Samples from Lake Vistytis included 93 specimens ranging from 250 to 500 mm in length and from 265 to 2870 g in mass. The feeding intensity varied from 70 to 88% in different months, averaging 78.5% (Table 1). The food spectrum in the tench of Lake Vistytis consists of 54 components, which are larvae and pupae of chironomids (28 species and forms, or 52%), gastropod and caddisfly larvae (7 species each), bivalves, biting midge larvae, leeches, and megalopteran larvae (2 species each), and single species of oligochaetes, gammarids, mayfly nymphs, beetles, and butterfly larvae (Table 2).
Table 1
– Description of samples and main feeding features of studied tench specimen from Lake Vistytis and Krugly Pond in different months.
Parameter | Month | General |
May | June | July | August | September |
Lake Vistytis |
Sample size, ind. | 8 | 25 | 25 | 25 | 10 | 93 |
Standard length, mm | 343 ± 19 | 363 ± 111 | 353 ± 8 | 389 ± 11 | 331 ± 12 | 362 ± 6 |
Mass, g | 1061 ± 169 | 1237 ± 113 | 1281 ± 77 | 1560 ± 124 | 947 ± 99 | 1289 ± 56 |
Feeding intensity, % | 87.5 | 80.0 | 68.0 | 88.0 | 70.0 | 78.5 |
Consumption index, ‱ | 112 ± 38 | 28 ± 6 | 16 ± 3 | 13 ± 2 | 21 ± 6 | 28 ± 5 |
Bolus mass, mg | 8988 ± 3115 | 2428 ± 553 | 1983 ± 328 | 1861 ± 231 | 1983 ± 731 | 2739 ± 415 |
Food components number | 22 | 26 | 21 | 38 | 13 | 57 |
Individual food components number | 8 (6—10) | 6 (2—8) | 7 (3—10) | 8 (4—16) | 5 (4—7) | 7 (2—16) |
Krugly Pond |
Sample size, ind. | | 18 | 31 | 27 | 14 | 90 |
Standard length, mm | | 208 ± 6 | 209 ± 4 | 216 ± 5 | 227 ± 7 | 214 ± 3 |
Mass, g | | 264 ± 26 | 252 ± 14 | 272 ± 16 | 305 ± 28 | 269 ± 10 |
Feeding intensity, % | | 72.2 | 83.3 | 70.4 | 64.3 | 73.3 |
Consumption index, ‱ | — | 33 ± 8 | 42 ± 9 | 83 ± 34 | 65 ± 11 | 55 ± 11 |
Bolus mass, mg | | 894 ± 282 | 1089 ± 239 | 2010 ± 921 | 1932 ± 295 | 1430 ± 289 |
Food components number | | 29 | 47 | 44 | 34 | 61 |
Individual food components number | | 9 (1—15) | 6 (1—14) | 8 (1—14) | 10 (6—16) | 8 (1—16) |
Table 2
– Occurrence (%) of most abundant macroinvertebrate species* in bolus in tench of Vishtynetskoye Lake and Krugly pond in different months
Food component | Month | General |
May | June | May | June | May | |
Lake Vistytis |
Gammarus lacustris | — | 100.0 | 100.0 | 100.0 | 100.0 | 90.4 |
Asellus aquaticus | 57.1 | 80.0 | 94.1 | 86.4 | 71.4 | 82.2 |
Bithynia tentaculata | 42.9 | 60.0 | 76.5 | 86.4 | 85.7 | 72.6 |
Athripsodes aterrimus | 14.3 | 65.0 | 76.5 | 40.9 | 28.6 | 52.1 |
Sialis lutaria** | 57.1 | 35.0 | 17.6 | 40.9 | 57.1 | 37.0 |
Dreissena polymorpha | 14.3 | 10.0 | 41.2 | 59.1 | 42.9 | 35.6 |
Pisidium amnicum | 28.6 | 40.0 | 11.8 | 28.0 | 57.1 | 31.5 |
Ablabesmyia monilis** | — | 30.0 | 41.2 | 36.4 | — | 28.8 |
Microtendipes pedellus | — | 30.0 | 29.4 | 40.9 | 14.3 | 28.8 |
Leptocerus tineiformis | — | 25.0 | 35.3 | 27.3 | — | 23.3 |
Procladius sp. | 71.4 | 30.0 | — | 13.6 | — | 19.2 |
Krugly Pond |
Endochironomus stackelbergi | | 84.6 | 60.0 | 68.4 | 66.7 | 68.2 |
Glyptotendipes barbipes | | 61.5 | 48.0 | 31.6 | 44.4 | 45.5 |
Mallochohelea inermis | | 76.9 | 28.0 | 31.6 | 55.6 | 42.4 |
Cryptochironomus obreptans | | 76.9 | 20.0 | 42.1 | 44.4 | 40.9 |
Procladius choreus | | 84.6 | 16.0 | 47.4 | 33.3 | 40.9 |
Caenis horaria | | — | 32.1 | 36.8 | 77.8 | 34.8 |
Polypedilum tetracrenatum | | 23.1 | 20.0 | 42.1 | 22.2 | 27.3 |
Cricotopus algarum | | 61.5 | 12.0 | 31.6 | — | 25.8 |
Procladius ferrugineus | | 46.2 | 8.0 | 15.8 | 66.7 | 25.8 |
Ablabesmyia monilis** | | 61.5 | 24.0 | 15.8 | — | 25.8 |
Tanypus punctipennis | — | 38.5 | 20.0 | 31.6 | 11.1 | 25.8 |
Chironomus plumosus | | — | 28.0 | 26.3 | 44.4 | 24.2 |
Phriganea bipunctata | | 53.8 | 4.0 | 21.1 | 44.4 | 24.2 |
Cladopelma viridulum | | 38.5 | 8.0 | 42.1 | 11.1 | 24.2 |
Glyptotendipes cauliginellus | | 30.8 | 12.0 | 5.3 | 77.8 | 22.7 |
Sialis lutaria** | | 7.7 | 28.0 | 15.8 | 22.2 | 19.7 |
Epitheca bimaculata | | — | — | 31.6 | 66.7 | 18.2 |
Ablabesmyia phatta | | — | 36.0 | 15.8 | — | 18.2 |
Ilybius sp. | | — | 8.0 | 10.5 | 77.8 | 16.7 |
Notice: * – species with occurrence above 15%; ** – common species. |
Таблица 3 – Consumption index (‱) of groups, primary and secondary food components in tench of Lake Vistytis and Krugly Pond in different months
Food component
|
Month
|
General
|
May
|
June
|
May
|
June
|
May
|
Lake Vistytis
|
Groups
|
Chironomidae
|
107.04
|
3.41
|
0.54
|
0.31
|
0.02
|
11.42
|
Gammaridae
|
—
|
13.58
|
5.89
|
4.58
|
7.15
|
7.16
|
Isopoda
|
1.34
|
4.23
|
4.52
|
3.43
|
4.04
|
3.79
|
Gastropoda
|
1.36
|
3.73
|
2.29
|
1.76
|
3.03
|
2.53
|
Bivalvia
|
1.14
|
0.53
|
0.18
|
1.97
|
5.22
|
1.29
|
Trichoptera
|
0.01
|
1.15
|
2.18
|
0.81
|
0.89
|
1.15
|
Sialidae
|
—
|
1.08
|
0.12
|
0.37
|
0.59
|
0.54
|
Ceratopogonidae
|
0.15
|
—
|
—
|
<0.01
|
—
|
0.01
|
Coleoptera
|
—
|
—
|
0.03
|
—
|
—
|
0.01
|
Ephemeroptera
|
0.01
|
—
|
—
|
<0.01
|
—
|
<0.01
|
Varia*
|
0.68
|
0.15
|
<0.01
|
0.01
|
0.51
|
0.11
|
Total
|
111.73
|
27.86
|
15.75
|
13.26
|
20.45
|
28.02
|
Species**
|
Chironomus plumosus
|
52.42¹
|
0.40
|
—
|
—
|
—
|
5.15²
|
Chironomus tentans
|
50.27¹
|
0.75
|
—
|
—
|
—
|
5.02²
|
Gammarus lacustris
|
—
|
13.58¹
|
5.89¹
|
4.58¹
|
7.15¹
|
7.16¹
|
Asellus aquaticus
|
1.34
|
4.23¹
|
4.62¹
|
3.43¹
|
4.04¹
|
3.79¹
|
Microtendipes pedellus
|
—
|
1.75¹
|
0.03
|
0.11
|
0.01
|
0.52
|
Athripsodes aterrimus
|
0.01
|
1.02
|
1.92¹
|
0.35
|
0.79
|
0.91
|
Dreissena polymorpha
|
0.07
|
0.05
|
0.14
|
1.59¹
|
0.38
|
0.57
|
Bithynia tentaculata
|
1.36
|
3.73²
|
2.28²
|
1.42¹
|
3.03¹
|
2.40²
|
Pisidium amnicum
|
1.07
|
0.48
|
0.03
|
0.33
|
3.84¹
|
0.72
|
Krugly Pond
|
Groups
|
Anisoptera
|
|
10.30
|
—
|
53.52
|
22.92
|
20.56
|
Chironomidae
|
|
10.00
|
26.58
|
8.69
|
9.00
|
15.77
|
Trichoptera
|
|
8.98
|
0.97
|
10.02
|
7.08
|
5.98
|
Coleoptera
|
|
—
|
3.84
|
2.06
|
13.61
|
3.90
|
Ceratopogonidae
|
|
1.91
|
4.56
|
2.78
|
3.31
|
3.36
|
Sialidae
|
—
|
0.05
|
4.35
|
0.03
|
0.94
|
1.79
|
Ephemeroptera
|
|
—
|
0.93
|
0.99
|
8.20
|
1.75
|
Gastropoda
|
|
0.31
|
0.84
|
4.44
|
—
|
1.66
|
Bivalvia
|
|
1.04
|
—
|
—
|
—
|
0.20
|
Varia***
|
|
0.49
|
0.08
|
0.09
|
0.18
|
0.15
|
Total
|
|
32.99
|
42.15
|
82.62
|
65.24
|
55.12
|
Species**
|
Tanypus punctipennis
|
|
4.14¹
|
5.50¹
|
1.51
|
0.01
|
3.34²
|
Mallochohelea inermis
|
|
1.90¹
|
4.43¹
|
2.74¹
|
3.26¹
|
3.29¹
|
Endochironomus stackelbergi
|
|
1.51¹
|
14.60¹
|
0.35
|
0.31
|
5.97¹
|
Procladius choreus
|
|
1.21¹
|
0.04
|
0.39
|
0.08
|
0.38
|
Cryptochironomus obreptans
|
—
|
1.24²
|
0.22
|
0.38
|
0.44
|
0.50
|
Epitheca bimaculata
|
|
—
|
—
|
51.89¹
|
16.80
|
17.23
|
Caenis horaria
|
|
—
|
0.31
|
0.99
|
8.20¹
|
1.75
|
Ilybius sp.
|
|
—
|
1.11
|
1.64
|
13.61²
|
2.75
|
Chironomus plumosus
|
|
—
|
2.37
|
4.46
|
6.20²
|
3.03
|
Notice: * - includes leeches, oligochaetes and butterflies; ** - primary food components are marked with «¹», secondary – with «²»; *** - includes damselflies and beetles.
Samples from Krugly Pond included 90 specimens ranging from 161 to 267 mm in length and from 112 to 469 g in mass. The feeding intensity varied from 70 to 83% in different months, averaging 73.3% (Table 1). The food spectrum in the tench of Krugly Pond consists of 61 components, which include 37 species (or 61%) of larvae and pupae of chironomid, 6 species of dragonfly larvae, 5 species of caddisfly larvae, 4 species of biting midges larvae, 2 species of gastropods and 2 species of beetles, single species of bivalves, mayfly nymphs, megalopteran larvae, stonefly nymphs and damselfly larvae (Table 2).
The most occurring food components in the tench of Lake Vistytis are amphipods, isopods, and gastropods. Their occurrence rates were 90%, 82% and 74%, respectively. Chironomids, caddisflies, megalopteran larvae and bivalves occurred in 41—70% of the samples. Other groups were detected in less than 10% of the fish. The most frequently occurring species in the tench of Lake Vistytis were Gammarus lacustris (90%) and Asellus aquaticus (82%), followed by Bithynia tentaculata (73%), the caddisfly Athripsodes aterrimus (52%), the megalopteran Sialis lutaria (37%), the bivalve Dreissena polymorpha (37%), Pisidium sp. (30%), and the chironomids Microtendipes pedellus and Ablabesmyia monilis (29% both) (Table 2).
Chironomids and biting midges larvae are the most frequently occurred food components in the tench of Krugly Pond are were found in 97% and 67% of samples respectively. The occurrence of caddisfly, mayfly and dragonfly nymphs, beetles and megalopteran larvae varied from 20 to 39% during the survey period. The total occurrence of the other groups did not reach 10%. The most frequent species found in the food of tench in Krugly Pond were chironomids Endochironomus stackelbergi (68%), Glyptotendipes barbipes (46%), Cryptochironomus obreptans (41%), Procladius choreus (41%) and biting midge Mallochohelea inermis (42%). The occurrence of the other species did not reach 30% (Table 2).
The highest consumption indices are met for chironomids (11‱) and amphipods (7‱) in Lake Vistytis. Isopods, gastropods, bivalves, and caddisfly larvae are consumed at the 1 to 4‱ level. The consumption rate of the other groups of benthic invertebrates did not reach 1‱. The primary food components in the tench of Lake Vistytis ignoring seasonal changes are the amphipod Gammarus lacustris and the isopod Asellus aquaticus. The secondary components were larvae of the chironomid Chironomus plumosus and Ch. tentans, and bivalve Bithynia tentaculate (Table 3).
The consumption indices of dragonfly larvae has highest values in the tench of Krugly Pond and averaged at 21‱. The chironomid consumption index is the second highest and equals 16‱. The other groups’ mean consumption indices did not reach 6‱. The primary forage species in Krugly Pond are the chironomid Endochironomus stackelbergi and the ceratopogonid Mallochohelea inermis. The secondary food components were the chironomids Tanypus punctipennis and Chironomus plumosus (Table 3).
The food spectrum in the tench of Krugly Pond contains 61 components, which is slightly wider than that of Lake Vistytis, where it consists of 54 components. The number of chironomid species and forms differed significantly and were equal to 37 in pond versus 28 in lake. Common food components found in the tench of the compared waters include the caddisflies Athripsodes aterrimus and Phryganea bipunctata; the chironomids Ablabesmyia monilis, Chironomus plumosus, Cricotopus sylvestris, Dicrotendipes nervosus, Glyptotendipes cauliginellus, Microtendipes pedellus, and Tanypus punctipennis. The megalopteran Sialis lutaria and the chironomid Ablabesmyia monilis are also common species that are highly frequent (Table 2).
The most frequent food components in the tench of Lake Vistytis are amphipods and isopods, but the occurrence of chironomid larvae does not exceed 70%, while in the Krugly Pond, chironomid larvae are the most frequently consumed (97%) food items.
Primary food components in the tench of Krugly Pond include not only the chironomid larvae Tanypus punctipennis, Endochironomus stackelbergi, Procladius choreus, Cryptochironomus obreptans, and Chironomus plumosus, but also the ceratopogonid Mallochohelea inermis, the dragonfly larvae Epitheca bimaculata, the mayfy Caenis horaria, and the Ilybius beetles in different months.
Tench in both Krugly Pond and Lake Vistytis ceased nutrition in October.
The highest consumption index in the tench of Lake Vistytis was observed in May, when it was equal to 112‱. It decreased significantly to 28‱ in June and then reached its minimal values in July and August when it was estimated 16‱ and 13‱ respectively. Some increase in that value to 21‱ was recorded in September. The bolus mass changed in the same manner. It equals 8.99 g in May, and 1.98—2.43 g in other months. The food spectrum width, e.g., the number of components found in the digestive tract, exhibited different patterns. It contains 22 components in May, 26 in June, and 21 in July and reaches its maximum of 38 components in August when the consumption index has the lowest values. The lowest monthly spectrum width containing 13 components is recorded in September. The individual food spectrum contained 5 to 8 components in different months and had no clear seasonal pattern (Table 1).
The amphipods Gammarus lacustris were met in 100% of the samples from June to September, but was absent in the food of the tench in May. The isopods Asellus aquaticus were met in the majority of the samples in May (57%), further this part become even greater – 80–94% in summer and 71% in September. Gastopods, mainly Bithynia tentaculata, are met in under half of the samples in May (43%), after which their occurrence increased to 60–91%. Chironomids are found in all nonempty guts in May. The occurrence of these species had lowered to 65–80% in summer, and even 14% in September. In contrast, caddisfly larvae, which are most frequently presented with 2 species – Athripsodes aterrimus and Leptocerus tineiformis, were rare in May (14%) and frequently met in food in the latter months (59–82%). The bivalves Dreissena polymorpha and Pisidium amnicum were present nearly half of the intestinal samples (43–47%) until August, after which their occurrence reached 64% in August, and 86% in September. The megalopteran Sialis lutaria was detected in 71% of the samples in May. Later, their occurrence was estimated to be 18 to 57%. Other notable groups were leeches, which were met in a one-fifth of the feeding tenches in June, and oligochaetes, which were met in 29% of the samples in May (Table 2).
The consumption of the chironomid reached a maximum of 107‱ in May when they formed 96% of the bolus mass. Almost all of it were backed by 2 species, which are primary food components in May – Chironomus plumosus (52.4‱) and Chironomus tentans (50.3‱). There was a dramatic decrease in chironomid consumption in June – it decreased 30-fold to approximately 3‱, the group contribution to bolus mass decreased to 12%, the genus Chironomus representatives ceased to prevail in food, and their consumption almost stopped, which caused with their emergence (Shcherbina, 1989).
The primary food components consumed in June were Gammarus lacustris, which had a consumption index of 14‱, or 49% of general consumption, the isopod Asellus aquaticus and the bivalve Bithynia tentaculata, which had a consumption index of 4‱ each. Chironomids are almost absent in tench food from July to September when they contain 0–0.5‱, or less than 3.4% of the food mass, while crustaceans kept to be the primary food components. The caddisflies Athripsodes aterrimus became the primary food component in July. Bithynias were consumed as secondary food in June and July, and then became primary food along with the bivalves Dreissena polymorpha and Pisidium amnicum in August and September (Tables 2, 3).
Analysis of similarity using Bray–Curtis distance detected slight seasonality of food composition (R = 0.294, p = 0.001), which was also confirmed with multiway permutational analysis of variance with month as a grouping factor (pseudo-F = 7.88, p = 0.001) (Fig. 3). May differed from the other months with significantly higher consumption level and clear dominants, and also food composition. In terms of the Morisita-Horn overlap index the food composition in May differed significantly from that in other months (< 0.06). The other months had similar food compositions (0.87—0.96).
The consumption index in the tench of Krugly Pond amounted to 33‱ in June when water was freed from ice. This indicator increased during summer and reached a maximum of 83‱ in August. After that, index decreased to 65‱ in September. The bolus mass changes followed consumption index one, as the studied fishes were quite same sized. The mass was 0.89 g in June, 2.01 g in August and 1.93 g in September. The minimal count of food components was 29; it was registered in June, and the maximum was 47 in July. The average individual food spectrum varied from 6 to 10 components in different months (Table 1).
Chironomids are the most frequently encountered food component. It was met in all the samples in all the months except July, when it was met at 92% of guts. Ceratopogonid larvae are also abundant food component which are met at from 58% of intestinal samples in August to 85% in June. Caddisfly (28–62%), megalopteran (8–28%) and damselfly (4–22%) larvae were also found in food during the whole season. Mayfly nymphs (36–78%) and beetles (24–78%) were met in food of the tench from July to September. Gastropods are met from June to August (4–11%). The occurrence of all food groups except for chironomids increased in September compared to August (Table 3).
Endochironomus stackelbergi was the only species that was met during the whole season in most of the studied tench intestines. Most of occurred food components are chironomids from other genera, and the mayfly Caenis horaria (32–78%, not met in June), the biting midge Mallochohelea inermis (28–77%), the dragonfly Epitheca bimaculata (32–67%, not met in June–July), the caddisfly Phriganea bipunctata (4–54%), the megalopteran Sialis lutaria (8–28%), and the beetle from the Ilybius genus (8–78%, not met in June) (Table 3).
Dragonfly nymphs (10‱), chironomid (10‱) and caddisfly (9‱) larvae constituted tench food in Krugly Pond in June. Dragonflies fell out of tench food in July, but the chironomid ratio increased 2.6 times and reached 63% of total consumption. A significant part of the food mass was backed by ceratopogonids (11%), megalopterans (10%) and beetles (9%) during July. The consumption index of dragonflies was estimated to be 53‱, or 65% of the bolus mass, in August. Chironomid larvae and caddisfly nymphs were consumed at 9 and 10‱ respectively, during the same month. Over a third (35%) of the bolus mass was still backed with dragonfly nymphs in September. The second most abundant food component was beetles, while a significant mass was due to chironomid larvae (14%), ephemeropteran nymphs (13%) and trichopteran nymphs (11%) (Table 2).
Analysis of similarity using Bray–Curtis distances revealed the absence of seasonality of food composition in the tench of Krugly Pond (R = 0.053, p = 0.077). The same conclusion is made with permutational multiway analysis of variance, with month as the grouping factor (pseudo-F = 2.909, p = 0.001) (Fig. 4).
The larvae of the ceratopogonid Mallochohelea inermis were the primary food component in the tench of Krugly Pond throughout the whole season. The larvae of Tanypus punctipennis and Endochironomus stackelbergi chironomids joined that rank in June and July, the larvae of Procladius choreus did in June, the dragonfly Epitheca bimaculata – in August, and the mayfly Caenis horaria – in September. Secondary food components were detected in June, which were larvae of Cryptochironomus obreptans, and in September, when they were Ilybius beetles and Chironomus plumosus (Table 3).