Contrasting batch fecundity estimates of albacore (Thunnus alalunga), an indeterminate spawner, by different laboratory techniques

Abstract

A range of methods can be applied to estimate the batch fecundity of species with an indeterminate reproductive strategy. The traditional Hydrated Oocyte (HO) method based on direct counts of hydrated oocytes is the easiest and most accurate method but the main problem with this method is the shortage of hydrated ovaries in sampled fish such as tuna species. Batch fecundity estimates of albacore Thunnus alalunga resulting from counts of migratory nucleus (MG) oocytes using the application of the Weibel and Gomez (W&G), Physical Disector (PD), Oocyte Packing Density (OPD), and HO methods were compared using the last method as “control”. Postovulatory follicles (POFs) were also counted using the PD method. Correction factors due to shrinkage were considered in the application of the different methods. Our results showed the highest batch fecundity estimates were obtained with the design-based PD method. The outputs from the assumption-based W&G and the theoretical OPD methods were closest to the HO method. Annotations of POFs instead of MG oocytes gave markedly lower values. The new OPD method was used to estimate batch and relative fecundity on a larger sample of fish (selected according to their length). The relationships between batch and relative fecundity estimates of albacore and the associated biological metrics (length, body weight and ovary weight) were investigated. Batch fecundity estimates ranged from 0.42 to 2.16 million oocytes with a mean relative batch fecundity of 136 oocytes per gram of body weight. The batch fecundity was shown to increase with fish size (length and weight) and gonad weight, while relative batch fecundity (g−1) was related only to gonad weight.