Hematological parameters in largemouth bass ( with formalin-preservation: comparison between wild tournament-caught and captive-raised fish.

Background: Largemouth bass () are an economically important freshwater fish species that have been investigated for both the short and long-term effects of stress, secondary to angling. Limited data has been published on the hematological parameters of this species and blood sample stability is a notable limitation of hematologic field studies. A relatively novel technique using 10% neutral buffered formalin preserves heparinized whole blood and maintains blood cell stability beyond one month in striped bass. The objective of this study was to evaluate the differences in hematological parameters between tournament-caught and captive-raised largemouth bass using whole blood preservation with neutral buffered formalin.

Methods: Two populations of largemouth bass ( = 26 wild;  = 29 captive) underwent coccygeal venipuncture to collect heparinized whole blood for packed cell volume, total solids, and manual differential. Formalin preservation of heparinized whole blood facilitated manual hemocytometer analysis. Results were compared between the populations (tournament-caught, and captive-raised) with Wilcoxon rank sum test, a Hotelling's test, and Bonferroni simultaneous 95% confidence intervals to determine significance.

Results: The mean packed cell volume (44.9 ± 5.4%) and total solids (7.2 ± 1.1 g/dL) were significantly higher, while the total leukocyte count (7.08 ± 1.86 × 10/µL) was significantly lower in the wild tournament-caught population of largemouth bass, as compared to the captive-raised counterparts (PCV 34.4 ± 7.2%; TS 5.2 ± 1.0 g/dL; WBC 16.43 ± 8.37 × 10/µL). The wild population demonstrated a significantly distinct leukogram characterized by a neutropenia (24.1 ± 12.7%), lymphocytosis (67.7 ± 13.0%), and monocytopenia (8.3 ± 2.9%), while the erythrocyte and thrombocyte counts were not significantly different between populations.

Discussion: Numerous factors have been demonstrated to influence hematologic parameters in fish including age, size, sex, temperature, environmental oxygen level, population density, and infection. The wild population endured stress during angling capture, live-well hypoxia, transport, and extended air exposures at weigh in, which may have caused a stress leukopenia as well as osmoregulatory dysfunction and subsequent hemoconcentration. Further evaluation of seasonal impact as well as increased sample size is warranted to enhance our understanding of largemouth bass hematology.

Conclusion: This study concluded that wild largemouth bass captured via tournament angling have higher packed cell volume and total solids, and lower total leukocyte counts, compared to captive-reared individuals. Through the completion of this study, we demonstrated the successful use of 10% neutral buffered formalin to preserve heparinized whole blood for precise hemocytometer cell counts in a new teleost species, the largemouth bass.