Summer Habitat Utilization and Ecology of Cutthroat Trout Fry (Salmo clarki) in Cascade Mountain Streams

Emergent cutthroat trout fry (Salmo clarki) were observed in the margins, backwaters, and side channels, collectively called "lateral habitats," of three study streams with different riparian vegetation. Most fry remained in these lateral habitats until the end of their first summer. The abundance of cutthroat fry was proportional to the area of lateral habitat in each of the study streams. Average size and growth rate of fry were related to the effect of site elevation on stream temperature and the influence of riparian vegetation on the availability of invertebrate food. Lateral habitats are characterized by slow, shallow-water, abundant detritus and benthic invertebrate assemblages of high density. Stream margins and backwaters provide gradients of depth and velocity, cover, and access to food that are appropriate to the habitat requirements of fry. Because fry populations are closely related to the abundance and quality of lateral habitats in small streams, these habitats should be included in the assessment of habitat requirements of cutthroat trout.

Retinal Structure of Sympatric and Allopatric Populations of Cutthroat Trout (Salmo clarki clarki) and Dolly Varden Char (Salvelinus malma) in Relation to their Spatial Distribution

Retinal structures of cutthroat trout (Salmo clarki clarki) and Dolly Varden char (Salvelinus malma) were compared to determine if these features were consistent with the different light regimes in which the species live, depending on their sympatry or allopatry in lakes. Cone cell density in sympatric trout was over twice that of sympatric char and rod cell density was slightly less. The cross-sectional size of cone cells in sympatric trout was approximately 30% less than in sympatric char. The ratios of photoreceptor cells to ganglion cells in sympatric trout and char retinas were approximately 40:1 and 80:1, respectively. There were no differences between sympatric and allopatric trout with respect to these structural features. Estimates of cone cell density, cone size, and number of photoreceptor cells per ganglion cell for allopatric char were significantly different and intermediate between those for sympatric trout and char. All four populations had similar cone cell mosaics with four paired cones surrounding each single cone. Results are considered in relation to differences in vertical distribution and feeding of sympatric and allopatric populations in three study lakes.

Developmental success of hybrids between two taxa of salmonid fishes with moderate structural gene divergence

The developmental rate and stability of Yellowstone cutthroat trout (Salmo clarki bouvieri) (Y), westslope cutthroat trout (Salmo clarki lewisi) (W), and their hybrids (female × male) were used to examine the evolutionary divergence of gene regulatory systems between these taxa. The Nei's structural gene divergence between these subspecies has been estimated as D = 0.34. Pure W × W developed significantly faster than Y × Y as detected by hatching time and degree of yolk sac resorption. In addition, W × W embryos had greater activities of malate dehydrogenase, lactate dehydrogenase, phosphoglucomutase, and glucose-6-phosphate isomerase from hatching to yolk sac resorption than Y × Y embryos. The two hybrids did not differ significantly from their maternal parental cross in hatching time. W × Y hybrids did, however, hatch significantly sooner than Y × W hybrids. There were no consistent differences between the parental crosses and hybrids in the amount and rate of increase in activity of the four enzymes. Hybrids had significantly greater meristic counts for two out of four characters than the parental crosses. Heterotic effects were observed in that hybrids were longer and showed a faster rate of yolk sac resorption than their maternal parental cross. Moreover, the Y × W hybrid had significantly greater developmental stability as measured by fluctuating asymmetry of four bilateral meristic characters than the Y × Y cross. The lack of perturbation in the development of hybrids indicates that despite their structural gene divergence, few gene regulatory incompatibilities affecting the developmental process exist between these cutthroat trout taxa.

Genetic Identification of Cutthroat Trout, Salmo clarki, in Glacier National Park, Montana

Trout populations in 29 lakes in Glacier National Park were identified by meristic and electrophoretic analyses to assess the extent of introgressive hybridization between introduced nonnative trout and the indigenous cutthroat trout, Salmo clarki lewisi. Native cutthroat trout remain in 16 lakes draining to the North and Middle forks of the Flathead River; no native trout were found east of the Continental Divide. Introduced Yellowstone cutthroat trout, Salmo clarki bouvieri, occur in six headwater lakes. Hybrid populations, including both S. c. lewisi × bouvieri and S. clarki × S. gairdneri, inhabit six lakes. Hybridization between native and introduced trouts has been minimal, apparently due to strong selective pressures favoring the indigenous genotype. Close agreement was observed between the meristic and electrophoretic results.

Linkage Relationships Reflecting Ancestral Tetraploidy in Salmonid Fish

ABSTRACT Fifteen classical linkage groups were identified in two salmonid species (Salmo trutta and Salmo gairdneri) and three fertile, interspecific hybrids (S. gairdneri × Salmo clarki, Salvelinus fontinalis × Salvelinus namaycush and S. fontinalisx Salvelinus alpinus) by backcrossing multiply heterozygous individuals. These linkage relationships of electrophoretically detected, protein coding loci were highly conserved among species. The loci encoding the enzymes appeared to be randomly distributed among the salmonid chromosomes. Recombination frequencies were generally greater in females than in males. In males, certain linkage groups were pseudolinked with other linkage groups, presumably because of facultative multivalent pairing and directed disjunction of chromosomes. Five such pseudolinkage groups were identified and they also appeared to be common among species and hybrids. Duplicate loci were never classically linked with each other, although some exhibited pseudolinkage and some showed evidence of exchanging alleles. Gene-centromere recombination frequencies estimated from genotypic distributions of gynogenetic offspring were consistent with map locations inferred from female intergenic recombination frequencies. These linkage relationships support the contention that all extant salmonids arose from a common tetraploid progenitor and that this progenitor may have been a segmental allotetraploid.

Genetic Structure of Anadromous Cutthroat Trout (Salmo clarki clarki) Populations in the Puget Sound Area: Evidence for Restricted Gene Flow

Genetic relationships among anadromous populations of coastal cutthroat trout (Salmo clarki clarki) in the Puget Sound area (United States) were investigated by electrophoretic methods between successive year classes within streams (populations), among streams within drainages, between drainages within each of two regions separated by Puget Sound, and between the two regions. Average allele frequencies for fish from the two regions differed by approximately 0.10 at several polymorphic loci, suggesting that gene flow between the two regions is restricted. Despite this divergence, the sampled populations were all very similar genetically (Nei's I > 0.97), possibly reflecting a common ancestral invasion following Pleistocene glaciation. The total gene diversity (expected heterozygosity) was 0.101 and was partitioned as follows: between regions, 1.24%; between drainages, 1.01%; among streams, 2.41%; between year classes, 1.17%; within individual year class samples, 94.2%. In addition, stream effects were significant in 7 of 12 intralocus ANOVA comparisons, suggesting that populations were structured genetically at this level of subdivision. This latter result may be reflecting the homing instinct of spawning adults.

Use of Small, Temporary, Floodplain Tributaries by Juvenile Salmonids in a West Coast Rain-Forest Drainage Basin, Carnation Creek, British Columbia

Seasonal movement of trout (Salmo clarki and S. gairdneri) into and out of three tributaries which drain areas ranging from 15 to 100 ha within the lower Carnation Creek catchment basin were monitored periodically from 1972 to 1985. The number of trout entering the three tributaries relative to total trout was as high as the number of coho salmon (Oncorhynchus kisutch) entering these tributaries relative to total coho. The percentage of the salmonid population represented by trout was highest in the two largest tributaries and lowest in the smallest. Trout were most clearly associated with nonvegetated sand and gravel bottom portions of the three tributaries. Coho were associated with this habitat too, but they also frequented portions of the tributaries that were vegetated and had a mud substrate. In the two largest tributaries, trout were represented by more age classes than were coho salmon. The paper considers some of the implications of use of small drainages by trout to habitat managers.