Repeat Spawners Research Articles

Genetic parentage reveals the (un)natural history of Central Valley hatchery steelhead.

Populations composed of individuals descended from multiple distinct genetic lineages often feature significant differences in phenotypic frequencies. We considered hatchery production of steelhead, the migratory anadromous form of the salmonid species Oncorhynchus mykiss, and investigated how differences among genetic lineages and environmental variation impacted life history traits. We genotyped 23,670 steelhead returning to the four California Central Valley hatcheries over 9 years from 2011 to 2019, confidently assigning parentage to 13,576 individuals to determine age and date of spawning and rates of iteroparity and repeat spawning within each year. We found steelhead from different genetic lineages showed significant differences in adult life history traits despite inhabiting similar environments. Differences between coastal and Central Valley steelhead lineages contributed to significant differences in age at return, timing of spawning, and rates of iteroparity among programs. In addition, adaptive genomic variation associated with life history development in this species varied among hatchery programs and was associated with the age of steelhead spawners only in the coastal lineage population. Environmental variation likely contributed to variations in phenotypic patterns observed over time, as our study period spanned both a marine heatwave and a serious drought in California. Our results highlight evidence of a strong genetic component underlying known phenotypic differences in life history traits between two steelhead lineages.

DNA and scale reading to identify repeat spawning in Atlantic salmon: Unique insights into patterns of iteroparity.

Iteroparity represents an important but often overlooked component of life history in anadromous Atlantic salmon. Here, we combined individual DNA profiling and scale reading to identify repeat spawners among ~8000 adult salmon captured in a fish trap in the river Etne, Norway, in the period 2015-2019. Additionally, 171 outward migrating kelts were captured in the spring of 2018-2020 and identified using molecular methods to estimate weight loss since ascending the river to spawn. The overall frequency of repeat spawners identified using molecular methods and scale reading combined was 7% in females and 3% in males (5% in total). Most of these (83%) spent one full year reconditioning at sea before returning for their second spawning, with a larger body size compared with their size at first spawning, gaining on average 15.9 cm. A single female migrating back into the river for a fifth breeding season was also identified. On average, kelts lost 40% bodyweight in the river, and more female than male kelts were captured during outward migration. The date of arrival in the upstream fish trap was significantly but moderately correlated between maiden and second entry to the river for alternate and consecutive spawners. The estimated contribution from repeat spawners to the total number of eggs deposited in the river each year varied between 2% and 17% (average 12%). Molecular-based methods marginally underestimated the number of repeat spawners compared with scale reading (5% vs 7%) likely due to a small number of returning spawners not being trapped and sampled. Differences between the methods were most evident when classifying the spawning strategy (alternate or consecutive-year repeat spawners), where the scale method identified proportionally more consecutive-year repeat spawners than the molecular method. This unique data set reveals previously unstudied components of this life history strategy and demonstrates the importance of repeat spawners in population recruitment.

Guiding downstream migrating Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) of different life stages in a large river using bubbles

AbstractSalmonid repeat spawners are precious individuals for wild populations due to their high fecundity and previous spawning experience, making them important in environmental policy. However, repeat spawners rarely exist above hydropower dams in regulated rivers as the mortality of post‐spawners (kelts) when passing through turbines during downstream migration is very high. To mitigate this problem, there are different technical solutions that potentially guide fish toward available fishways. Bubble barriers represent one alternative to costly physical guiding structures, but the efficiency of bubbles for guiding downstream migrating kelts has not been tested. In this study, we evaluate a 100 m long bubble barrier in guiding salmonids—both smolts and kelts—away from the main current and toward an alternative fishway in Ume River, a large regulated river in northern Sweden. We used both acoustic telemetry and sonar to measure the guiding effect of the bubble barrier for downstream migrating fish. We found that more than twice as many salmonids chose the alternative fishway when the bubble barrier was turned on. This was true both for smolts and kelts, suggesting that bubble barriers can be used to guide salmonids of different life stages in rivers with flow rates over 500 m3 s−1. Indeed, our study indicates that bubble barriers are low‐cost structures that could be rapidly applied in many regulated rivers to support salmonid migration.

Reproduction parameters of hatchery reared grayling (Thymallus thymallus) females

European grayling (Thymallus thymallus) is an endangered salmonid fish species and its populations have decreased considerably in European waters in recent decades. One of the conservation measures applied to grayling is stocking with hatchery-reared fish. To advance the knowledge of grayling reproduction in the hatchery, husbandry data were systematically collected in the period from 2006 to 2015 during grayling spawning seasons and analyzed to quantify grayling female reproductive parameters such as fecundity, egg size, and embryo survival to the eyed egg stage. Data was collected for first time spawners (3 years old) and repeat spawners reared in the hatchery. Statistically significant differences were found for the number of eggs between first time spawners, producing on average 3600 eggs, and repeat spawners, producing on average 8000 eggs. Absolute fecundity was positively correlated to grayling female total length and weight for both first time and repeat spawners. However, relative fecundity did not correlate with these parameters. A positive correlation was observed between female total length and mean egg weight for repeat spawners, while the correlation was negative for first time spawners. Heavier eggs tended to have a higher rate of survival until the eyed egg stage, though the correlation was stronger for first time spawners than for repeat spawners. Due to the higher reproductive success of repeat spawners, it could be concluded that larger and older grayling females should be included in hatchery reproduction.

Maternal spawning experience and thermal effects on offspring viability of chub mackerel and their influence on reproductive success

Maternal effects on offspring viability significantly contribute to the productivity and resilience of exploited marine fish populations. The temporal trend of biomass of chub mackerel (Scomber japonicus) in the western North Pacific showed cyclic fluctuation patterns, but it has been rapidly increasing since 2011. Although there are several assumptions regarding the mechanisms of the population dynamics of this species in these waters, there is a contradiction in the effects of thermal environmental conditions on recruitment success. We explored the effects of maternal age and/or previous spawning experience on egg and offspring traits of captive chub mackerel in response to thermal conditions. Moreover, from the relationship between the growth rate and temperature experienced by captive fish, the experienced temperatures of wild specimens were assessed. The sizes of eggs spawned from females aged 1–3 years were negatively correlated with temperature, but the effects of maternal age on egg size and nutrition were significant with larger and nutrient-rich eggs spawned from 2- or 3-year-old females (repeat spawners) than those from 1-year-old females (first-time spawners) at a given temperature. Such temperature- and maternal-age-dependent egg traits reflected the standard length of first-feeding larvae. The effects of maternal age on the starvation tolerance of larvae after hatching and body sizes of first-feeding larvae were also evident in larvae derived from 3-year-old females showing longer starvation tolerance and larger body heights than those from 1-year-old females. Further, larvae derived from 3-year-old females grew significantly faster than those from 1-year-old females at the tested temperatures. The process of ontogenetic development and growth rate of captive specimens until age 25 days were positively correlated with temperature. Meanwhile, the estimated temperatures experienced by wild specimens until age 25 days ranged from 18.0 to 20.3°C. These findings suggested that there were opposing thermal responses between maternal and offspring traits of chub mackerel, but the temperatures experienced by the main recruit fish may be restricted within the range of temperatures available for spawning. We discuss the mechanisms of reproductive success of this species in accordance with the maternal spawning experience and thermal effects on life history traits.

Feeding after spawning and energy balance at spawning are associated with repeat spawning interval in steelhead trout

Consecutive and skip repeat spawning (1- or ≥2-year spawning interval) life histories commonly occur in seasonally breeding iteroparous fishes. Spawning interval variation is driven by energetic status and impacts fisheries management. In salmonids, energetic status (either absolute level of energy reserves or the rate of change of energy reserves, i.e., energy balance) is thought to determine reproductive trajectory during a critical period ∼1 year prior to initial spawning. However, information on repeat spawners is lacking. To examine the timing and the aspects of energetic status that regulate repeat spawning interval, female steelhead trout (Oncorhynchus mykiss) were fasted for 10 weeks after spawning and then fed ad libitum and compared to ad libitum fed controls. Plasma growth hormone (GH) and insulin-like growth factor-I (IGF-I) levels were measured to assess long-term energy balance. Plasma estradiol levels showed that some fish in both groups initiated a consecutive spawning cycle. In fasted fish, GH was lower at spawning in consecutive versus skip spawners. In consecutive spawners, GH was higher at spawning in fed versus fasted fish. These results suggest that fish with a less negative energy balance at spawning initiated reproductive development in the absence of feeding, but that feeding during the post-spawning period enabled initiation of reproduction in some fish with a more negative energy balance at spawning. Thus, both energy balance at spawning and feeding after spawning regulated reproductive schedules. These results show that the critical period model of salmonid maturation applies to regulation of repeat spawning, and that the reproductive decision window extends into the first 10 weeks after spawning.

Iteroparity and its contribution to life-history variation in Atlantic salmon

Evolution of iteroparity is shaped by the trade-off between current and future reproduction. We studied variation in iteroparity among 205 050 individual Atlantic salmon caught in 179 rivers spanning 14° of latitude. The proportion of repeat spawners (iteroparous individuals) averaged 3.8% and ranged from 0% to 26% across rivers. Females were more often repeat spawners than males and had lower cost of reproduction in terms of lost body mass between spawning events. Proportion of repeat spawners for a given sea age at maturity, and the ratio of alternate to consecutive repeat spawners, increased with increasing population mean sea age at maturity. By combining smolt age, sea age at maturity, and age at additional spawning events, we identified 141 unique life-history types, and repeat spawners contributed 75% of that variation. Our results show that repeat spawners are important for life-history variation and suggest that the association between mean sea age and the frequency of repeat spawning is adaptive rather than a pleiotropic side effect arising from selection on sea age.

Variability in straying behaviour among repeat spawning anadromous brown trout (Salmo trutta) followed over several years

Abstract Natal homing, a strategy in which individuals return to breed in the same river as they were born, is prevalent among salmonids. However, some individuals may not return to their natal river, and instead stray to non-natal rivers. To date, there is limited documentation on patterns of straying among iteroparous salmonids that have returned to spawn over multiple years. In this study, 21538 out-migrating juvenile anadromous brown trout (Salmo trutta L.) were tagged over two years with passive integrated transponder (PIT) tags, and followed as they returned to either their natal or three non-natal rivers in a fjord system, over multiple years. In total, 206 individuals were detected returning for more than one year, of which 43% were strayers. A divergence in migratory strategies among strayers was documented: a smaller proportion of strayers were only detected in non-natal rivers, while the majority (67%) displayed high variability in their migratory patterns as they were detected in both their natal and non-natal rivers, sometimes within the same reproductive season. This study documents high variability in the migratory patterns of repeat spawners and high probability for them to stray, and suggests that straying is an important life-history strategy possibly affecting individual fitness.

Time series covering up to four decades reveals major changes and drivers of marine growth and proportion of repeat spawners in an Atlantic salmon population.

Wild Atlantic salmon populations have declined in many regions and are affected by diverse natural and anthropogenic factors. To facilitate management guidelines, precise knowledge of mechanisms driving population changes in demographics and life history traits is needed.Our analyses were conducted on (a) age and growth data from scales of salmon caught by angling in the river Etneelva, Norway, covering smolt year classes from 1980 to 2018, (b) extensive sampling of the whole spawning run in the fish trap from 2013 onwards, and (c) time series of sea surface temperature, zooplankton biomass, and salmon lice infestation intensity.Marine growth during the first year at sea displayed a distinct stepwise decline across the four decades. Simultaneously, the population shifted from predominantly 1SW to 2SW salmon, and the proportion of repeat spawners increased from 3 to 7%. The latter observation is most evident in females and likely due to decreased marine exploitation. Female repeat spawners tended to be less catchable than males by anglers.Depending on the time period analyzed, marine growth rate during the first year at sea was both positively and negatively associated with sea surface temperature. Zooplankton biomass was positively associated with growth, while salmon lice infestation intensity was negatively associated with growth.Collectively, these results are likely to be linked with both changes in oceanic conditions and harvest regimes. Our conflicting results regarding the influence of sea surface temperature on marine growth are likely to be caused by long‐term increases in temperature, which may have triggered (or coincided with) ecosystem shifts creating generally poorer growth conditions over time, but within shorter datasets warmer years gave generally higher growth. We encourage management authorities to expand the use of permanently monitored reference rivers with complete trapping facilities, like the river Etneelva, generating valuable long‐term data for future analyses.

Effect of egg size on the growth rate and survival of wild walleye pollock Gadus chalcogrammus larvae

AbstractTo clarify the survival process of walleye pollock larvae of the Japan Pacific stock in relation to the individual egg size, we estimated the original relative egg diameter from the diameter of the hatch check formed on larval sagittal otoliths. Significant positive correlations were obtained between egg diameter and embryonic body length, yolk sac volume, and lapillar otolith diameter in 3‐year groups (2013–2015 year classes) collected with a plankton net in Funka Bay. Embryos before hatching were without a check; however, checks were observed on all larval otoliths just after hatching. The check diameters showed significant positive regressions to age at collection (days after hatching) in 2014 and 2015 year classes, however not in the 2013 year class. Diameters of eggs collected with a ring net in Funka Bay gradually increased from 2013 to 2015 year classes. During the 2013–2015 spawning seasons, the proportion of adult females of the 2007 and 2009 year classes, which had relatively high abundances, gradually increased to higher proportions with age (43%, 57%, and <73%, respectively), indicating that the mechanism by which the eggs gradually became larger. The low water temperature experienced by the spawners ca. 2 months earlier, advancement in spawner age, and increase of repeat spawners (at least twice in their life time) would contribute to the increase in egg size and growth and survival rate of walleye pollock larvae.

Changes in Red Color Sensitivity over the Spawning Cycle of Female Three-spined Stickleback (Gasterosteus aculeatus).

Male red nuptial coloration is a primary mating signal for three-spined sticklebacks (Gasterosteus aculeatus), and the retinae of both sexes are especially sensitive to red during the breeding season. Red sensitivity is an important aspect of female mate choice in this species, but only when they are ready to spawn and not over the entire breeding period. Here, we aimed to determine if the red sensitivity of female sticklebacks change over their repeat spawning cycle. To this end, we assessed retinal opsin mRNA levels and behavioral red sensitivity in females over this cycle. Both methods indicated that females were more sensitive to red during spawning than in the inter-spawning intervals. Relative expression levels of red color opsin genes (lws) and optical motor sensitivity were high during spawning, decreased after the spawning period, and then increased again 72-96 h later when they were ready to spawn again. Thus, female sticklebacks altered their color sensitivity according to need, but the underlying mechanism remains unclear.

Life history contrasts in nutritional state and return probability of post-spawned Atlantic salmon

Using nonlethal tissue biopsies, we investigated somatic energy, lipid composition, and condition in post-spawned Atlantic salmon (Salmo salar) (kelts, n = 69) returning to sea in the spring and explored contrasts as a function of previous migration and spawning history, length and sex. Using mark–recapture analysis, we also explored linkages between spawning history and probability of future repeat spawning. Lipid density was significantly higher in previously spawned individuals compared to first-time spawned fish and in females compared to males. Fatty acid composition differed significantly among spawning histories. Return rates to consecutive spawning the following year were consistent with spawning history-dependent differences in lipid density, as first-time spawned kelts had lower return rates compared to consecutive repeat spawned kelts. We suggest that spawning history related contrasts in nutritional state in post-spawned Atlantic salmon may be a carry-over effect of differences in the feeding habitat as affected by spawning-history-dependent migration strategies. Alternatively, it may represent an adaptive response to increased survival and recovery potential with age.

Spawning runs of naturalised rainbow and brown trout from Lake Otamangakau, central North Island, New Zealand

ABSTRACT This study describes some of the characteristics of the spawning runs of rainbow and brown trout from Lake Otamangakau, into Te Whaiau Stream. More than 55,000 fish undergoing spawning migrations were enumerated in a fish trap between 1994 and 2013. During that period, the spawning run size increased for both species, but particularly for rainbow trout, which exhibited a concurrent decline in average weight. The timing of the runs was consistent across both species, sexes, and maturation statuses but was affected by the average weight of the fish and the run size. The level of iteroparity revealed that 40% and 55% of the runs were made up of previous spawner rainbow and brown trout, respectively. Detections of PIT-tagged fish at the trap indicated that rainbow and brown trout could spawn up to six and eight times, respectively. Remarkably, repeat spawning fish of both species and sexes returned within a few days of the same date each year over consecutive spawning events. Rainbow and brown trout grew substantially after spawning up to the 3rd and 4th spawning, respectively. The results of this study are relevant for informing management of the popular Lake Otamangakau trout fishery, in particular achieving a balance between satisfactory CPUE and large fish size.

Low impact of first-time spawners on population growth in a brown trout population

For species with individual variation in reproductive success, experience in breeding and the distribution of different breeders is important for population productivity and viability. Human impacts, such as climate change and harvesting, can alter this distribution and thus population dynamics. Here, we investigated the effect of spawning experience on population growth in a population of migratory brown troutSalmo truttasubject to stressors including migration barriers, harvesting, and climate change. We described the population dynamics with a structured integral projection model that differentiates between first-time and repeat spawners. We then took a scenario-based approach to test to which extent spawning experience has a positive effect on the population growth of brown trout by running 3 different model simulations: a baseline scenario with no changes to the reproductive output of the population, a non-selective scenario in which the reproductive output of all spawners was reduced, and a selective scenario where the reproductive output of only first-time spawners was reduced. We found that the reproductive output of repeat spawners is more important than that of first-time spawners for population growth, in line with other studies. Moreover, the contribution of first-time spawners to the population growth through their own survival is more important than their contribution to growth through reproduction. To ensure the continued existence of the study population, survival of first-time spawners and reproductive success of repeat spawners should be prioritised. More generally, including breeding experience adds more mechanistic detail, which ultimately can aid management and conservation efforts.

STUDY OF THE CAUSES OF VARIATION IN THE PROPORTIONS OF RE-SPAWNING INDIVIDUALS IN ATLANTIC SALMON POPULATIONS (SALMO SALAR (L., 1758))

The Atlantic salmon belonging to populations of four rivers in the Murmansk Кegion – the Kola, the Tuloma, the Ponoy and the Varzuga, was the object of the study. The paper investigates the reasons for the population dynamics of re-spawning individuals in the spawning salmon populations in the said rivers by searching for statistically significant relationships between the proportion of re-spawning individuals and abiotic factors. Correlation analysis was used as the mathematical research tool. The authors considered their own and literature data on the timing of downstream migration of kelts and their return to repeat spawning, as well as the age structure of the re-spawning subpopulation. Comparison of interannual changes in the proportion of re-spawning individuals in different populations did not reveal any synchronism in the dynamics of this indicator. This fact suggests there exist factors specific to each population that regulate the number of re-spawning salmon individuals. Further analysis showed that in some cases the proportion of re-spawning individuals demonstrated a weak correlation with water temperature in the sea, in other cases – with temperature and water level in the river. In particular, water level has great influence on the proportion of re-spawning individuals in the Kola and Varzuga rivers, but this influence is alternating: while in the Kola River the number of re-spawning individuals in Atlantic salmon populations is increased by a high level of flood water, in the Varzuga River, on the contrary, it is decreased by this factor. The explanation lies in the different hydrological characteristics of the rivers: in the shallow Varzuga, high flood leads to ice clogging, which can cause a mass death of kelts. The number of re-spawning salmon depends on the overall state of reproduction: the higher the risk of anthropogenic impact, the smaller the relative number of re-spawning individuals, and the simpler the age structure of the spawning stock.

The various ways that anadromous salmonids use lake habitats to complete their life history

Despite the preponderance of exorheic lakes in rivers home to anadromous salmonids, little research has focused on how salmon, trout, and char use lakes as part of their anadromous life histories. The literature on this subject has so far revealed that some parr move into lakes to feed and grow before smoltification but that smolts moving through lakes tend to have high mortality in disproportion to what is observed in other habitats they migrate in or through. Adults have been observed using lakes for behavioural thermoregulation prior to spawning, and kelts of iteroparous species often exploit lakes to overwinter before returning to sea to recondition. We summarized knowledge on lakes as salmonid habitat and identified knowledge gaps about the use of lakes by anadromous salmonids related to whether lakes are barriers that structure genetics of populations, whether mortality in lakes is compensatory or additive, and whether systems with lakes have higher rates of repeat spawning among iteroparous salmonids. Human activities that alter lakes require further study to understand how changes in temperature, oxygen, ice, or circulation affect navigation and fate.

Can Energy Depletion of Wild Atlantic Salmon Kelts Negotiating Hydropower Facilities Lead to Reduced Survival?

Repeat spawners constitute an important component of Atlantic salmon populations, but survival of post-spawning individuals (kelts) are often compromised by anthropogenic structures such as hydropower plants (HPPs). Potential effects of HPPs include migration delays and associated increased energy depletion, which potentially results in increased overall mortality. We combined a detailed 3D hydraulic model with high-resolution 3D tracking of tagged kelts (length 73–104 cm) to obtain estimates of kelt movement through water. These estimates were then used in an energetics model to estimate hourly energy expenditure while negotiating the HPP area. Hourly kelt energy expenditure varied between 0.8 and 10.1 kJ × h−1 and was dependent on kelt length. Degree of additional energy depletion can amount to several percent of remaining energy content (our study indicates 4–5 percentage points) potentially leading to reduced post-spawning survival. In turn, this can nullify the iteroparous breeding strategy and jeopardize long-term stability and persistence of Atlantic salmon populations inhabiting HPP rivers.

Synergistic Combination of Exogenous Hormones to Improve the Spawning and Post-spawning Survival of Female Yellow Catfish.

Multiple repeat spawners make large contributions to long-term population stability and aquaculture breeding programs. A high percentage of female yellow catfish (Pelteobagrus fulvidraco) died for spawning failure or incomplete spawning after artificial spawning by traditional synthetic hormones including human chorionic gonadotropin (hCG), luteinizing hormone releasing hormone (LHRH), and domperidone (DOM). The present study was designed to compare the efficacy of different combinations of exogenous hormones for inducing ovulation in yellow catfish using hCG, LHRH, DOM, and carp pituitary extraction (CPE). We found a optimal strategy for exogenous hormones administration, the mixture of LHRH/CPE for the first injection and LHRH/CPE/DOM for the second injection, could greatly improve the rates of spawning success, weight of ovulated eggs and survival rate after spawning. Interestingly, a population of female yellow catfish with defective reproductive duct could not spawn and showed high mortality after induced by a combination of hCG/LHRH/DOM, whereas a synergistic combination of hCG, LHRH, DOM, and CPE could efficiently induce spawning and reduce mortality in the defective yellow catfish, in which a significant decrease of Vitellin and E2 levels. Altogether, our findings provide an effective combination of exogenous hormones to improve spawning and post-spawning survival of female yellow catfish.

Effects of non-spawning season (yolkless to early yolk formation stage) temperature on growth and reproductive characteristics in hatchery-reared, repeat-spawning walleye pollock Gadus chalcogrammus

The effects of water temperature during the non-spawning season (yolkless to early yolk formation stage of the ovary) on the growth and reproductive characteristics of 4-year-old hatchery-reared walleye pollock Gadus chalcogrammus (repeat spawners) were examined. Fish were kept in tanks under two temperature regimes (non-controlled: 6.4–17.2 °C; cooled: 5.1–15.7 °C) from 42 to 46 months post-hatching (non-spawning period) and showed differences in growth (in terms of weight and condition factor) shortly before the spawning period (mostly at the early yolk formation stage of the ovary). The fish spawned spontaneously from 47 to 51 months post-hatching at 5 °C. Growth differences were related to egg diameter but not to the timing of spawning. Larger eggs were spawned from fish that had greater increases in weight and condition during the non-spawning season. In contrast, the main spawning period did not vary according to the non-spawning season treatment. These results were compared with the spawning records of 2- and 3-year-old walleye pollock from two previous years, showing that the timing of spawning advances as fish size increases with age. In addition, spawning experience and the amount of energy acquired before the spawning period likely influence egg size.

Response of Out‐Migrating Adult American Shad Guided by Spill and Ultrasound as They Approach Rivière‐des‐Prairies Hydroelectric Generating Station (Québec, Canada)

AbstractFish entrainment and mortality at hydropower generating stations constitute a major concern for power producers and government agencies. The American Shad Alosa sapidissima is particularly at risk at higher latitudes because of the large percentage of repeat spawners and the vulnerability of many populations. Total or partial turbine shutdowns at powerhouses and concurrent increased spillage are generally considered an effective way to enable fish to bypass the turbines and to reduce associated mortality, but this strategy is unappealing to hydropower producers because of the loss of power generation revenue. Two approaches were tested to repel out‐migrating adult American Shad from the turbine intakes of the Rivière‐des‐Prairies Generating Station powerhouse and guide them toward the adjacent spillway. The first approach consisted of shutting down the powerhouse and increasing spillage. The second approach consisted of testing the effectiveness of an ultrasonic field to deter American Shad from getting close to turbine intakes and to guide them toward the partially opened spillway. On average, fish density observed in front of the 54‐MW powerhouse was reduced by 93% as a result of a powerhouse shutdown lasting about 1 h and a concurrent spillage increase. Similarly, our results showed a decrease in shad abundance of 78–90% in front of the turbine intakes when a pulsed ultrasonic field (125 kHz) was activated. Measurements of sound pressure levels (SPLs) in the forebay suggested that the deterrent effect could be perceived by American Shad more than 100 m away, before they could sense higher water velocities near the powerhouse intakes. These results indicate that the combination of high‐SPL pulsed ultrasounds (125 kHz) and the concurrent opening of adjacent spillway gates could guide postspawn American Shad away from powerhouse intakes.