# Testicular Neoplasms and Other Abnormalities in Common Carp Cyprinus carpio from the Lower Colorado River, United States

## TL;DR

Older common carp in the Colorado River show testicular tumors and abnormalities, possibly due to aging, low temperatures, and PCB exposure.

## Contribution

The study identifies novel testicular abnormalities and potential environmental risk factors in wild carp.

## Key findings

- Testicular neoplasms and abnormalities were observed in carp from Willow Beach.
- Carp at this site had the highest PCB levels and an unusual thermal regime.
- Advanced age, low temperature, and PCB exposure may contribute to reproductive abnormalities.

## Abstract

Common carp are used as an indicator species in the Colorado River and elsewhere for fish health and reproductive studies. Carp were collected at one site, Willow Beach, downstream of Lake Mead and Hoover Dam, at multiple times since 2003. Tumors, including seminoma, spermatogenic seminoma, and mixed germ cell–stromal neoplasms, were observed. Other testicular abnormalities, including Sertoli cells, pigmented macrophage aggregates, and intersex (testicular oocytes), were also found. Carp collected at this site in 2003 ranged in age from 35 to 54 years. They also had the highest total PCB (polychlorinated biphenyls) body burden compared to other sites within the Colorado River watershed. Additionally, this site has an unusual thermal regime when compared to other sites studied in Lake Mead and upstream sites, in that temperatures varied little over the seasons (amplitude around 1.5 °C) and barely reached 15 °C, which is considered the lowest temperature for spawning.

Abnormalities were observed in the testes of common carp Cyprinus carpio collected from Willow Beach, Arizona, USA, a site on the lower Colorado River, downstream of Lake Mead and Hoover Dam. Testicular tissue collected from this site in 2003 exhibited numerous large, pigmented macrophage aggregates (MAs) and a novel, previously undescribed hypertrophy and proliferation of putative Sertoli cells. In testes samples collected in 2007, numerous testicular MA, testicular oocytes, and proliferations of Sertoli cells were observed. Three carp collected in 2007 also had raised nodules within the testes, and, microscopically, seminoma, spermatogenic seminoma, and mixed stromal cell–germ cell neoplasms were diagnosed. Several risk factors for these adverse effects were identified. Carp collected at this site in 2003 ranged in age from 35 to 54 years and had the oldest mean age of the thirteen sites sampled within the Colorado River basin. This site also has an unusual thermal regime when compared to other sites studied in Lake Mead and upstream sites, in that temperatures varied little over the seasons (amplitude around 1.5 °C) and barely reached 15 °C. Additionally, carp from this site had the highest total polychlorinated biphenyl (PCB) body burden. Hence, advanced age, low water temperature, and exposure to PCBs and other environmental contaminants may contribute to the observed abnormalities, highlighting the complex environmental factors initiating pre-neoplastic and neoplastic changes in wild carp.