A rapid method for estimation of oocyte size and potential fecundity in Atlantic cod using a computer-aided particle analysis system

Abstract

Estimation of individual reproductive investment is central in many types of fish biology studies. Such work seeks to obtain quick and reliable reports on fecundity and maturity in fish with numerous eggs. Typically, fecundity has often been estimated using the gravimetric method, in which fecundity is the product of the number of oocytes per gram of ovary tissue (oocyte density) and the weight of the ovary in grams. This paper presents a method for estimating oocyte density using an image analysis system. The image analyser rapidly determines the average diameter of vitellogenic (yolk containing) oocytes in a sample, and then, the mean diameter is converted into oocyte density using a precise calibration curve. To establish the calibration curve pre-spawning ovaries from wild and laboratory-reared Northeast Arctic cod were sampled for gravimetric counting of vitellogenic oocyte density and mean vitellogenic oocyte diameter. The latter was measured by an image analysis system, while oocyte density was found by manual counting of pre-weighed subsamples. A power regression, where mean oocyte diameter (OD) was the independent variable and oocyte density (NG) was the dependent variable, explained 99% of the variation, measured as r2. It was concluded that potential fecundity (Fp, i.e. standing stock of vitellogenic oocytes in pre-spawning individuals) of cod could be estimated from total ovary weight (OW) and mean oocyte diameter using the formula:Fp=2.139×1011×OD−2.700(μm)× OW(g),300μm≤OD≤850μm.It took approximately 5min to measure mean oocyte diameter, including the manual preparation of the sample for analysis and the data processing afterwards. Thus, this was a highly time-efficient system compared to previous protocols for potential fecundity estimation. We have named this procedure using an image analyser to estimate potential fecundity the auto-diametric fecundity method. Furthermore, the accurate and precise measurement of oocyte size is shown to have direct practical implications for the assessment of maturity stage and predictions of start of spawning.