No abstract

SynopsisAn effect of ploidy on thermal tolerance in juvenile trout was assessed in a series of tests comparing time to chronic lethal maximum (CLMax). Diploid and triploid fish were produced from a common spawn for three different groups each of brook trout Salvelinus fontinalis and of rainbow trout Oncorhynchus mykiss. One or two CLMax tests were performed per group, on between 15 and 50 individuals per ploidy within groups. The tests involved exposure of fish to a progressive 2°C day )1 water temperature increase and recording of the time at which each individual fish reached loss of equilibrium (LE). The time to LE data were rank transformed and analyzed as a randomized complete block design. Although relative performance varied among trials, the analysis indicated overall differences due to ploidy were small and nonsignificant among both brook trout and rainbow trout. Size proved to be significantly correlated with time to LE in the brook trout trials, but not in the rainbow trout trials. Two of the six groups included a large proportion of fish which had received a heat shock following fertilization, but were not successfully triploidized. In both cases, thermal tolerance of the heat-shocked diploids was similar to that of the non-heat shocked control diploids, indicating no persistent effect of the heat shock on thermal tolerance.

The range of brook trout Salvelinus fontinalis in the southern Appalachian Mountains has been greatly reduced by environmental disturbance and introduction of rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta. In addition, hatchery‐reared brook trout originating from northern populations have been widely stocked in the region's streams. To an undetermined extent northern brook trout have established themselves or interbred with the genetically distinct native Southern Appalachian strain. Genetic analyses have been performed on a portion of North Carolina's wild brook trout populations to identify those of unaltered native origin. However, information for many streams is lacking, and an assessment of the relative abundance or rarity of genetically unaltered native populations cannot yet be made. The objective of this project was to determine the phylogenetic origin of brook trout in 23 streams within the Pigeon River system, Haywood County, North Carolina. Fish were collected by electroshocking, and muscle tissue samples were obtained by nonlethal biopsy. The samples were analyzed by cellulose acetate gel protein electrophoresis for loci encoding creatine kinase and up to five additional enzymes. Allele frequencies for these enzymes indicated that 15 of the populations were of unaltered native Southern Appalachian origin, 2 were of hatchery‐derived northern origin, and 6 were of mixed genetic origin. This information has been incorporated into a North Carolina Wildlife Resources Commission trout species distribution database used in refining stream and fisheries management policies.

Monosex female stocks are widely used in the commercial production of rainbow trout Oncorhynchus mykiss. The potential for commercial production of brook trout Salvelinus fontinalis, however, is constrained by the lack of published protocols for producing the sex‐reversed males required to create monosex female stocks. Immersion and immersion plus feeding treatments with 17α‐methyltestosterone (MT) and 17α‐methyldihydrotestosterone (MDHT) were applied to genotypically female gynogenetic brook trout to induce phenotypic sex reversal. The fry were exposed to a 6‐h immersion in a solution of MT or MDHT on day 10 following completion of hatch and/or to a steroid‐treated diet for 60 d beginning at first feeding. Immersion dosages were 0.5 or 1.0 mg/L, and feeding dosages were 1.0 or 2.0 mg/kg of feed for MT and 0.5 or 1.0 mg/kg for MDHT. Phenotypic sex of the fish was determined 19 or 22 months after first feeding. Control gynogenetic fish were 100% phenotypic females. Treatments with MT had minimal effect: most fish remained female, with only a low incidence of phenotypic males (1–3% in four of the treatments), intersex fish, or sterile fish. In contrast, a substantial number of phenotypic males were observed in several of the MDHT treatments, with the highest proportion (45%) occurring in the 0.5 mg/L immersion plus 0.5 mg/kg feeding treatment. Sperm was obtained from 29 males from five MDHT treatment groups and one MT treatment group examined at maturity (22 months) and was used in progeny tests of these males. The progeny were 100% female, confirming the male parents to be genotypically female. These protocols may be used to create sex‐reversed brook trout males for the production of monosex female progeny, although additional trials are ongoing to test similar MDHT immersion dosages applied once or multiple times, with or without feeding treatments, to identify protocols with increased efficacy.