...problems with field-collected European pilchard (Sardina pilchardus, known regionally as the Iberian sardine), method was developed in which the time elapsed from spawning (POF age) was estimated from the size of POFs (i.e., from the cross-sectional area in histological sections). The potential effect of the preservative type and embedding material on POF size and the effect of ambient water temperature on POF resorption 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% 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 identifying daily classes of spawners and the effect of other factors (such as temperature and laboratory processing) in applications of the DEPM to S. pilchardus and other fish species.

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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 Goldberg, 1980). The study of POF degeneration is important in fishery studies because it permits the assignment of spawning females to daily classes to provide estimates of the daily fraction 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 parameters, are used to estimate daily specific 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 number 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 degeneration is continuous and DEPM samples are usually obtained opportunistically 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 sacrificing 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 European 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 (Goldberg 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 metabolic rates of poikilotherms, like fish, may be directly affected by ambient temperature. As a result, POF degeneration may be faster at higher temperatures and may lead to biased estimates of the spawning fraction and biomass when spatial variation in ambient water temperature is large within a survey area. This dependence can be assessed under laboratory conditions through inspection of specimens spawning under different temperature regimes (Fitzhugh and Hettler, 1995). Alternatively, the effect of temperature on POF degeneration can be studied in the wild by the examination of individuals caught at variable environmental conditions (Ganias et al., 2003; Roumillat and Brouwer, 2004).

Because of the above unresolved issues of the POF method (that can be species specific), the spawning fraction...

NOTE: All illustrations and photos have been removed from this article.



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