Evidence for predatory control of the invasive round goby

We coupled bioenergetics modeling with bottom trawl survey results to evaluate the capacity of piscivorous fish in eastern Lake Erie to exert predatory control of the invading population of round goby Neogobius melanostomus. In the offshore (>20 m deep) waters of eastern Lake Erie, burbot Lota lota is a native top predator, feeding on a suite of prey fishes. The round goby invaded eastern Lake Erie during the late 1990s, and round goby population size increased dramatically during 1999–2004. According to annual bottom trawl survey results, round goby abundance in offshore waters peaked in 2004, but then declined during 2004–2008. Coincidentally, round goby became an important component of burbot diet beginning in 2003. Using bottom trawling and gill netting, we estimated adult burbot abundance and age structure in eastern Lake Erie during 2007. Diet composition and energy density of eastern Lake Erie burbot were also determined during 2007. This information, along with estimates of burbot growth, burbot mortality, burbot water temperature regime, and energy densities of prey fish from the literature, were incorporated into a bioenergetics model application to estimate annual consumption of round goby by the adult burbot population. Results indicated that the adult burbot population in eastern Lake Erie annually consumed 1,361 metric tons of round goby. Based on the results of bottom trawling, we estimated the biomass of yearling and older round goby in offshore waters eastern Lake Erie during 2007–2008 to be 2,232 metric tons. Thus, the adult burbot population was feeding on round goby at an annual rate equal to 61% of the estimated round goby standing stock. We concluded that the burbot population had high potential to exert predatory control on round goby in offshore waters of eastern Lake Erie.     

Arctic charr in sympatry with burbot: ecological and evolutionary consequences

The trophic niche and parasite infection of Arctic charr (Salvelinus alpinus) were explored in two lakes with sympatric burbot (Lota lota) and two lakes without burbot in subarctic Norway. The CPUE of burbot and charr were similar in one lake, but burbot had a low population density in the other. Burbot were benthivorous in both lakes. Other co-occurring species like brown trout (Salmo trutta), Atlantic salmon parr (Salmo salar), grayling (Thymallus thymallus) and minnow (Phoxinus phoxinus) were also benthivores. At high densities, benthivorous burbot forced the whole Arctic charr population to utilise mainly the limnetic trophic niche. In contrast, at low burbot density or without burbot present, Arctic charr were primarily benthivorous in the littoral zone. Thus, a clear interactive segregation in diet was observed between Arctic charr and burbot at high burbot densities. There was also a high predation pressure from burbot on young Arctic charr along the benthic zones. The extensive use of zooplankton as prey caused a high parasite infection pressure of copepod transmitted Diphyllobothrium spp. larvae, with the potential for high negative impact on the Arctic charr population. As the benthivore trophic niche was occupied by burbot, the ecological opportunities for polymorphism with benthivorous ecotypes or morphs of Arctic charr were probably prevented. Therefore, the sympatry with burbot seems to have large ecological and evolutionary consequences for this Arctic charr population compared with neighbouring lakes where burbot is absent.     

Diet, prey selection, and growth of larval and juvenile burbot Lota lota (L.)

The diet, prey selection, and growth of larval and juvenile Oneida Lake burbot, Lota lota (L.), was investigated from the time of first hatch in mid-April until their disappearance from the pelagic zone at the beginning of June. Mean length increased exponentially from 3–20 mm on 18 April to 15–00 mm on 29 May. The number and size of prey in burbot stomachs, and the prey size most selected by burbot increased as they grew. Prey width, not prey length, limited the size of prey ingested. The smallest burbot ate and selected highly for the largest prey they could ingest, but older burbot did not. As burbot grew, prey selection was highest first for the rotifer Asplanchna , followed by copepod nauplii, then cyclopoid copepods. Daphnia and calanoid copepods and cyclopoid copepods were selected to similar degrees by 29 May. For a given size, however, cyciopoid copepods were selected more strongly than any other prey type from 18 to 29 May. There were no significant differences in prey selection among offshore, nearshore, and onshore habitats, nor among depths in the offshore habitat.     

Effects of food abundance, density and climate change on reproduction in the sparrowhawk Accipiter nisus

The reproductive success of predators depends on abiotic environmental conditions, food abundance and population density, and food abundance, density and their interactions may respond to changes in climatic conditions. Timing of reproduction by five of the eight numerically most common prey of the sparrowhawk Accipiter nisus advanced significantly since 1971, during a period of temperature increase. There was no evidence that mean laying date or any other reproductive parameter of sparrowhawks changed consistently during the study period 1977–1997. Laying date advanced and percentage of unsuccessful female sparrowhawks decreased with beech mast in the current year, an index of food abundance for avian prey. Mean laying date of sparrowhawks was advanced in warmer springs, and although mean clutch size was not larger in warm than in cold springs, mean brood size of successful pairs and breeding success increased in such springs, showing that sparrowhawks enjoyed a fitness gain when reproducing early. The timing of sparrowhawk reproduction with respect to the peak in abundance of fledgling prey increased, from a good match between mean timing of fledging by prey and maximum demand for food by the predator in 1977, to reproduction occurring later than the peak in fledging prey availability in 1997. The size of the breeding population of sparrowhawks was not predicted by mean spring temperature, the size of the breeding population the previous year or beech mast crop. The size of the post-breeding population was predicted by size of the breeding and post-breeding population the previous year and by the proportion of unsuccessful females the current year. These findings imply that sparrowhawks did not respond to change in climate, although climate changed the timing of reproduction by the main prey species.     

Predator odor recognition and antipredatory response in fish: does the prey know the predator diel rhythm?

We studied in a laboratory experiment using stream tanks if two percid prey fish, the perch (Perca fluviatilis) and the ruffe (Gymnocephalus cernuus), can recognize and respond to increased predation risk using odors of two piscivores, the pike (Esox lucius) and the burbot (Lota lota). Burbot is night-active most of the year but pike hunts predominantly visually whenever there is enough light. Perch is a common day-active prey of pike and dark-active ruffe that of burbot. We predicted that besides recognizing the predator odors, the prey species would respond more strongly to odors of the predator which share the same activity pattern. Both perch and ruffe clearly responded to both predator fish odors. They decreased movements and erected the spiny dorsal fins. Fin erection showed clearly the black warning ornamentation in the fin and thus erected fin may function besides as mechanical defense also as warning ornament for an approaching predator. No rapid escape movements were generally observed. Both perch and ruffe responded more strongly to pike odor than to burbot. There were no clear differences in response between day and night. In conclusion, we were able to verify clear predator odor recognition by both prey fish. Both perch and ruffe responded to both predator odors and it seemed that pike forms a stronger threat for both prey species. Despite of diel activity differences both perch and ruffe used the same antipredatory strategies, but the day-active perch seemed to have a more flexible antipredatory behavior by responding more strongly to burbot threat during the night when burbot is active.     

The response of burbot (Lota lota) to change in lake trout (Salvelinus namaycush) abundance in Lake Opeongo,Ontario

The purpose of this study was to test the hypothesis that the burbot and lake trout compete with or prey on each other, and that a change in the abundance of lake trout has triggered a response in the burbot population. Burbot growth, length-weight relationship and population size did not change, and it appears that at these population levels, the changes in lake trout abundance had no detectable effect on burbot. It is suggested that the burbot population in Lake Opeongo is limited at the larval stage by competition with, or predation by, a planktivore, the lake herring.     

Ontogenetic niche shifts and resource partitioning in a subarctic piscivore fish guild

The feeding ecology of three piscivorous fish species (perch (Perca fluviatilis), pike (Esox lucius) and burbot (Lota lota)), was studied in the subarctic Pasvik watercourse (69 °N), northern Norway and Russia. All three species primarily occupied the benthic habitats in the watercourse. Perch and burbot exhibited distinct ontogenetic niche shifts in food resource use, perch changing from a dominance of zooplankton to zoobenthos to fish, and burbot from zoobenthos to fish. Fish prey dominated the diet of all the investigated size-classes of pike, but small-sized pike (<20 cm) were not represented in the sample. Fish prey size was positively related to predator size in all three species. Whitefish (Coregonus lavaretus) was the dominant prey of pike and large-sized burbot and perch. Nine-spined sticklebacks (Pungitius pungitius) was also an important prey and appeared to be a dietary stepping-stone enhancing the transition from invertebrate feeding to consumption of large-sized whitefish prey for all three predators. A cluster analysis separated the different size groups of the three predator species into five functional feeding groups, most of them containing two or all three species. Within these feeding groups, and especially among the piscivorous size-classes, there was a strong and significant interspecific overlap in prey selection, and the dietary similarities between the species were in general much larger than the intraspecific similarities between ontogenetic stages. All three piscivorous species are important top predators in the aquatic food web of the watercourse, and their ontogenetic diet shifts and resource partitioning patterns generate a substantial food web complexity in this subarctic ecosystem.     

Piscivory and prey selection of four predator species in a whitefish dominated subarctic lake

According to logistic regressions derived for pike Esox lucius and burbot Lota lota , the probability of ingesting fishes in Lake Muddusjärvi, northern Finland, was 50% at 19·3 and 22·1 cm L _T, whereas Arctic charr Salvelinus alpinus and brown trout Salmo trutta shifted to piscivory at the lengths of 25·7 and 26·4 cm L _T. The specialist piscivores, pike and burbot, consumed more prey species and took a wider range of prey sizes than Arctic charr and brown trout. The prey length for all predators increased in relationship to predator length. Whitefish Coregonus lavaretus was the dominant prey species in the lake and in the diet of all the piscivorous species. The whitefish population was divided into three forms, of which the slow-growing, and the most numerous densely rakered whitefish form (DR), was selected by all predator species. This form also had the smallest average size and widest habitat range, utilizing both pelagic and epibenthic habitats. Two sparsely rakered whitefish forms (LSR and SSR) occupied only epibenthic habitats and had lower relative densities than DR. These forms, LSR and SSR, had a minor importance in the diet of predator species.     

Test of Competitive Interactions for Space Between Two Benthic Fish Species, Burbot Lota lota, and Stone Loach Barbatula barbatula

The substrate preferences, growth rates and foraging efficiency of two small benthic fish species, juvenile burbot, Lota lota, and stone loach, Barbatula barbatula, were compared in combined outdoor mesocosm and indoor laboratory experiments. Both species preferred the same stony substrate when alone, but significant differences in habitat selection were found between the two species under food deprivation and competition conditions. In burbot, preference for the stony habitat was reinforced under food-deprivation conditions and became even stronger when a potential competitor, the stone loach, was present. In contrast, stone loach switched to the gravel substrate when either starving or in the presence of a heterospecific competitor. Growth rates and foraging efficiency of burbot were significantly highest in the stony substrate and decreased with finer substrates. In stone loach, neither growth rates nor foraging efficiency were significantly different among the different substrates. The results provide an example of habitat partitioning by means of different competition styles, with a stenoecious, dominant style of the burbot and an euryoecious, evasive style of stone loach allowing coexistence of two sympatric fish species by graded interactions at an individual level in the littoral zone of a large lake.     

Lake Whitefish and Lake Herring Population Structure and Niche in Ten South-Central Ontario Lakes

Synopsis This study compares simple fish communities of ten oligotrophic lakes in south-central Ontario. Species densities and population size structure vary significantly among these lake communities depending on fish species present beyond the littoral zone. Lake whitefish are fewer and larger in the presence of lake herring than in their absence. Diet analysis indicates that lake whitefish shift from feeding on both plankton and benthic prey when lake herring are absent to a primarily benthic feeding niche in the presence of lake herring. When benthic round whitefish are present, lake whitefish size and density decline and they move lower in the lake compared to round whitefish. Burbot are also fewer and larger in lakes with lake herring than in lakes without herring. Burbot, in turn, appear to influence the population structure of benthic coregonine species. Lower densities of benthic lake whitefish and round whitefish are found in lakes containing large benthic burbot than in lakes with either small burbot or where burbot are absent. Predation on the pelagic larvae of burbot and lake whitefish by planktivorous lake herring alters the size and age structure of these populations. As life history theory predicts, those species with poor larval survival appear to adopt a bet-hedging life history strategy of long-lived individuals as a reproductive reserve.     

Effects of prey density on growth and survival of white sucker,Catostomus commersoni,and pumpkinseed,Lepomis gibbosus,larvae

Synopsis Growth and survival of white sucker and pumpkinseed larvae were examined in the laboratory in relation to prey density. Mortality of both species was greatest during the transition from endogenous to exogenous nutrition. Mortality prior to yolk absorption was substantial for white suckers and was not related to prey density whereas, pumpkinseed mortality during the same period was low. After yolk absorption, however, pumpkinseed suffered considerably higher mortality rates than white sucker larvae and, in both species, mortality and growth were directly related to prey density. The minimum prey density supporting 10% survival of pumpkinseed larvae was estimated at 0.16 plankters ml^-1 whereas, for white suckers it was 0.15 plankters ml^-1. Significant growth of both white sucker and pumpkinseed larvae, however, occurred only at prey densities higher than 0.25 plankters ml^-1. We suggest that since egg size and yolk reserve are greater in sucker larvae, suckers are better adapted to survive short term declines in prey abundance during the transition to exogenous feeding than pumpkinseed larvae. The reproductive strategies of each species seems to reflect this, with suckers spawning over a short time interval, but producing young with large yolk. Pumpkinseed, in contrast, spawn intermittently over an extended period but produce young with relatively small volk reserves.     

In situ estimation of gastric evacuation and consumption rates of burbot (Lota lota) in a summer‐warm lowland river

The burbot (Lota lota) is the only cold‐stenothermal gadoid inhabiting freshwaters with high temperature amplitudes. Summer temperatures up to 25°C have been reported as being far above the thermal preferendum of this species. Thus this study aimed to determine gastric evacuation, daily food consumption and energy uptake of burbot at high temperatures under in situ conditions. These data are prerequisites for bioenergetics modelling of the adaptive physiological behaviour of burbot in summer‐warm freshwaters. The study was conducted in the lower Oder River, Germany between May and October 2003 covering a temperature range between 4.6 and 23.4°C. A total of 1683 burbot was caught in five successive 24‐h fisheries. The mean index of stomach fullness showed a significant decrease with rising temperature. Highest stomach fullness values were reached at 4.6°C in October. A negative correlation was observed between gastric evacuation and temperature. In contrast to the data reported in the literature gastric evacuation was found to be high at 4.6°C, which may indicate an ongoing compensatory adaptation to the long period of high temperatures in July and August, when a significant reduction of the mean daily ration was observed. Balancing the energy flux probably resulted in a decrease in the hepatosomatic index at high temperatures and an increase during the cool period, when the liver energy was restored. The endogenous energy pool of the liver may determine the survival of burbot during summer months. The ability of burbot to actively forage during winter and to rapidly digest high amounts of food at cold temperatures was considered an adaptation to increase survival chances in an environment with high environmental, especially thermal, heterogeneity as is typical for a summer‐warm lowland river. Limitations of the energy budget have to be expected with extended periods of high temperatures.     

Diel and seasonal patterns of food intake and prey selection by Coregonus sp. in re-oligotrophicated Lake Lucerne, Switzerland

Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Lucerne in Switzerland was high during dusk when the bulk of potential prey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seasons reflected changes in prey availability. During the intensive feeding and growing period (May-September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non-evasive species, Daphnia spp. and Bythotrephes longimanus , were the most numerous and frequent organisms in the diet during the major part of the growing season. Smaller ( Bosmina spp.) as well as evasive species (cyclopoid copepods) were consumed in large numbers when larger, non-evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus , while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not only selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure. The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the prey and the present trophic state of the lake.     

Life history and ecological characteristics of humper and lean ecotypes of lake trout stocked in Lake Erie

Through combined effects of over-exploitation and predation by invasive non-native sea lamprey Petromyzon marinus (Linneaus 1958), the lake trout Salvelinus namaycush (Walbaum 1972) population in Lake Erie collapsed and was considered extirpated by 1950; annual stocking occurred in 1969 to bolster and maintain this population as natural recruitment ceased. While various hatchery strains of lean ecotype lake trout remain the focus of Lake Erie’s stocking, between 2004 and 2011 a humper-like ecotype was stocked to aid re-establishment efforts. We assessed ecotype performance via adult fish collected during the summer-stratified period (2004–2017) and evaluated if life history and ecological characteristics varied between stocked ecotypes. Ecotype-specific differences in survival, length-at-age, accumulation of weight with length, and diet were observed. Lean lake trout grew faster (length-at-age) and matured at larger sizes than the humper ecotype. Humper lake trout exhibited higher sea lamprey wounding and lower survival rates than lean lake trout. Humpers also consumed more benthic prey fish than lean lake trout, suggesting this ecotype may exhibit a more benthic orientation than leans. Our study represents the first evaluation of how a non-lean ecotype lake trout performs outside their native ecosystem and provides fishery managers with valuable information relevant to re-establishing lake trout populations in the Great Lakes.

     

Spatial structure in length at age of cod in the Barents Sea

Cod Gadus morhua population in the Barents Sea was found to be spatially structured with regard to length-at-age. Results were based on data collected during research surveys in the Barents Sea between 1982 and 1997. The identified spatial structure was most pronounced for age groups 2–4 years and decreased for the older age groups with higher potential for migration. A positive linear correlation between mean length-at-age and mean geographical temperature was established for age groups 2–4 years. This correlation was shown to be strongest when based on mean temperatures during 3 year periods ending with the year of capture. The spatial structure in length-at-age was shown to follow the temperature gradient of the Barents Sea. A large part of the observed area effects could be explained by temperature variation between areas. Evidence is also presented which indicates that the predictability and sensitivity of the dependence of length-at-age on temperature increases under extreme environmental conditions, i.e. in the northern and eastern areas of the Barents Sea.     

Burbot (Lota lota) exhibit plasticity in life-history traits in a small drainage at the southwestern-most extent of the species' native range

Little is known about the life-history traits exhibited by burbot (Lota lota) throughout their circumpolar range. Monitoring burbot movements between lentic and lotic habits and collection of demographic data (length, age, sex, and maturity) were used to answer the following questions in the Torrey Creek drainage of west-central Wyoming, USA: (a) is there plasticity in the life-history traits of the burbot population, (b) do Trail Lake origin and Torrey Creek origin burbot interchange during the spawning period, and (c) is there a difference in growth and age at sexual maturity between burbot captured in Trail Lake and Torrey Creek? Results indicated that burbot in Trail Lake and Torrey Creek exhibit plasticity in their life history traits. Directional movement of PIT-tagged burbot in Trail Lake and Torrey Creek was monitored nearly continuously by tandem stream-width antennas. Thirty-five percent of lentic-origin burbot migrated upstream into Torrey Creek, and 11% of lotic-origin burbot migrated downstream of the antennas near the Torrey Creek inlet to Trail Lake. Migratory activity of burbot was highest during the late winter and early spring at a time that coincided with spawning. Additionally, Torrey Creek-origin burbot were smaller and younger than Trail Lake-origin burbot. In addition to documenting migration, the sampling of small, sexually mature burbot in Torrey Creek suggests that stream-resident burbot reach maturity earlier than adfluvial and lacustrine burbot in Trail Lake. Furthermore, high catch rates of age-0 burbot indicate that Torrey Creek upstream from Trail Lake provides nursery habitat to the burbot population. Life history trait plasticity and spawning and nursery habitats documented in this study should be considered when selecting conservation actions for this unique burbot population.     

The impact of raptors on the abundance of upland passerines and waders

The issue of predator limitation of vertebrate prey populations is contentious, particularly when it involves species of economic or conservation value. In this paper, we examine the case of raptor predation on upland passerines and waders in Scotland. We analysed the abundance of five wader and passerine species on an upland sporting estate in southern Scotland during an eight-year period when hen harrier, peregrine and merlin numbers increased due to strict law enforcement. The abundance of meadow pipit and skylark declined significantly during this time. Golden plover also showed a declining trend, whereas curlew increased significantly and there was a near significant increase in lapwings. Contrasting the local population trends of these species with trends on nearby areas revealed higher rates of decline for meadow pipit and skylark at the site where raptors increased, but no differences in trends for any of the three wader species. There was a negative relationship between the number of breeding harriers and meadow pipit abundance the same year and between total annual raptor numbers and meadow pipit abundance. Predation rates of meadow pipit and skylark determined from observations at harrier nests suggested that predation in June was sufficient to remove up to 40% of the June meadow pipit population and up to 34% of the June skylark population. This 'quasi-natural' experiment suggests that harrier predation limited the abundance of their main prey, meadow pipit, and possibly the abundance of skylark. Thus, high densities of harriers may in theory reduce the abundance of the prey species which determine their breeding densities, potentially leading to lower harrier breeding densities in subsequent years. We found no evidence to suggest that raptor predation limited the populations of any of the three wader species. We infer that concerns over the impact of natural densities of hen harriers on vulnerable upland waders are unjustified.     

Effects of temperature and a piscivorous fish on diel winter behaviour of juvenile brown trout (Salmo trutta)

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Phenotypic variation of an alien species in a new environment: the body size and diet of American mink over time and at local and continental scales

Introduced species must adapt their ecology, behaviour, and morphological traits to new conditions. The successful introduction and invasive potential of a species are related to its levels of phenotypic plasticity and genetic polymorphism. We analysed changes in the body mass and length of American mink (Neovison vison) since its introduction into the Warta Mouth National Park, western Poland, in relation to diet composition and colonization progress from 1996 to 2004. Mink body mass decreased significantly during the period of population establishment within the study area, with an average decrease of 13% from 1.36 to 1.18 kg in males and of 16% from 0.83 to 0.70 kg in females. Diet composition varied seasonally and between consecutive years. The main prey items were mammals and fish in the cold season and birds and fish in the warm season. During the study period the proportion of mammals preyed upon increased in the cold season and decreased in the warm season. The proportion of birds preyed upon decreased over the study period, whereas the proportion of fish increased. Following introduction, the strictly aquatic portion of mink diet (fish and frogs) increased over time, whereas the proportion of large prey (large birds, muskrats, and water voles) decreased. The average yearly proportion of large prey and average‐sized prey in the mink diet was significantly correlated with the mean body masses of males and females. Biogeographical variation in the body mass and length of mink was best explained by the percentage of large prey in the mink diet in both sexes, and by latitude for females. Together these results demonstrate that American mink rapidly changed their body mass in relation to local conditions. This phenotypic variability may be underpinned by phenotypic plasticity and/or by adaptation of quantitative genetic variation. The potential to rapidly change phenotypic variation in this manner is an important factor determining the negative ecological impacts of invasive species. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 681–693.     

Spatial and temporal variations in the White-tailed Eagle Haliaeetus albicilla breeding diet revealed by prey remains

ABSTRACT

Capsule: Despite its population increase, the White-tailed Eagle Haliaeetus albicilla has not become food-limited, and does not pose a serious threat to birds of conservation concern in the region east of the Baltic Sea.

Aims: We aimed to test variations in the main prey groups and dietary niche breadth of White-tailed Eagles through the years and along an environmental gradient as well as to evaluate the extent of predation on bird species of conservation concern.

Methods: Prey remains were collected during nestling ringing from successful nests located in Lithuania.

Results: The frequency of the prey groups varied along an environmental gradient, with a general pattern of increasing avian prey consumed by pairs breeding in areas with less abundant aquatic habitats. The frequency of alternative prey (i.e. terrestrial birds, mammals) and dietary niche breadth did not increase between 2005 and 2018 and during which time the White-tailed Eagle population nearly doubled. Instead, the occurrence of Northern Pike Esox lucius remains, which was the most frequent single prey species, increased throughout the study period. Bird species of conservation concern constituted only a small fraction of all the prey identified.

Conclusions: The White-tailed Eagle has not become more dependent on alternative prey despite its population increase.