Trophic plasticity of invasive juvenile largemouth bass Micropterus salmoides in Iberian streams

Biological invasions are a major factor for biodiversity loss, particularly in freshwater environments. Largemouth bass Micropterus salmoides is native to North America and is invasive on the Iberian Peninsula, primarily to provide angling opportunities in reservoirs. However, this species is a threat to the endemic Iberian fauna via predation and competition. Currently, there is little information on largemouth bass in European streams. Thus, we assessed the trophic plasticity and body condition of young largemouth bass in both invasive (the regulated Bullaque River) and native (Murray Creek) streams. Abundance of juvenile largemouth bass, percentage of full stomachs and body condition were higher in Bullaque River. Largemouth bass preyed on benthic invertebrates much more heavily in the Bullaque River, whereas fishes were the most important prey in Murray Creek. Prey richness, diet diversity and trophic niche breadth were higher in the Bullaque River population. Largemouth bass preferred water-column fishes as prey and avoided consuming benthic fishes in Murray Creek, whereas water-column fishes were avoided in Bullaque River. These results demonstrate that largemouth bass display substantial trophic plasticity which possibly facilitates its success as invasive species. Regulated Iberian streams may provide both suitable food and habitat resources with minimal predation pressure, and hence may serve as recruitment sources for this invasive fish.     

Predation as a determinant of size structure in populations of crucian carp (Carassius carassius) and tench (Tinea tinea)

Predation is an important structuring force in many fish assemblages and may have large, direct lethal effects on prey fish populations but also may affect growth rate and survival indirectly through shifts in behavioral patterns. Some species appear more sensitive to predation and earlier studies in a limited set of Finnish ponds suggests that crucian carp (Carassius carassius) is very sensitive to predation, which results in changes in population size-structure and density. In this study we tested the effect of piscivore presence or absence on crucian carp populations by using data from a large number (>600) of Swedish lakes across a wide geographical area and ranging in size from small ponds to large lakes (0.02–4400 ha). We also included a second species, tench (Tinea tinea), in our analyses. The population size structures of crucian carp and tench were clearly related to the presence or absence of piscivores. In lakes and ponds without piscivores. populations were dominated by small-bodied individuals. When piscivores were present, populations consisted almost exclusively of large individuals. Densities of crucian carp and tench were much higher in ponds and lakes without piscivores. There were no differences in size structure or relative abundance when catches from electrofishing were compared with gill net catches, suggesting that our results are not an effect of a behavioral shift in the presence of piscivores.     

Variation in DVM behaviour of juvenile and adult pearlside (Maurolicus muelleri) linked to feeding strategies and related predation risk

In the Norwegian fjord Masfjorden, different developmental stages of the mesopelagic planktivore Maurolicus muelleri form vertically separate sound scattering layers (SSLs) and in late autumn display different diel vertical migration (DVM) behaviour. Post‐larvae and juvenile fish perform normal crepuscular DVM, whereas the large majority of adults remain at depth throughout the diel period. In this study we examined the stomach contents of juvenile and adult fish caught at different times and depths during a 24‐h period in autumn. The different DVM behaviour of these two SSLs in addition to a shallow layer believed to be composed of post‐larvae is explained with a model for visual foraging in aquatic environments that uses gradients in vertical light intensity and copepod density and size as input variables. Field data revealed that vertically migrating juveniles distributed at a higher ambient light intensity and on average consumed 25 times more copepods than non‐migrating adult fish. The model showed that juveniles experienced a 15 times higher prey encounter rate and a higher level of predation risk than non‐migrating adults, and that the energetic benefits for post larvae and juveniles from prolonged feeding in a nearly constant and brighter environment outweigh the associated predation risk. The model also suggests that the visual detection range of piscivore predators is relatively more limited by the turbid surface water than that of their prey, which provide the post‐larva and juvenile life‐stages of M. muelleri a window of reduced visual predation near the surface.     

The effects of mechanically reducing northern pike density on the sport fish community of West Long Lake, Nebraska, USA

Abstract  Removal of piscivores may affect population size structure, abundance and growth rates of the remaining fish community. A total of 572 northern pike, Esox lucius L., was removed from 25-ha West Long Lake, Nebraska from 2003 to 2006. The northern pike population was reduced from 36 fish ha⁻¹ and 22 kg ha⁻¹ in 2002 to 20 fish ha⁻¹ and 9 kg ha⁻¹ in 2006. The northern pike population shifted to dominance by smaller individuals. Relative abundance increased for bluegill, Lepomis macrochirus Rafinesque, but remained similar between years for largemouth bass, Micropterus salmoides (Lacepède), and yellow perch, Perca flavescens (Mitchill). Bluegill and yellow perch size structure increased probably because of reduced predation by northern pike. The effects of reduced northern pike density on largemouth bass were less evident. The largemouth bass population may be returning to higher density with smaller size structure observed before pike were introduced. The sport fish community responded positively to the removal of northern pike in this small lake.     

Maximum prey size estimation of longtooth grouper,Epinephelus bruneus,using morphological features,and predation experiments on juvenile cannibalism

In fish culture, cannibalism, which induces critical damage, is separated into two types: type I, which occurs in early larval stages when size heterogeneity is low; and type II, which starts in later larval and/or early juvenile stages following type I when size heterogeneity is high. In type II, the cannibal's maximum prey size is limited by its gape size relative to the prey fish body size. Ratios of prey to cannibal body size changes are associated with allometric factors and feeding behaviour with growth. In a previous study on grouper, the maximal prey size was estimated morphologically (i.e., the relationship between gape size and body height). This method is very simple and useful; however, it does not consider allometric factors and feeding behaviour changes; therefore, it probably causes a large estimation error. To overcome this, we present a new method for estimating maximal prey size to determine the size ranges for size grading of longtooth grouper, Epinephelus bruneus, juveniles using predation experiments. It is shown that grouper type II cannibalism is affected by two factors; in early juvenile stages by morphology and in later (TL over 50 mm) by prey size selectivity.     

Piscivory in larval perch (Perca fluviatilis): mechanisms structuring larval roach (Rutilus rutilus) cohorts

Abstract – Perch ( Perca fluviatilis ) can act as a piscivore from larval stage VI (body size 10.3 mm) on newly hatched larval roach ( Rutilus rutilus ), bream ( Abramis brama ) and smaller siblings of its own cohort. Consumption rates at this stage were approx. 0.5 prey/perch*h at 21°C. Larval perch predation was strongly gap-limited, and the maximum size of roach consumed by perch (perch length interval 10.3–62.0 mm) under experimental conditions followed the linear regression, P_prey-max.=0.478*L_Pred.+1.829 ( r ²=0.99, P <0.001, n =12). Under experimental conditions, predatory 0⁺ perch substantially affected the size distributions of 0⁺ roach prey cohorts, since smaller prey individuals were predated more frequently than larger ones. In both unimodal and bimodal size distributions of prey roach, the distributions changed according to the maximum prey size consumed by the added predatory perch. Unimodal prey distributions were positively skewed when piscivorous perch were added compared with controls without predators. According to the size distributions of lake-living 0⁺ roach and 0⁺ perch and the relative size difference between prey and predator, the vulnerability of 0⁺ roach cohort to 0⁺ perch predation changed from June to September. Prey vulnerability was extremely sensitive to the relative size difference between predator and prey. Therefore differences in hatching time and growth rates between the two species will strongly influence the potential for predator-prey interactions. ^Note     

Diet dynamics of the adult piscivorous fish community in Spirit Lake, Iowa, USA 1995–1997

Abstract – Diets of adults of six important piscivorous fish species, black crappie Pomoxis nigromaculatus , largemouth bass Micropterus salmoides , northern pike Esox lucius , smallmouth bass Micropterus dolomieui , walleye Stizostedion vitreum , and yellow perch Perca flavescens were quantified in Spirit Lake, Iowa, USA from May to October in 1995–1997. Forty-one prey taxa were found in the diets of these species, including 19 species of fish. The most important prey taxa overall were yellow perch, amphipods and dipterans. Diets of northern pike and walleye were dominated by yellow perch. Largemouth bass diets included large percentages of both yellow perch and black bullhead Ameiurus melas . Smallmouth bass diets included large percentages of both yellow perch and crayfish. Black crappie and yellow perch diets were dominated by invertebrates, primarily amphipods and dipterans. There were pronounced differences in diets among species, among size classes within species and over time. Most of the dominant prey taxa we documented in the diets of piscivorous species were in accordance with previous studies, but a few deviated significantly from expectations. Many of the temporal diet changes were asynchronous among piscivorous species and size classes, suggesting different responses to common prey resources over time.     

The effect of depth on the interaction between perch (Perca fluviatilis) and minnow (Phoxinus phoxinus)

We tested the hypothesis that small fishes avoid piscivores by increasing their use of shallow water areas, thereby reducing the capture success of the piscivore. Experiments were conducted in 600-1 aquaria, using perch (Perca fluviatilis) as the piscivore and minnow (Phoxinus phoxinus) as the prey. Two size classes of minnows, presented alone and together, were placed together with perch in aquaria with and without shallow water areas. The capture success of perch and use of depth by minnows changed as predicted by our hypothesis. In addition, we observed a decrease in the shoal size of minnows when perch were present. Perch selected large minnows over small minnows when both size classes were present, probably because large minnows were a more energetically profitable prey size.     

Predation dynamics of mackerel on larval and juvenile anchovy: is capture success linked to prey condition?

We tested whether the predation dynamics of chub mackerel Scomber japonicus and spotted mackerel S. australasicus on young anchovy Engraulis japonicus relates to individual growth characteristics of the prey and could account for the growth-selective survival predicted by recruitment hypotheses. Juvenile and adult mackerel were sampled along with their young anchovy prey field in 2004 (juvenile mackerel and larval anchovy) and 2005 (adult mackerel and juvenile anchovy) off the Pacific coast of Honshu, Japan. The recent 5-day mean growth rate of larval and juvenile survivors and prey found in the stomach of mackerel was estimated from the otolith microstructure. No significant difference was found between the recent growth of larval or juvenile survivors and that of preyed individuals. We conclude that despite a relatively small body size, the high activity level and predation skills displayed by mackerel prevent fast-growing larvae and early juveniles from benefitting in terms of the expected survival advantage over slow-growers. Hence, growth-selective predation mortality of larval fish would depend on the feeding ecology of the predator rather than predator size. Selection for fast growth is more likely to occur under predation pressure from invertebrate organisms and small pelagic fish specialized on zooplankton, such as herring and anchovy.     

Whole-lake influences of littoral structural complexity and prey body morphology on fish predator–prey interactions

Abstract –  We used a longline tethering method to examine the relationship between prey refugia, prey body morphology and the location and magnitude of predation mortality within an individual lake and among three lakes that differed in coarse woody habitat (CWH) and aquatic macrophyte abundances. Predation events were lowest in the macrophyte and/or CWH refuges, peaked at or just beyond the refuge edge and declined in pelagic portions of the lakes. Predation risk at the refuge edge and just beyond was positively correlated with the abundance of littoral refuge. In contrast, predation events within the refuge and in the pelagic zone were negatively correlated with littoral refuge abundance. Deep-bodied and spiny prey morphologies were less vulnerable to predation than fusiform prey lacking fin spines. The structural complexity of littoral zones and prey fish body morphology may influence the outcome of predator–prey interactions and ultimately determine fish species assemblages in lakes.     

Diet overlap in a piscivore community

Abstract— We examined prey selection of largemouth bass ( Micropterus salmoides ), smallmouth bass ( M. dolomieu ), and yellow perch ( Percaftavescens ) by comparing diet overlap in a small, unexploited lake in Michigan, USA from 1988 to 1990. Niche hypervolume principles were applied to diet data as a means of assessing diet space for each species and the community as a whole. Largemouth bass occupied the largest proportion of community diet space (70.2%), followed by smallmouth bass (44.2%), and yellow perch (21.7%). The majority of community diet space (58.8%) was occupied by a single species, and 41.2% was shared by ≥ 2 species. Diet overlap was assessed by measuring the amount of diet space of one species occupied by the other species. Our analyses demonstrated that diet partitioning in a three species piscivore community is reflected in different use of the prey resource by co-occurring species. Niche overlap of largemouth bass with smallmouth bass and yellow perch is strongly asymmetric, largely due to the ability of largemouth bass to effectively consume prey of terrestrial origin.     

Density‐dependent diet composition of juvenile Atlantic salmon (Salmo salar)

Abstract – Decreases in body growth with increasing population density may be caused by reduced prey delivery rate. However, changes in food quality because of an increasing inclusion of suboptimal prey in the diet may also contribute to such effects. Here, we test for density‐dependent diet composition by creating spatial variation in Atlantic salmon young‐of‐the‐year (YOY) density in three replicate streams and obtain detailed information on individual positions (±1 m) and diet. Diet breadth with respect to prey size increased with increasing local density for the two most common prey types (Chironomidae and Ephemeroptera). For the largest prey type (Ephemeroptera), there was also an increase in mean prey size with increasing density, suggesting that YOY salmon preferentially utilise the smaller prey among those available. According to optimal foraging theory, changes in diet with increasing local density are likely to come at an energetic cost and hence may contribute to the commonly observed density‐dependent growth of juvenile salmonids.     

Satiation effects in piscivore prey size selection

Abstract –  We present a functional response model that primarily evaluates effects of satiation in piscivore prey-size selection. The model also includes other passive processes, such as prey-size-dependent encounter rate and prey-size-dependent capture success, where capture success decreases and encounter rate increases with prey size. The model generates a wide variety of outcomes, where small, intermediate or large prey is positively selected for. These very different selectivity patterns are generated without any active prey choice included in the model. The results stress the importance of controlling for satiation and other passive processes in empirical studies on prey-size selection, especially if the aim is to test active prey choice in piscivores.     

Projecting future habitat quality of three midwestern reservoir fishes under warming conditions

Rising temperatures caused by climate change are likely to affect cool‐water and warm‐water fishes differently. Yet, forecasts of anticipated temperature effects on fishes of different thermal guilds are lacking, especially in freshwater ecosystems. Towards this end, we used spatially explicit, growth rate potential (GRP) models to project changes in seasonal habitat quality for a warm‐water piscivore (largemouth bass Micropterus salmoides), a cool‐water piscivore (walleye Sander vitreus) and a hybrid piscivore (saugeye S. vitreus × S. canadensis) in two Midwestern reservoirs. We assessed habitat quality for two periods (early and middle 21st century) under two realistic greenhouse gas emission scenarios (a mid‐century emissions peak and a rapid continuous increase in emissions). Largemouth bass were projected to experience enhanced or slightly reduced habitat during all seasons, and throughout the mid‐21st century. By contrast, walleye habitat was projected to decline with anticipated warming, except during the spring in the smaller of our two study reservoirs and during the fall in the larger of our two study reservoirs. Saugeye habitat was projected to either increase modestly or decline slightly during the spring and fall and declines in habitat quality and quantity that were smaller than those for walleye were identified during summer. Collectively, our findings indicate that climate warming will differentially alter habitat suitability for reservoir piscivores, favouring warm‐water species over cool‐water species. We expect these changes in habitat quality to impact the dynamics of reservoir fish populations to varying degrees necessitating the consideration of climate when making future management decisions.     

Marine Fish Larvae Feeding: Formulated Diets or Live Prey?

In the rearing of larval marine fish, any diet that reduces dependance on live prey production is of technical and economic interest. Weaning juveniles with a completely developed digestive tract to a conventional diet, (i.e., "late weaning") can be successful in any marine fish species. For example, weaning one-month-old sea bass (20 mg) to the study's reference diet, Sevbar, resulted in over 85% survival (40% from hatching) and 1.25 g fish at day 90 (at 19 C)
In contrast, "early weaning" of larvae to special microdiets during the first month is still difficult. The best way to reduce live prey utilization in sea bass is to wean larvae at about 3–4 mg in size (day 20). If weaning could be accomplished 15 days earlier, Artemia savings could be as high as 80%. However, this introduces risks relative to growth retardation (30% weight loss) and lower juvenile quality, including greater size variability and skeletal abnormalities. Similar results have been obtained with commercial microparticles (Fry Feed Kyowa) and experimental microbound diets (MBD) made from raw materials (alginate MBD) or preferably from freeze-dried protein sources (zein MBD).
Total replacement of live prey is still impossible in marine fish. Sea bass larvae fed formulated diets exclusively from first feeding (0.3 mg larval wet weight), or even from their second week of life onwards, exhibited low survival and poor growth. Better results can be obtained when formulated diets are used in combination with live prey from first feeding, although the optimal ratio of live prey to formulated diet is still to be specified.     

Differentiated predation risk on hatchery-reared juvenile masu salmon by white-spotted charr with different body sizes

Predation after release is one of the major concerns of hatchery fish propagation. However, size-specific interaction between predator and prey on the survival of hatchery-released salmonid fish is largely unknown. To understand the size-selective predation risk, 24-h predation experiments were conducted on masu salmon Oncorhynchus masou in tanks. Four ranges of fork length (FL) were examined for masu salmon as a prey, in combination with three ranges of FL for white-spotted charr Salvelinus leucomaenis as a predator. The results show that not only predator and prey sizes, but also interaction between prey size and predator size, strongly affected the survival rate of masu salmon. Predation on masu salmon with the FL exceeding 40% of the FL of white-spotted charr was rare in the experiment. A logistic regression suggests that 37% relative FL of masu salmon to white-spotted charr results in the 50% survival of masu salmon. Our results suggest that adjusting relative size of hatchery fish to the size of local fish predators at the time of hatchery release will have a significant impact on the survival of hatchery fish in the wild. From this perspective, site-specific, adaptive management might be important to improve the effectiveness of hatchery fish propagation.     

Size-dependent predation on marine fish larvae by Ctenophores, Scyphomedusae, and Planktivorous fish

Size-dependent predation rates on marine fish larvae by the ctenophore Mnemiopsis leidyi , scyphomedusa Chrysaora quinquecirrha , and planktivorous fish Anchoa mitchilli were estimated via experiments in 3.2 m³ me-socosms. Predation rates on larvae of the goby Gobio-soma bosci were estimated in relation to 1) length of larval prey, 2) presence or absence of alternative < 1 mm zooplankton prey, and 3) a predator-prey interaction between the scyphomedusa and ctenophore. The consumption rate of larvae by the three predators generally declined as larval length increased. The ctenophore ate fewer (1.0 d^-1 per predator) larvae than did the medusa (7.8 d ^-1 per predator) or bay anchovy (4.0 d ^-1 per predator), but it consumed larvae in all size classes tested (3.0–9.5 mm SL). Predation by bay anchovy and the medusa was more size-dependent: 3.0–5.0 mm larvae were most vulnerable. Fewer larvae were eaten by the ctenophore and bay anchovy when zooplankton alternative prey was available, but predation on larvae by the medusa was not influenced by alternative zooplankton prey. Consumption rate of fish larvae by the medusa was reduced 20–25% when the ctenophore was present as its alternative prey. An encounter-rate model was developed and its parameters estimated from the experimental results. Model simulations indicated that the relationship between larval size and vulnerability is dependent on size, swimming speed, and behavior of both predators and larvae, and that bigger or faster-growing fish larvae are not always less vulnerable to predation.     

Modification of 0+ perch foraging behaviour by indirect cues of predation risk

Abstract –  Intake rate and prey size selection of 0+ perch, Perca fluviatilis , from Lake Wallersee (Austria) was studied at different illumination (day light: 400 lx, twilight: 2 lx) during 5 days of habituation to novel surroundings in aquaria. The hypothesis was tested that high illumination and novelty of surroundings (transfer from holding to test aquaria) as indirect cues of predation risk influences 0+ perch foraging behaviour. Significantly lower total intake and lower proportion of large Daphnia magna (two prey size groups, 2.9 and 1.2 mm, were used) were observed at higher illumination and under novel surroundings. Habituation to novelty caused an increase in consumption of large prey and decrease in consumption of small prey. During the whole period of habituation, more large prey was ingested at twilight than at daylight; no light-induced difference in ingestion of small prey was found. Foraging 0+ perch responded to indirect nonspecific cues of predation risk by reduction of intake of large prey, which are costly in terms of handling time. This allows fish to be more vigilant without ceasing their foraging activity even in potentially dangerous situations. In the lake, young perch are most vulnerable to abundant piscivorous fish and birds during the day; in twilight perch can afford a more time-consuming foraging activity.     

Relationships Among Total Weight, Body Shape, Visceral Components, and Fillet Traits in Palmetto Bass (Striped Bass Female Morone saxatilis× White Bass Male M. chrysops) and Paradise Bass (Striped Bass Female M. saxatilis× Yellow Bass Male M. mississippiensis)

Relationships among total weight (W), and linear measures of body shape, visceral component weights, and fillet weight (Y) in market-size (>454 g) palmetto bass (Morone saxatilis female ×M. chrysops male, N= 138) and paradise bass (M. saxatilis female ×M. mississippiensis male, N= 134) were determined with the allometric equation: Y = aW^b. Allometric analysis was used to compare traits of palmetto bass and paradise bass, and to identify factors influencing fillet yield. Paradise bass, an all female hybrid, had deeper, thicker, shorter bodies, and smaller heads than palmetto bass females. Male and female palmetto bass had similar body shapes. Values of growth coefficients (b) for body shape traits (range 0.21–0.48) indicated that shape was proportional across the weight range of fish used. Mean visceral fat and ovary weight were higher in paradise bass than in palmetto bass females suggesting the reproductive cycle was more advanced in paradise bass females. Whole fillet (skin and ribs intact) and skinless fillet (ribs intact) were larger for paradise bass than for palmetto bass, but trimmed fillet (skin and ribs removed) was not different between fish. Relative increases of whole and skinless fillet weights were greater than total weight in both groups indicating that the percentage of body mass attributed to fillet increases slightly as total weight increases. Therefore, small increases in fillet yield can be achieved by rearing fish to a larger size. Stepwise regression of whole, skinless, and trimmed fillet weight on body shape traits resulted in three parameter models with r²-values of 0.27–0.29 in palmetto bass, and of 0.37–0.43 in paradise bass. Addition of visceral components as independent variables in the models increased r²-values to 0.31–0.36 for palmetto bass and to 0.45–0.52 for paradise bass. Low phenotypic variation in fillet yield (CV = 3–5%) and poor predictability of yield from measures taken on live fish limit the potential for improving yield through individual selection. Identification of superior species or strain crosses or rearing fish to a larger size appear to be the best strategies for improving fillet yield of Morone hybrids.     

An experimental study of the effects of smallmouth bass on the behavior, growth and survival of prey fish

ABSTRACT The effects of smallmouth bass on the behavior, growth and survival of the prey fish (goldfish, ayu and Japanese dace) were examined experimentally in artificial ponds. In most cases, goldfish aggregated both when smallmouth bass were present and absent. Ayu and Japanese dace aggregated more frequently in the presence of smallmouth bass. All prey fish fed predominantly on benthic algae or aquatic insects and their dietary composition was not affected by smallmouth bass. In goldfish and Japanese dace, the total number of feeding acts exhibited by each individual was significantly less in the presence of smallmouth bass than in the absence and, in goldfish, the growth rate was less when smallmouth bass were present. In contrast, the presence of smallmouth bass did not affect either the total number of feeding acts or the growth rate of ayu. However, ayu were preyed on by smallmouth bass most frequently among the three prey species. Smallmouth bass tended to prey on smaller individuals, but ayu were not smaller than goldfish or Japanese dace.