Since the delisting of the Presque Isle Bay (PIB) Area of Concern in 2013, the use of hepatocellular and orocutaneous tumors in Brown Bullhead (Ameiurus nebulosus) as indicators of ecosystem health has been questioned. Specifically, the following questions have been raised: 1) are the occurrence of liver and external tumors in Brown Bullhead indicative of a negative impact to the ecosystem health Presque Isle Bay; 2) are liver and external tumors unique to Brown Bullhead; and 3) are there other indicators that could be used to evaluate the ecological health of Presque Isle Bay? To address these questions, the prevalence of hepatocellular, orocutaneous, and gonadal tumors and intersex within Smallmouth Bass (Micropterus dolomieu), Bowfin (Amia calva), Yellow Bullhead (Ameiurus natalis), and Brown Bullhead sampled from PIB were compared to the prevalence in fishes sampled from the reference site Long Point Bay (LPB), Ontario. Condition factor (Ktl), hepatosomatic index (HSI), and gonadosomatic index (GSI) in these fishes were also assessed.
Hepatocellular tumors were observed in Brown Bullhead, Bowfin, and Yellow Bullhead at both locations, and in all cases, age was a significant (p < 0.05) factor in predicting hepatocellular tumor prevalence. The prevalence of hepatocellular tumors was low in both Brown Bullhead and Yellow Bullhead, and prevalence was not significantly (p > 0.05) different between locations. The prevalence of hepatocellular tumors in the present study were comparable to prevalence data used in deciding to the delist PIB. The prevalence of hepatocellular tumors was elevated in Bowfin at PIB and LPB, although prevalence was not significantly (p > 0.05) different between locations.
Orocutaneous tumors were observed in both Brown Bullhead and Yellow Bullhead at PIB and LPB. Age was a significant (p < 0.05) factor in predicting external tumor prevalence in Brown Bullhead. The prevalence of external tumors in Brown Bullhead remains higher in PIB compared to LPB, although the probability of a Brown Bullhead having a tumor is not significantly (p > 0.05) different between the two locations. In contrast, the prevalence of orocutaneous tumors in Yellow Bullhead was low in PIB and LPB. One stromal cell tumor was observed in a male Smallmouth Bass sampled from PIB. The development of neoplasia in fishes is likely multifactorial, resulting from both extrinsic and intrinsic factors. It remains unclear what influence risk factors such as viruses and chemicals have on inducing neoplasia in fishes at PIB and LPB. Laboratory investigations would strengthen the linkage between chemicals and neoplasia in fishes. The presence of viruses in fishes with neoplasia should be investigated as well. Intersex, as indicated by the presence of testicular oocytes, was observed in Smallmouth Bass sampled from PIB and LPB.
The prevalence of intersex was significantly (p < 0.05) higher in male Smallmouth Bass sampled in 2013 than in 2014 and 2015. The prevalence of intersex was also significantly (p < 0.05) higher in male Smallmouth Bass sampled from PIB in 2013 than from LPB in 2014, LPB in 2015, and PIB in 2015. Both urban and agricultural land uses have been associated with high prevalence of intersex in male Smallmouth Bass, which may explain the elevated prevalence observed in both PIB and LPB. Given the complexity of the mixtures of compounds found in the environment, including in PIB and LPB, it is likely that multiple compounds are responsible for inducing intersex in fishes. The effects of specific endocrine disrupting chemicals (EDCs) and mixture of EDCs, including gene expression, on Smallmouth Bass health needs further study.
Mean Ktl were not statistically (p > 0.05) different in any fishes sampled from PIB compared to LPB. Based on Ktl, fishes in PIB and LPB appear healthy. I did observe significant (p < 0.05) differences in mean HSI and GSI in fishes sampled from PIB compared to LPB. The differences in mean HSI and GSI may be related to the timing of the sampling and collection methods, or other pathological differences opposed to exposure to xenobiotics. Research focused on establishing “healthy” ranges of Ktl, HSI, and GSI for fishes would also be beneficial.