Intraspecific variation in Gape–prey size Relationships and Feeding Success During Early Ontogeny in Red Drum, Sciaenops Ocellatus

The relationship between predator gape and prey consumption in laboratory-reared larva and field-caught early juvenile red drum, Sciaenops ocellatus, was investigated in light of the hypothesis that feeding success varies throughout the early life history intervals of marine fishes. We expected the feeding ability of red drum to be more strongly constrained by mouth gape in smaller fish and expected this ability to improve with gape size. To test this hypothesis, field-caught, early juvenile red drum were examined to determine the relationship between gape size and prey size consumed. In field-caught early juveniles, gape (height and width) and prey size consumed (length and width) increased linearly with standard length (SL); however, mean width of prey consumed was only 20–47% of gape width. Furthermore, when regressed on SL, gape width yielded a higher slope than prey width. To further test this hypothesis on less developed, pre-metamorphic fish, age-specific differences in gape, number of prey and size of prey consumed prior to metamorphosis were determined from laboratory-reared red drum larvae. Similar patterns were observed for gape height– and gape width–SL relationships in laboratory-reared red drum larvae. Size of consumed prey increased from three days from hatching (dfh) to 18dfh. The percentage of feeding larvae also increased from 3% at 3dfh to 97% at 18dfh. In both field-caught, early juvenile red drum and laboratory-reared larvae, there was little evidence that the size of prey consumed was constrained by mouth gape. It is hypothesized that besides gape size, the development of other features of the feeding mechanism (e.g., hyoid and opercular series) influences prey-capture performance prior to settlement in marine fishes.     

The food and feeding relationships of larval and 0+ year juvenile fishes in lowland rivers and connected waterbodies. I. Ontogenetic shifts and interspecific diet similarity

Many fish species are zooplanktivorous at the onset of exogenous feeding, but distinct shifts in their foraging biology may occur with development. This study investigated the food and feeding relationships of the larvae and 0+ year juveniles of 13 fish species in 'main river', 'marina' and 'pond' macrohabitats in the lower River Trent, England. Young larvae had narrow diet spectra, with feeding diversity and the importance of species-specific traits increasing with development. Two main ontogenetic shifts in diet were differentiated, corresponding with the transition from finfold to finformed larvae, and from the larval to the juvenile period. Dietary overlap was generally greatest among ecologically similar species, with competition and resource partitioning most likely where prey availability was lowest. The greater availability of zooplankton in connected waterbodies compared with main river channels provides suitable prey for a wide range of fish species, and may enhance recruitment success.     

Trophic relationships and seasonal utilization of salt-marsh creeks by zooplanktivorous fishes

Synopsis In a high salinity estuary at North Inlet, South Carolina, co-occurrence and possible competition among adults of four dominant zooplanktivorous fishes were minimized by seasonal adjustments in lateral and vertical distributions as well as in dietary preferences. In winter, Atlantic silversides, Menidia menidia, occupied the entire water column while other planktivores were rare or absent from the estuary, and they consumed large prey such as mysid shrimps and fish larvae. An immigration of bay anchovies, Anchoa mitchilli, in the spring resulted in a redistribution of species with Atlantic silversides shifting to the surface waters and bay anchovies dominating the lower half of the water column. Both fishes consumed mostly copepods in the spring, but each favored a different species. There was little similarity in the large prey items consumed by the two fishes. Striped anchovies, Anchoa hepsetus, arrived in mid-summer and were most abundant at the surface while bay anchovies continued to dominate the bottom waters. Atlantic silversides were rare in all summer collections. The diets of the two anchovies were similar, but vertical separation during the period of maximum zooplankton abundance probably minimized competition. Rough silversides, Membras martinica, which were obligate surface dwellers, shared the upper water column with striped anchovies, but the two species had very different diets during their period of co-occurrence. Although seasonal changes in fish diets reflected shifts in zooplankton composition and all fishes consumed a variety of prey types, preferences for some prey taxa and total avoidance of others were indicated. Electivity indices indicated an especially strong selection for fiddler crab megalopae by all fishes in the summer and fall. All fishes, except rough silversides, which fed almost exclusively on copepods and crab zoeae, consumed large prey items when they were available. Fine scale partitioning of the food resources was apparent in the selection of different copepod and insect species by the fishes. Spatial and temporal separation in the distribution and/or dietary preferences of the zooplanktivores fishes probably reduces the potential for resource competition. Given the high abundances and selectivity of the planktivores, significant impacts on some zooplankton populations probably result.     

A Review of Predation Impact by 0+ Fish on Zooplankton in Fresh and Brackish Waters of the Temperate Northern Hemisphere

To assess potential differences in predation impact on zooplankton communities by small (larva) and large 0+ juvenile fish, 18 studies were reviewed from fresh water and the brackish Baltic Sea of the northern hemisphere temperate region. These case studies were performed either in the field or in mesocosm experiments. Larva stocks were found to exert only minor impact on small zooplankton species such as rotifers, copepodids and small cladocerans. In contrast, stocks of 0+ juveniles were found to have the potential to depress populations of large cladocerans and copepods, especially during late summer and autumn. However, studies where both 0+ juvenile fish consumption and zooplankton dynamics and production were exactly quantified are still very rare, and therefore final evaluation of this interaction cannot be made. In addition, papers were summarized that describe differences in morphological and physiological performance between larva and 0+ juvenile fish. The greater impact of 0+ juvenile fish on large zooplankton may be explained by their larger mouth gape and by their better developed abilities to detect and consume their prey items. However, this partly is lessened by the lower energy requirements of juvenile fish compared with identical biomasses of fish larvae, although larva bioenergetics remains only fragmentarily understood. Consequently, selective predation by fish larvae on particular small zooplankton prey may be more important than has been detected so far.     

Feeding ecology of larval and juvenile black seabream (Acanthopagrus schlegeli) and Japanese seaperch (Lateolabrax japonicus) in Tolo Harbour, Hong Kong

Black seabream, Acanthopagrus schlegeli, and Japanese seaperch, Lateolabrax japonicus, are important commercial species in the coastal waters of western Pacific Ocean, including Japan, Korea and China. In Hong Kong, larvae and juveniles of these two species occur in bays and estuaries during late winter and spring. This study reports on the ontogenetic changes in food habits in larvae and juveniles of these species in an artificial rocky shore area. Copepods and cladocerans were the most numerous food items for black seabream. There was a shift to larger and benthic prey as the fishes grew. Japanese seaperch <2.1cm fed predominantly on copepods and cladocerans, while larger prey were added as fish size increased. Japanese seaperch >6.0cm were piscivorous. Maximum prey width increased with fish standard length and mouth gape width in both species. Overall, black seabream showed greater diet breadth than did Japanese seaperch. In black seabream, diet breadth increased with fish size. In Japanese seaperch, diet breadth increased with size for fishes <4.0cm, then decreased as the fishes became piscivorous. Prey selectivity in black seabream was determined using information on prey availability in plankton samples. In general, preference was stronger for cypris larvae, Penilia avirostris and decapod larvae than for copepods and podonids. In recent years, overfishing and environmental degradation have led to the decline of fish populations in Tolo Harbour. Absence of fishes with empty gut indicates that inner Tolo Harbour is still an important nursery area for these two commercial species.     

Diel variations in the assemblage structure and foraging ecology of larval and 0+ year juvenile fishes in a man‐made floodplain waterbody

This study investigated diel variations in zooplankton composition and abundance, and the species composition, density, size structure, feeding activity, diet composition and prey selection of larval and 0+ year juvenile fishes in the littoral of a man‐made floodplain waterbody over five 24 h periods within a 57 day period. There was a significant difference in the species composition of diurnal and nocturnal catches, with most species consistently peaking in abundance either during daylight or at night, reflecting their main activity period. There were no consistent diel patterns in assemblage structure or the abundance of some species, however, most likely, respectively, due to the phenology of fish hatching and ontogenetic shifts in diel behaviour or habitat use. There were few clear diel patterns in the diet composition or prey selection of larval and 0+ year juvenile roach Rutilus rutilus and perch Perca fluviatilis, with most taxa consistently selected or avoided irrespective of the time of day or night, and no obvious shift between planktonic and benthic food sources, but dietary overlap suggested that interspecific interactions were probably strongest at night. It is essential that sampling programmes account for the diel ecology of the target species, as diurnal surveys alone could produce inaccurate assessments of resource use. The relative lack of consistent diel patterns in this study suggests that multiple 24 h surveys are required in late spring and early summer to provide accurate assessments of 0+ year fish assemblage structure and foraging ecology.     

Transitional ecological requirements for early juveniles of two sympatric stichaeid fishes,Cebidichthys violaceus andXiphister mucosus

Synopsis The distribution patterns, diets, and substratum (refuge) requirements of early juveniles of two sympatric stichaeid fishesCebidichthys violaceus andXiphister mucosus, were investigated in a rocky intertidal habitat at Diablo Canyon, California. Monthly investigations were conducted at low tide for four consecutive months, to assess ontogenetic differences in distribution, diet, and refuge requirements within and between the two species. Distinct differences in vertical zonation were exhibited by both stichaeids throughout the study. Interspecific zonation patterns were similar to those recorded for adults of both species. Diet analyses showed that early juveniles of both stichaeids were zooplanktivorous, differing markedly from the primarily herbivorous diets of adults. Changes in diet were largely due to the selection of larger prey taxa as both fishes, and their mouth size, grew over the study period. Predation by both fishes on water-column planktors (calanoid copepods, zoea and polychaete larvae) was greatest following initial intertidal settlement and habitat establishment by early juvenile fishes. Greater dependence on substrate-oriented and/or benthic prey (harpacticoid copepods, gammarid amphipods and mysid shrimp) was exhibited by both fishes as they grew in size. Affinities for sand, gravel, and pebbles during monthly field surveys were similar for both species throughout the study. As the fishes grew, their substratum preferences changed in relation to the substrata which provided the best refuge. Results from laboratory experiments indicated that young stichaeids select very specific substrata based on fish age (size) and substratum suitability (i.e. adequate refuge). Similarities in diet and substratum preferences, and changes in those preferences over time, appear to be the result of morphological similarities (body size and shape and mouth gape) for both species at a given age.     

Prey selection by age-0 walleye pollock, Theragra chalcogramma, in nearshore waters of the Gulf of Alaska

Juvenile walleye pollock, Theragra chalcogramma, is the dominant forage fish on the continental shelf of the Gulf of Alaska, yet little is known about the feeding habits of this important interval of pollock life history. The taxonomic composition and size of prey found in the stomachs of age-0 juveniles collected at three nearshore locations in the Gulf of Alaska in September 1990 were compared to the composition and size of zooplankton collected in concurrent plankton tows. The maximum length of prey consumed increased dramatically over the length range of pollock examined (58–110 mm) from approximately 7 mm to 30 mm, due mainly to the consumption of large euphausiids and chaetognaths by the bigger individuals. The maximum width of prey changed little over this size range although there was a general increase in prey width with increasing predator size. The minimum prey length and width did not change with increasing fish size. Juvenile pollock generally selected the larger prey sizes relative to what was available. Juvenile pollock showed a marked preference for adult euphausiids and decapod larvae and an avoidance of copepods and chaetognaths relative to the numbers collected in net tows. These results are discussed relative to the feeding ecology of these juvenile fishes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Utilization of the exotic cladoceran Daphnia lumholtzi by juvenile fishes in an Illinois River floodplain lake

Daphnia lumholtzi comprises a substantial component of the zooplankton community during mid‐ to late‐summer in Lake Chautauqua, a floodplain lake along the Illinois River near Havana, Illinois. In order to quantify the utilization of D. lumholtzi by juvenile fishes, diet analyses were conducted for seven juvenile fish species collected from Lake Chautauqua during the 2001 annual drawdown period. Freshwater drum Aplodinotus grunniens and emerald shiner Notropis atherinoides demonstrated negative selectivity for D. lumholtzi relative to native zooplankton species whereas four species of fish (bluegill Lepomis macrochirus, white bass Morone chrysops, white crappie Pomoxis annularis and black crappie Pomoxis nigromaculatus) consumed substantial amounts of D. lumholtzi. Although selectivity values for D. lumholtzi varied among these fish species, positive selection for D. lumholtzi increased similarly among larger size classes of each fish species, and corresponded with ontogenetic shifts in diet. Mean body length of D. lumholtzi consumed by 20–69 mm L_T juvenile fishes ranged from 0·75 to 0·99 mm with a calculated total length range of 2·0–2·6 mm. Results from this study provide evidence that high abundances of D. lumholtzi in mid‐ to late‐summer provide an additional food source for several juvenile fish species during a time when abundances of large native cladoceran species (i.e. Daphnia) are low, and juvenile fishes are searching for larger prey associated with ontogenetic shifts from zooplankton to macroinvertebrates and fishes. Because zooplankton production is typically lower in rivers than in lakes, survivorship of juvenile fishes produced in floodplain lakes may be higher in riverine systems if they are not reliant on zooplankton as a primary food resource. Therefore, high abundances of D. lumholtzi may benefit juvenile fishes in managed floodplain lakes, such as Lake Chautauqua, by increasing growth and facilitating the transition from zooplanktivory to insectivory or piscivory.     

Relationship between gape size and feeding selectivity of fish larvae from a Neotropical reservoir

Larvae feeding selectivity of Iheringichthys labrosus , Hypophthalmus edentatus and Plagioscion squamosissimus was assessed, examining the role of mouth gape in prey selection. Fish larvae were sampled in the Itaipu Reservoir (Brazil–Paraguay). Iheringichthys labrosus and H. edentatus larvae, with small and similar gape sizes, exhibited slightly different diets; I. labrosus preferred cladocerans ( Bosmina hagmanni , Bosmina huauriensis and Bosminopsis deitersi ) and the rotifer Brachionus calyciflorus . Hypophthalmus edentatus , however, primarily ingested the cladocerans B. hagmanni , Ceriodaphnia cornuta , Daphnia gessneri and Diaphanosoma spinulosum . Plagioscion squamosissimus , with a greater gape size, preferred Calanoida. The mechanistic processes that determine food selectivity of fish larvae in temperate aquatic systems were similar in the Neotropical system. The trophic spectrum of these species is characterized by small- to intermediate-sized prey. Plagioscion squamosissimus larvae, which have larger mouths, exploit primarily larger prey differing from the most abundant species or size classes; consequently, their diet is quite different from I. labrosus larvae and modestly similar to H. edentatus larvae, opportunistic feeders that they eat more abundant prey.     

The foraging ecology of larval and juvenile fishes

Knowledge of the foraging ecology of fishes is fundamental both to understanding the processes that function at the individual, population and community levels, and for the management and conservation of their populations and habitats. Furthermore, the factors that influence the acquisition and assimilation of food can have significant consequences for the condition, growth, survival and recruitment of fishes. The majority of marine and freshwater fish species are planktivorous at the onset of exogenous nutrition and have a limited ability to detect, capture, ingest and digest prey. Improvements in vision, development of fins and associated improvements in swimming performance, increases in gape size and development of the alimentary tract during ontogeny often lead to shifts in diet composition. Prey size, morphology, behaviour and abundance can all influence the prey selection of larval and juvenile fishes. Differences in feeding behaviour between fish species, individuals or during ontogeny can also be important, as can inter- and intraspecific interactions (competition, predation risk). Temporal (diel, seasonal, annual) and spatial (microhabitat, mesohabitat, macrohabitat, regional) variations in prey availability can have important implications for the prey selection, diet composition, growth, survival, condition and, ultimately, recruitment success of fishes. For fish populations to persist, habitat must be available in sufficient quality and quantity for the range of activities undertaken during all periods of development. Habitats that enhance the diversity, size ranges and abundance of zooplankton should ensure that sufficient food resources are available to larval and juvenile fishes.     

Prey selection by juvenile cyprinids from running water

SUMMARY. 1. The juveniles of dace ( Leuciscus leuciscus (L.)) and roach ( Rtuihts rtuihis (L.)) and yearling minnows ( Phoxtnus phoxinus (L.)) exhibited generally similar patterns of feeding behaviour and preferences for particular prey types.
2. Factors affecting prey selection included predator and prey size, prey activity and environmental conditions.
3. All three species preferred prey sizes approximately 0.6x their maximum gape.
4. Prey motion was essential to induce attack, and prey were usually selected in proportion to their relative activities.
5. Specialization on one prey type was observed in still water, and was always directed at the most active prey type. In flowing water no specialization was observed, and size selection was suppressed in the juvenile fish. Yearling minnows, however, were size selective in both still and flowing water.
6. The presence of macrophyte cover did not significantly change the pattern of prey selection, but did reduce the predation rale.
7. These results are discussed in the context of previous studies of prey selection and optimal foraging.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Diet changes in prey size selectivity in larvae of perch <Emphasis Type="Italic">Perca fluviatilis</Emphasis>

A hypothesis on change in prey size selectivity in relation to illumination level was tested on the basis of data on weight and size composition of the content of the digestive tract of larvae of perch Perca fluviatilis and on zooplankton in the layer 0–6 m (Wallersee Lake, Austria). Larvae foraging in the twilight-night period had almost two times more food in the intestine than those foraging in the daytime. The size composition of perch larvae and concentration and size composition of zooplankton hardly differed in the daytime and twilight-night samples. For the first time, it is shown on field material that more intensive feeding of larvae at twilight is related to selection of significantly larger prey than in daytime feeding. In the day-time, the larvae consumed more prey but their maximum size did not exceed 0.6 of the diameter of mouth opening of the fish; at twilight it was over 0.8. In case of feeding on so large prey, not only the weight of the consumed feed increases but the time used for capture and swallowing also considerably increases. The larvae which in the period of investigation did not yet form schools, which perform the principal defensive function, were especially vulnerable for predators feeding on relatively large prey. The decrease of the part of small-sized prey at twilight is not related to their lesser availability due to a low illumination. It is assumed that feeding on energetically more valuable but less available prey is shifted to the period of low illumination when the larvae are less exposed to predation risk. The obtained results are discussed from positions of the triotroph concept (Manteifel’, 1961).     

Resource seasonality and fish diets in an Illinois stream

Synopsis The purpose of this study was to evaluate the intensity of competition for food among 9 species of stream fishes that primarily eat aquatic invertebrates. The taxonomic and size composition, and numerical abundance of aquatic invertebrates were monitored for one year using drift and benthic samples. Diet data were obtained from stomachs of fishes captured at the same time and place that invertebrate sampling was done. Diet characteristics examined included taxonomic and size composition, number of prey per fish, and diet breadth. Drifting invertebrates were more abundant early in the year (March–June) than later (July–January). The summer-early fall scarcity of invertebrates was especially notable among those>3.6 mm long, which comprised the bulk of prey found in fish stomachs. Average prey size eaten by a fish species was positively correlated with fish mouth size, but interspecific overlap in prey size was extensive. Cyprinids as a group (5 species) ate proportionally fewer small (< 3.6 mm long) prey from July to January than did the centrarchids and stonecat. Taxonomic compositions of available invertebrates and fish diets varied markedly among sampling dates, but the use of prey taxa by fishes was not correlated with the availability of those taxa. Use of aquatic prey taxa was generally similar among fish species, but cyprinids as a group ate proportionally more terrestrial prey from July to January than did the centrarchids and stonecat. Diet breadths for all species increased as food levels declined, indicating that these fishes experienced resource depression. Food scarcity was evidently more severe for cyprinids since their stomachs contained few prey through the summer and fall relative to the centrarchids and stonecat. Though the fish species studied probably compete for food in the summer and fall, this competition did not account for the community structure observed.     

Diet and prey selection of six species of tuna baitfish in three coral reef lagoons in the Solomon Islands

The diets and prey selection of six species of tuna baitfish (the engraulids Stolephorus devisi and S. heterolobus, three dussumierids of genus Spratelloides, and the apogonid Archamiazosterophora. were examined at two heavily fished and one unfished site in the Solomon Islands. All species ate almost exclusively nektonic zooplankton, especially crustaceans. Calanoid copepods were the most importantpreyofallspeciesexcept A. zosterophora, whichateagreater biomassoflarvalfish. Baitfishcould be divided into two groups according to prey taxa: the apogonid A. zosterophora and the engraulids S. devisi and S. heterolobus ate similar prey in similar proportions; the three dussumierid species (Spratelloides) ate smaller zooplankton and a greater proportion of calanoids than the other species. The size of prey eaten differed greatly between baitfish species. A. zosterophora ate larger prey than other species. The two Stolephorus species ate similar-size prey, which were larger than the prey of the three species of Spratelloides. Among the Spratelloides, S. delicatulus ate smaller prey than the other two species. All species differed in the size of calanoids, carids and brachyurans they ate. In all cases, prey differed more by the minimum dimension (width) than by volume. There were significant relationships between fish length and prey width and volume, except for Archamia. However, in terms ofvolume, the prey of Stolephorus increased with fish size at a much greater rate than in Spratelloides species. Baitfish diets were compared to the zooplankton available. Prey selection by the three Spratelloides species and two Sfolephorus species was density-dependent for all common prey types, although Stolephorus positively selected hyperiid amphipods and brachyurans. Archamia showed strong negative selection for calanoids and selected larger zooplankters, including fish larvae and carids, avoiding smaller, more common, prey. Overall baitfish density and zooplankton density did not correlate at either of the heavily fished sites. Nor were there consistent differences for any species in diet or changes in prey selection between the heavily fished and unfished sites. This suggests that, in these lagoons, commercial baitfish catches are not directly influenced by the composition or density of prey.     

Backwater areas as nursery habitats for fishes in pool 13 of the Upper Mississippi River

Samples of larval and juvenile fishes were collected at two depths weekly during spring and summer 1983 near the mouths of backwater areas in Pool 13 of the Upper Mississippi River. The study was conducted to determine the relative value of these habitats as nursery areas for fishes present and to note any interactions that might occur between the backwaters which are being rapidly lost to siltation, and the main channel. The larvae and juveniles collected represented 13 families divided into 27 lower taxa. Cyprinidae, Clupeidae, and Sciaenidae made up 90% of the total catch. Both larvae and juveniles were more abundant near the surface than near the bottom. Densities differed greatly among the three backwater areas studied. Larval fishes were grouped on the basis of their relative abundance in the backwaters or main channel. Overall, more larvae were captured in the backwaters than in main-channel habitats, indicating that backwaters were more productive. In the main channel, densities were greater downstream from the mouths of the backwaters than upstream-possibly indicating that (1) larval fish drifted out of the backwater areas, (2) water rich in nutrients or zooplankton that flowed into the main channel created productive downstream sites that were used as nursery areas, or (3) adult fishes selected downstream sites as spawning areas. Juvenile forms were more abundant in the backwater areas then in the main-channel habitats, some bottom-dwelling fishes excepted. The backwater areas were judged to be important nursery areas for larval and juvenile fishes, and seemed to benefit downstream main-channel sites. Any loss of these habitats would be detrimental to the Mississippi River as a whole.     

Feeding and fate of wild larval razorback sucker

The razorback sucker (Xyrauchen texanus) is disappearing throughout its native range in the Colorado River basin of western North America. The largest remaining wild population in Lake Mohave, Arizona-Nevada, has shown no recruitment since the 1950s. Although annual spawning is successful and larvae are seasonally abundant, no juveniles have been collected in recent decades. To evaluate the potential role of food availability in determining fate of larvae, fish and zooplankton samples were taken in 1985 from the reservoir and an adjacent, isolated backwater in which larvae were naturally produced. Food availability and primary dietary constituents were similar in both habitats. Reservoir larvae selectedBosmina spp. (Cladocera) and apparently avoided Copepoda, while larvae from the backwater selectedBosmina, but avoided Rotifera. Larvae from both places showed evidence of selection for certain sizes of zooplankters, but preferred sizes differed between habitats. These differences were neither attributable to larval size nor zooplankton community structure. Nutritional factors such as type, number, or size of available foods do not explain disappearance of larval razorback suckers from Lake Mohave, since larvae survive to far greater ages and size in the backwater. Predation by introduced fishes appears a significant cause of larval mortality.     

Limited contribution of predation by zooplanktivorous cyprinids to 0+ fish mortality

Predation by zooplanktivorous cyprinids was hypothesized as substantially contributing to larval fish mortality. To test this assumption, potential predators were sampled by electric fishing at sites and times of high fish larvae densities in the lower Oder River, Germany, in 2002 and 2003. At ten sites with high prey fish densities, all fishes ≥1+ and 0+ pike – all considered potentially predating on fish larvae – were captured, anaesthetised, and fixed. In the laboratory, 982 guts, respectively, stomachs were dissected and checked for fish larvae and larvae remains. Predation on 0+ fish was detected in the primary piscivorous fishes Esox lucius L. and Aspius aspius (L.), and facultative piscivorous Perca fluviatilis L., whilst none of the other nine species examined consumed fish. These findings implicate that predation by planktivorous cyprinids did not explain the typically strong decline in larval fish densities in the first weeks after hatching.     

Size-dependent predator–prey relationships between perch and their fish prey

Size-dependent interactions between piscivorous perch Perca fluviatilis (age ≥1 year) and their fish prey age 0 year perch, pikeperch Sander lucioperca and roach Rutilus rutilus in the biomanipulated Bautzen Reservoir indicated that the highest ratio of prey total length ( L _T) to predator L _T was 59%. Perch L _T and prey fish L _T were positively and linearly related. Perch L _T was strongly related with both gape width and gape height. Within the range 80–110 mm L _T, the gape height of perch exceeded gape width, while beginning at 120 mm L _T the gape width exceeded gape height. The minimum, maximum and mean prey L _T and prey body depths of all three prey species increased with increasing predator size, but the increases in mean sizes of perch and pikeperch as prey were less than that of roach. The low limit of the 'predation window' observed in this study coupled with results of previous studies on perch in the Bautzen Reservoir indicated that perch had a major impact on the population dynamics of both perch and pikeperch.