Coho Salmon Populations Research Articles

Correction: An Investigation into the Poor Survival of an Endangered Coho Salmon Population

There was an error in the Competing Interests section. The correct competing interest information is as follows: Dr. David Welch is the president of Kintama Research. Kintama Research is an environmental consultancy that designed and deployed large parts of the current array. Their operating costs partly come from the Gordon and Betty Moore Foundation. The Gordon and Betty Moore Foundation was not directly involved in the study design, analysis, or writing of the paper, nor the decision to submit the manuscript for publication. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials, as detailed online in the PLoS ONE Author Guidelines.

An Investigation into the Poor Survival of an Endangered Coho Salmon Population

To investigate reasons for the decline of an endangered population of coho salmon (O. kisutch), 190 smolts were acoustically tagged during three consecutive years and their movements and survival were estimated using the Pacific Ocean Shelf Tracking project (POST) array. Median travel times of the Thompson River coho salmon smolts to the lower Fraser River sub-array were 16, 12 and 10 days during 2004, 2005 and 2006, respectively. Few smolts were recorded on marine arrays. Freshwater survival rates of the tagged smolts during their downstream migration were 0.0–5.6% (0.0–9.0% s.e.) in 2004, 7.0% (6.2% s.e.) in 2005, and 50.9% (18.6% s.e.) in 2006. Overall smolt-to-adult return rates exhibited a similar pattern, which suggests that low freshwater survival rates of out-migrating smolts may be a primary reason for the poor conservation status of this endangered coho salmon population.

Selection on breeding date and body size in colonizing coho salmon, Oncorhynchus kisutch

Selection during the colonization of new habitat is critical to the process of local adaptation, but has rarely been studied. We measured the form, direction, and strength of selection on body size and date of arrival to the breeding grounds over the first three cohorts (2003-2005) of a coho salmon (Oncorhynchus kisutch) population colonizing 33 km of habitat made accessible by modification of Landsburg Diversion Dam, on the Cedar River, Washington, USA. Salmon were sampled as they bypassed the dam, parentage was assigned based on genotypes from 10 microsatellite loci, and standardized selection gradients were calculated using the number of returning adult offspring as the fitness metric. Larger fish in both sexes produced more adult offspring, and the magnitude of the effect increased in subsequent years for males, suggesting that low densities attenuated traditional size-biased intrasexual competition. For both sexes, directional selection favoured early breeders in 2003, but stabilizing selection on breeding date was observed in 2004 and 2005. Adults that arrived, and presumably bred, early produced stream-rearing juvenile offspring that were larger at a common date than offspring from later parents, providing a possible mechanism linking breeding date to offspring viability. Comparison to studies employing similar methodology indicated selection during colonization was strong, particularly with respect to reproductive timing. Finally, female mean reproductive success exceeded that needed for replacement in all years so the population expanded in the first generation, demonstrating that salmon can proficiently exploit vacant habitat.

Identification, development, and characterization of three molecular markers associated to spawning date in Coho salmon ( Oncorhynchus kisutch)

In salmonids, spawning date is an important productive and life-history trait determining the time of fertilization, date of progeny emergence, and even the probability of survival and the growth rate of the small fry. Recent evidences support a polygenic basis with several Quantitative Trait Loci (QTL) affecting the phenotypic expression of this trait in salmon species. Utilizing females from two populations of Coho salmon ( Oncorhynchus kisutch) obtained by divergent selection for early and late spawning date, we have identified three RAPD markers amplified with primers UBC-075, UBC-229 and UBC-249 associated with spawning date. Polymorphic fragments were characterized and converted in two codominant and one dominant SCAR PCR-based marker Oki075 ( AY926350), OkiCAT229UCH ( AY926352) and Oki249 ( AY929856). The SCARs Oki075 and Oki249 have been mapped at different chromosomes using FISH. We identified a 52 bp repetitive motif minisatellite like on marker Oki075 and a microsatellite di-nucleotide TG/AC motif on marker OkiCAT229UCH. These three SCAR markers show significant association with spawning date in Coho salmon, however it was not possible to discard a genetic drift effect, therefore more study is necessary to verify the associations presented here.

Long-term changes in the fine-scale population structure of coho salmon populations (Oncorhynchus kisutch) subject to extensive supportive breeding

The long-term viability of a metapopulation depends partly on the gene flow among sub-populations. Management approaches such as translocations and supportive breeding between closely related populations may affect gene flow and overall structure, and therefore viability. Here, we examined temporal changes in the fine-scale population structure of coho salmon (Oncorhynchus kisutch) by comparing archived (1938) and modern (2001-2005) populations in six rivers within a single conservation unit (Puget Sound, Washington) sampled before and after an extended period of between-river transfers and releases of millions of cultured salmon. Genotype frequencies at eight microsatellite loci showed that current populations descended from historical Puget Sound populations, but populations in different rivers that exchanged fish for hatchery propagation share more of their ancestry recently than they did historically. Historically, populations in different rivers were isolated by geographic distance, but that relationship is no longer significant. Allelic richness among all populations declined significantly, suggesting that genetic drift has increased because of a population bottleneck. Populations in different rivers and within the same river have become more diverged, providing further evidence for a widespread bottleneck. Previously, we observed that genetic distance significantly decreased with the number of fish exchanged; however, some populations apparently resisted introgression. Altered gene flow and lost diversity may affect the complexity, and therefore resiliency of sub-populations within a conservation unit. Plans for artificial culture need to maintain existing genetic diversity and avoid disrupting the fine-scale structure by using local populations for parents whenever possible.

Using an unplanned experiment to evaluate the effects of hatcheries and environmental variation on threatened populations of wild salmon

Efforts to conserve depleted populations of Pacific salmon ( Oncorhynchus spp.) often rely on hatchery programs to offset losses of fish from natural and anthropogenic causes, but their use has been contentious. We examined the impact of a large-scale reduction in hatchery stocking on 15 populations of wild coho salmon along the coast of Oregon (USA). Our analyses highlight four critical factors influencing the productivity of these populations: (1) negative density-dependent effects of hatchery-origin spawners were ∼5 times greater than those of wild spawners; (2) the productivity of wild salmon decreased as releases of hatchery juveniles increased; (3) salmon production was positively related to an index of freshwater habitat quality; and (4) ocean conditions strongly affect productivity at large spatial scales, potentially masking more localized drivers. These results suggest that hatchery programs’ unintended negative effects on wild salmon populations, and their role in salmon recovery, should be considered in the context of other ecological drivers.

Interlocus variance of FST provides evidence for directional selection over an olfactory receptor gene in Coho salmon ( Oncorhynchus kisutch) populations

Coho salmon ( Oncorhynchus kisutch) utilize olfactory cues to recognize and home to natal streams during spawning migrations. Chemically distinct river systems may promote directional selection for appropriately tuned olfactory receptor repertoires among Coho populations. Here, we use F ST outlier methods to test for a signal of selection over olfactory receptor gene-linked markers, characterized in Coho populations from four geographically proximate, but ecologically distinct rivers. We report evidence for directional selection over one such marker, OkiOR3001, and document substantially higher levels of genetic structure among Coho populations from Oregon coastal lakes than previously observed with putatively neutral microsatellites.

Genetic structure, migration, and patterns of allelic richness among coho salmon (Oncorhynchus kisutch) populations of the Oregon coast

Genotypic data from eight microsatellite loci are used to infer population structure, effective population size, migration rates, and patterns of allelic richness among wild and hatchery populations of Oregon coastal coho salmon ( Oncorhynchus kisutch ). Corroborating the results of a previous study, we found relatively weak genetic structure among coho from different river basins, although some geographically and ecologically defined clades are supported. Contemporary migration rates among basins appear to be high and asymmetrical. Hatchery populations tended to resemble the wild populations from which they were founded, but presented significantly lower levels of allelic richness. Allelic richness was also low in Oregon coastal lake populations and peaked in the central region of the evolutionarily significant unit among wild river populations. We suggest that the observed patterns may reflect both current source–sink dynamics and post-Pleistocene colonization events.

Changes in fish communities following recolonization of the Cedar River, WA, USA by Pacific salmon after 103 years of local extirpation

AbstractMigration barriers are a major reason for species loss and population decline of freshwater organisms. Significant efforts have been made to remove or provide passage around these barriers; however, our understanding of the ecological effects of these efforts is minimal. Installation of a fish passage facility at the Landsburg Dam, WA, USA provided migratory fish access to habitat from which they had been excluded for over 100 years. Relying on voluntary recruitment, we examined the effectiveness of this facility in restoring coho (Oncorhynchus kisutch) salmon populations above the diversion, and whether reintroduction of native anadromous species affected the distribution and abundance of resident trout (O. mykiss and O. clarki). Before the ladder, late summer total salmonid (trout only) density increased with distance from the dam. This pattern was reversed after the ladder was opened, as total salmonid density (salmon + trout) approximately doubled in the three reaches closest to the dam. These changes were primarily due to the addition of coho, but small trout density also increased in lower reaches and decreased in upper reaches. A nearby source population, dispersal by adults and juveniles, low density of resident trout and high quality habitat above the barrier likely promoted rapid colonization of targeted species. Our results suggest that barrier removal creates an opportunity for migratory species to re‐establish populations leading to range expansion and potentially to increased population size. Copyright © 2008 John Wiley & Sons, Ltd.

Estimates of Natural Selection in a Salmon Population in Captive and Natural Environments

Captive breeding is a commonly used strategy for species conservation. One risk of captive breeding is domestication selection--selection for traits that are advantageous in captivity but deleterious in the wild. Domestication selection is of particular concern for species that are bred in captivity for many generations and that have a high potential to interbreed with wild populations. Domestication is understood conceptually at a broad level, but relatively little is known about how natural selection differs empirically between wild and captive environments. We used genetic parentage analysis to measure natural selection on time of migration, weight, and morphology for a coho salmon (Oncorhynchus kisutch) population that was subdivided into captive and natural components. Our goal was to determine whether natural selection acting on the traits we measured differed significantly between the captive and natural environments. For males, larger individuals were favored in both the captive and natural environments in all years of the study, indicating that selection on these traits in captivity was similar to that in the wild. For females, selection on weight was significantly stronger in the natural environment than in the captive environment in 1 year and similar in the 2 environments in 2 other years. In both environments, there was evidence of selection for later time of return for both males and females. Selection on measured traits other than weight and run timing was relatively weak. Our results are a concrete example of how estimates of natural selection during captivity can be used to evaluate this common risk of captive breeding programs.

Comparisons of FST and QST of Growth‐Related Traits in Two Populations of Coho Salmon

AbstractUnderstanding the mechanisms driving population divergence may be an important step in achieving conservation goals. Estimates of population divergence at neutral markers (FST) can be compared with divergence at quantitative traits (QST) to determine whether population differentiation is being driven by neutral processes such as genetic drift or by selection. We examined growth‐related fitness traits, specifically length, weight, and condition factor, at various life history stages in coho salmon Oncorhynchus kisutch. Fish from two populations originating in different evolutionarily significant units and with different histories of selection were reared in a common environment, and weight and length measurements were collected for the alevin and juvenile stages during the first year of growth. Divergence at neutral markers was estimated from 10 microsatellite loci typed in each of the two populations (FST = 0.089, 95% confidence interval = 0.056‐0.0123). While QST tended to be greater than FST across all sample dates, this relationship was only significant when juvenile traits were combined for all sample dates (QST = 0.406, 95% confidence interval = 0.305‐0.468). These results suggest that diversifying selection is driving the differentiation between the two populations examined for the juvenile growth traits measured.

Large-scale frequency dynamics of alternative male phenotypes in natural populations of coho salmon (Oncorhynchus kisutch): patterns, processes, and implications

Using over 20 years of annual spawner survey data collected from 46 natural populations of coastal Oregon coho salmon (Oncorhynchus kisutch), we examined large-scale spatial and temporal dynamics of alternative male pheno types (age-3 hooknoses and age-2 jacks). Fluctuations in jack and hooknose abundance were synchronous among populations (i.e., region-wide synchrony), similar to patterns seen previously in Oregon hatchery populations and indicative of oceanic processes operating at large spatial and temporal scales. By contrast, regional-scale synchrony in the ratio of jack to hooknose males was likely attributable to the influences of the freshwater environment. Moreover, within-population variation in phenotype ratio was lower by brood year than by return year (i.e., weak cohort relationship), contrary to previous patterns in hatchery populations. The spatial and temporal patterns of phenotype ratio observed in natural populations but not in hatchery populations, where freshwater effects were statistically controlled, indicate that the frequency dynamics of coho male alternative phenotypes are influenced more strongly by freshwater than by marine processes. The environmentally induced variability in phenotype ratio suggests that there may be constant perturbations of fitness functions for alternative phenotypes, such that the system should be less stable than predicted from the status-dependent selection model with fixed fitness functions.

ASSESSMENT OF HARBOR SEAL PREDATION ON ADULT SALMONIDS IN A PACIFIC NORTHWEST ESTUARY

The populations of many native species have increased or expanded in distribution in recent decades, sometimes with negative consequences to sympatric native species that are rarer or less adaptable to anthropogenic changes to the environment. An example of this phenomenon from the Pacific Northwest is predation by locally abundant pinnipeds (seals and sea lions) on threatened, endangered, or otherwise depleted salmonid (Oncorhynchus spp.) populations. We used survey sampling methodology, acoustic telemetry, and molecular genetics to quantify the amount of harbor seal (Phoca vitulina) predation on a depressed run of coho salmon (O. kisutch) and to determine whether some seals consumed a disproportionately higher number of salmonids than others. Based on a probability sample totaling 759.5 h of observation, we estimated that seals consumed 1161 adult salmonids (95% CI = 503-1818 salmonids) during daylight hours over an 18.9-km estuarine study area in Oregon during an 84-d period in fall 2002. Simultaneous tracking of 56 seals via an acoustic telemetry array indicated that a small proportion of marked seals (12.5%) exhibited behavior that was consistent with specialization on salmonids. These seals spent the majority of their time in the riverine portion of the study area and did so disproportionately more at night than day. Genetic analysis of 116 salmonid structures recovered from 11 seal fecal samples suggested that coho salmon accounted for approximately one-half of total salmonid consumption. Though subject to considerable uncertainty, the combined results lead us to infer that seals consumed 21% (range = 3-63%) of the estimated prespawning population of coho salmon. We speculate that the majority of the predation occurred upriver, at night, and was done by a relatively small proportion of the local seal population. Understanding the extent and nature of pinniped predation can provide important inputs into risk assessments and other modeling efforts designed to aid the conservation and recovery of salmonids in the Pacific Northwest. Such understanding may also help inform management actions designed to reduce the impact of pinniped predation on salmonids, which potentially range from short-term lethal removal programs to long-term ecosystem restoration and protection efforts.

Evaluating the Feasibility of Reestablishing a Coho Salmon Population in the Yakima River, Washington

AbstractHistorical returns of coho salmon Oncorhynchus kisutch to the Yakima River basin were estimated to range from 45,000 to 100,000 fish annually but declined to zero by the 1980s after decades of overexploitation of fishery, water, and habitat resources. In 1996, the Yakama Nation and cooperators initiated a project to determine the feasibility of reestablishing a naturally spawning coho salmon population in the Yakima River. The project explored the feasibility of successful coho salmon recolonization in the Yakima River by introducing stocks that had been reared in hatcheries for multiple generations. After 10–20 years of outplanting, we compared data for adult returns of known natural origin (i.e., returns from parents that spawned in the wild) and returns from hatchery releases. We found that fish of natural origin returned at a significantly larger size than those of hatchery origin. The mean egg mass and mean egg size of natural‐origin females were greater than those of hatchery‐origin females, but the differences were statistically significant for only one of three sample years. Natural‐origin adults returned 2–9 d later and spawned 5 d later than their hatchery‐origin counterparts. Preliminary indices of smolt‐to‐adult survival for natural‐origin fish were 3.5–17.0 times the survival indices of hatchery‐origin fish. The number of returns to the historical spawning habitats in upriver areas generally increased. Spawning surveys demonstrated the existence of robust and sustainable spawning aggregates in various locations in the basin. Hatchery releases from the local brood source (Yakima River returns) had significantly higher smolt‐to‐smolt survival than releases from out‐of‐basin (non‐Yakima River) hatchery broodstock, but some of these observed differences in survival may be partially attributable to differences in smolt size. We concluded that hatchery‐origin coho salmon with a legacy of as many as 10–30 generations of hatchery influence demonstrated an ability to reestablish themselves in the Yakima River (i.e, as a naturalized, nonnative population) after as few as 3–5 generations of outplanting in the wild.

Assessing genetic diversity of protected coho salmon (Oncorhynchus kisutch) populations in California

California coho salmon (Oncorhynchus kisutch) are under legal protection owing to significant declines in abundance over the last decades. Previously, California coho salmon were characterized as having low genetic diversity and weak population subdivision, attributable potentially to homogenization by out-of-basin hatchery releases. Here, diversity at seven highly polymorphic microsatellite DNA markers is assessed within and among 32 collections of coho salmon from 16 California watersheds. In 71% of local populations, genotypic composition deviates significantly from that expected under the assumption of random mating. We develop and apply methods to adjust for two potential causes of deviation from random mating expectations: (i) Wahlund effects, owing to heterogeneous collections of individuals, and (ii) the "Allendorf–Phelp's effect", owing to closely related juveniles in samples. Such population-level "adjustments" reduce within-region and increase among-region variance; after adjustment, we find strong concordance of genetic and geographic distances. We conclude that stock transfers have had minimal impact on population structure and that California coho salmon populations likely comprise small numbers of endemic breeders, potentially experiencing high levels of genetic drift and inbreeding.

Coho salmon dependence on intermittent streams

In February 2006, the US Supreme Court heard cases that may affect whether intermittent streams are jurisdictional waters under the Clean Water Act. In June 2006, however, the cases were remanded to the circuit court, leaving the status of intermittent streams uncertain once again. The presence of commercial species, such as coho salmon (Oncorhynchus kisutch), can be an important consideration when determining jurisdiction. These salmon spawn in the upper portions of Oregon coastal stream networks, where intermittent streams are common. In our study of a coastal Oregon watershed, we found that intermittent streams were an important source of coho salmon smolts. Residual pools in intermittent streams provided a means by which juvenile coho could survive during dry periods; smolts that overwintered in intermittent streams were larger than those from perennial streams. Movement of juvenile coho into intermittent tributaries from the mainstem was another way in which the fish exploited the habitat and illustrates the importance of maintaining accessibility for entire stream networks. Loss of intermittent stream habitat would have a negative effect on coho salmon populations in coastal drainages, including downstream navigable waters.

Changes in run timing and natural smolt production in a naturally spawning coho salmon (Oncorhynchus kisutch) population after 60 years of intensive hatchery supplementation

Supplementing natural fish populations with artificially propagated (hatchery) fish is a common practice. In evaluating supplementation, it is important to assess the relative fitness of both hatchery-produced and naturally produced fish when they spawn together in the wild and to evaluate how the absolute fitness of the natural population changes after many generations of supplementation. We evaluated the relative fitness of naturally produced and hatchery-produced coho salmon (Oncorhynchus kisutch) in Minter Creek, Washington, USA. We also evaluated long-term changes in natural smolt production in this stream after several decades of intensive hatchery supplementation. Total smolt production was estimated to be 14 660 and 19 415 in 2002 and 2003, respectively, compared with the average value of 28 425 from 1940 to 1955. We found no significant difference in relative fitness between hatchery and natural fish, probably because the natural population consists largely of fish produced from the hatchery a generation or two previously. There has been a long-term trend for adults to return to the stream earlier in the spawning season. We estimated standardized selection differentials on run timing, with results indicating stabilizing selection with an optimum run timing later than the mean contemporary run timing but earlier than the historical mean run timing.

Effects of species, culture history, size and residency on relative competitive ability of salmonids

The relative competitive ability of juvenile farm and wild salmonids was investigated to provide insight into the potential effects of introduction of cultured salmon on wild Pacific salmonid (Oncorhynchus) species. Aquarium experiments involving equal contests (i.e. size matched, simultaneously introduced individuals) indicated that two wild coho salmon Oncorhynchus kisutch populations were competitively equal to a farm coho salmon population. In equal contests between farm Atlantic salmon Salmo salar (Mowi strain) and these wild coho salmon populations or coastal cutthroat trout Oncorhynchus clarki clarki, Atlantic salmon were subordinate in all cases. When Atlantic salmon were given a residence advantage, however, they were competitively equal to both wild coho salmon populations, but remained subordinate to coastal cutthroat trout. When Atlantic salmon were given a 10–30% length advantage, they were competitively equal to one wild coho salmon population but remained subordinate to the other. In equal contests in semi‐natural stream channels, both wild coho and farm Atlantic salmon grew significantly more in the presence of the other species than when alone. It appears that coho salmon obtain additional food ration by out competing Atlantic salmon, whereas Atlantic salmon were stimulated to feed more in the presence of coho salmon competitors. These results suggest that wild coho salmon and cutthroat trout should out compete farm Atlantic salmon of a similar size in nature. As the relative competitive ability of Atlantic salmon improves when they have a size and residence advantage, should feral populations become established, they may exist on a more equal competitive footing owing to the long freshwater residence of Atlantic salmon.

Genetic improvement in Coho salmon (Oncorhynchus kisutch). I: Selection response and inbreeding depression on harvest weight

In this study the genetic response to selection and the effects of inbreeding on harvest weight in two populations (which spawned as even and odd years classes) of Coho salmon are described. Artificial selection was performed using the best linear unbiased prediction (BLUP) of breeding values obtained from an animal model. Both populations were also selected for early spawning date using independent culling levels; these results are presented in a companion paper. The estimation of genetic parameters was based on the phenotypic records of 12,208 animals randomly sampled at harvest time (random group). These fish represented a mean of 64% and 48% of all animals with phenotypic records in the even and odd year classes, respectively. The narrow sense heritability estimated for harvest weight was high and very similar in the two populations, 0.39±0.03 in the even year class and 0.40±0.04 in the odd year class. Due to the mating design, the genetic selection differentials were 2.3 times greater in males than in females. The mean genetic selection response obtained was high in both populations, 383.2 g or +1.26 σ and 302.4 g or +1.23 σ per generation in the even and odd year classes, respectively. This is equivalent, on the average, to an increase in weight of 13.9% per generation compared to the base population, or 10.2% per generation with respect to the difference between successive generations. After the 4th generation of selection the mean inbreeding level was 9.5% in the even year class and 4.4% in the odd year class, which are close to preliminary estimates based on the records of the random group plus a group of animals with high harvest weight (Gallardo, J.A., García, X., Lhorente, J.P., Neira, R., 2004b. Effects of nonrandom mating schemes to delay the inbreeding accumulation in cultured populations of Coho salmon (Oncorhynchus kisutch). Can. J. Fish. Aqua. Sci. 61, 547–553.). The estimated effects of inbreeding depression on harvest weight were negative, but not significantly different from 0 in either the even year class (bf=−7.04 g/ΔF, s.e.=3.9; Mean=4118 g; inbreeding depression per 10% of ΔF was −1.7%; P=0.06) or the odd year class (bf=−4.8 g/ΔF, s.e.=0.33; Mean=3243 g; inbreeding depression per 10% of ΔF was −1.5%; P=0.33). Other programs in salmon have selected for more than one character, however, there are still no estimates of economic weights published in salmon breeding programs, which are necessary to calculate the total genetic merit for these animals.

Genetic improvement in coho salmon (Oncorhynchus kisutch). II: Selection response for early spawning date

Phenotypic records of within-year spawning date in two cultivated populations of coho salmon, named as even (n=2272) and odd (n=1347) year classes are analyzed. Both populations were selected for harvest weight and early spawning for four generations, using independent culling levels. Harvest weight was selected using breeding values obtained from an animal model in males and females, while early spawning was selected phenotypically only in females. A post-selection analysis to estimate breeding values in both characters allowed comparison of the phenotypic response (R) and genetic selection response (GR) to selection for early spawning, narrow sense heritability (h2), and genetic correlation with harvest weight (rg). Mean spawning dates were 13 and 15 days earlier after four generations of selection in the even and odd year class, respectively. This represents a phenotypic response to selection of −2.74±0.7 (P=0.03) and −3.23±1.3 (P=0.09) days per generation in the even and odd year classes, respectively. The heritability estimates, by regression of the selection differential on phenotypic change, were h2=1.78±0.37 (P<0.05) for the even year class and h2=1.90±0.42 (P<0.05) for the odd year class. In both cases, calculated values were greater than the highest possible value for heritability (h2=1), which indicates that the response is overestimated due to a positive environmental effect which was not quantified. The estimates of narrow sense heritability using an animal model were high in both populations (even year class h2=0.40±0.05, odd year class h2=0.44±0.06). Breeding value analysis showed the genetic selection response was −0.62 and −1.13 days per generation in the even and odd year classes, respectively. The genetic correlation between spawning date and harvest weight was low in both year classes (even rg=0.25±0.13; odd rg=−0.02±0.24). The genetic selection differentials (GS) were lower in males than in females (even year class GSf=−1.12 y GSm=−0.13; odd year class GSf=−1.46; GSm=−0.36), which reflects selection practiced only on females.