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The number of herbivores in populations of ectothermic vertebrates decreases with increasing latitude. At higher latitudes, fish consuming plant matter are exclusively omnivorous. We assess

whether omnivorous fish readily shift to herbivory or whether animal prey is typically preferred. We address temperature as the key factor causing their absence at higher latitudes and

discuss the potential poleward dispersion caused by climate changes. A controlled experiment illustrates that rudd (Scardinius erythrophthalmus) readily utilize plant matter at water

temperatures above 20 °C and avoid its consumption below 20 °C. Field data support these results, showing that plant matter dominates rudd diets during the summer and is absent during the

spring. Utilizing cellulose requires the enzyme cellulase, which is produced by microorganisms growing at temperatures of 15–42 °C. Water temperatures at higher latitudes do not reach 15 °C

year-round; at our latitude of 50°N~150 days/year. Hence, the species richness of omnivorous fish decreases dramatically above 55° latitude. Our results provide support for the hypothesis

that strict herbivorous specialists have developed only in the tropics. Temperatures below 15 °C, even for a short time period, inactivate cellulase and cause diet limitations for omnivorous

fish. However, we may expect increases in herbivory at higher latitudes caused by climate change.

A specialization of herbivory in aquatic ecosystems was previously considered a sporadic phenomenon with irrelevant impacts on the water communities1,2. However, recent studies indicate that

herbivory in aquatic ecosystems has been overlooked by scientists in comparison to terrestrial ecosystems3,4,5,6, and the impact of herbivory by both vertebrates and invertebrates on

aquatic ecosystems is substantial7,8,9. Fishes have the following two methods of digesting barely degradable high molecular weight polysaccharide material: i) predominantly chemical

processing by acid hydrolysis in the stomach (e.g., Acanthuridae, Kyphosidae, Pomacentridae) or ii) mechanical processing by breakdown of plant matter using a muscular stomach (e.g.,

Acanthuridae, Sparidae) or pharyngeal teeth (e.g., Cichlidae, Cyprinidae, Scaridae)10,11,12.

A specialization in herbivory is often developed by fish and other ectothermic aquatic and terrestrial vertebrates at lower latitudes13,14. In contrast, ectothermic vertebrates (mainly fish)

at higher latitudes that use plant matter are omnivores, consuming both plant and animal sources in their diets. At these latitudes, either a herbivorous specialization has not developed or

evidence of this type of specialization is lacking in the fossil record14,15,16. The quantity of plant matter in the diet of omnivores and the number of ectothermic species capable of

consuming plant matter rapidly decrease poleward16,17,18,19, which can potentially be explained by a number of theories as follows: (i) short-term evolution and the inability of fish to

migrate along latitudinal gradients20, (ii) availability of a readily palatable plant diet at lower latitudes21, (iii) absence of plant diet at higher latitudes during the winter22, and (iv)

constraints in the digestion of plant matter at low temperatures14,18,23. The last theory is supported by observations that marine herbivorous fish have spread to higher latitudes due to

global warming24,25.

Of the European native fish species found in lentic water, herbivory is most developed in rudd (Scardinius erythrophthalmus). The proportion of plant-based diet consumed (mainly macrophytes)

is markedly higher for rudd than for other species, e.g., roach (Rutilus rutilus), even when a nutrient-rich diet is available26. Recent studies examining macrophyte characteristics and the

causes of selective herbivory in rudd assert that the greatest factors are C:N ratio and phenolic concentration, both of which are negatively correlated with herbivory rates. Lesser

importance is assigned to dry matter content and the concentration of total soluble proteins15,27,28. The shared evolution of rudd and macrophytes also seems to be a relevant factor28.

Although rudd are shown to prefer an animal diet in experimental conditions (aquarium, mesocosm)7,15, natural observations indicate a preference for a plant-dominated diet in the wild29,30.

The percentage of plant matter in rudd diet usually increases with size and age31. However, the key environmental factor driving these preferences is still unknown. According to previous

findings, temperature seems to play an important role, as it is positively correlated with the intake of plant matter by omnivorous fish14,26,30,32. This trend has also been observed in

other fish species33. It is often a consequence of changes in other environmental factors, such as plant availability22 and the increased cellulolytic activity mediated by symbiotic

microorganisms10,30.

This study focuses on the food preferences of omnivorous fish using rudd as a model organism in two oligotrophic lakes exhibiting differences in both macrophyte occurrence and animal food

availability. Our principal conclusions were drawn from mesocosm experiments testing rudd preferences for animal prey versus plant matter in the presence of different concentrations of food

and under a range of temperature conditions (16, 20, 24 °C). The primary aim of the study was to determine if omnivorous fish readily shift to herbivory, or whether animal prey is always

preferred. These results will enhance our understanding of the key factors causing the absence of ectothermic herbivores at higher latitudes. Additionally, recent climate change models

predict rapid warming, particularly at higher latitudes. The results of this study can provide important insights into how many herbivorous fish species may disperse to new areas as water

temperatures increase and fish species gain advantages in trophic competition by utilizing resources that are not available to other aquatic animals, including native species.

In both lakes, macrophytes and macroalgae (Chara and Vaucheria, henceforth referred to as macrophyte coverage for simplicity) occurred down to a depth of 12 m, but the abundance in the two

locations differed noticeably. In Milada Lake in September, dense coverage by submerged macrophytes reached 91% at the primary rudd habitat depth of 0–3 m, with a mean wet mass of 2,720 g 

m−2 (Table 1). In May, the coverage was lower (60.1%), but macrophytes were still present with a mean mass of 1,275 g m−2. The species occurring at this depth were as follows: Potamogeton

pectinatus (September: 40%; May: 30%), Myriophyllum spicatum (20%; 12%), Chara sp. (19%; 11%), Vaucheria (11.5%; 7%), Potamogeton trichoides (0.2%; 0%), Spharganium emersum (0.15%; 0%),

Myriophyllum verticillatum (0.05%; 0%), Potamogeton crispus (0.05%; 0.1%) and Elodea canadensis (0.04%; 0%). In Most Lake, there was only a sparse macrophyte coverage of 1.6% and 0.1% at a

depth of 0–3 m, and a mean mass of only 51 g m−2 and 11 g m−2 in September and May, respectively. The species occurring at this depth were Myriophylum spicatum (September: 0.8%; May: 0.03%),

Potamogeton pectinatus (0.3%; 0.04%), Chara sp. (0.2%; 0.03%), Spharganium emersum (0.2%; 0%) and Potamogeton crispus (0.1%; 0%). The shorelines of both lakes comprise stones covered by

periphyton layers of similar densities, but an accurate biomass measurement was not taken.

Invertebrates living in the fine sediment, on the surface of the stones and macrophytes had a mean biomass 5.4 g m−2 and 4.2 g m−2 in Milada and Most Lakes, respectively. In both lakes, the

following genera occurred in order of descending biomass: waterlouse (Asellus aquaticus), dragon fly larvae (Odonata), chironomid larvae (Chironomus spp.) and caddisfly larvae (Trichoptera).

Additionally, zebra mussels (Dreissena polymorpha) occurred at high densities in both studied lakes and were the only potential rudd food source that was more abundant in Most Lake than in

Milada Lake (Table 1). However, this food source was not found in rudd stomachs in either of the studied lakes (Table 2).

The mean density of zooplankton at 0–20 m depth was 36.3 ind. L−1 and 27.4 ind. L−1 in Milada and Most Lakes, respectively. Copepods (Copepoda) and large Daphnia were more abundant in Milada

Lake, while small Daphnia were more abundant in Most Lake (Table 1).

GCA revealed that plant matter dominated the diet of rudd in September 2013 and 2014 (surface water temperature: 19.1–21 °C) in both lakes. For rudd older than one year in Milada Lake, 92.5%

of the food consumed was plant matter in the form of macrophytes, and the rest was animal prey (Table 2). In Most Lake, plant matter also dominated but consisted of periphyton (68%) and

detritus (25%). Animal prey accounted for 7% of the gut contents. In September in Milada Lake, juvenile rudd were strict herbivores consuming macrophytes, whereas in Most Lake they were

strict zooplanktivores. In contrast to the warmer month of September, in May 2015 (surface water temperature: 13.1–14.2 °C), only animal prey was found in rudd digestive tracts. In Milada

Lake, diet consisted solely of the aerial stage of aquatic insects. In Most Lake, it consisted of the aerial stage of aquatic insects, benthic invertebrates and zooplankton (Table 2).

The results of SIA indicated a lower proportion of plant matter in rudd diet in September than was suggested by GCA (Table 2 and Fig. 1). The SIAR stable isotope mixing model showed 51% and

62% plant matter consumed by rudd in Milada and Most Lakes, respectively. In contrast, SIA showed a higher percentage of animal prey consumption (Milada: 49%, Most: 38%) than the GCA (7.5%,

7%) (Table 2 and Fig. 1). No clear trend was observed between fish size and δ15N or fish size and the proportion of plant matter in gut contents (Supplementary Figs S1 and S2). Additionally,

there were no statistically significant differences in δ13C and δ15N between sampling years 2013 and 2014 (see Supplementary Table S1, and Fig. S3 for SIA biplots).

Probability proportion of plant matter and animal prey in assimilated diet of rudd in Milada and Most Lakes according to stable isotope analysis.

Plant matter is presented by the categories macrophytes (Milada Lake), periphyton and detritus (both Most Lake). Animal prey is presented by the categories zooplankton, benthos, and zebra

mussel (Dreissena polymorpha). The credibility intervals are 95, 75 and 25%. (see Supplementary Fig. S3 for SIA Biplots).

Under experimental conditions reflecting the natural conditions in Milada Lake (an animal prey to plant matter ratio of 1:400), plant matter dominated rudd diets at 24 °C (99.8%) and 20 °C

(88.8%). In contrast, plant matter constituted an average of only 1.6% of gut contents at 16 °C (Fig. 2 and Table 3). The percentage of plant matter at 24 °C and 20 °C decreased with

changing diet ratios. In the presence of a ratio of animal prey to plant matter of 1:10, plant matter comprised 52.5% and 44.5% of the gut contents, respectively. At 16 °C and a ratio of

1:10, the proportion of plant matter remained very low (1.4%). The final experiment used a ratio of animal prey to plant matter of 1:1, which differed the most from natural conditions of all

the treatments. Under these conditions, the percentages of plant matter at 24 °C and 20 °C decreased to 31.9% and 15.1%, respectively. At 16 °C and a 1:1 ratio, the proportion of plant

matter was only 1.1% (Table 3 and Fig. 2).

Proportion of plant matter in gut content of experimental rudd in given temperatures and three different diet ratios.

The diet ratios of animal prey vs. plant matter were 1:1 (grey), 1:10 (blue) and 1:400 (green). Box and whiskers plots: upper and lower quartiles (boxes), median values (line inside the

boxes), maximum and minimum values (whiskers), and outliers (circles) are shown.

Both temperature and the ratio of available food sources significantly affected the diet preferences of rudd. The impact of temperature was greater (F2,15 = 10.35, p