Male blue catfish, Ictalurus furcatus, mature at 4-7 years, cannot be hand-stripped for sperm, therefore making collection a lethal procedure. Sperm are also highly variable in quality/quantity. To better understand testes development and maturity, we followed male blue catfish for 34 months (2.3 to 4.9 years old; y/o) to observe changes in body morphometrics, plasma androgens, osmolality, ions, testicular development, and gene expression. Catfish (n = 5) were sampled every other month from Aug. 2021-Feb. 2024. Weight and length were recorded and gonadosomatic index (GSI) was calculated. Blood samples were also taken for androgen and ionic analyses. Next, testes were dissected, weighed, and a sample (~1 g) was taken for histological analysis. The spermatogenic maturity index (SMI) was then calculated based on histological analysis at different developmental stages. Additional testis samples were analyzed for transcriptomic analyses between different ages and seasons.
We reported that body weight and length of fish ranged from 0.6±0.2 to 3.2±0.2 kg and 35.2±2.1 to 58.9±1.9 cm, respectively. Testes weight and GSI were highly variable within/ among sampling points. Largest testes weight was 6.95 g (4.7 y/o) and GSI was 39.9% (3.7 y/o). Age-related changes in testicular development were significant, with highest SMI values (0.31-0.73) reported for 3.11 to 4.9 y/o fish and the lowest for 2.3 (0.13) and 2.11 (0.12) y/o fish. Spermatozoa were observed in 3.3 y/o and 4.2 to 4.9 y/o fish, with the highest ratio of spermatozoa were detected at 4.2 y/o (70%) (Fig. 1). Androgens levels spiked for 3.11 y/o males in July 2023. An increase in osmolality was observed for 3.11 to 4.5 y/o fish, then decreased. K+, Na+, and Cl- were highest ion levels throughout development. Transcriptomics revealed 6,317 differentially expressed genes between 2.9, 3.1, 3.9, and 4.2 y/o fish. Most differential genes were expressed in 3.1 (2,348) and 4.2 (2,689) y/o fish, when compared to 2.9 y/o. Genes related to spermatogenesis/sperm development (cfap61, hsf5, rsph9, dmrt2a, LOC128611646, usp21, gstt1b) were differentially expressed, based on age and season. These findings improve our understanding of reproduction for an important aquaculture species.