Size-mediated control of perch–midge coupling in Lake Erie transient dead zones

Transient ecosystem-level disturbances such as oxygen depletion (hypoxia) in aquatic systems modulate species distributions and interactions. In highly eutrophic systems, hypoxic areas (“dead zones”) have expanded around the world, temporarily preventing many demersal predators from accessing their food resources. Here, we investigate how yellow perch (Perca flavescens), an exploited, cool-water mesopredator, interact with their dominant invertebrate prey in benthic habitat–non-biting midge (chironomid) larvae–as bottom-water hypoxia develops in central Lake Erie (United States–Canada) during summer. We apply linear mixed-effects models to individual-level data from basin-wide field surveys on size-based interactions between perch and midge larvae under varying habitat conditions and resource attributes. We test if 1) midge populations (larval body size and density) differ among habitat states (unstratified normoxia, stratified normoxia, and stratified hypoxia); and 2) size-based perch–midge interactions (predator–prey mass ratio or PPMR) differ among habitat states with varying temperature and midge density. Midge populations remained highly abundant after bottom-water oxygen depletion. Despite their high densities, midge larvae also maintained their body size in hypoxic water. In contrast, perch on average consumed relatively smaller (by up to ~64%) midges (higher PPMR) in warmer and hypoxic water, while prey size ingested by perch shrunk less in areas with higher midge density. Our analysis shows that hypoxia-tolerant midges largely allow perch to maintain their consumer–resource relationships in contracted habitats through modified size-mediated interactions in dead zones during summer, revealing plasticity of their trophic coupling in the chronically perturbed ecosystem.     

Seasonal and interannual effects of hypoxia on fish habitat quality in central Lake Erie

1. Hypoxia occurs seasonally in many stratified coastal marine and freshwater ecosystems when bottom dissolved oxygen (DO) concentrations are depleted below 2–3 mg O₂ L^?1. 2. We evaluated the effects of hypoxia on fish habitat quality in the central basin of Lake Erie from 1987 to 2005, using bioenergetic growth rate potential (GRP) as a proxy for habitat quality. We compared the effect of hypoxia on habitat quality of (i) rainbow smelt, Osmerus mordax mordax Mitchill (young‐of‐year, YOY, and adult), a cold‐water planktivore, (ii) emerald shiner, Notropis atherinoides Rafinesque (adult), a warm‐water planktivore, (iii) yellow perch, Perca flavescens Mitchill (YOY and adult), a cool‐water benthopelagic omnivore and (iv) round goby Neogobius melanostomus Pallas (adult) a eurythermal benthivore. Annual thermal and DO profiles were generated from 1D thermal and DO hydrodynamics models developed for Lake Erie’s central basin. 3. Hypoxia occurred annually, typically from mid‐July to mid‐October, which spatially and temporally overlaps with otherwise high benthic habitat quality. Hypoxia reduced the habitat quality across fish species and life stages, but the magnitude of the reduction varied both among and within species because of the differences in tolerance to low DO levels and warm‐water temperatures. 4. Across years, trends in habitat quality mirrored trends in phosphorus concentration and water column oxygen demand in central Lake Erie. The per cent reduction in habitat quality owing to hypoxia was greatest for adult rainbow smelt and round goby (mean: ?35%), followed by adult emerald shiner (mean: ?12%), YOY rainbow smelt (mean: ?10%) and YOY and adult yellow perch (mean: ?8.5%). 5. Our results highlight the importance of differential spatiotemporally interactive effects of DO and temperature on relative fish habitat quality and quantity. These effects have the potential to influence the performance of individual fish species as well as population dynamics, trophic interactions and fish community structure.     

Fifty years later: trophic ecology and niche overlap of a native and non-indigenous fish species in the western basin of Lake Erie

Since the introduction of white perch (Morone americana) into Lake Erie over 50 years ago, the population size of native yellow perch (Perca flavescens) has decreased up to 79 % and significant changes to the ecosystem have occurred. We examined long-term population estimates and used stable isotopes of carbon (δ ¹³C) and nitrogen (δ ¹⁵N) paired with stomach content analysis to quantify the trophic ecology and niche overlap of adult yellow perch and white perch in the western basin of Lake Erie. We found that changes in yellow perch abundance since 1979 appeared to be better correlated with changes in fishery exploitation rates than with food competition effects from white perch. At the time of this study, yellow perch were found to have higher δ ¹³C values, indicating greater utilization of benthic food resources than white perch, and white perch occupied higher trophic positions based on δ ¹⁵N. The diets of both species varied spatially and seasonally based on stable isotopes and stomach contents, likely driven by changes in prey abundance. Comparison of niche widths using stable isotope population metrics and Schoener diet similarity index suggested a low to moderate degree of niche overlap between species. Isotopic niches of white perch were generally larger than those of yellow perch demonstrating broader resource utilization by this non-indigenous species. We submit that isotopic niche overlap comparisons are more appropriate for studies seeking to understand interactions among populations over course temporal scales, while diet overlap indices, such as the Schoener index provide a means to study fine-scale interactions such as ontogenetic and seasonal diet shifts.     

Direct and indirect effects of hypoxia on benthos in Corpus Christi Bay, Texas, U.S.A.

Hypoxia (low oxygen conditions) has been found in the southeastern region of Corpus Christi Bay, Texas, U.S.A. every summer since 1988. The objectives of the current study were to determine direct and indirect effects of hypoxia on macrofauna. Direct physiological effects of hypoxia include reduction of benthic abundance, biomass, diversity, species richness and species evenness because of physiological intolerance. Indirect ecological effects of hypoxia include predation of emerging benthic fauna from the sediment. Macrofaunal community characteristics were compared vertically within sediments in caged and uncaged sediment samples in hypoxic and normoxic areas. Cage effects were determined with partial cages, which had reduced flow and no predator exclusion. Dissolved oxygen concentrations during the experiment was monitored in water column profiles and continuous measurement of bottom water in the hypoxic and normoxic areas. Hypoxia in Corpus Christi Bay in 1999 occurred as transient events, many of which were of short duration (less than 1 h) and moderate intensity (around 2 mg l^− 1). The macrobenthic community characteristics (i.e., abundance, biomass, species richness, diversity, and evenness) were directly affected by hypoxia as indicated by depressed levels and few deeper-dwelling organisms in the hypoxic area. Community structure was also different between the hypoxic and normoxic areas because of loss of species (presumably due to intolerance to low oxygen) in the hypoxic areas. Benthic invertebrates were found primarily in the surface in the hypoxic area, but there was no significant indication of indirect effects, i.e., increased predation pressure in the hypoxic area. The increased exposure to predation risk may be mitigated by predator avoidance of hypoxic areas. In conclusion, hypoxia in Corpus Christi Bay has negative direct effects on benthic organisms, but no indirect effects, such as increased predation pressure. The most significant finding is the interaction between hypoxia and vertical distributions of infauna, which drive hypoxia intolerant organisms to the surface and out of sediments.     

Yellow perch genetic structure and habitat use among connected habitats in eastern Lake Michigan

Maintenance of genetic and phenotypic diversity is widely recognized as an important conservation priority, yet managers often lack basic information about spatial patterns of population structure and its relationship with habitat heterogeneity and species movement within it. To address this knowledge gap, we focused on the economically and ecologically prominent yellow perch (Perca flavescens). In the Lake Michigan basin, yellow perch reside in nearshore Lake Michigan, including drowned river mouths (DRMs)—protected, lake‐like habitats that link tributaries to Lake Michigan. The goal of this study was to examine the extent that population structure is associated with Great Lakes connected habitats (i.e., DRMs) in a mobile fish species using yellow perch as a model. Specifically, we tested whether DRMs and eastern Lake Michigan constitute distinct genetic stocks of yellow perch, and if so, whether those stocks migrate between the two connected habitats throughout the year. To do so, we genotyped yellow perch at 14 microsatellite loci collected from 10 DRMs in both deep and littoral habitats during spring, summer, and autumn and two nearshore sites in Lake Michigan (spring and autumn) during 2015–2016 and supplemented our sampling with fish collected in 2013. We found that yellow perch from littoral‐DRM habitats were genetically distinct from fish captured in nearshore Lake Michigan. Our data also suggested that Lake Michigan yellow perch likely use deep‐DRM habitats during autumn. Further, we found genetic structuring among DRMs. These patterns support hypotheses of fishery managers that yellow perch seasonally migrate to and from Lake Michigan, yet, interestingly, these fish do not appear to interbreed with littoral fish despite occupying the same DRM. We recommend that fisheries managers account for this complex population structure and movement when setting fishing regulations and assessing the effects of harvest in Lake Michigan.     

Combined climate‐ and prey‐mediated range expansion of Humboldt squid (Dosidicus gigas), a large marine predator in the California Current System

Climate‐driven range shifts are ongoing in pelagic marine environments, and ecosystems must respond to combined effects of altered species distributions and environmental drivers. Hypoxic oxygen minimum zones (OMZs) in midwater environments are shoaling globally; this can affect distributions of species both geographically and vertically along with predator–prey dynamics. Humboldt (jumbo) squid (Dosidicus gigas) are highly migratory predators adapted to hypoxic conditions that may be deleterious to their competitors and predators. Consequently, OMZ shoaling may preferentially facilitate foraging opportunities for Humboldt squid. With two separate modeling approaches using unique, long‐term data based on in situ observations of predator, prey, and environmental variables, our analyses suggest that Humboldt squid are indirectly affected by OMZ shoaling through effects on a primary food source, myctophid fishes. Our results suggest that this indirect linkage between hypoxia and foraging is an important driver of the ongoing range expansion of Humboldt squid in the northeastern Pacific Ocean.     

Oxygen‐mediated plasticity confers hypoxia tolerance in a corallivorous polychaete

There is mounting evidence that the deoxygenation of coastal marine ecosystems has been underestimated, particularly in the tropics. These physical conditions appear to have far‐reaching consequences for marine communities and have been associated with mass mortalities. Yet little is known about hypoxia in tropical habitats or about the effects it has on reef‐associated benthic organisms. We explored patterns of dissolved oxygen (DO) throughout Almirante Bay, Panama and found a hypoxic gradient, with areas closest to the mainland having the largest diel variation in DO, as well as more frequent persistent hypoxia. We then designed a laboratory experiment replicating the most extreme in situ DO regime found on shallow patch reefs (3 m) to assess the response of the corallivorous fireworm, Hermodice carnaculata to hypoxia. Worms were exposed to hypoxic conditions (8 hr ~ 1 mg/L or 3.2 kPa) 16 times over an 8‐week period, and at 4 and 8 weeks, their oxygen consumption (respiration rates) was measured upon reoxygenation, along with regrowth of severed gills. Exposure to low DO resulted in worms regenerating significantly larger gills compared to worms under normoxia. This response to low DO was coupled with an ability to maintain elevated oxygen consumption/respiration rates after low DO exposure. In contrast, worms from the normoxic treatment had significantly depressed respiration rates after being exposed to low DO (week 8). This indicates that oxygen‐mediated plasticity in both gill morphology and physiology may confer tolerance to increasingly frequent and severe hypoxia in one important coral predator associated with reef decline.     

Physiological biomarkers of hypoxic stress in red swamp crayfish Procambarus clarkii from field and laboratory experiments

The crayfish industry in Louisiana is the largest in the United States, with crayfish frequently harvested from waters that experience episodic or chronic hypoxia (dissolved oxygen [DO]≤ 2 mg/l). We examined physiological biomarkers (hemolymph lactate, glucose, and protein concentrations) of hypoxic stress in the red swamp crayfish Procambarus clarkii from chronically hypoxic natural habitats and laboratory hypoxia experiments. P. clarkii from normoxic and hypoxic areas in the Atchafalaya River Basin were sampled monthly from April to July 2010. Laboratory experiments subjected P. clarkii to severe hypoxia (1 mg/l DO), moderate hypoxia (2 mg/l DO), or normoxic conditions (control: DO>7.5 mg/l) for 12, 24, and 48 h. P. clarkii from normoxic and hypoxic natural habitats did not display significantly different hemolymph lactate or glucose concentrations; however, mean hemolymph protein concentration was significantly lower in crayfish from hypoxic areas. P. clarkii exposed to severe hypoxia in laboratory experiments had significantly higher hemolymph lactate and glucose concentrations for all three exposure times, whereas large differences in protein concentrations were not observed. These results suggest that elevated hemolymph lactate and glucose concentrations are responses to acute hypoxia in P. clarkii, while differences in protein concentrations are the result of chronic hypoxic exposure.     

Hypoxic refuges,predator–prey interactions and habitat selection by fishes

Localized hypoxic habitats were created in Delta Marsh, Manitoba, Canada to determine the potential of regions of moderate hypoxia to act as refuges for forage fishes from piscine predators. Minnow traps and giving‐up density (GUD) plates (plexiglas plates covered with trout crumble and fine gravel) were used to assess habitat use and perceived habitat quality for forage fishes, respectively, while passive integrated transponder tags provided data on habitat use by predator species to assess the level of predation risk. Data were collected both before and after a hypoxia manipulation (2–3 mg l^?1 dissolved oxygen, DO) to create a before–after control–effect style experiment. Fathead minnows Pimephales promelas were more abundant and consumed more food from GUD plates in hypoxic bays after the DO manipulation, indicating hypoxic locations were perceived as higher quality, lower‐risk habitats. The frequency of predator visits was not consistently affected. The duration of visits, and therefore the total time spent in these habitats, however, was significantly shorter. These predator data, combined with the prey information, are consistent with the hypothesis that hypoxic regions function as predator refuges. The refuge effect is not the result of predator exclusion, however; instead predators are rendered less capable of foraging and pose less of a threat in hypoxic locations.     

Growth responses of age 0 white perch and yellow perch from field-enclosure experiments

In this paper, we investigate the potential for competition between age 0 white perch Morone americana and yellow perch Perca flavescens in field-enclosure experiments. In 1986 and 1987, different densities of white perch and yellow perch were stocked into cages in the western basin of Lake Erie and their growth responses determined. Fish were weighed and measured pre- and post-stocking. In 1986 when fish were food-limited, individual growth of both species were negatively affected by increased total fish density; yellow perch to a greater extent. In 1987, experiments were carried out later in the year at lower temperatures. Yellow perch were shown to feed and continue to grow at lower water temperatures than white perch. We conclude that competition in summer may be mitigated in the fall because of differences in thermal optima between these two species.     

Evidence of predatory control of yellow perch (Perca flavescens) recruitment in Lake Erie, U.S.A.

Predation can be a major factor in recruitment success of yellow perch ( Perca flavescens Mitchitl). Trawl catches of age-0 yellow perch in western Lake Erie declined from 870·3 per trawling h in June to 3·3 per trawling h in late July 1988. Coincident with the decline in relative abundance of age-0 yellow perch we found large numbers of age-0 yellow perch in the stomachs of small walleyes ( Stizostedion vitreum Mitchill). From this evidence we hypothesized that predation by walleyes may have caused the demise of the 1988 year-class of yellow perch. We used a population and bioenergetics modelling approach to estimate the impact of walleye predation on the abundance of age-0 yellow perch. Modelling showed that 6·8 × 10⁹ age-0 yellow perch that had attained 18 mm total length ( t.l. ), were eaten by small (age-2 and younger) walleye from June through July 1988. We estimated that walleye ate 28·4–89·7% of the yellow perch reaching 18 mm t.l. during 1988. The majority of this predation (77% of total) was by the abundant age-2 cohort of walleyes. We concluded that, even in a large system such as Lake Erie, predation can play a major role in structuring year-class strength of yellow perch and, thus, management of percid fisheries should be conducted on a fish-community basis.     

Effects of Hypoxic Exposure during Feeding on SDA and Postprandial Cardiovascular Physiology in the Atlantic Cod,Gadus morhua

Some Atlantic cod in the Bornholm Basin undertake vertical foraging migrations into severely hypoxic bottom water. Hypoxic conditions can reduce the postprandial increase in gastrointestinal blood flow (GBF). This could subsequently postpone or reduce the postprandial increase in oxygen consumption (MO₂), i.e. the SDA, leading to a disturbed digestion. Additionally, a restricted oxygen uptake could result in an oxygen debt that needs to be compensated for upon return to normoxic waters and this may also affect the ability to process the food. Long-term cardio-respiratory measurements were made on fed G. morhua in order to understand how the cardio-respiratory system of feeding fish respond to a period of hypoxia and a subsequent return to normoxia. These were exposed to 35% water oxygen saturation for 90 minutes, equivalent to the time and oxygen level cod voluntarily endure when searching for food in the Bornholm Basin. We found that i) gastric and intestinal blood flows, cardiac output and MO₂ increased after feeding, ii) gastric and intestinal blood flows were spared in hypoxia, and iii) there were no indications of an oxygen debt at the end of the hypoxic period. The magnitude and time course of the measured variables are similar to values obtained from fish not exposed to the hypoxic period. In conclusion, when cod in the field search for and ingest prey under moderate hypoxic conditions they appear to stay within safe limits of oxygen availability as we saw no indications of an oxygen debt, or negative influence on digestive capacity, when simulating field observations.     

A trophic bottleneck?: The ecological role of trout‐perch Percopsis omiscomaycus in Saginaw Bay,Lake Huron

Trout‐perch are abundant in many North American aquatic systems, but the ecological roles of trout‐perch as predators, competitors and prey remain relatively understudied. To elucidate the ecological role of trout‐perch in Saginaw Bay (Lake Huron, North America), the spatial and temporal diet composition was quantified and the frequency of occurrence of trout‐perch in diets of piscivorous walleye and yellow perch was evaluated. From May through November 2009–2010, trout‐perch and their potential predators and prey were collected monthly from five sites in Saginaw Bay using bottom‐trawls. Trout‐perch were abundant components of the Saginaw Bay fish community, and in 2009, represented 13.5% of fish collected in trawls, with only yellow perch (38%) and rainbow smelt (19.1%) being more common. Trout‐perch primarily consumed Chironomidae (84.0% of diet biomass) and exhibited strong, positive selection for Chironomidae and Amphipoda, suggesting that their diet preferences overlap with the economically important yellow perch and juvenile walleye. Energy content of trout‐perch averaged 4795 J g^?1 wet and was similar to yellow perch (4662 J g^?1 wet) and round goby (3740 J g^?1 wet). Thus, they may provide a comparable food source for larger piscivorous fish. However, despite their high energy density, abundance, and spatial overlap with other fish prey species, trout‐perch were very rare in diets of piscivorous walleye and yellow perch in Saginaw Bay, indicating that trout‐perch are a weak conduit of energy transfer to higher trophic levels.     

Growth, ingestion rates and metabolic activity of walleye in lakes with and without lake herring

Growth efficiencies, ingestions rates and activity levels of walleye Sander vitreus were compared in lakes with and without lake herring Coregonus artedi . Yellow perch Perca flavescens were the main prey in lakes without lake herring. Walleye were sampled in September and October from 38 lakes in Ontario in 1998 and 1999, using multimesh monofilament gillnets. Ingestion rates were estimated from annual increments in somatic mercury and body mass, and the mercury content of yellow perch and lake herring. Walleye had higher growth efficiencies, and lower ingestion and activity rates in lakes with lake herring. Lake herring grow larger than yellow perch and therefore could provide more profitable prey for larger walleye. The results are consistent with optimal foraging theory that predicts that walleye feeding on optimal prey sizes should grow more efficiently, if the ratio of feeding benefit (energy) to cost (search and seizure) is a function of the ratio of predator and prey size.     

Novel molecular approach demonstrates that turbid river plumes reduce predation mortality on larval fish

Turbidity associated with river plumes is known to affect the search ability of visual predators and thus can drive ‘top‐down’ impacts on prey populations in complex ecosystems; however, traditional quantification of predator–prey relationships (i.e. stomach content analysis) often fails with larval fish due to rapid digestion rates. Herein, we use novel molecular genetic methods to quantify larval yellow perch (YP) in predator stomachs in western Lake Erie to test the hypothesis that turbidity drives variation in larval predation. We characterize predator stomach content DNA to first identify YP DNA (single nucleotide polymorphism) and then quantify larval YP predation (microsatellite allele counting) in two river plumes differing in turbidity. Our results showed elevated larval YP predation in the less turbid river plume, consistent with a top‐down impact of turbidity on larval survival. Our analyses highlight novel ecological hypothesis testing using the power of innovative molecular genetic approaches.     

Zebra mussels affect benthic predator foraging success and habitat choice on soft sediments

The introduction of zebra mussels (Dreissena spp.) to North America has resulted in dramatic changes to the complexity of benthic habitats. Changes in habitat complexity may have profound effects on predator-prey interactions in aquatic communities. Increased habitat complexity may affect prey and predator dynamics by reducing encounter rates and foraging success. Zebra mussels form thick contiguous colonies on both hard and soft substrates. While the colonization of substrata by zebra mussels has generally resulted in an increase in both the abundance and diversity of benthic invertebrate communities, it is not well known how these changes affect the foraging efficiencies of predators that prey on benthic invertebrates. We examined the effect of zebra mussels on the foraging success of four benthic predators with diverse prey-detection modalities that commonly forage in soft substrates: slimy sculpin (Cottus cognatus), brown bullhead (Ameirus nebulosus), log perch (Percina caprodes), and crayfish (Orconectes propinquus). We conducted laboratory experiments to assess the impact of zebra mussels on the foraging success of predators using a variety of prey species. We also examined habitat use by each predator over different time periods. Zebra mussel colonization of soft sediments significantly reduced the foraging efficiencies of all predators. However, the effect was dependent upon prey type. All four predators spent more time in zebra mussel habitat than in either gravel or bare sand. The overall effect of zebra mussels on benthic-feeding fishes is likely to involve a trade-off between the advantages of increased density of some prey types balanced against the reduction in foraging success resulting from potential refugia offered in the complex habitat created by zebra mussels.     

Physiological refugia: swamps,hypoxia tolerance and maintenance of fish diversity in the Lake Victoria region

In Lake Nabugabo, Uganda, a satellite of Lake Victoria, approximately 50% of the indigenous fishes disappeared from the open waters subsequent to the establishment of the introduced predatory Nile perch, Lates niloticus. This pattern is similar to the faunal loss experienced in the much larger Lake Victoria. Several of these species persisted in wetland refugia (e.g. ecotonal wetlands, swamp lagoons); however, deep swamp refugia (habitats lying well within the dense interior of fringing wetlands), are available only to a subset of the basin fauna with extreme tolerance to hypoxia. Although air-breathers are common in deep swamp refugia; we also documented a surprisingly high richness and abundance of non-air-breathing fishes. We describe several mechanisms that may facilitate survival in deep swamp refugia including high hemoglobin concentration, high hematocrit, large gill surface area and a low critical oxygen tension (P(c)). In addition, swamp-dwelling fishes showed lower PO(2) thresholds for onset of aquatic surface respiration than the lake-dwelling fishes. This suggests higher tolerance to hypoxia in the swamp fishes because they are able to withstand a lower oxygen tension before approaching the surface. We suggest that physiological refugia may be important in modulating the impact of Nile perch and indigenous fishes in the Lake Nabugabo region; this highlights the need to evaluate relative tolerance of introduced predators and indigenous prey to environmental stressors.     

Bridge under troubled water: Turbulence and niche partitioning in fish foraging

The coexistence of competing species relies on niche partitioning. Competitive exclusion is likely inevitable at high niche overlap, but such divide between competitors may be bridged if environmental circumstances displace competitor niches to enhance partitioning. Foraging‐niche dimension can be influenced by environmental characteristics, and if competitors react differently to such conditions, coexistence can be facilitated. We here experimentally approach the partitioning effects of environmental conditions by evaluating the influence of water turbulence on foraging‐niche responses in two competing fish species, Eurasian perch Perca fluviatilis and roach Rutilus rutilus, selecting from planktonic and benthic prey. In the absence of turbulence, both fish species showed high selectivity for benthic chironomid larvae. R. rutilus fed almost exclusively on zoobenthos, whereas P. fluviatilis complemented the benthic diet with zooplankton (mainly copepods). In turbulent water, on the other hand, the foraging‐niche widths of both R. rutilus and P. fluviatilis increased, while their diet overlap simultaneously decreased, caused by 20% of the R. rutilus individuals turning to planktonic (mainly bosminids) prey, and by P. fluviatilis increasing foraging on littoral/benthic food sources. We show that moderate physical disturbance of environments, such as turbulence, can enhance niche partitioning and thereby coexistence of competing foragers. Turbulence affects prey but not fish swimming capacities, with consequences for prey‐specific distributions and encounter rates with fish of different foraging strategies (pause‐travel P. fluviatilis and cruise R. rutilus). Water turbulence and prey community structure should hereby affect competitive interaction strengths among fish species, with consequences for coexistence probability as well as community and system compositions.     

Interactions Between Nile Perch, Lates niloticus, and Other Fishes in Lake Nabugabo, Uganda

The introduction of the predatory Nile perch, Lates niloticus, into the Lake Victoria basin coincided with a dramatic decline in fish species richness and diversity. This study focused on interactions between Nile perch and indigenous fishes in Lake Nabugabo, Uganda, a small satellite lake of Lake Victoria. We evaluated how the foraging impact of juvenile Nile perch on prey fishes varied with the size of the predator. We also evaluated the role of wetland ecotones in minimizing interaction between Nile perch and indigenous fishes. Wetland ecotones in Lake Nabugabo were characterized by complex structure (e.g., dense vegetation) and lower dissolved oxygen levels than non-wetland (exposed) areas. Nile perch (8.6–42.2cm, TL) were 3.7 times more abundant in offshore exposed areas than in inshore areas near wetland ecotones, and the proportion of Nile perch using wetland and exposed areas was independent of their body size. However, species richness was higher in waters at wetland ecotones than in exposed areas. Nile perch (5–35cm, TL) exhibited a shift in diet at approximately 30cm TL from feeding primarily on invertebrates to piscivory. Although the shift to piscivory occurred at approximately the same body size for Nile perch from both wetland and exposed habitats, the shift to piscivory was less abrupt in Nile perch captured near wetland ecotones. Nile perch from wetland areas consumed a greater diversity and a larger percentage of fish prey than those from exposed sites. However, the low abundance of Nile perch in wetland ecotones suggested that interaction between predator and prey in these areas is much reduced.     

Increased parental care cost for nest-guarding fish in a lake with hyperabundant nest predators

Although parental care increases offspring survival, providingcare is costly, reducing parental growth and survival and, thereby,compromising future reproductive success. To determine if anexotic benthic predator might be affecting parental care bynest-guarding smallmouth bass (Micropterus dolomieu), we comparednest-guarding behavior and energy expenditures in two systems,one with a hyperabundant recently introduced predator, the roundgoby (Neogobious melanostomus). In Lake Erie, USA, smallmouthbass vigorously defended their nests from benthic round gobies.In Lake Opeongo, Canada, smallmouth bass were exposed to fewerand predominantly open-water predators and were less activein their nest defense. From scuba and video observations, wedocumented that nest-guarding smallmouth bass chased predators(99% of which were round gobies) nine times more frequentlyin Lake Erie than in Lake Opeongo. This heightened activityresulted in a significant decline in weight and energetic contentof guarding males in Lake Erie but no change in Lake Opeongomales. Bioenergetic simulations revealed that parental careincreased smallmouth bass standard metabolic rate by 210% inLake Erie but only by 28% in Lake Opeongo. As energy reservesdeclined and offspring became increasingly independent, malesin both lakes consumed more prey and spent more time foragingaway from their nests; however, nest-guarding smallmouth bassconsumed few prey and, in Lake Erie, rarely consumed round gobies.Therefore, increased parental care costs owing to the presenceof round gobies could affect future growth, reproduction, andsurvival if smallmouth bass approach critically low energy reserves.