LOWER FITNESS OF HATCHERY AND HYBRID

RAINBOW TROUT COMPARED TO NATURALIZED POPULATIONS

L. M. Miller, T.Close, and A. R. Kapuscinski. 2004. Lower fitness of hatchery and hybrid rainbow trout compared to naturalized populations in Lake Superior tributaries. Blackwell Publishing Ltd.. Molecular Ecology. 13, 3379-3388.

Abstract: We have documented an early life survival advantage by naturalized populations of anadromous rainbow trout Oncorhynchus mykiss over a more recently introduced hatchery population and outbreeding depression resulting from interbreeding between the two strains. We tested the hypothesis that offspring of naturalized and hatchery trout, and

reciprocal hybrid crosses, survive equally from fry to age 1+ in isolated reaches of Lake

Superior tributary streams in Minnesota. Over the first summer, offspring of naturalized

females had significantly greater survival than offspring of hatchery females in three of

four comparisons (two streams and 2 years of stocking). Having an entire naturalized

genome, not just a naturalized mother, was important for survival over the first winter.

Naturalized offspring outperformed all others in survival to age 1+ and hybrids had

reduced, but intermediate, survival relative to the two pure crosses. Averaging over years

and streams, survival relative to naturalized offspring was 0.59 for hybrids with naturalized

females, 0.37 for the reciprocal hybrids, and 0.21 for hatchery offspring. Our results indicate that naturalized rainbow trout are better adapted to the conditions of Minnesota’s tributaries to Lake Superior so that they outperform the hatchery-propagated strain in the same manner that many native populations of salmonids outperform hatchery or transplanted fish. Continued stocking of the hatchery fish may conflict with a management goal of sustaining the naturalized populations.

Quotes from the text: “In the US State of Minnesota, there are concerns about negative interactions between hatchery and naturalized populations of rainbow trout Oncorhynchus mykiss in the state’s waters of Lake Superior. Rainbow trout from various Pacific coast sources were translocated into Lake Superior beginning in the late 1800s and many naturalized anadromous populations now spawn in tributaries throughout the lake. Naturalized rainbow trout have provided a popular recreational fishery in Lake Superior for almost a century, but catch rates in Minnesota have declined since peaking in the 1960s. In the late 1960s, the Minnesota Department of Natural Resources (MNDNR) introduced a hatchery strain of rainbow trout to create new fishing opportunities. The MNDNR once stocked this hatchery strain along much of the Minnesota shore of Lake Superior but now stocks only a few streams because of concerns about potential negative impacts of hatchery fish on naturalized populations.

As a consequence of stocking hatchery fish, outbreeding depression due to loss of local adaptation may arise from either or both the use of nonlocal fish that have evolved in different environments from the resident population or from genetic changes in captive populations due to adaptation to the hatchery environment, i.e. domestication. Either case could result in hybrids between hatchery and wild fish having lower fitness than individuals from the resident populations.

We made crosses within and between naturalized and hatchery rainbow trout for 2 consecutive years… These matings produced four cross types, two pure strain crosses (NxN and HxH) and two reciprocal hybrid crosses (NxH and HxN).

We have documented an early life survival advantage by naturalized populations of anadromous rainbow trout over a more recently introduced hatchery population and outbreeding depression resulting from interbreeding between the two strains.

Having an entire naturalized genome, not just a naturalized mother, was important for survival over the first winter, a time of harsh environmental conditions in northern Minnesota streams. Our results therefore indicate that naturalized rainbow trout are better adapted to the conditions of Minnesota’s tributaries to Lake Superior, so they outperform the hatchery-propagated strain in the same manner that many native populations of salmonids outperform hatchery fish. Continued stocking of hatchery fish may disrupt this apparent local adaptation, reducing the fitness of naturalized populations.

Family sizes were consistently low for crosses with hatchery females. For age 1+, they ranged from 0 to 3 in HxH crosses and 0-7 in HxN crosses of the 1999 year-class, and from 0 to 6 in both crosses of the 2000 year class. Naturalized female crosses had higher means and greater variance, especially the NxN crosses. Family sizes for pure naturalized crosses ranged from one to 24 for the 1999 year-class and 0-14 for 2000 year-class. In addition, the NxN cross had five and six families larger than any HxH or HxN family for the 1999 and 2000 year-classes, respectively.

Breeding competition probably interacts with survival differences to limit the introgression of hatchery genes into naturalized populations. Studies involving native salmonid populations have documented inferior mating success by hatchery fish. In a meta-analysis of experimental studies in seminatural conditions, Fleming and Petersson (2001) found sex differences in reproductive inferiority, with hatchery males less successful than hatchery females in breeding competition with wild fish. Fleming et al. (2000) showed that this male bias extended to breeding competition in the wild. We found that NxH hybrids had higher survival than HxN hybrids, but NxH hybrids would be uncommon if hatchery males are reproductively inferior. HxN hybrids would be more common but we found them less likely to survive. As a result, there would be fewer hybrid adults and less chance to initiate introgressive backcross matings with naturalized fish, than would be expected if the sexes mated randomly in hybrid crosses.

The smaller size of NxN offspring at age 1+ was unexpected, considering the generally equal size of all cross types at age 0+. It is possible that offspring with a hatchery parent (HxH, HxN, and NxH) needed to obtain a larger threshold size to survive the winter than did the NxN offspring.

The viability of these naturalized populations could also be compromised by continued stocking of translocated or hatchery-propagated fish. The potential for rapid adaptation implies that we should be cautious of using a population’s non-native status to justify indiscriminant stocking over naturalized populations. If the goal is to maintain the naturalized population, then on-going stocking of hatchery fish or transplants may reduce the fitness and viability of the established population.