The spawning ecology of coastal cutthroat trout (Oncorhynchus clarki clarki) and steelhead (O. mykiss irideus) in the Stone Lagoon watershed and the potential for hybridization

The potential for and existence of hybridization between coastal cutthroat trout (Oncorhynchus clarki clarki) and steelhead (Q. mvkiss irideus) was examined in the Stone Lagoon/McDonald Creek watershed of northern California. Hybridization potential was assessed by contrasting the temporal and spatial spawning characteristics of the two species. To determine run size and timing, traps and weirs were installed to intercept the spawning migration. Spawning surveys were conducted to determine redd locations within the watershed, measure microhabitat characteristics at redd sites and observe spawning behavior of coastal cutthroat trout and steelhead. The spawning microhabitat examined included riffle depths and surface velocities at active redd sites. Sixty-eight juvenile salmonids were examined for hybridization with protein electrophoresis (35 in 1992 and 33 in 1993). The electrophoretic analysis identified true hybrids and also tested our ability to field identify juveniles as either cutthroat, steelhead or hybrid through external morphological features (maxillary length, spotting pattern, slash coloration, white tip on dorsal fin and presence or absence of parr-like marks on anterior dorsal surface). During the 1992 season, 924 spawners were trapped: 831 cutthroat, 89 steelhead and four possible hybrids. During the 1993 season, 2,339 spawners were trapped: 2,231 cutthroat, 79 steelhead and 29 possible hybrids. The number of wild cutthroat trapped increased from 43 in 1992 to 376 in 1993. Stream entry dates of wild cutthroat, introduced cutthroat and wild steelhead were significantly different in both 1992 and 1993. Spawning surveys revealed that redd locations in the main channels of both forks of McDonald Creek overlapped, while only cutthroat redds were found in several smaller tributaries. There were no significant differences in riffle depths at cutthroat and steelhead redd sites (P(Tt) = 0.08). However, there were significant differences of average surface velocities at cutthroat and steelhead redd sites (P(Tt) = 0.02). The electrophoretic data confirmed our ability to visually identify juvenile coastal cutthroat (90% correctly identified) and juvenile steelhead (71% correctly identified). Our ability to visually identify juvenile hybrids was limited (37% correctly identified). Substantial temporal and spatial overlap of coastal cutthroat and steelhead spawning existed in McDonald Creek and may jepardize species integrity of the cutthroat population. Utilization of McDonald Creek tributaries may provide the spatial separation required by coastal cutthroat to maintain species integrity.