Coho Salmon in Karst-dominated Streams

Study area.-All the study streams are on the northern quarter of Prince of Wales Island (Figure 1) and traverse a mixture of karst and nonkarst geology. Some streams are fed directly by resurgence from limestone caves, whereas others are fed by overland flow. Hecata Limestone and Bay of Pillars are the major karst formations (Wissmar et al. 1997). Many of the streams flow along contact zones between limestone and nonlimestone formations (Table 1). For example, Jasen and Hot Calder creeks flow along a contact zone between a graywacke formation and the massive Hecata limestone underlying Mt Calder. Other streams, such as Flicker Creek, flow through mixed geologies that include the massive Hecata limestone in the upper reaches, and sandstones, graywacke and conglomerates in the lower reaches (Table 1). Wissmar et al. (1997) provide a detailed discussion Karst topography is a unique geological feature formed by the differential dissolution of limestone (Harding and Ford 1993). The landscape typically has an irregular surface topography with sinkholes, caves, underground streams, and upwelling water (Ford and Williams 1989). Karst topography occurs in several distinct areas of southeast Alaska (Soja 1990; Busch 1994) and is best represented on the northern end of Prince of Wales Island and islands along its west coast (Baichtel 1993). Dissolution of limestone increases the alkalinity of streams as they flow through karst terrain, and higher alkalinity has been associated with increased growth rates for brown trout Salmo trutta (Campbell 1961; Neophitou and O’Hara 1986). However, no studies have linked productivity of aquatic ecosystems to karst landscape or examined its influence on salmon populations in southeast Alaska. We provide the first description of the influence of karst terrain on salmon populations. Our objectives were to describe the range of water chemistry with respect to carbonate buffering (pH, alkalinity), to determine existing habitat conditions available to salmonid populations, and to reAbstract.-Karst topography is a unique and distinct landscape and its geology may have important implications for salmon productivity in streams. The relationship between salmonid communities and water chemistry and the influence of habitat was examined in a set of streams on north Prince of Wales Island, southeast Alaska. Streams in karst landscapes showed higher alkalinities (1,500-2,300 μeq/L) than streams not influenced by karst landscapes (750-770 μeq/L). A significant, positive relationship was observed between alkalinity and density of coho salmon parr Oncorhynchus kitsutch. Backwater pools supported higher densities of coho salmon than did other habitat units. Both coho salmon fry and parr tended to be larger in most karst-influenced streams than in nonkarst streams. Although past timber harvest practices in the riparian areas of several of the streams appeared to influence stream habitat and water temperature, streams flowing through karst landscapes had a distinct water chemistry. Furthermore, these streams appeared to support more fish than nonkarst streams. MASON D. BRYANT* AND DOUGLAS N. SWANSTON