Emergent trophic interactions following the Chinook salmon invasion of Patagonia

Predation events during ontogeny may have long‐term consequences for fish population abundance and variability. We used a stage‐based matrix model to evaluate Walleye Sander vitreus and Brown Trout Salmo trutta predation on Chinook Salmon Oncorhynchus tshawytscha parr of the Muskegon River stock and the relative influence of parr predation on the long‐term population dynamics and recruitment of Chinook Salmon in Lake Michigan. The model predicted the number of Chinook Salmon individuals in each stage (fry, smolts, and lake age 0 [recruits] through lake age 4) and forecasted population trajectories based on demographic data (e.g., survival, growth, and fecundity). The relative influence of parr predation was compared with influences of environmental stochasticity in the egg stage and Alewife Alosa pseudoharengus abundance (prey for lake‐stage salmon) on Chinook Salmon fecundity, recruitment, and population growth. To simulate environmental stochasticity and the influence of Alewife abundance, we varied Chinook Salmon stage‐specific survival rates, growth rates, maturity schedule, and carrying capacity. Relative to a baseline recruitment scenario, removal of stocked Brown Trout resulted in a significant increase in parr survival and long‐term Chinook Salmon abundance. Walleye predation on parr had little apparent influence on Chinook Salmon population dynamics. Predation on parr during out‐migration was positively correlated with variation in Chinook Salmon population stability and was negatively correlated with population growth, suggesting that Brown Trout have a significant negative effect on Chinook Salmon recruitment and long‐term population stability. The negative effects of variation in egg survival rates and Alewife abundance on Chinook Salmon recruitment and population growth rates were similar to the negative effects from parr predation scenarios. Our study suggests that management decisions to promote Great Lakes Chinook Salmon populations may require evaluation of trout stocking practices in nursery habitats.

Nearly all ecosystems are contaminated with highly toxic methylmercury (MeHg), but the specific sources and pathways leading to the uptake of MeHg within and among food webs are not well understood. In this study, we report stable mercury (Hg) isotope compositions in food webs in a river and an adjacent forest in northern California and demonstrate the utility of Hg isotopes for studying MeHg sources and cross-habitat transfers. We observed large differences in both δ(202)Hg (mass-dependent fractionation) and Δ(199)Hg (mass-independent fractionation) within both food webs. The majority of isotopic variation within each food web could be accounted for by differing proportions of inorganic Hg [Hg(II)] and MeHg along food chains. We estimated mean isotope values of Hg(II) and MeHg in each habitat and found a large difference in δ(202)Hg between Hg(II) and MeHg (∼2.7‰) in the forest but not in the river (∼0.25‰). This is consistent with in situ Hg(II) methylation in the study river but suggests Hg(II) methylation may not be important in the forest. In fact, the similarity in δ(202)Hg between MeHg in forest food webs and Hg(II) in precipitation suggests that MeHg in forest food webs may be derived from atmospheric sources (e.g., rainfall, fog). Utilizing contrasting δ(202)Hg values between MeHg in river food webs (-1.0‰) and MeHg in forest food webs (+0.7‰), we estimate with a two-source mixing model that ∼55% of MeHg in two riparian spiders is derived from riverine sources while ∼45% of MeHg originates from terrestrial sources. Thus, stable Hg isotopes can provide new information on subtle differences in sources of MeHg and trace MeHg transfers within and among food webs in natural ecosystems.

The impacts of non‐native species are hypothesised to be proportional to the functional distinctiveness of invaders in their invaded ecosystems. Throughout the Patagonia region of southern South America, Chinook salmon (Oncorhynchus tshawytscha) have recently established non‐native populations, and their anadromous, semelparous life cycle could be functionally unique such that marine‐derived nutrients are delivered to streams which have historically lacked such a resource linkage with the ocean. We tested the hypothesis that salmon subsidise biofilm‐associated algae in streams throughout the Aysén province of southern Chile. Using spatial and temporal variation in the presence of salmon among multiple streams and across two spawning seasons, we found strong evidence of salmon‐subsidised algae in three out of four streams examined that have spawning salmon populations. The biofilm of subsidised streams had enriched stable isotopic ratios of nitrogen and carbon, indicating that marine‐derived nutrients were incorporated by biofilms. This nutrient uptake translated into increased algal biomass and percent of total biofilm biomass composed of algae, indicating that the incorporation of marine‐derived nutrients stimulated autotrophic production of biomass. In one stream, the incorporation of marine‐derived nutrients by biofilm occurred in only one of the two studied spawning seasons. Incorporation occurred in a year with low flows of water throughout salmon spawning (4.59 m3/s) and did not occur in a year with much higher flows (41.6 m3/s), suggesting that inter‐annual variation in discharge can mediate the subsidising effect of salmon. These results indicate that Chinook salmon have bridged the historical gap between productive marine ecosystems and nutrient‐poor stream ecosystems in Patagonia. Anadromous salmon can be a significant source of nutrients in nutrient‐limited catchments, and their ongoing expansion in southern South America is likely to entail ecological impacts in stream and riparian food webs.

The goal of this study was to determine the food web pathways supporting juvenile Chinook (Oncorhynchus tshawytscha) salmon in the Columbia River estuary through multiple stable isotope analysis (δ13C, δ15N, δ34S). Using this method, we distinguished the role of various organic matter sources in Chinook food webs and interpreted the dynamics of their use both spatially and temporally within the estuary. Our results indicate that subyearling Chinook are associated with fluvial, anthropogenic, estuarine, and marine organic matter sources, with hatchery food and vascular plant detritus being the most dominant sources in juvenile Chinook food webs. Although freshwater phytoplankton is involved in many food web pathways to subyearling Chinook, increased phytoplankton production from the impounded river has not replaced the loss of autochthonous marsh production to fish. Our results indicate that large-scale ecosystem alteration may have decreased the availability and quality of food webs in the estuary and potentially diminished the ability of the Columbia to support Chinook salmon.

We examined PCB concentration data for seven species of Lake Michigan fishes to determine what trends were apparent °20 yr after PCB restrictions became effective. Total PCB concentrations in all seven species, lake trout (Salvelinus namaycush), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), alewife (Alosa pseudoharengus), and bloater chub (Coregonus hoyi) declined and appeared to stabilize in the mid‐to‐late 1980s. Concentrations in two species, chinook and coho salmon, appear to have increased slightly since the late 1980s. All species are currently well below the high PCB levels that existed when PCB use was curtailed in the 1970s. We believe stabilizing concentrations are the result of large pools of PCBs that are being recycled in the environment. Atmospheric and sediment PCB inputs to the lake probably constitute current PCB sources. Increasing concentrations in chinook and coho salmon are likely the result of changing growth dynamics caused by alterations in the mid‐trophic levels of the food web. Median stable PCB concentrations estimated in this analysis are below the current FDA action level of 2 mg/kg, but not appreciably below this threshold. Improvements beyond these levels may result if management practices that maximize fish growth rates are implemented. Detection of future improvements in PCB levels may require samples in the range of 1000‐2000 fish because of the high variability in PCB concentrations among individuals.

We studied some of the relationships between federally listed fall Chinook Salmon, Oncorhynchus tshawytscha, endemic Sand Roller, Percopsis transmontana, and non-native Smallmouth Bass, Micropterus dolomieu, in Lower Granite Reservoir on the Snake River. Because of its recent reappearance and population increase, the Sand Rollers could be filling the role of a “native invader” in the reservoir food web. We speculated that Sand Rollers could either negatively affect fall Chinook Salmon by potentially competing with them for resources in shoreline habitats or, alternatively, benefit the salmon by providing a buffer against Smallmouth Bass predation. Nighttime beach seining showed that habitat use by fall Chinook Salmon and Sand Rollers overlapped completely in spring when both species were present along shorelines. Diet data from stomach samples also showed high overlap, but data on stable isotopes of 13C and 15N suggested that each species could be obtaining much of their dietary energy from different reservoir locations. Although habitat and diet overlap are evidence of competition, diel and spatial partitioning of resource use between fall Chinook Salmon and Sand Rollers may act to reduce potential competition. Analyses of Smallmouth Bass diets showed that fall Chinook Salmon and Sand Rollers comprised the majority of prey fish consumed by bass. Across years, as Smallmouth Bass increased their consumption of Sand Rollers (range 0.219 to 0.392 fish smallmouth-1 day-1), they decreased their consumption of fall Chinook Salmon (range 0.114 to 0.050 fish smallmouth-1 day-1). The greatest effect Sand Rollers may have on fall Chinook Salmon in Lower Granite Reservoir is to serve as a buffer against Smallmouth Bass predation.

A study was carried out on two important recreational fishing rivers in southern Chile, the Petrohué and the Puelo, to assess the population structure of the salmonid species inhabiting both rivers. Five river sectors were surveyed on four dates between April 2016 and February 2017. In the Petrohué River, 2400 fish were sampled (42.9% rainbow trout Oncorhynchus mykiss; 23.8% brown trout Salmo trutta, and 33.3% Chinook salmon Oncorhynchus tshawytscha), while in the Puelo River, 1972 fish were examined (51.6% rainbow trout; 30.4% brown trout and 18% Chinook salmon). Fry and fingerling stages accounted for the highest proportion of fish collected by electrofishing. In the Petrohué and Puelo rivers, rainbow trout fry and fingerlings represented 96.3 and 99.2% of the fish sampled, respectively; 96.8 and 97.1% in the case of brown trout, while for Chinook salmon, parr and pre-smolts represented 97% of the fish examined in the Petrohué River and 98.3% in the Puelo River. Rainbow and brown trout shared the same habitats and food items in both rivers, and the two oldest trout recorded 5+ and 6+years. Parr and pre-smolt Chinook salmon were present mainly in the winter and spring seasons and were aged 1+ and 2+, respectively. There were no differences in the food items recorded from the juvenile fish sampled in both rivers. The mature, returning adult Chinook salmon aged between 1.5+ and 1.6+ years were devoid of food and were recorded over the autumn season with the highest abundance in the Petrohué River. The study did not identify any adverse competition or negative interactions among the three main salmonid species sharing the same habitat.

The modern sport fishery for salmonids in Wisconsin waters of Lake Michigan was begun during 1963–1969 with the stocking of rainbow trout Oncorhynchus mykiss, lake trout Salvelinus namaycush, brook trout S. fontinalis, brown trout Salmo trutta, coho salmon O. kisutch, and chinook salmon O. tshawytscha. The fishery grew rapidly during 1969-1985 as angler effort increased 10-fold, catch rate doubled, and catch increased 20-fold. The stocking and catch became increasingly dominated by chinook salmon, with coho salmon and lake trout of secondary importance, and brown, rainbow, and brook trout of least importance. Trolling dominated the fishery, particularly by launched-boat anglers and, more recently, by moored-boat anglers. Charter boat trolling grew the most continuously and had the highest catch rates. The catch by trollers was dominated by chinook and coho salmon and lake trout. Pier, stream, and shore anglers fished less overall, but had catch rates that were similar to launched-boat anglers. Th...

Carbon-based balanced trophic structure and flows in the offshore Lake Ontario food web before (1987–1991) and after (2001–2005) invasion-induced ecosystem change

The decline of wild Chinook salmon (Oncorhynchus tshawytscha) in the Pacific Northwest is concerning due to their critical role in the culture, economy, and ecology of the region, and the endangered species status of some of the evolutionarily significant units. Decline in Chinook stocks has been partially attributed to increases in pinniped abundance. The northwest coast of Washington State, USA, provides year-round habitat to early marine-phase Chinook salmon from multiple stocks and habitat for increasingly abundant Steller sea lions (Eumetopias jubatus). We estimated the Chinook salmon biomass consumed by Steller sea lions along the northwest coast of Washington using diet data obtained via DNA metabarcoding from scat and a prey consumption model. Between December 2020 and August 2021, Steller sea lions consumed 284 metric tons (95% PI: 191–417 t) of Chinook salmon. A set of experimental models were used to estimate the consumption of age-0 Chinook salmon, and the base model estimated 146 t (95% PI: 93–221 t) of ocean age-0 Chinook—or approximately 2,064,418 (95% PI: 1,431,524–2,932,922) individual ocean age-0 Chinook— were consumed during the study period. While precise consumption values should be interpreted with caution due to high uncertainty highlighted by sensitivity analyses, our results suggest that Steller sea lions contribute to the low marine survival rates of early marine-phase Chinook salmon at a higher rate than previously estimated. The high uncertainty in model estimates, compounded by assumptions and limitations arising from data gaps, highlights the need for further research on both predator and prey populations in the region.

Spatial and temporal variation in marking rates and severity of sea lamprey attacks on salmonines in Lakes Michigan and Huron

The behavioral interaction between underyearling Chinook salmon Oncorhynchus tshawytscha and brown trout Salmo trutta, both exotics in an unnatural sympatry in New Zealand, was tested during spring and summer in a simulated stream containing natural food. Both species were highly territorial and actively defended preferred drift-feeding sites and resting areas in the pool. Species dominance differed with season and was influenced by prior residence. In spring, Chinook salmon were larger (because of their earlier emergence) and socially dominant, but they did not dominate when brown trout had prior residence. In summer, interspecific differences in size no longer existed, and brown trout were always socially dominant, although dominance required more time to establish when Chinook salmon were prior residents. Most aggressive attacks of both species were initiated and won by the dominant fish in both spring and summer. Yearling brown trout in the pool in summer did not influence the interaction bet...

The sea louse (Lepeophtheirus salmonis Krøyer) is a major health problem for both farmed and wild salmonids. This paper investigates louse epidemiology and management in the salmon-farming zone of western Scotland. Based on a review of the marine ecology of wild salmon (Salmo salar L) and sea trout (Salmo trutta L), and catch and farm production statistics, best estimates were made for numbers of wild and farm hosts present in coastal waters in March-June 2000. Applying data for ovigerous female louse infections and fecundity, the sources and risks of larval transmission to wild salmon and sea trout were modelled. Farm salmon in the second spring of production were the primary host group (98% of fish), while numbers of wild salmonids (< 1%) and escaped farm salmon (2%) were relatively insignificant. Farm salmon produced 97% of louse eggs at high levels (eight ovigerous lice per fish), and 78% at low levels (one per fish). Wild salmonids produced < 1% of eggs under both scenarios, but escaped farm salmon produced 3% and 21%, respectively. All hosts potentially cross-infect one another, but farm salmon are more likely to infect wild and farm smolts, and also other farm salmon. Monitoring of lice on sea trout in June 1998-2000 by the Association of West Coast Fisheries Trusts corroborated the model's conclusions. Localised epizootics occurred every year and coincided with the presence of ovigerous lice on local farms. In areas of mixed-year class production on farms, epizootics were evident every spring, but occurred every second spring in areas of single-year class production. In 1998-2000 at least 14-40% of sea trout were infected with potentially lethal infestations of lice. Ovigerous louse levels of < 0.005 per fish were required on farm salmon in the spring of 2000 to produce less eggs than those emitted by wild salmonids. With the industry's continued expansion, and thus increased numbers of farm salmon, a target of zero ovigerous lice will be required on farms to minimise impacts on wild salmonids. Due to the limited long-term efficacy and availability of louse medicines, management strategies are discussed which will improve control, including single-year class production over large areas, alternate S1-S1/2 smolt inputs, and 11-month production cycles.

Foraging ecology of walleye and brown trout in a Great Lakes tributary

Background and Research Aims Native vertebrate scavenger communities may be affected by free-ranging dogs ( Canis lupus familiaris), but little information is available on this subject. We aimed to evaluate the influence of free-ranging dogs on a community of native vertebrate scavengers and to propose management and conservation actions. Methods Between November 2014 and June 2016, we systematically monitored 37 ungulate carcasses in the buffer zone of Antisana National Park, Andes of Ecuador at an elevation range between 3000 and 4150 m asl. We monitored carcasses with camera traps operating 24 hours per day. Results We obtained 1010 independent events and recorded seven vertebrate scavenger species. Free-ranging dogs were present at carcasses for 40.39% of the total activity time, but the most frequently recorded species were the Andean fox ( Lycalopex culpaeus) and the Carunculated Caracara ( Phalcoboenus carunculatus). The scavenging birds were diurnal, the Andean fox was mostly nocturnal, and the free-ranging dogs were active in all periods of the day. Native scavenger species changed their peak of activity to times when the free-ranging dogs were less active, probably to avoid agonistic encounters. The presence of free-ranging dogs negatively influenced the number of independent events of the native species. Conclusion The presence of free-ranging dogs showed a negative relationship with the residence time and the number of independent events of native scavenging species. Furthermore, their presence modified the hour of the peak in the activity patterns of the scavenger community. Implications for Conservation The presence of free-ranging dogs may have important implications for the conservation of the Vulnerable Andean Condor ( Vultur gryphus) in Ecuador. To achieve the conservation goals of native vertebrate scavengers, particularly of Andean Condors, it is necessary to develop environmental education programs, provide technical training in responsible dog ownership, and conduct campaigns to sterilize and control dogs.