Diet Of Brown Trout Research Articles

Robert M. McDowall—taxonomist and biogeographer

Robert (Bob) McDowall died in Christchurch, New Zealand on 20 February 2011 after a short illness. Bob was a widely published author and an acknowledged world authority on the taxonomy and biogeography of freshwater fishes. In a preface to Bob’s recent magnum opus on the osteology of the galaxiids and allied genera (McDowall and Burridge 2011), the series editor, Peter Bartsch called him “a complete zoologist” and listed his knowledge and experience ranged from “taxonomy, biogeography and systematics to reproductive biology, behaviour, ecology, evolution, fisheries and conservation biology”. So, how did this talented and productive fisheries scientist come to acquire a profound knowledge of not only the freshwater fish fauna of New Zealand but of the southern hemisphere? Bob was born on 15 September 1939 in Palmerston North in 1939, the second youngest in a family of five. Bob attended Victoria University of Wellington (1958–62), completing an M. Sc. in zoology. As a young scientist, Bob recognised he had available to him a virtually unstudied fish fauna, the native freshwater fishes of New Zealand. The fauna is sparse (~ 40 species) and characterised by a high degree of diadromy. Bob soon realised that this fauna offered an opportunity to “explore pattern and process, cause and effect, evolution and biogeography, in a way that would have been much more difficult in areas with more speciose faunas” (McDowall 2010). One of the earliest papers he wrote on the origins of the New Zealand freshwater fish fauna (McDowall 1964) was probably instrumental in his gaining the opportunity to study at the Museum of Comparative Zoology, Harvard University USA (1965–68) where he was exposed to the teachings of the likes of Ernst Mayr, P J Darlington, Giles Mead, Alfred Romer and other luminaries of that era. His Ph. D. was on the systematics and phylogeny of the New Zealand galaxiids. The family Galaxiidae comprises a group of southern hemisphere fishes whose wide geographic range and the diversity of habitats they have colonised are somewhat akin to the northern hemisphere salmonids. The galaxiids were destined to become a major research focus for Bob. Upon completion of his doctorate and return to New Zealand, Bob was instructed to commence research on the diet of brown trout, Salmo trutta, something he regarded of much lesser importance than understanding the ecology of native species. With a dogmatism that often characterised his dealings with authority, he ignored this directive and established an extensive field Environ Biol Fish DOI 10.1007/s10641-011-9877-0

Evaluación del lavado gástrico como herramienta para el análisis de la dieta en trucha común

Evaluation of the gastric lavage as a tool for the analysis of the brown trout diet The efcacy of the method of extracting stomach content, by means of the procedure of stomach washing through a pump, based on Seaburg's original model (1957), has been studied. The effectiveness of the method is not related to the size of thesh, but rather to the prey's own morphological characteristics, the degree of repletion of the stomach and the extent of digestion of the food. Thus, the effectiveness of the method is inversely related to the degree of repletion. Furthermore, bigger prey having rigid structures, such as Aeshnidae and Caddisies with cases are more difcult to expel. However, the observed differences in the effectiveness of expulsion between preys with similar characteristics are due to the extent of their digestion. Nevertheless, the average effectiveness of the method is high and in excess of 80 %. It is therefore a relatively simple and effective methodology and the reason why its use is appropriate for obtaining stomach content samples in young and adult brown trout.

The importance of size and growth rate in determining mercury concentrations in European minnow (Phoxinus phoxinus) and brown trout (Salmo trutta) in the subalpine lake, Øvre Heimdalsvatn

The alien fish species, European minnow (Phoxinus phoxinus), was recorded in the mountain lake, Ovre Heimdalsvatn, southern Norway, for the first time in 1969. After some years minnows were observed in the diet of brown trout (Salmo trutta). In 2006, brown trout had significantly higher δ15N values than European minnow, indicating that brown trout holds the top trophic position in the lake food web. The δ15N signatures of brown trout were significantly correlated to fish size, age and mercury concentration, while δ15N signatures in minnows did not show any change with fish size, age and mercury concentrations, indicating that European minnow remain at the same trophic level throughout the life history. The mercury concentrations (mg kg−1 w.w.) in brown trout and minnows were in the range 0.014–0.093 and 0.014–0.16, respectively. For both species, positive relationships between total mercury concentrations and fish length, weight and age were found. Although European minnow was found as prey in brown trout stomachs, minnow had significantly higher mercury concentrations than brown trout. It is most likely that the higher mercury concentrations in the European minnow are due to the high metabolic rate, with more energy spent on maintenance than on growth as a consequence of its small size and very low annual growth rate compared to brown trout. Changes in population densities of both brown trout and European minnow may influence their habitat use, feeding preferences and metabolism, and may thus potentially lead to changes in the mercury loads of both species.

The brown trout (Salmo trutta) in the lake, Øvre Heimdalsvatn: long-term changes in population dynamics due to exploitation and the invasive species, European minnow (Phoxinus phoxinus)

Studies of the brown trout (Salmo trutta) population in the Norwegian subalpine lake, Ovre Heimdalsvatn, over a 50-year period have revealed major changes in population dynamics. In 1958, the population density was high, with individuals stagnating in growth at lengths below 30 cm. After heavy exploitation during the years 1958–1969, the number of older fish declined substantially, and growth rates increased significantly. Since 1969, the European minnow (Phoxinus phoxinus) have been observed in the lake, with increasing densities from 1977–1978 to 1999–2000. The age structure of the brown trout population has changed markedly from the 1960s to the period 1993–2006. Annual recruitment significantly declined, from an average number of 3746 individuals in age-class 4 during the period 1958–1966 to an average of 1668 during the period 1993–2006. However, due to lower exploitation rates, the number of old fish was significantly higher in the latter period. The summer diet of brown trout has changed substantially from a dominance of the large crustaceans Lepidurus arcticus and Gammarus lacustris to a high occurrence of European minnows, while L. arcticus has become practically absent from the diet. There was a negative relationship between brown trout biomass and annual length increment. However, despite a brown trout biomass at the same level during the years 1993–2006 as in the 1960s, annual individual growth rates have significantly declined. The reduced recruitment and reduced annual growth rates of the brown trout, as well as changes in the diet, are most likely related to the introduction and establishment of the invasive species, the European minnow.

Winter ecology of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) in a subarctic lake, Norway

We studied habitat choice, diet, food consumption and somatic growth of Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) during the ice-covered winter period of a subarctic lake in northern Norway. Both Arctic charr and brown trout predominantly used the littoral zone during winter time. Despite very cold winter conditions (water temperature 20 cm. The winter diet of brown trout <20 cm was dominated by insect larvae, whereas large-sized trout mainly was piscivorous, feeding on juvenile Arctic charr. Piscivorous feeding behaviour of trout was in contrast rarely seen during the summer months when their encounter with potential fish prey was rare as the small-sized charr mainly inhabited the profundal. The study demonstrated large differences in the ecology and interactions of Arctic charr and brown trout between the winter and summer seasons.

Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.

A Bioenergetic Analysis of Factors Limiting Brown Trout Growth in an Ozark Tailwater River

We examined prey utilization and energy consumption by brown trout, Salmo trutta, in a cold tailwater (Little Red River, Arkansas, USA; LRR) having low biodiversity and low availability of fish as prey. Stomach content analysis and age estimation were performed on thirty brown trout (10 each of three size classes for a total of 710 trout) collected monthly from an upstream and downstream site over a 1-year period. Diet diversity was low at both sites, as 80% and 70% of all prey consumed by upstream and downstream brown trout, respectively, were isopods. Piscivory ( < 0.5% of individuals sam- pled) and consumption of terrestrial invertebrates were rare. There was no relation between diet diversity and trout age, and a very small ontoge- netic shift in brown trout diet. Second, we inves- tigated brown trout growth rates relative to prey consumption and temperature. Temperatures and availability of prey were less than required for maximal trout growth. However, prey availability limited trout growth directly, but sub-optimal temperatures probably buffered the effect of this reduced energy consumption by reducing meta- bolic energy expenditures. Brown trout growth was 54.8-57.0% of the maximum predicted by a bioenergetics model. Instantaneous growth rates for age 1 and adult brown trout were slightly higher for those downstream (0.195) versus those upstream (0.152). Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna and subsequent lack of establishment of cold-tolerant forage fish species due to the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major barrier to brown trout growth.

Diet of brown trout in relation to variation in abundance and size of pelagic fish prey

The seasonal diet of a predator, brown trout Salmo trutta[total length (LT) 17–69 cm] and simultaneous density and size‐structure of prey populations, vendace Coregonus albula and smelt Osmerus eperlanus(4–16 cm LT), in a large boreal lake were analysed and compared in 2001 and 2002. The upper LT limit for consumed prey was c. 40% of the predator LT. All brown trout, however, preferred small (&lt;10 cm LT) and avoided large (≥10 cm LT) prey. The results also suggested that equal densities of similar‐sized (4–10 cm LT) fish of the two prey species led to random foraging on these species by brown trout, but if either one of the prey species predominated (&gt;50%) in the lake, brown trout shifted to foraging on this species almost exclusively. Brown trout diets thus reflected the density dynamics of the two alternative prey species.

Indirect behavioral effects of a piscivore on trophic interactions in stream enclosures

Abstract:The indirect behavioral effect of piscivorous pike (Esox lucius) on the growth and diet of brown trout (Salmo trutta) as well as the consequences of the pike-trout interaction for benthic macroinvertebrates and periphytic algae were studied in nine 2 × 3 m field stream enclosures. The indirect effect of pike on trout behavior was studied by holding trout density relatively constant, which was done by replacing consumed trout during the course of the experiment. Three treatments were established, each replicated three times: fishless controls and two fish treatments, one with 15 brown trout (2.5 trout m-2) and one with 15 trout and 1 northern pike (0.17 pike m-2). Growth of brown trout was lower in the presence of pike than in the absence of pike. Pike affected the biomass of the leech, Erpobdella, whose biomass was greater in the presence of pike than in its absence. The biomass of grazing heptageniids was lower in the trout only treatment than in the other two treatments. Moreover, the number of grazing Baetis observed over algal-cultured ceramic tiles was lower in the two different treatments with trout than in the fishless controls. Periphytic chlorophyll-a biomass reflected trout's effect on Baetis as chlorophyll-a biomass was similar in all enclosures with trout (with and without pike), and these enclosures had higher biomasses of chlorophyll-a than the fishless controls.

Distribution of polycyclic aromatic hydrocarbons in the food web of a high mountain lake, Pyrenees, Catalonia, Spain

We investigated the contents of polycyclic aromatic hydrocarbons (PAHs) in the food web organisms included in the diet of brown trout from a remote mountain lake. The preferential habitat and trophic level of the component species have been assessed from the signature of stable isotopes (delta13C and delta15N). Subsequently, the patterns of accumulation and transformation of these hydrocarbons in the food chain have been elucidated. Most of the food web organisms exhibit PAH distributions largely dominated by phenanthrene, which agrees with its predominance in atmospheric deposition, water, and suspended particles. Total PAH levels are higher in the organisms from the littoral habitat than from the deep sediments or the pelagic water column. However, organisms from deep sediments exhibit higher proportions of higher molecular weight PAH than those in other lake areas. Distinct organisms exhibit specific features in their relative PAH composition that point to different capacities for uptake and metabolic degradation. Brown trout show an elevated capacity for metabolic degradation because they have lower PAH concentrations than food and they are enriched strongly in lower molecular weight compounds. The PAH levels in trout highly depend on organisms living in the littoral areas. Fish exposure to PAH, therefore, may vary from lake to lake according to the relative contribution of littoral organisms to their diet.

Prey Selection by Trout in a Spring-Fed Stream: Terrestrial Versus Aquatic Invertebrates

ABSTRACT We evaluated the prey sources that contributed to the diets of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) in Valley Creek, Minnesota during the summer of 2003. We collected drift before dawn and in the afternoon on five dates in June, July, and August 2003 in three sequential riffles. Immediately following drift sampling, we collected brown and rainbow trout, 13–20 cm in length, in pools immediately below the riffles where drift was collected. Terrestrial invertebrates numerically comprised about 3% of the drift across times and dates but made up approximately 10% by number of the overall diet of brown and rainbow trout. Trout were size-selective, preferring larger invertebrates by both length (7.0–12.9 mm) and biomass. Gammarus pseudolimnaeus made up about 75% of the diet of brown trout and about 60% of the diet of rainbow trout based on biomass. Diet overlap was >60% for both trout in relation to total individuals, length, and biomass of invertebrate prey.

Increasing turbidity significantly alters the diet of brown trout: a multi‐year longitudinal study

The stomach contents of adult brown trout Salmo trutta from Lake Sorell, Tasmania, were analysed during 6 years of increasing turbidity to follow changes in the diet associated with dramatic changes in water clarity. Brown trout were sampled from 1996, when turbidity was 26 NTU, to 2001 when turbidity was 141 NTU. The mean relative volume of stomach contents decreased progressively to 2001, by which time it was only one sixth of that in 1996, and the mean diversity of prey in stomachs decreased from an average of more than six species per stomach in 1996 to one species in 2001. The species composition of stomach contents shifted from domination by the phreatoicid isopod Colubotelson sp., to the galaxiid fish Galaxias auratus and the amphipod Austrochiltonia australis, and then the cladoceran Daphnia carinata. To give an indication of diet changes over a typical yearly cycle in the current turbid state of the lake, a sample was taken from each season from December 2000 to September 2001. Two basic diets were found during the year; brown trout specialized on D. carinata in summer and autumn, and G. auratus in winter and spring. Mean diversity of prey was less than two species per stomach in all samples from 2000 to 2001, except for the sample from spring 2001 when it was 2·2 species per stomach, and the mean relative volume of stomach contents was more than three times greater in winter than any other season. The ways in which high turbidity may have influenced the changes in the brown trout diet observed since 1996 and the patterns evident during the seasons of 2000–2001 are discussed.

Seasonal and spatial variation in the diet of brown trout (Salmo trutta L.) in the subarctic River Laxá, north-east Iceland

This study examines seasonal and spatial patterns in the diet composition, stomach fullness and condition of a landlocked population of brown trout (2.5–61.3 cm) in the lake-fed River Laxa in NE-Iceland. The stomach contents consisted predominantly of benthic invertebrates, such as blackfly larvae Simulium vittatum (58%), chironomids (24%) and the freshwater snail Radix peregra (7%). The seasonal and spatial patterns of S. vittatum in the stomach contents were consistent with published studies on the life cycle and production of S. vittatum in the River Laxa. Close to the Lake Myvatn outlet (0–16 km), S. vittatum constituted 63% of the stomach content volume, and exhibited two peaks (i.e., generations) in its seasonal contribution to the stomach contents, whereas further downstream (16–35 km) S. vittatum constituted 45% of the volume and showed only clear evidence of one generation. Seasonal fluctuations in S. vittatum availability appear to affect the stomach fullness and the condition factor of the trout, especially close to the lake outlet where the fluctuations are pronounced.

Seasonal Changes and Selectivity in the Diet of Brown Trout in the River Nera (Central Italy)

ABSTRACT The stomach contents of brown trout from the middle region of the Nera River, Central Italy, were compared with the composition of the macroinvertebrate community of the same river stretch. The epibenthic fauna, identified mostly at the species level, was composed mainly of Ephemeroptera (Serratella ignita, Baetis spp., Rhithrogena semicolorata), Plecoptera (Dinocras cephalotes), Trichoptera (Hydropsyche sp., Micrasema spp.) and Diptera. We found that the dietary composition of brown trout was not simply proportional to the environmental density of the prey. Ephemeroptera dominated the spring and summer diets; Trichoptera prevailed in winter. The electivity index showed a negative selection towards Baetis spp. and chironomids and positive values of selection for Trichoptera. Dimensions, mobility, degree of exposure, and visibility of prey, were important factors in shaping brown trout's diet. Nevertheless species of the genera Micrasema and Lepidostoma, despite their reduced dimensions and scarce mobility, became in winter the favorite prey. This was probably due to the wintering habitat choice of the trout, under the river banks and hidden in the vegetation.

Interspecific competition between stream‐dwelling brown trout and Alpine bullhead

Density and composition of benthic invertebrates and the diet of brown trout Salmo trutta and Alpine bullhead Cottus poecilopus were studied at two sites in one Norwegian stream. The sites were separated by an impassable waterfall, and brown trout density was five to 10 times higher at the upper, allopatric site than downstream where it lived in sympatry with the Alpine bullhead. Benthic invertebrate communities did not differ between sites; however, the size distribution of chironomids and trichopterans were skewed towards lighter individuals at the sympatric site. Diet composition suggested that sympatric brown trout foraged more on invertebrate drift and from the surface than allopatric brown trout. Alpine bullhead diet did not differ significantly from brown trout diet, except that the Alpine bullhead fed on heavier individual prey within a few taxa and did not consume chironomid pupae or surface insects. The collected data support the hypothesis that brown trout living in sympatry with Alpine bullhead feed at locations with higher predation risk, which is a probable explanation for their lower population density.

Invertebrate accessibility and vulnerability in the analysis of brown trout (Salmo trutta L.) summer habitat suitability

AbstractWater discharge regulation can affect food availability, accessibility and vulnerability and thus, the trophic habitat suitability for lotic salmonids. To analyse brown trout habitat suitability, we therefore combined the relative importance of the food availability (overall abundance of benthic and drifting invertebrates), with the potential food vulnerability (accessibility, conspicuousness and ease of handling), the latter depending on both physical habitat characteristics (flow patterns and structural complexity of habitat) and invertebrate traits (size and other biological traits). We considered the trophic patterns of trout at two spatial scales: the reach scale (unregulated reach versus regulated reach) and the macrohabitat scale (e.g. riffles and pools).Discharge regulation reduced trout abundance, biomass, and temperature‐independent growth rates. In the regulated reach, trout had a lower total prey consumption, a higher consumption of terrestrial invertebrates and a higher diet diversity than in the unregulated reach, indicating that trout were food‐limited. However, the potential availability of food supplies per individual trout was similar for the two reaches. Thus, trout prey consumption in the regulated reach should have been predominantly affected by the decrease in both the availability of large invertebrates in the drift and their vulnerability in the total food supplies.There were no macrohabitat‐specific differences in the total prey consumption and in the potential food availability within each reach. However, brown trout diets differed between the macrohabitats of each reach, in relation to differences in potential invertebrate vulnerability.Therefore, the potential vulnerability of invertebrates to predation was more relevant in the ecological evaluation of salmonid habitat suitability than the total food availability. The analysis at the macrohabitat scale provided a better understanding of the switches in brown trout diet and enabled a finer and more realistic analysis of trout feeding patterns. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Traits of brown trout prey in relation to habitat characteristics and benthic invertebrate communities

A trait database based on habitat, behavioural and morphological characteristics of invertebrates enabled invertebrates known to have differential vulnerability to salmonid predation to be discriminated. Among all of these traits the most important were associated with invertebrate spatial proximity to salmonids (macrohabitat preference), potential conspicuousness and accessibility (e.g. flow exposure, mobility, tendency to drift, trajectory and agility) and handling efficiency (body flexibility and size). Data on benthos assemblages and brown trout salmo trutta diets collected over two consecutive summers in different macrohabitat types (e.g. pools, runs and riffles) of two Pyrenean streams demonstrated that: (i) decrease in torrential flow characteristics of habitats was associated with changes in invertebrate body shape and movement, and a decrease in body flexibility and tendency to drift; (ii) brown trout tended to feed on the most vulnerable invertebrates (e.g. those living in exposed microhabitats and with a high tendency to drift, agility and aggregation), except in faster flowing habitats. The results suggest that the potential vulnerability of benthic invertebrates increased with increases in flow (torrential) characteristics. Also, the database compiled on traits explained differences among brown trout diets, especially in slow flowing areas.

Effect of habitat type on growth and diet of brown trout, Salmo trutta L., in stream enclosures

AbstractThe effect of habitat on the growth and diet of brown trout, Salmo trutta L., stocked at the same densities in nine stream enclosures, comprising three habitat types of different quality, were tested. The habitats, which were created based on microhabitat preference data, were a shallow water habitat lacking cobbles (habitat 1), a deeper, mixed cobble‐bottomed (128‐384 mm diameter) habitat (habitat 2) and a large cobble‐bottomed (256‐384 mm) habitat of intermediate depth (habitat 3). Brown trout were found to have greater increases in total biomass in habitats 2 and 3 than in habitat 1. The pattern for length did not follow that of biomass as trout had greater increases in total length in habitat 2 than in the other two habitats. Biomass of food in trout diets reflected habitat‐specific fish biomass changes, with a greater total biomass of prey as well a greater biomass of the leech, Erpobdella, in habitats 2 and 3 than in habitat 1. There were no habitat‐specific differences in the biomass of benthic or drifting invertebrates in the enclosures, with the exception of a tendency for an effect of habitat on the biomass of Erpobdella. Although there may have been habitat‐specific differences in food resources that were not detected, it is believed that the higher biomass growth in habitats 2 and 3 may have reflected differences in cover afforded by the deeper water and coarser substrates and/or improved foraging opportunities facilitated by the larger volumes of water in the deeper habitats in which the trout could search for prey.

A seasonal analysis of the diet and feeding dynamics of brown trout, Salmo trutta L., in a small nursery stream

. The diet of brown trout, Salmo trutta L., in the Owendoher Stream, a tributary of the River Dodder, was examined at monthly intervals from June 1981 to May 1982. The diets of the three main age groups, 0+, 1 + and 2+/3+ fish, have been separately described and compared. Ephemeropteran nymphs, larval and adult chironomids as well as a variety of other adult insects were the most frequently consumed foods. While 0+ trout largely concentrated on aquatic organisms, adult insects, in particular those of terrestrial origin, became progressively more important in the diet with increasing age of the fish. Daily ration calculations indicated that food intake was highest during the summer and early autumn months when it was well in excess of maintenance requirements. This corresponded to a period of good growth. For the remainder of the year food intake was not much higher than maintenance requirements. In fact, 1+ and older (2+/3+) fish probably experienced an energy deficit on occasions during the winter which was reflected in their poor condition and weight loss. The relationship between growth and production in the system are further discussed.

Predation and the prey community of a headwater stream

SUMMARY 1. Predatory, net‐spinning larvae of the caddis Plectrocne‐mia conspersa (Curtis) were abundant in the acid headwaters of some southern English streams where fish were absent, but were scarce or absent downstream where brown trout (Salmo trutta L.) occurred.2. Field enclosure experiments showed that both underyearling and older brown trout reduced the density of P. conspersa. However, whereas small trout affected the overall density of caddis, older fish reduced that of large larvae only.3. Although P. conspersa is itself an important invertebrate predator there was little evidence of an indirect effect of brown trout on the prey of P. conspersa. Perhaps the diets of brown trout and P. conspersa are so similar that fish simply replaced the caddis as top predator.