Reproductive Phenology of Wild and Hatchery-Reared Rio Grande Silvery Minnow


Date: 2024/01/09

Author(s): Archdeacon T.P., Diver T.A., Grey R.E.

Publication: Prepared by U.S. Fish and Wildlife Service, New Mexico Fish and Wildlife Conservation Office. Prepared for U.S. Bureau of Reclamation, Albuquerque Area Office, 34 p.


The study of reproductive phenology through ovarian development can help answer a wide range of questions needed for the management of imperiled species. For fishes, baseline reproductive timing may be used to guide water management strategies to enhance recruitment or stimulate spawning. The reproductive cycle in gonochoristic fish is categorized into five ovarian phases within a continuum: immature fish that are not and have never been reproductively active; the developing phase signals the beginning of the reproductive season and is characterized by gonadal growth and development in preparation for spawning; the spawning capable phase characterized by advanced gamete development and oocyte maturation; the regressing phase signaling the end of the reproductive season, and the regenerating phase (e.g., resting) where mitotic proliferation occurs prior to the next spawning cycle.

The peak of Rio Grande Silvery Minnow spawning is generally the middle of May and has been inferred from the appearance of larvae and eggs in the river. Our specific objectives were to document the reproductive and spawning seasons of both wild and hatchery-stocked female Rio Grande Silvery Minnow through macroscopic and histological examination of ovarian tissue to improve water and other management decisions. We used a time-series of weekly to monthly ovary collections made from February through September, 2017-2019, to describe the reproductive cycle of both wild and hatchery-reared Rio Grande Silvery Minnow.

The proportion of spawning-capable females was low before April and after mid-June. The highest proportion of spawning-capable females occurred during the month of May. A subsequent peak in regressing females followed in July and August, with most females in the regenerating phase by July. We observed few differences between hatchery-reared and wild fish. Instead, inter-annual variation among years was more significant and likely reflected environmental conditions and management actions. Channel drying in the San Acacia Reach in 2018 began before Rio Grande Silvery Minnow were fully capable of spawning and assured a year-class failure in that reach. Water management decisions should consider the intended effects (stimulate spawning or enhance recruitment), the reproductive readiness of the population, and antecedent environmental and water storage conditions to achieve the desired management outcome.

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