Methods of Assessing Ovarian development in Fishes: a Review

An important component of many studies of fish reproductive biology is the assessment of the stage of gonad development of individual fish. The methods in use vary from highly detailed to cursory, but there are few reviews of their reliability or usefulness. This review examines histology, measurements of oocyte size, staging based on the appearance of whole oocytes, staging based on the external appearance of the ovary, and gonad indices. Histology is the most accurate technique, but it is time- consuming and expensive. Staging based on the appearance of whole oocytes can be a useful alternative but may be inaccurate with oocytes in transitional stages of development. Staging based on the external appearance of the ovary is the simplest and most rapid method, but it may be subjective and its accuracy is uncertain. Oocyte size may be used as a predictor of developmental stage if the size ranges of the various stages are known, but the sizes of different oocyte stages may overlap, which complicates this approach. Oocyte size may be used on its own to measure development but gives little information on the physiological status of the ovaries. Gonad indices (gonad size relative to body size) provide a useful insight into changes in ovary size and complement results obtained using staging methods. However, gonad indices, like oocyte size, may be biased when samples of fish of different body sizes are compared.