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Map of Lake Chelan, showing the two lake basins, principal sampling sites (stars), and hydroacoustic transects (dotted lines). The inset shows the location of the lake in north-central Washington, USA.
Source publication
The 2007 Lake Chelan Fishery Plan charges fishery managers to prioritize the conservation and restoration of native species, while maintaining healthy recreational fisheries. Achieving these goals requires an accurate understanding of the relationships between the major predators and their prey in the lake. The aim of this study was to provide guid...
Contexts in source publication
Context 1
... Chelan is a deep (maximum depth 453 m), glacially-formed lake located in the Cascade Range in north-central Washington (48° N, 120° W; Figure 1). The lake is long and narrow (length 81 km, maximum width < 3 km), and is composed of two basins joined by a narrow channel. ...
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... targeted sampling was conducted in June, July, and August 2006, and in September 2008. Fish were captured with horizontal sinking gill nets at five fixed sites in the lake, two in Wapato Basin and three in Lucerne Basin (Figure 1). At each sampling site, each of four depth strata were sampled with one small-mesh net (2.5, 3.2, 3.8, 5.1, 6.4, and 7.6 cm stretched mesh) and one large-mesh net (8.9, 10.2, 11.4, 12.7, and 15.2 cm stretched mesh). ...
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... surveys were conducted during February, May, and November 2005 as well to characterize seasonal kokanee migration patterns and horizontal-vertical distribution. Each survey consisted of 21 transects in a zig-zag pattern across the full length of the lake (Figure 1). Hydroacoustic sampling was conducted from a 7-m aluminum boat with a Biosonics DE 6000 200-kHz system. ...
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... trout consumed substantial proportions of cyprinid prey in both lake basins, and most of this consumption occurred during spring (Table 4). The lengths of ingested prey fishes were < 40% of lake trout lengths (n = 54; Figure 10). ...
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... pikeminnow consumed large proportions of invertebrate prey as well as fish in both lake basins (Figure 11; Table A2). Kokanee represented 26% and unidentified salmonids 40% of the diet of large (401-610 mm FL) northern pikeminnow in the Wapato Basin. ...
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... the Lucerne Basin, kokanee were absent from the diet of northern pikeminnow and unidentified salmonids contributed < 1% of the diet (Table A2). The lengths of ingested prey fishes were < 45% of northern pikeminnow lengths (n = 25; Figure 12). ...
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... diet of burbot was composed primarily of Mysis, crayfish, and fish ( Figure 13; Table A2). Many burbot inverted their stomachs upon capture, and relatively few non-empty stomachs were collected (n = 60). ...
Context 8
... of the large size of this specimen, unidentified salmonid prey represented 40% of the diet of large (451-700 mm TL) burbot in Wapato Basin. The lengths of ingested prey fishes were < 30% of burbot lengths (n = 2; Figure 12). ...
Context 9
... bass diets contained large proportions of non-salmonid fishes, including northern pikeminnow (20%), suckers (17%), and peamouth (15%), as well as crayfish (16%; Figure 14). No salmonid prey were found in smallmouth bass stomachs (Table A2), although most samples were collected during summer when kokanee densities were low in the Wapato Basin. ...
Context 10
... salmonid prey were found in smallmouth bass stomachs (Table A2), although most samples were collected during summer when kokanee densities were low in the Wapato Basin. The lengths of ingested prey fishes were < 47% of smallmouth bass lengths (n = 5; Figure 12). ...
Context 11
... isotope signatures indicated the general structure of the Lake Chelan aquatic food web ( Figure 15). Invertebrates were depleted in δ 15 N, indicating low trophic positions, and their δ 13 C values grouped into two distinct clusters. ...
Context 12
... mixing model analysis indicated that the diet of Chinook salmon was composed largely of Mysis relicta (Figure 16). The median estimate of the Mysis diet proportion in the annual diet was 62% (49-72%; 5 th and 95 th percentile estimates, respectively). ...
Context 13
... fishes represented most of the remainder of the diet for the three largest lake trout size classes. Detailed results are included in the appendix (Figures A1-A4). ...
Context 14
... trout growth patterns differed between lake basins ( Figure 17). The von Bertalanffy growth parameter L ∞ was greater in Lucerne Basin (707 mm FL) than in Wapato Basin (657 mm FL), indicating that lake trout achieved a larger maximum length in Lucerne Basin. ...
Context 15
... trout mortality was estimated from catch-at-age curves ( Figure 18). The instantaneous annual mortality rate (Z) for lake trout was 0.4413 for Lucerne Basin (n = 37 fish ages 7-12 y, r 2 = 0.55) and 0.3072 for Wapato Basin (n = 86 fish ages 7-12 y, r 2 = 0.84). ...
Context 16
... consumption rates were estimated for size-structured unit populations of 1,000 lake trout ages 2-16 in each lake basin (Figure 19). Lake trout in Lucerne Basin consumed 2,527 kg of prey annually per unit population, including 1,549 kg of fish prey and 965 kg of Mysis. ...
Context 17
... classes were defined by fork length (lake trout and northern pikeminnow) or by total length (burbot). Figure 12. Relationship between lengths of piscivorous burbot, northern pikeminnow, and smallmouth bass and lengths of prey fishes ingested by these species. ...
Context 18
... prey lengths were < 30% of predator lengths for burbot (n = 2), < 45% for northern pikeminnow (n = 25), and < 47% for smallmouth bass (n = 5). Figure 17. Lake trout length at age in Lucerne and Wapato Basins of Lake Chelan. ...
Context 19
... represent best fit von Bertalanffy growth relationships for each basin. Figure 18. Age-frequency distributions for lake trout captured in gill nets in the two basins of Lake Chelan. ...
Context 20
... are corrected for bias as indicated in the text. Figure 19. Annual prey consumption by size structured "unit populations" of 1,000 lake trout in Lucerne and Wapato Basins of Lake Chelan. ...
Context 21
... diet composition of Lake Chelan fishes Diet proportions by weight Table A2 concluded. Seasonal diet composition of Lake Chelan fishes Diet proportions by weight Figure A1. Diet composition of lake trout 180-400 mm (FL), in the Lucerne Basin, estimated by a stable isotope mixing model. ...
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Citations
... Lake trout are a common predator in Lake Ontario that are frequently present in our study site (unpublished data). While all tagged bloater are within the general prey size range of lake trout (Schoen & Beauchamp, 2010), the smaller bloater are vulnerable to predation by a wider range of lake trout (or other predators). Although it is difficult to assess predation without predation tags or more extensive detection data, we have stringently removed potential detections of predators that consumed tagged bloater. ...
• Determining the movement and fate of fishes post‐stocking is challenging due to the difficulty in monitoring them, particularly immediately after release. Bloater (Coregonus hoyi; Salmonidae) is a deepwater cisco that has been extirpated from Lake Ontario for several decades and is presently the focus of binational restoration stocking efforts; however, there is limited information to evaluate the efficacy of these efforts. The aim of this study was to examine the initial post‐release survival, 3D movement, and behaviour of hatchery‐reared bloater stocked in Lake Ontario to expand knowledge of post‐stocking ecology of fish and inform stocking practices for deepwater ciscoes.
• In total, 74 hatchery‐reared bloater were tagged with acoustic transmitters with depth and temperature sensors in 2016, 2017, and 2018 and passively monitored on an array of 105 69‐kHz acoustic receivers deployed in north‐eastern Lake Ontario. Several spatial metrics analysed movements after release to investigate immediate post‐stocking survival and behaviour for the first time in a pelagic freshwater forage fish.
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... The reference material for carbon was Vienna Pee Dee Belemnite and atmospheric N2 for nitrogen. The integrative nature of stable isotopes enabled relatively high-precision estimates of trophic position with relatively low sample sizes (McIntyre et al. 2006;Schoen and Beauchamp 2010;Lowery and Beauchamp 2010). ...
... Additionally, adfluvial bull trout and lake trout Salvelinus namaycush fill very similar niches in lake food webs. Both exhibit an affinity for eating kokanee and other salmonids, and may continue to feed on these prey even when availability declines (Jeppson and Platts 1959;Leathe and Graham 1982;Johnson and Martinez 2000;Beauchamp and Van Tassell 2001;Beauchamp and Shepard 2008;Schoen and Beauchamp 2010;Ellis et al. 2011). When kokanee represent the primary forage base for bull trout, the abundance and availability of kokanee can influence the growth, reproductive success, and degree of cannibalism exhibited by the bull trout population (Beauchamp and Van Tassell 2001). ...
... We estimated the upper and lower bounds of relative predation risk in each basin based on the uncertainty in the lake trout consumption and kokanee abundance estimates. We assumed lake trout predation was a good indicator of overall predation risk, because other piscivores either had little or no kokanee in their diets (i.e., burbot Lota lota, northern pikeminnow, and smallmouth bass Micropterus dolomieu) or were very low in abundance (i.e., Chinook salmon O. tshawytscha) (Schoen and Beauchamp 2010). We performed statistical analyses using R version 3.1.1 ...
... We estimated the upper and lower bounds of relative predation risk in each basin based on the uncertainty in the lake trout consumption and kokanee abundance estimates. We assumed lake trout predation was a good indicator of overall predation risk, because other piscivores either had little or no kokanee in their diets (i.e., burbot Lota lota, northern pikeminnow, and smallmouth bass Micropterus dolomieu) or were very low in abundance (i.e., Chinook salmon O. tshawytscha) (Schoen and Beauchamp 2010). ...
... We considered whether the difference in lake trout CPUE could be an artifact of reduced gill net efficiency on the steeper slopes of the deep basin. However, this was unlikely because the catch rates of other littoral and profundal fishes were either greater in the deep basin or similar in both basins, suggesting net efficiency did not differ substantially (also see Schoen and Beauchamp 2010). Indeed, our comparison of gill net catch rates probably underestimated the positive effect of Mysis availability on lake trout density in the shallower basin, for two reasons. ...
In many food webs, species in similar trophic positions can interact either by competing for resources or boosting shared predators (apparent competition), but little is known about how the relative strengths of these interactions vary across environmental gradients. Introduced Mysis diluviana shrimp interact with planktivorous fishes such as kokanee salmon (lacustrine Oncorhynchus nerka) through both of these pathways, and effective management depends on understanding which interaction is more limiting under different conditions. An "environmental matching" hypothesis predicts the ecological impacts of Mysis are maximized under cool conditions near its thermal optimum. In addition, we hypothesized Mysis is more vulnerable to predation by lake trout in relatively shallow waters, and therefore Mysis enhances lake trout density and limits kokanee through apparent competition more strongly in shallower habitats. We tested whether these hypotheses could explain food web differences between two connected lake basins, one relatively shallow and the other extremely deep. The shallower basin warmed faster, thermally excluded Mysis from surface waters for 75% longer, and supported 2.5-18 times greater seasonal production of cladoceran zooplankton than the deeper basin, standardized by surface area. Mysis consumed 14-22% less zooplankton in the shallower basin, and lower ratios of total planktivore consumption to zooplankton production (C:P) indicated less potential for resource competition with kokanee, consistent with environmental matching. Lake trout diets contained more Mysis in the shallower basin and at shallower sampling sites within both basins. The catch rate of lake trout was seven times greater and the predation risk for kokanee was 4-5 times greater in the shallower basin than in the deeper basin, consistent with stronger apparent competition in shallower habitats. Understanding how the strengths of these interactions are mediated by temperature and depth would enable managers to select appropriate strategies to address the unique combinations of conditions in hundreds of affected systems.
Read More: http://www.esajournals.org/doi/abs/10.1890/14-1822.1
... Lake trout stomach contents were identified to species for prey fishes and to order and life stage for invertebrates, and the blotted wet weight of each prey type was recorded. The lengths of prey fish were measured or were estimated from the lengths of diagnostic bones when possible (see Schoen and Beauchamp 2010). A subset of salmonid prey specimens (n = 21) was unidentifiable to the species level based on bone morphology; those specimens were analyzed genetically by the Molecular Genetics Facility, School of Aquatic and Fishery Sciences, University of Washington. ...
... A subset of salmonid prey specimens (n = 21) was unidentifiable to the species level based on bone morphology; those specimens were analyzed genetically by the Molecular Genetics Facility, School of Aquatic and Fishery Sciences, University of Washington. Prey DNA samples were extracted, amplified using polymerase chain reaction, and sequenced via the methods of Buser et al. (2009), with modifications described by Schoen and Beauchamp (2010). Phylogenetic relationships were assigned using MEGA4 software (Tamura et al. 2007). ...
... We calculated diet proportions by weight (Chipps and Garvey 2007) for four size-classes of lake trout subdivided by season. Within these groups, diet composition differed between the two lake basins (Schoen and Beauchamp 2010), so we estimated lakewide diet proportions as the mean of the diet proportions in each basin weighted by the relative abundance of lake trout in each basin. We assumed that the density of each lake trout size-class was proportional to the catch per unit effort (CPUE) for that size-class in sinking gill nets. ...
Introduced long-lived predators often cause significant impacts on their prey, but these impacts can be masked from detection due to high "predatory inertia": time lags in population growth and dietary ontogeny. We evaluated whether predation by introduced lake trout Salvelinus namaycush could explain the 88% decline in escapement of kokanee Oncorhynchus nerka during 2005-2009 in Lake Chelan, Washington. We quantified the strength and trend of predation impacts with field sampling, a hydroacoustic assessment of kokanee production, and bioenergetics and age-structured population models of lake trout. Lake trout consumption of kokanee exceeded kokanee production, indicating strong predation impacts at the start of the decline. Fully piscivorous lake trout (>550 mm fork length) were responsible for 83% of this predation. The population model predicted that a pulse of strong stocked cohorts crossed this piscivorous size threshold, causing the biomass of fully piscivorous lake trout to expand by roughly 70-300% during 2004-2009 and driving predation pressure to peak levels. Together, these results suggested that lake trout predation was a large and growing source of kokanee mortality during the decline. Counterintuitively, predation pressure was projected to increase even if the numbers of harvestable lake trout declined, as strong cohorts grew to piscivorous size while succumbing to mortality. Angler catch rates of lake trout declined by 40% during 2004-2007, as was predicted by the population model; this decline in catch masked the rise in predation pressure. This analysis demonstrates the potential for introduced predators exhibiting high predatory inertia to cause strong, latent impacts on prey that would be unexpected based on harvest trends and prior dynamics alone. Forward-looking monitoring and modeling analyses are clearly advantageous for managers who seek to maintain ecosystems in long-term "balance" by detecting and reversing incipient changes in predation.
In the Great Lakes, an Alewife Alosa pseudoharengus diet has been reported to cause thiamine deficiency in salmonines as a result of high thiaminase activity. The ontogeny of thiamine deficiency and its relationship to Alewife consumption have not been determined in Chinook Salmon Oncorhynchus tshawytscha within this system. Using stable isotope mixing models along with muscle thiamine, we assessed ontogenetic changes in the diet and corresponding muscle thiamine status of Lake Ontario Chinook Salmon ranging in TL from 100 to 1,000 mm. Thiamine levels were highest in young of the year collected in a tributary of Lake Ontario but declined once Chinook Salmon entered Lake Ontario. In Lake Ontario, the diets of juvenile (200–400 mm; 97–99%) and adult (>400 mm; 97–100%) Chinook Salmon consisted almost entirely of Alewives as estimated using mixing models. This Alewife diet was associated with an ontogenetic decline in muscle thiamine among Chinook Salmon. The asymptotic decline in thiamine concentration with increasing size plateaued at 720–758 pmol/g of muscle. The proportion of individuals below a thiamine threshold of 500 pmol/g (associated with a loss of equilibrium) increased with TL up to 799 mm and then declined between 800 and 1,000 mm. At these muscle concentrations, no outward signs of thiamine deficiency were observed. Calculation of total mass accumulation of body stores of thiamine (muscle concentration times body weight) showed an unexpected and significant increase with size. Although it appears that dietary Alewives affected thiamine status, there were undetermined factors, such as incomplete destruction of thiamine by thiaminase, gut synthesis of thiamine, gill absorption of water thiamine, and/or other factors, that contributed to the overall thiamine status of these Chinook Salmon.
Aim
Despite a long‐standing research interest in the association between the biodiversity (i.e. taxonomic and functional composition) and trophic structure of communities, our understanding of the relationship remains limited. Community assembly theory predicts that niche partitioning will result in communities with a diverse array of functional traits, which in turn may facilitate a correspondingly diverse array of trophic interactions that define the trophic niche of those communities. The aim of our study is to test this prediction.
Location
North America.
Methods
We built a database composed of functional traits and stable isotope values (δ ¹³ C and δ ¹⁵ N) for 63 freshwater fish communities containing 109 species in 34 lentic and 29 lotic ecosystems. First, using linear mixed models (i.e. an alpha‐diversity approach), we tested whether the taxonomic diversity of communities was positively associated with their functional diversity and if their functional diversity was positively associated with their trophic diversity. Second, we assessed the taxonomic, functional and trophic similarity of communities using multiple regression on distance matrices ( MRM ) and their respective ‘turnover’ and ‘nestedness‐resultant’ components to test if the taxonomic similarity of communities was positively correlated with their functional similarity and if their functional similarity was positively associated with their trophic similarity (i.e. a beta‐diversity approach).
Results
We found that the functional diversity of communities increased as their taxonomic diversity increased. Similarly, the trophic diversity of communities increased as their functional diversity increased. The pairwise taxonomic and functional similarity of communities were also positively associated, but there was a weak relationship between the functional and trophic similarities of communities.
Main conclusions
Our study demonstrates that communities with similar functional characteristics can have disparate food web structures, suggesting that additional site‐specific factors influence community variation in trophic niche geometry. Determining the relative importance of functional characteristics and site‐specific factors in shaping trophic interactions is crucial for a better understanding of how future species loss and species introductions will affect food web structure and ecosystem functioning.
Aquatic ecosystems are structured by environmental gradients, including temperature,
depth, and water clarity. These gradients can mediate the strength of trophic interactions by
influencing the distribution, physiology, and behavior of predators and prey. My dissertation
addressed the following questions: 1) What factors determine the foraging success of pelagic
piscivores during encounters with prey? and 2) How do temperature and depth mediate direct
and indirect trophic interactions in lakes? First, I conducted feeding experiments to quantify the
effects of turbidity and prey shoaling behavior on the foraging efficiency of Chinook salmon
during encounters with Pacific herring. Chinook salmon successfully consumed herring during
only 1-4% of encounters, limited by the rates of attacks per encounter and capture success (prey
consumed per attack). Capture success declined with increasing turbidity and prey shoaling.
These results indicate post-encounter processes can strongly limit feeding rates of pelagic
piscivores and they provide the necessary parameters to incorporate these processes into foraging
models. Second, I conducted field sampling in Lake Chelan, Washington to quantify how the
strengths of trophic interactions between zooplankton, mysid shrimp, kokanee, and lake trout
changed along environmental gradients. Bioenergetics and population models revealed strong
predation impacts of lake trout on kokanee, which were initially masked from detection by
ecological time lags. Mysids influenced kokanee through two negative indirect interactions,
which differed in strength between contrasting lake basins. Mysids competed for zooplankton
prey more strongly in a deeper, cooler basin due to their low thermal optimum. However,
mysids provided greater energetic support to lake trout diet in a shallower basin, where lake trout
were greater in density and inflicted greater predation risk on kokanee. A diel vertical migration
model predicted mysids were more vulnerable to lake trout predation at shallower sites within
the lake, and this prediction was supported by stable isotope analysis of lake trout diets. These
findings revealed a mechanism by which mysids could cause greater food-web impacts in
shallower systems where they are more vulnerable to predation. In conclusion, my results show
strong associations between the physical environment, the behaviors of individual predators and
prey, and the dynamics of populations and food webs.
