Young-of-the-year Research Articles

Spatial distribution of four freshwater fish species in different types of artificial European water bodies

Spatial distribution of young-of-the-year (YOY) and older roach, rudd, perch and ruffe was compared in two artificial lakes with macrophytes present and absent, and a valley reservoir, using gillnets. Almost all species of interest and both age categories preferred benthic habitats. The depth distribution in benthic habitats was relatively consistent across water bodies with the highest fish densities found in the shallowest depths. In the macrophyte-rich lake, YOY roach and perch utilize the 3–6 m benthic layer the most, whereas the fish preferred the 0–3 m benthic layer in the macrophyte-poor lake and reservoir. No differences were found in the depth distribution in pelagic habitats sampled by pelagic gillnets for YOY fish between the water bodies. Older fish usually utilized the surface water layer. Macrophytes influenced the depth distribution of YOY fish in benthic habitats, where their density maximum shifted deeper in the macrophyte-rich lake when fewer macrophytes were present in the shallowest benthic depth. In lakes, YOY fish utilized a wider depth spectrum due to the deeper thermocline when compared to the reservoir. Oxygen and temperature stratification are the main factors influencing fish distribution, whereas macrophyte presence particularly influences the depth distribution of YOY fish in benthic habitats.

Development of a Captive Rearing Protocol for Threatened & Endangered Appalachian Crayfish

Crayfish are the third most threatened taxa worldwide. In 2016, two Appalachian endemic species were federally listed by the United States Fish & Wildlife Service: Cambarus callainus (Big Sandy Crayfish) and Cambarus veteranus (Guyandotte River Crayfish), which are listed as threatened and endangered respectively. Both species were listed due to limited and declining ranges caused by various anthropogenic activities, especially those causing stream sedimentation. Captive propagation can be used as a tool for crayfish conservation by helping to restore the native range of a species or improving their fecundity within their current range. Over a ten week period 120 young-of-the-year (YOY) Cambarus chasmodactylus (New River Crayfish), a surrogate species for C. callainus and C. veteranus, were raised in individual cells to compare (1) growth and (2) survival on two different diets. They were fed every other day, with half (60) raised on trout diet pellets (TD) and the other half raised on blood worms (BW). Results showed that the YOY grew more on BW (17.3% growth) than TD (13.6% growth), and there was increased survival with BW (84.5% survival) versus TD (70.5% survival). These findings may be due in part to the fact that blood worms are easier for the crayfish to eat and may have increased nutritional value over TD. This study suggest a BW diet is more effective in captive rearing of crayfishes than TD, and future studies should explore additional dietary needs.

Estimating the effective number of breeders from single parr samples for conservation monitoring of wild populations of Atlantic salmon Salmo salar.

This study assesses whether the effective number of breeders (Nb ) can be estimated using a time and cost-effective protocol using genetic sibship reconstruction from a single sample of young-of-the-year (YOY) for the purposes of Atlantic salmon Salmo salar population monitoring. Nb was estimated for 10 consecutive reproductive seasons for S. salar in the River Nivelle, a small population located at the rear-edge of the species distribution area in France, chronically under its conservation limit and subjected to anthropogenic and environmental changes. Subsampling of real and simulated data showed that accurate estimates of Nb can be obtained from YOY genotypes, collected at moderate random sampling intensity, achievable using routine juvenile electrofishing protocols. Spatial bias and time elapsed since spawning were found to affect estimates, which must be accounted for in sampling designs. Nb estimated in autumn for S. salar in the River Nivelle was low and variable across years from 23 (95% C.I. 14-41) to 75 (53-101) and was not statistically correlated with the estimated number of returning adults, but it was positively correlated with the estimated number of YOY at age 9months. Nb was found to be lower for intermediate levels of redd aggregation, suggesting that the strength of the competition between males to access females affects reproductive success variance depending on redd spatial configuration. Thus, environmental factors such as habitat availability and quality for spawning and YOY development predominate over demographic ones (number of returning adults) in driving long-term population viability for S. salar in the River Nivelle. This study showcases Nb as an integrated parameter, encompassing demographic and ecological information about a reproductive event, relevant to the assessment of both short-term effects of management practices and long-term population conservation status.

Intensified environmental and density-dependent regulation of white perch recruitment after an ecosystem shift in the Hudson River Estuary

Long-term monitoring data were used to test whether the invasion of zebra mussels (Dreissena polymorpha) in the Hudson River Estuary in 1991 altered the influence of density dependence and environmental conditions on life-stage transitions, growth, and partial migration in white perch (Morone americana). During the post-invasion period (1992–2013), we estimated standing stocks of white perch eggs, yolk-sac larvae (YSL), post-yolk-sac larvae (PYSL), young-of-the-year (YOY), and adults as well as indices of YOY growth and spatial distribution. A series of linear and nonlinear functions were employed to model life-stage transitions, while the effects of six environmental and density-dependent variables on YOY growth and partial migration were quantified. Comparisons to pre-invasion observations (1974–1991) indicated that egg–YSL, PYSL–YOY, and YOY–yearling transitions changed significantly after the invasion, while PYSL abundance developed a stronger negative effect on YOY growth. The PYSL–YOY transition became more sensitive to density dependence and freshwater flow from 1992 to 2013, which is consistent with diminished abundance and increased environmental sensitivity of the forage base in the Hudson River Estuary reported after the zebra mussel invasion.

Fisheries-independent surveys identify critical habitats for young scalloped hammerhead sharks (Sphyrna lewini) in the Rewa Delta, Fiji

Sharp declines in numerous shark populations around the world have generated considerable interest in better understanding and characterising their biology, ecology and critical habitats. The scalloped hammerhead shark (SHS, Sphyrna lewini) is subject to a multitude of natural and anthropogenic threats that are often exacerbated within the coastal embayments and estuaries used during SHS early life stages. In this study, we describe the temporal and spatial distribution, age class composition, and reproductive biology of SHS in the Rewa Delta (RD), Fiji. A total of 1054 SHS (including 796 tagged individuals; 101 of which were recaptured) were captured from September 2014 to March 2016 in the RD. A majority of the captures in this area were neonates and young-of-the-year (YOY) (99.8%). Significant seasonality in patterns of occurrence of both neonates and YOY individuals suggests a defined parturition period during the austral summer. Between the seven sampling sites in the RD we also found significant differences in SHS neonate catch per unit of effort, and average total length of individuals. According to the data, the RD is likely to represent an important nursery area for SHS up to one year of age.

Natal origin identification of Pacific bluefin tuna (Thunnus orientalis) by vertebral first annulus

Pacific bluefin tuna (Thunnus orientalis) has two spawning grounds, and each is used at different times: May–June in the Nansei Islands of western North Pacific Ocean and July–August in the Sea of Japan. Although previous studies developed body-length based methods to differentiate between individuals originating in each spawning ground, these methods are only applicable to the young-of-the-year (YOY). In this study, we evaluate a simple technique for discerning the natal grounds of Pacific bluefin tuna greater than one year in age by observing the first annulus on the vertebrae of fish caught around Japan. We found a significant linear relationship between fork length and vertebral radius, and determined that the first annulus forms in boreal winter. Both the back-calculated fork length at the time of first annulus formation and the measured fork length of YOY in February and March had a bimodal frequency distribution, and the peaks of the two frequency distributions had similar means. The larger fork length individuals appear to have originated near the Nansei Islands, whereas the smaller fork length individuals appear to have originated in the Sea of Japan. This is because PBF hatched around the Nansei Islands will have longer growth time for the YOY to develop the first annulus at longer distance from the vertebrae center. Similarly, we observed bimodal distributions for the vertebral first annulus radius of mature Pacific bluefin tuna (>120cm) in both spawning grounds. Our technique offers improved sample treatment efficiency and lower costs. In addition, this method may help reveal the contribution rate of each spawning ground.

Dispersal and density-dependent growth of Atlantic salmon (Salmo salar) juveniles: clumped versus dispersed stocking

Dispersal from nesting sites and habitat selection are essential for the fitness of young individuals and shape the distribution, growth, and persistence of populations. These processes are important to consider when releasing young, hatchery-origin fishes into the wild to restore extirpated or depleted populations. By manipulating the density of released young-of-the-year (YOY) Atlantic salmon (Salmo salar), we evaluated the effects of clumped- (releasing all the fish at one location) and dispersed-stocking (releasing the fish evenly over a complete reach) treatments on juvenile habitat use, dispersal, growth, and survival. Across 14 river reaches, clump-stocked YOY density decreased and growth rate increased with distance downstream, whereas dispersed-stocked YOY densities and growth were relatively constant. Overall, density, spatial variance in density, growth, and survival did not differ between these two stocking treatments, likely due to the greater-than-expected mobility of fish in clumped-stocking reaches; YOY dispersed up to 1600 m, with 41% moving over 200 m downstream. As predicted from previous work, growth rate of individual fish was density-dependent, following a negative power curve. Our results provide insights into how the growth and survival of released individuals are altered via stocking treatments, ultimately shaping their distribution and growth rate.

Empirically based models of oceanographic and biological influences on Pacific Herring recruitment in Prince William Sound

Abundances of small pelagic fish can change dramatically over time and are difficult to forecast, partially due to variable numbers of fish that annually mature and recruit to the spawning population. Recruitment strength of age-3 Pacific Herring (Clupea pallasii) in Prince William Sound, Alaska, is estimated in an age-structured model framework as a function of spawning stock biomass via a Ricker stock-recruitment model, and forecasted using the 10-year median recruitment estimates. However, stock size has little influence on subsequent numbers of recruits. This study evaluated the usefulness of herring recruitment models that incorporate oceanographic and biological variables. Results indicated herring recruitment estimates were significantly improved by modifying the standard Ricker model to include an index of young-of-the-year (YOY) Walleye Pollock (Gadus chalcogrammus) abundance. The positive relationship between herring recruits-per-spawner and YOY pollock abundance has persisted through three decades, including the herring stock crash of the early 1990s. Including sea surface temperature, primary productivity, and additional predator or competitor abundances singly or in combination did not improve model performance. We suggest that synchrony of juvenile herring and pollock survival may be caused by increased abundance of their zooplankton prey, or high juvenile pollock abundance may promote prey switching and satiation of predators. Regardless of the mechanism, the relationship has practical application to herring recruitment forecasting, and serves as an example of incorporating ecosystem components into a stock assessment model.

Identification of young-of-the-year great hammerhead shark Sphyrna mokarran in northern Florida and South Carolina.

Two sharks, visually identified in the field as young-of-the-year (YOY) scalloped hammerhead Sphyrna lewini, were identified as great hammerhead Sphyrna mokarran based on nuclear-encoded single nucleotide polymorphisms (SNP) and sequences of mtDNA. Individuals were captured and released in Bulls Bay, SC, and Saint Joseph Bay, FL, in 2013 and 2014, respectively. These findings indicate S. mokarran may be pupping in or around these areas and highlight new regions that may be a productive focus for future research on early life history of S. mokarran.

Philometra rubra (Nematoda: Philometridae) — first description of the male from striped bass (Morone saxatilis) and implications for re-introduction of an extirpated population

Philometra rubra (Leidy, 1856) is a pathogenic parasitic nematode infecting striped bass (Morone saxatilis (Walbaum, 1792)) in eastern North America. Identification of philometrids is difficult, male P. rubra have never been found, and transmission is poorly understood. To re-establish the “Extirpated” striped bass population in the St. Lawrence Estuary (SLE), Quebec, young-of-the-year (YOY) striped bass from the Miramichi River, New Brunswick, were collected and released into the SLE. The present study was undertaken to find Philometra in striped bass from the Miramichi River, for identification, description, and examine transmission and lesions. Philometra rubra was identified in 100% of 32 YOY (intensity = 26–125) and 100% of nine 1-year-old (intensity = 4–25 in five striped bass) striped bass collected in summer 2011. The swim bladder and body cavity were sites of infection in YOY with no tissue response observed histopathologically. Gravid P. rubra females occurred in the body cavity and swim bladder, with males predominately in the latter site. Males were short-lived, being absent in 1-year-old bass. Adult male and adult female P. rubra are described. Males of P. rubra can be distinguished from other described species based on spicule, gubernaculum, and body-length measurements. This parasite may have implications for the re-introduction program and conservation of striped bass in the SLE and in other populations.

Maturation at a young age and small size of European smelt (Osmerus eperlanus): A consequence of population overexploitation or climate change?

Age of fish at maturation depends on the species and environmental factors but, in general, investment in growth is prioritized until the first sexual maturity, after which a considerable and increasing proportion of resources are used for reproduction. The present study summarizes for the first the key elements of the maturation of European smelt (Osmerus eperlanus) young of the year (YoY) in the North-eastern Gulf of Riga (the Baltic Sea). Prior to the changes in climatic conditions and collapse of smelt fishery in the 1990s in the Gulf of Riga, smelt attained sexual maturity at the age of 3–4 years. We found a substantial share (22%) of YoY smelt with maturing gonads after the collapse of the smelt fisheries. Maturing individuals had a significantly higher weight, length and condition factor than immature YOY, indicating the importance of individual growth rates in the maturation process. The proportion of maturing YoY individuals increased with fish size. We discuss the factors behind prioritizing reproduction overgrowth in early life and its implications for the smelt population dynamics.

Decrease in young-of-the-year yellow perch growth rates following Bythotrephes longimanus invasion

Bythotrephes longimanus, an invasive zooplankter from Eurasia, has caused severe declines in native zooplankton communities in Rainy and Kabetogama lakes in northern Minnesota. Both lakes have experienced a 40–60% decrease in peak summer zooplankton biomass following B. longimanus establishment around 2006–2007. In these lakes, yellow perch (Perca flavescens) are a key fishery species, and young-of-the-year (YOY) yellow perch are mainly planktivorous during their first summer. This led to concern that their growth could be detrimentally affected by the depletion of zooplankton forage. We used seining data to compare growth rates of YOY yellow perch before (2001–2005) and after (2008–2012) B. longimanus establishment in Rainy and Kabetogama lakes. Nearby Lake Vermilion, assumed to have been unaffected by B. longimanus during this time period, was used as a reference for natural variation in YOY growth in the region. YOY yellow perch length was modeled as a linear function of cumulative growing degree days (GDD) throughout the summer, and the slope of the relationship was compared between pre- and post-B. longimanus time periods for the three study lakes. The two lakes with B. longimanus showed similar decreases in YOY yellow perch growth rate relative to GDD, whereas Lake Vermilion showed no evidence of a decline in growth rates during this period. The reduction in growth rates resulted in an approximate 10% decrease in mean length of YOY yellow perch at the end of the summer after B. longimanus establishment, which could lead to further effects of this invasive zooplankter at higher trophic levels.

Who needs the hotspot? The effect of temperature on the fish host immune response to Tetracapsuloides bryosalmonae the causative agent of proliferative kidney disease

Proliferative kidney disease (PKD) of salmonids, caused by Tetracapsuloides bryosalmonae may lead to high mortalities at elevated water temperatures. However, it has not yet been investigated how temperature affects the fish host immune response to T. bryosalmonae. We exposed YOY (young of the year) rainbow trout (Oncorhynchus mykiss) to T. bryosalmonae at two temperatures (12 °C and 15 °C) that reflect a realistic environmental scenario and could occur in the natural habitat of salmonids. We followed the development of the parasite, host pathology and immune response over seven weeks. We evaluated the composition and kinetics of the leukocytes and their major subgroups in the anterior and posterior kidney. We measured immune gene expression profiles associated with cell lineages and functional pathways in the anterior and posterior kidney. At 12 °C, both infection prevalence and pathogen load were markedly lower. While the immune response was characterized by subtle changes, mainly an increased amount of lymphocytes present in the kidney, elevated expression of Th1-like signature cytokines and strong upregulation of the natural killer cell enhancement factor, NKEF at week 6 P.E. At 15 °C the infection prevalence and pathogen burden were ominously greater. While the immune response as the disease progressed was associated with a Th2-like switch at week 6 P.E and a prominent B cell response, evidenced at the tissue, cell and transcript level. Our results highlight how a subtle, environmentally relevant difference in temperature resulted in diverse outcomes in terms of the immune response strategy, altering the type of interaction between a host and a parasite.

Native brook trout and invasive rainbow trout respond differently to seasonal weather variation: Spawning timing matters

Abstract Salmonids have been introduced globally, and native and invasive salmonids co‐exist in many regions. However, their responses to seasonal weather variation and global climate change are poorly known. The aim of this study was to compare effects of inter‐annual variation in seasonal weather patterns on native brook trout (BKT) (Salvelinus fontinalis) versus invasive rainbow trout (RBT) (Oncorhynchus mykiss) abundance using summer electrofishing data (May through September) spanning 28 years in the Great Smoky Mountains National Park, U.S.A. (c. 200 stream sites per species). In particular, we tested if different spawning timing between BKT (autumn) and RBT (late winter) would result in heterogeneous population responses to high seasonal precipitation, which would negatively affect early life stages with impaired swimming ability. As predicted, young‐of‐the‐year (YOY) abundance of autumn‐spawning BKT was most strongly impacted by total precipitation between February and March, and RBT YOY abundance was most strongly impacted by peak precipitation between April and May. Despite the presence of these different key seasonal drivers, inter‐annual variation in YOY density of these two species was positively correlated because precipitation in April and May also impacted the abundance of BKT YOY. Adult abundance was less responsive to weather variation than YOY abundance, and was most strongly correlated with YOY abundance in the previous year, indicating the importance of flow‐driven population control influences on early life stages affecting population sizes into subsequent years. Adult BKT densities were not affected by any weather covariate, whereas adult RBT densities were correlated with four weather covariates in competing models. As a result, there was no correlation in the inter‐annual variation in adult density in these two species. The differing responses of BKT and RBT to long‐term seasonal weather patterns suggest that they will likely respond differently to global climate change. In particular, winter precipitation will likely be the key environmental driver of differences in their population responses.

Fine-scale partitioning of genomic variation among recruits in an exploited fishery: causes and consequences.

Conservation and management of exploited species depends on accurate knowledge of how genetic variation is partitioned across a fishery, especially as it relates to recruitment. Using double-digest restriction-site associated DNA sequencing, we surveyed variation in 7,382 single nucleotide polymorphisms (SNPs) in red snapper (Lutjanus campechanus) young-of-the-year (YOY) sampled at six localities and in adults sampled at two localities in the northern Gulf of Mexico. Significant genetic heterogeneity was detected between the two adult samples, separated by ~600 km, and at spatial scales less than five kilometers among samples of YOY. Genetic differences between YOY samples and between YOY samples and adult samples were not associated with geographic distance, and a genome scan revealed no evidence of loci under selection. Estimates of the effective number of breeders, allelic richness, and relatedness within YOY samples were not consistent with sweepstakes recruitment. Instead, the data demonstrate, at least within one recruitment season, that multiple pulses of recruits originate from distinct groups of spawning adults, even at small spatial scales. For exploited species with this type of recruitment pattern, protection of spawning adults over wide geographic areas may be critical for ensuring productivity and stability of the fishery by maintaining larval supply and connectivity.

Freshwater polychaetes (Manayunkia speciosa) near the Detroit River, western Lake Erie: Abundance and life‐history characteristics

Abstract Freshwater polychaetes are relatively rare and little-studied members of the benthos of lakes and rivers. We studied one polychaete species (Manayunkia speciosa) in Lake Erie near the mouth of the Detroit River. Abundances at one site were determined between 1961 and 2013 and life‐history characteristics at two sites were determined seasonally (March–November) in 2009–2010 and 2012–2013. Life‐history characteristics included abundances, length‐frequency distributions, presence/absence of constructed tubes, sexual maturity, and number and maturation of young-of-the-year (YOY) in tubes. Long-term abundances decreased in successive time periods between 1961 and 2003 (mean range = 57,570 to 2583/m2) but few changes occurred between 2003 and 2013 (mean = 5007/m2; range/y = 2355–8216/m2). Seasonal abundances varied substantially between sites and years, but overall, abundances were low in March–April, high in May–August, and low in September–November. Although reproduction was continuous throughout warmer months, en masse recruitment, as revealed by length–frequency distributions, occurred in a brief period late‐June to mid-July, and possibly in early-September. All life-history characteristics, including tube construction, were dependent on water temperatures (> 5 °C in spring and

Life history of the coal fields crayfish Cambarus theepiensis Loughman, Foltz, Garrison and Welsh, 2013 (Decapoda: Astacoidea: Cambaridae) in southwestern West Virginia, USA

Data of the life history of Cambarus ( Puncticambarus ) theepiensis Loughman, Foltz, Garrison and Welsh, 2013 was collected monthly from May 2012 through April 2013 from two sites within the Twelvepole Creek watershed in southwestern West Virginia, USA. Mature females showed signs of glair development in early May. Egg extrusion was noted in early June, with first- through third-stage juveniles occurring from July to August, before becoming free-living juveniles in late August and September. Evidence for possible overwintering with young was found as a mature female with free-living juveniles was collected in early April. Mature female total carapace length (TCL) ranged from 29.7-52.4 mm. Support for the hypothesis that form change occurs in female cambarid crayfishes was found in female C. theepiensis . Form I males were collected throughout the year, but reached their highest densities in May through August. Size cohorts, determined through histograms, demonstrate six size cohorts existed between both streams, following appearance of young of the year (YOY) in late summer. Cambarus theepiensis appears to have life history characteristics similar to other Cambarus species, displaying K-selected life history traits.

Young-of-the-year fish as a prospective bioindicator for aquatic environmental contamination monitoring

Toxic metals (Hg, Cd, Pb) and fifteen perfluoroalkyl substances (PFASs) were determined in different fish samples at two locations on the Elbe River in the Czech Republic. The muscle tissue of the two adult fish species most commonly used as bioindicators in central Europe and whole body homogenates of various species of young-of-the-year (YOY) fish were used. The purpose of this study was to evaluate the potential to replace adult fish muscle tissue with YOY fish for contamination monitoring. All of the toxic metals and five of the fifteen PFASs were found in the YOY fish samples while only mercury and PFOS were detected in the muscle tissue of adults. The concentration of total mercury (THg) in the YOY fish homogenates ranged between 0.014 and 0.062 μg g−1. Of the spectrum of analysed pollutants, only the THg concentrations were lower in YOY fish homogenates than in adult muscle tissue. The cadmium concentration varied from 0.004 to 0.024 μg g−1 and the lead concentration varied from 0.032 to 0.396 μg g−1 in YOY fish homogenates, while in most of the adult samples, Cd and Pb were below the detection limit of the analytical methods employed. The PFOS concentrations in YOY fish homogenates were comparable to the concentrations frequently found in adult liver tissue. These results show that mixed shoals of YOY fish can be successfully used for aquatic bio-monitoring. Interspecific variability in the concentrations of the target pollutants in YOY fish whole body homogenates is usually lower than the intraspecific variability of the concentrations of the pollutants in adult fish muscle. YOY fish were found to be a suitable bioindicator and have several advantages compared to adult fish.

Brandt's cormorant diet (1994–2012) indicates the importance of fall ocean conditions for northern anchovy in central California

AbstractEffective ecosystem‐based management requires a comprehensive understanding of the functional links in the system. In many marine systems, forage species constitute a critical link between primary production and upper trophic level marine predators. As top predators, seabirds can be indicators of the forage species they consume and the ocean processes that influence these populations. We analyzed the diet and breeding success for the years 1994, 2003, 2005, and 2007–2012 of the Brandt's cormorant (Phalacrocorax penicillatus), a piscivorous diving seabird, breeding in central California, to evaluate the extent to which cormorant diet composition relates to prey availability, and how diet composition relates to breeding success and ocean conditions. Cormorant diet was primarily composed of young‐of‐the‐year (YOY) northern anchovy (Engraulis mordax), YOY rockfish (Sebastes spp.), and several species of small flatfish (order Pleuronectiformes). YOY rockfish consumption was positively related to their abundance as measured in a late spring pelagic midwater trawl survey. Northern anchovy appeared to be the most important prey as its consumption was positively related to cormorant breeding success. More northern anchovy were consumed in years where warm‐water conditions prevailed in the fall season before cormorant breeding. Thus, warm ocean conditions in the fall appear to be an important contributing factor in producing a strong year‐class of northern anchovy in central California and consequently a strong‐year class of Brandt's cormorant on the Farallon Islands.

FishMORPH - An agent-based model to predict salmonid growth and distribution responses under natural and low flows.

Predicting fish responses to modified flow regimes is becoming central to fisheries management. In this study we present an agent-based model (ABM) to predict the growth and distribution of young-of-the-year (YOY) and one-year-old (1+) Atlantic salmon and brown trout in response to flow change during summer. A field study of a real population during both natural and low flow conditions provided the simulation environment and validation patterns. Virtual fish were realistic both in terms of bioenergetics and feeding. We tested alternative movement rules to replicate observed patterns of body mass, growth rates, stretch distribution and patch occupancy patterns. Notably, there was no calibration of the model. Virtual fish prioritising consumption rates before predator avoidance replicated observed growth and distribution patterns better than a purely maximising consumption rule. Stream conditions of low predation and harsh winters provide ecological justification for the selection of this behaviour during summer months. Overall, the model was able to predict distribution and growth patterns well across both natural and low flow regimes. The model can be used to support management of salmonids by predicting population responses to predicted flow impacts and associated habitat change.