Health of white sucker within the St. Louis River area of concern associated with habitat usage as assessed using stable isotopes

Abstract

In Spring 2011, 200 adult white sucker were collected in four areas of the St. Louis River area of concern (AOC), located in Minnesota and Wisconsin, USA. The areas included the upper AOC as a reference area, the upper estuary, St. Louis Bay and Superior Bay. Grossly visible abnormalities were documented and preserved for microscopic analyses, as were five to eight representative pieces of liver tissue. A piece of dorsal muscle was preserved for stable isotope analyses and otoliths removed for age determination. The incidence of raised skin lesions (mucoid plaques) was high (31 %), however, microscopically only 4.5 % of the white suckers had neoplasia (papillomas). The remaining lesions were epidermal hyperplasia. Superior Bay had the lowest percentage of skin/lip lesions (10 %), while St. Louis Bay had the highest (44 %). St. Louis Bay also had the highest incidence of skin neoplasms (12 %). No hepatocellular neoplasms were documented, however bile duct tumors were observed in 4.5 % of the suckers. Foci of cellular alteration were observed in fish from all sites except the upper AOC. Stable isotope data indicated that most of the suckers relied on the St. Louis River AOC for the majority (>75 %) of their diet, indicating they were resident within the AOC and not in Lake Superior. The amount of diet obtained from the upper estuary was a significant predictor of skin lesion incidence. Hence, habitat use within the AOC appears to be an important risk factor for skin and possibly, liver lesions.

Fig. 1

Sampling sites within the St. Louis River area of concern (AOC) included Superior Bay (C), St. Louis Bay (AB), the upper estuary (D) and the upper AOC (E)

Fig. 2

a Age distribution of white sucker collected throughout the St. Louis River area of concern. b Age distribution of white sucker with grossly visible raised skin or lip lesions

Fig. 3

Sex ratios observed in white sucker collected at four sites: Superior Bay (C), St. Louis Bay (AB), upper estuary (D), upper AOC (E) within the St. Louis area of concern (AOC)

Fig. 4

Skin lesions of white sucker collected within the St. Louis area of concern. a Slightly raised mucoid lesion (arrow) on the body surface. b Raised, white to creamy lesion (a) on the caudal fin of a male white sucker with breeding tubercles (arrows). c Slightly raised lip lesion (arrow)

Fig. 5

Microscopic pathology observed in white sucker collected within the St. Louis River area of concern. a Lip papilloma composed of proliferating epithelial cells (a) within the epidermis, leading to folds of tissue extending into the underlying layers (arrow). Scale bar 100 μm. b Body surface papilloma with folds of proliferating cells (arrow). Scale bar 100 μm. c Normal epidermis of white sucker skin contains a layer of columnar cells (a) along the basement membrane, flattened, squamous epithelial cells (b) toward the superficial surface with club cells (c) and goblet cells (arrows) present. Scale bar 50 μm. d Section of skin containing normal epidermis (a) and an area of hyperplastic epidermis (b). Epithelial and fright/alarm cells (arrows) are present in both areas. Scale bar 100 μm. e Hyperplastic lesions contained a basal layer of columnar cells (a) and hyperplasia epithelial cells (b). Scale bar 50 μm. f Cell within the papillomas vary in size and shape from thin elongated cells (a) to larger, plumper cells (b) which in some cases were binucleated (arrow). Scale bar 50 μm. Hematoxylin and eosin stain

Fig. 6

Microscopic pathology observed in the liver of white sucker collected within the St. Louis area of concern. a Encysted cestode (a) within the liver tissue (b) separated from hepatocytes by a layer of inflammatory and necrotic cells (black arrow). Macrophage aggregates (white arrows) were commonly observed within hepatic tissue. Scale bar 100 μm. b Sporoplasm of a myxozoan parasite (a) within a distended bile duct with areas of epithelial hyperplasia (black arrows). Scale bar 50 μm. c Focal area of bile duct proliferation (a) within hepatic tissue (b) with cross-sections of myxozoan sporoplasms (arrows). Scale bar 50 μm. d Foci of cellular alteration (a) within hepatic tissue (b). Scale bar 50 μm. e Section of a cholangiocarcinoma (a) illustrating well differentiated proliferating bile ductules containing bile (arrows). Scale bar pleomorphic cells, 50 μm. Hematoxylin and eosin stain

Fig. 7

Distribution of δ¹³C and δ¹⁵N values in muscle tissue of white sucker collected at four sites Superior Bay (C), St Louis Bay (AB), upper estuary (D), and upper AOC (E) within the St. Louis area of concern (AOC). The lower estuary includes sites C and AB

Fig. 8

Results of the dual stable isotope mixing model for white sucker captured below Fond du Lac Dam. The data points represent the dietary contribution to each fish from the upper estuary (site D) and lower estuary (Superior Bay C, St. Louis Bay AB, combined). The sum of all three sources (upper estuary, lower estuary, Lake Superior) total 100 %; thus, for any point, the Lake Superior contribution is the remainder of the sum of the upper and lower estuary contributions. That is, the points in the lower left corner of the plot represent fish with a high dietary contribution from Lake Superior. The shading indicates the capture location of the fish (gray site C, open site AB, closed site D)