The reproductive biology of small fishes and the clutch concept: Combining macroscopic and histological approaches

Accurate estimates of reproductive parameters important in understanding life history evolution and conservation of small fishes are dependent upon careful assignment of ovarian phases. Proper assignment is based upon the stages of propagule (oocyte) development, oocyte maturation and the location of any clutch or portion thereof within the ovaries. Macroscopic inspection and assignment of ovarian developmental phases have often been used for small freshwater fishes. By contrast, histological methods for assignment of reproductive phases have been developed and are widely used for marine fishes, but they have rarely been used for small freshwater fishes. We review oocyte development, ovum maturation, and the ovarian cycling process using both macroscopic and histological approaches which incorporate the clutch concept. New terminology, including three actively spawning phases based on macroscopic appearance of the clutch (actively spawning‐maturation, actively spawning‐hydration, actively spawning‐ovulation), is proposed. We assert that combining histological and macroscopic methodologies, shown to be complementary, will allow a more accurate and precise understanding of the reproductive biology of small fishes—especially freshwater fishes experiencing effects of habitat change and loss of habitat. Accurate estimates of reproductive parameters important in understanding life history evolution and conservation of small fishes are dependent upon careful assignment of ovarian phases. We review oocyte development and the ovarian cycling process using both macroscopic and histological approaches which incorporate the clutch concept. We suggest new terminology that combines histological and macroscopic methodologies and will allow a more accurate and precise understanding of the reproductive biology of small fishes.