984). 



Another source of potential bias in 

 the POF method is the duration of 

 follicular degeneration, which may be 

 temperature-dependent (Hunter and 

 Macewicz, 1985), because the meta- 

 bolic rates of poikilotherms, like fish, 

 may be directly affected by ambient 

 temperature. As a result, POF de- 

 generation may be faster at higher 

 temperatures and may lead to biased 



