A spatially intensive survey in 1989 of 52 sites in the Red River drainage in southwest Oklahoma and surveys in all years from 1978 to 1987 on four sites in the drainage provided evidence that construction of Altus Dam on the North Fork of the Red River caused major changes in fish community structure in the river above the dam. Pre‐impoundment data on the fish communities were scanty, but the inferences they allowed were similar to those obtained by comparing fish assemblages in the North Fork above the dam with assemblages elsewhere in the drainage, particularly along Salt Fork, which had similar habitat characteristics. Twenty‐five species were collected in the North Fork above Altus Dam, compared to 33 in the Salt Fork and 34 in the North Fork below the dam. The speckled chub Macrhybopsis (formerly Hybopsis) aestivalis and the chub shiner Notropis potteri were absent in the North Fork above Altus Dam but fairly common in similar streams elsewhere in the area. The plains minnow Hybognathus placitus and the Red River shiner Notropis bairdi were among the most common fish species found in southwest Oklahoma, but were not collected above Altus Dam in the 1989 survey and were collected only intermittently and in small numbers in the long‐term survey. We speculate that these two species have repeatedly been extirpated and have been reestablished as bait‐bucket introductions since the dam was closed. Upstream of the reservoir, the sand shiner Notropis stramineus and the emerald shiner Notropis atherinoides replaced the plains minnow and the Red River shiner as dominant species, and several reservoir species were more common. Significant negative association at two long‐term sites suggested that the sand shiner and Red River shiner were filling similar niches.

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An epizootic of pigmented subcutaneous spindle cell tumors affected nearly 25% of the adult gizzard shad (Dorosoma cepedianum) sampled from Lake of the Arbuckles in central Oklahoma over a 2 year period. Grossly, the tumors were primarily distributed over the head, trunk and fins as superficial raised masses that were almost always darkly pigmented. Histologically, they were located in the dermis, had a variable amount of connective tissue, and consisted of cells in a variety of forms and arrangements. Most tumors were composed of fusiform or spindle cells arranged in wavy bundles, whirling patterns or interwoven fascicles. Pigmentation was attributed to large dense deposits of melanin or to scattered individual melanin-containing cells. Immunohistochemical detection of proliferating cell nuclear antigen revealed a high proliferative activity in the spindle cells. Electron microscopy showed that the tumors were composed of several cell types, including host reactive cells, melanocytes in stages of maturity, and fibroblast-like cells. Tumor cells had neither cell-to-cell junctions nor an external lamina. Although the cell of origin of the tumors was not identified, evidence points toward melanocytes or, possibly, nerve sheath cells. However, an origin from fibroblasts or some other poorly differentiated cell cannot be ruled out. The etiology of the tumors was not determined. Fractionation of lake water and sediment samples followed by GC-MS analysis revealed no carcinogenic compounds. A retroviral etiology is unlikely because assays for reverse transcriptase in tumor homogenates were negative, and no evidence of viral particles was found in specimens examined by electron microscopy.

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