These physical changes make it relatively easy to identify sexually mature males. However, secondary female sex characteristics such as plumpness of the abdomen and redness of the vent are extremely subjective and can be misleading. Sampling the eggs and sperm of the brood fish eliminates the guesswork in determining the stage of sexual development. Milt can usually be stripped from males of most species when they are ready for spawning by applying gentle pressure to the abdomen between the pelvic fins and the vent. Sperm viability usually can be determined by observing motility with a microscope.
Several methods are available to determine the developmental stage of the eggs in the fish's ovary.
The diameter and appear-ance of the egg and the position of the nucleus in the egg are visual indicators of development. The ster-oid assay procedure determines the physiological response of the eggs to hormones. Both require that an egg sample be collected. An egg sample can also be taken by making a small incision along the belly or side of the fish. This technique is commonly used for sturgeon and paddlefish.