Michael J. Bradford

Empirical review of coho salmon smolt abundance and the prediction of smolt production at the regional level

Abstract Regional habitat and fisheries management planning requires estimates of the capacity of watersheds to produce salmonids. To predict the average abundance of smolts of coho salmon Oncorhynchus kisutch produced by streams and rivers, we related estimates of smolt abundance to habitat features derived from maps and discharge records. We assembled a database of 474 annual estimates of smolt abundance from 86 streams in western North America for this analysis. We found that only stream length and to a lesser extent latitude were useful in predicting mean smolt abundance, The frequency distribution of annual estimates of smolt abundance from individual streams tended towards a normal rather than the more usual lognormal distribution; the median coefficient of variation in abundance was 37%. Our results are consistent with the view that, on average, smolt abundance is limited by spatial habitat, but that there is significant annual variation in abundance probably due to variation in habitat quality cau...

Bet hedging and the diapause strategies of the cricket Allonemobius fasciatus

We investigated the roles of bet hedging and developmental plasticity in the diapause strategy of a partially bivoltine population ofthe cricket Allonemobius fasciatus. When reared under naturally changing environmental conditions, first-generation females produced increasing proportions of diapause eggs over their reproductive life-span, pre- sumably reflecting the decreasing probability over time that sufficient growing season re- mains for a second generation. The transition from direct-developing to diapause eggs occurred over 35 d for the population, suggesting a considerable degree of bet hedging. The analysis of individuals, however, revealed a much more rapid response and large between - female variability in the median (50%) diapause date. This rapid transition from nondia- pause to diapause eggs implies that interannual variability in season length has not resulted in the evolution of a substantial risk-spreading strategy for individual females. Our results highlight the need to study life history variation at the level of the individual rather than the population.

An Experimental Study of the Stranding of Juvenile Coho Salmon and Rainbow Trout during Rapid Flow Decreases under Winter Conditions

Abstract The stranding of juvenile coho salmon Oncorhynchus kisutch and rainbow trout O. mykiss on river bars caused by rapid decreases in river flow during the operation of hydroelectric facilities was investigated in an artificial stream channel. We conducted experiments with winter water temperatures (<4°C) and a gravel substrate, In daytime trials, many fish became stranded because they were concealed in the interstitial areas of the substrate and were reluctant to leave when water levels receded. Coho salmon were more likely to be stranded than rainbow trout. At night, instead of using the substrate as cover, fish were active in the water column and the incidence of stranding during flow reductions was greatly diminished. Stranding was less frequent at slow rates of dewatering. The addition of shallow, covered pools to the substrate did not alter the principal results. Our findings suggest that during winter months, fish losses from stranding will be minimized if flow reductions occur at night and at...

Parvicapsula minibicornis infections in gill and kidney and the premature mortality of adult sockeye salmon (Oncorhynchus nerka) from Cultus Lake, British Columbia

In recent years, large losses of migrating adult sockeye salmon (Oncorhynchus nerka) from the Fraser River, British Columbia, Canada, have resulted in reductions in catches and productivity. We investigated patterns of mortality and the occurrence of the myxosporean parasite Parvicapsula minibicornis in adult sockeye salmon from Cultus Lake, tributary to the Fraser River. Using data from a captive broodstock program, we found that early migrants to Cultus Lake had less severe P. minibicornis infections and were more likely to survive to maturity than those fish that arrived later after they had presumably held in the warmer Fraser River. We found P. minibicornis in kidneys and gills of spawners. In some fish, significant histopathology in the gills that included severe inflammation and hyperplasia of the gill lamellae was observed; the severity of the disease was correlated with the severity of P. minibicornis infections. Kidney and gill pathologies were more prevalent and more severe in fish that died be...

Quantitative genetics of the trade-off between fecundity and wing dimorphism in the cricket Allonemobius socius

Considerable evidence indicates that in general there is a phenotypic trade-off between wing dimorphism (a threshold trait) and fecundity in insects. However, the demonstration of a genetic basis to this trade-off has been shown in only one species, Gryllus firmus. In this paper we present evidence for another species, the cricket Allonemobius socius. There were significant negative phenotypic correlations between wing morph and fecundity during the first two periods following eclosion (0-12 days and 13-16 days) and also cumulative fecundity (i.e. macropterous females laid fewer eggs than micropterous females). Macropterous and microp-terous females did not differ in fecundity during the remaining two periods (17-20 days and 21-24 days). Both wing morph and age-specific fecundities had significant heritabilities. There was a significant negative genetic correlation between wing morph and fecundity during the first 12 days after eclosion. The standard errors of the estimates for the remaining periods were too large to permit a meaningful conclusion. Analysis of family mean data indicated that fecundity did not vary in a dichotomous manner with wing morph but was best described as a continuous function of the underlying continuously distributed trait.

Genetic and phenotypic sources of life history variation along a cline in voltinism in the cricket Allonemobius socius

Clinal variation in life histories can be genetically based, resulting from selection imposed by different environments, or it may be due to the differential expression of phenotypically plastic traits. We examined the cline in voltinism in the egg-diapausing cricket Allonemobius socius, with populations spanning the switch from a univoltine to a bivoltine phenology. A common garden experiment was employed, using environments that mimicked photoperiod and temperature conditions found in the field. There were only small differences in development time among populations, and the difference in phenology observed in the field is likely due to clinal variation in the length of the growing season. We found large genetically-based differences in the reaction norm for egg diapause that were further magnified by environmental cues. The synergism of genetic and environmental effects was an example of cogradient selection. In the zone of transition between phenologies, voltinism appeared to be a conditional strategy, rather than a genetic polymorphism. First-generation females from this area can lay either direct-developing or diapause eggs depending on the likelihood that a second generation will have sufficient time to develop. For this species, the cline in voltinism is the result of a combination of environmental effects on development, and genetic and environmental influences on egg diapause propensity.

AN EMPIRICAL MODEL OF DIAPAUSE STRATEGIES OF THE CRICKET ALLONEMOBIUS SOCIUS

We modeled the effect of heterogeneity in growing-season length on the diapause strategies of a partially bivoltine population of the striped ground cricket Allonemobius socius. First-generation females of this population can lay mixtures of direct-developing and diapause eggs, and bet-hedging theory predicts that in an unpredictably variable environment the proportion of diapause eggs laid will gradually increase over the season as the likelihood that a second generation will be able to complete development before winter decreases. We quantified variability in the thermal regime using long-term meteorological records, and constructed an analytical model using empirical components for growth and reproduction. The model predicted that females should switch to diapause egg production in early August, a finding that was corroborated by experimental and field observations. The form of the switch between egg types predicted by the model was a transition that spanned only a few days and was similar to that esti...

Biological Responses of Sockeye Salmon to the Fertilization of Chilko Lake, a Large Lake in the Interior of British Columbia

Abstract We evaluated the response of sockeye salmon Oncorhynchus nerka to a 5-year experimental fertilization of Chilko Lake, a large oligotrophic lake of the Fraser River, British Columbia watershed. To evaluate the effects of nutrient addition on smolt production, smolt size, and adult returns, we compared data from the treated years to 39 years of pretreatment data for Chilko Lake, as well as data from seven other untreated sockeye salmon populations that served as controls. By using the other populations to control for variation in survival outside of Chilko Lake, we found that the mean productivity (recruits per spawner) of fertilized broods was 73% higher than unfertilized broods, but the uncertainty surrounding this estimate was substantial: 90% confidence interval of−2% to 174%. Total phosphorus, primary productivity, and to a lesser extent, zooplankton biomass were greater during the fertilized years. Nutrient additions increased the mean size of age-1 smolts by 34% and that of age-2 smolts by 5...

A quantitative genetic analysis of phenotypic plasticity of diapause induction in the cricket Allonemobius socius

Although numerous studies have indicated that diapause is heritable and phenotypically plastic, none of them has examined the quantitative genetic basis of this plasticity. In this paper we report such an analysis for egg diapause in the cricket Allonemobius socius, the induction of which appears to be largely determined by the mother. We analysed the quantitative genetic basis of the phenotypically plastic response of female A. socius to age and environmental conditions. We measured the production of diapause eggs on four occasions over a 16-day period, and in two environments; one mimicking an ‘early’ period of the year and another mimicking a ‘late’ period. We analysed genetic variation in phenotypic plasticity using the character-state approach. Diapause proportion was heritable (h2 ranged from 0.17 to 0.49, being larger in the ‘early’ environment), and the genetic correlation between ages in proportion of diapausing eggs was close to 1 but showed a decrease with increased difference between ages. There were significant genetic correlations between environments for all ages. Because of the reduction in genetic correlation as the difference in ages increases, selection will be more effective at changing the overall shape of the reaction norm than causing local changes. Furthermore, the high genetic correlations may constrain the evolution of the reaction norm. When the two environments are converted into the estimated days in the year the two reaction norms form approximately a single curve as predicted from previous theoretical analysis of the optimal reaction norm.

Temporal and Spatial Trends in the Abundance of Coho Salmon Smolts from Western North America

Abstract Coast-wide trends in the catch of adult Pacific salmon (Oncorhynchus spp.) are often attributed to large-scale variation in marine survival caused by changing ocean conditions; however, the roles of the freshwater stages have not usually been assessed. I used a compilation of published coho salmon Oncorhynchus kisutch data to determine whether the spatial scale of common variation in smolt abundance is sufficient for the freshwater segment of the life cycle to contribute to regional trends in adult coho salmon abundance. I found positive covariation in annual smolt estimates for streams that were less than 20–30 km apart, but smolt abundances were, on average, uncorrelated for streams that were more separated by a greater distance. The proportion of variation of interannual fluctuations in coho salmon smolt abundance that was common to neighboring streams was small. Thus, it appears that large-scale influences on smolt production are likely diluted by the effects of watershed-specific landscape a...