Duration in captivity affects competitive ability and breeding success of male but not female steelhead trout (Oncorhynchus mykiss)

Female steelhead trout (Oncorhynchus mykiss) reared for 1 year (traditional, “S1”) and 2 years (experimental, “S2”) in captivity in the Methow River Basin, Washington, produced very similar numbers of offspring in replicate spawning channels over 3 consecutive years. In contrast, S1 anadromous males outcompeted S2 males for access to nesting females and, as a result, produced significantly more offspring. Male dominance was positively associated with body mass, but body mass did not account for differences between S1 and S2 males. Much smaller precocious S2 males that matured during rearing in hatchery raceways sired offspring in all six breeding groups. Contributions from precocious males were nearly always the result of sneak spawning events, although on rare occasions, precocious males were able to stimulate females to spawn in the absence of an anadromous male. Similarities in female breeding success suggest that S1 and S2 hatchery steelhead should exhibit similar productivity under natural conditions, but S1 male steelhead may result in greater rates of gene flow from hatchery to natural populations.

Plasticity in probabilistic reaction norms for maturation in a salmonid fish

The relationship between body size and the probability of maturing, often referred to as the probabilistic maturation reaction norm (PMRN), has been increasingly used to infer genetic variation in maturation schedule. Despite this trend, few studies have directly evaluated plasticity in the PMRN. A transplant experiment using white-spotted charr demonstrated that the PMRN for precocious males exhibited plasticity. A smaller threshold size at maturity occurred in charr inhabiting narrow streams where more refuges are probably available for small charr, which in turn might enhance the reproductive success of sneaker precocious males. Our findings suggested that plastic effects should clearly be included in investigations of variation in PMRNs.

Using 137Cs to measure and compare bioenergetic budgets of juvenile Atlantic salmon (Salmo salar) and brook trout (Salvelinus fontinalis) in the field

Through the 137Cs mass balance method, annual consumption rates were estimated for juvenile Atlantic salmon (Salmo salar) parr and precocious males as well as for brook trout (Salvelinus fontinalis) at four sites in the Ste-Marguerite River system, Québec. With explicit age analysis, consumption rates and growth rates were derived on an individual fish and age-class basis. These represent the first consumption estimates for Atlantic salmon in the wild. Precocious males had consumption rates 1.5 times greater than nonmaturing parr, while Atlantic salmon parr consumption rates were 2.7 times greater than brook trout. There was a strong positive relationship between individual annual consumption and growth rates for Atlantic salmon and brook trout at all sites. Subsequently the concept of field maintenance ration was introduced as the intercept of consumption over growth. Maintenance rations for Atlantic salmon parr ranged from 0.010 to 0.016 g·g-1·day-1 between sites. Brook trout had maintenance rations approximately half those of Atlantic salmon at 0.0059 g·g-1·day-1. Precocious male growth efficiencies were half those of nonmaturing parr despite higher feeding and growth rates. Brook trout growth efficiencies were significantly greater than those of Atlantic salmon parr. The lower growth efficiencies observed for Atlantic salmon are likely due to increased metabolic costs associated with higher activity. On average, Atlantic salmon parr spent 2.4-fold more energy in activity than brook trout. Atlantic salmon precocious males spent 1.7 times more energy in activity than parr.