Hieronder een voorbeeld van een recent wetenschappelijk onderzoek van enkele Turkse rivieren : het resultaat is totaly different van de amerikaanse vorser.
Conclusie : be careful with conclusions …
Turk J Vet Anim Sci
29 (2005) 417-428
© T†BÜTAK
417
Feeding Habits and Diet Composition of Brown Trout
(Salmo trutta) in the Upper Streams of River Ceyhan and
River Euphrates in Turkey
Cemil KARA
Department of Biology, Faculty of Science and Arts, Kahramanmaraß SŸtŸ Ümam University, 46100 Kahramanmaraß - TURKEY
E-mail: cemil@ksu.edu.tr
Ahmet ALP
Department of Fisheries, Faculty of Agriculture, Kahramanmaraß SŸtŸ Ümam University, 46100, Kahramanmaraß - TURKEY
E-mail: aalp@ksu.edu.tr
Received: 25.09.2003
Abstract: The feeding habits and diet composition of the stream dwelling resident brown trout Salmo trutta in the upper streams
of River Ceyhan and River Euphrates were investigated by examining the stomach contents of 611 specimens collected from May
2000 to April 2001. Analysis of monthly variations of stomach fullness indicated that feeding intensity was higher between February
and June than that for the spawning season that covered the period from November to January. A total of 42 prey taxa representing
Coleoptera, Trichoptera, Ephemeroptera, Plecoptera, Malacostraca, Diptera, Araneidae, Odonata, Gastropoda, Acridae, Acarii,
Heteroptera, fish and fish egg was identified in the diet. The index of relative importance (IRI%) revealed that five food items
together constituted more than 90% of the diet, with the most important being Gammarus sp. (49.72%), Hydropsychidae
(14.61%), an unidentified dipteran species (9.21%), Nemoura sp. (8.98%) and Isoperla sp. (6.90%). The Overlap Index (OI) values
indicated that the resident brown trout in the Stream Aksu, SšÛŸtlŸ and Hurman differed in terms of their diet compositions,
compared to the trout in other streams. The most important food item varied among the size classes of the brown trout, being
Rhithrogena sp., Nemoura sp., Gammarus sp. and fish for the classes of trout with 40-80 mm, 81-120 mm, 121-280 mm, and
>320 mm fork length, respectively. Diet composition of brown trout < 80 mm in length and >240 mm in length was different from
the other length groups (OI < 0.7). High value overlap index was observed between female and male (OI = 0.989), while no
significant dietary overlap was evident between immature and mature individuals (OI = 0.465 and OI = 0.472).
Key Words: Diet composition, brown trout, Salmo trutta, river Ceyhan, river Euphrates
Ceyhan ve FÝrat Nehirlerinin †st KollarÝndaki AlabalÝklarÝn (Salmo trutta)
Beslenme ve Besin KompozisyonlarÝ
…zet: MayÝs 2000 ve Nisan 2001 tarihleri arasÝnda Ceyhan ve FÝrat nehirlerinin Ÿst kollarÝndan yakalanan 611 adet alabalÝÛÝn mide
ierikleri incelenerek besin kompozisyonlarÝ ve beslenmeleri araßtÝrÝlmÝßtÝr. Mide Doluluk Ündekslerine gšre Þubat-Haziran aylarÝ
arasÝnda beslenmenin, Ÿreme sezonu olan KasÝm-Ocak aylarÝna gšre daha yoÛun olduÛu gšrŸlmŸßtŸr. Üncelenen mide ieriklerinde
42 takson tanÝmlanmÝß ve bunlarÝn Coleoptera, Trichoptera, Ephemeroptera, Plecoptera, Malacostraca, Diptera, Araneidae, Odonata,
Gastropoda, Acridae, Acarii, Heteroptera, balÝk ve balÝk yumurtasÝ gruplarÝna dahil olduÛu tesbit edilmißtir. KÝsmi …nemlilik Ündeksine
(% IRI) gšre 5 besinsel organizma grubu toplam besin kompozisyonunun % 90 dan fazlasÝnÝ olußturmuß ve bunlar Gammarus sp.
(% 49,72), Hydropsychidae (% 14,61), teßhis edilemeyen bir dipteran tŸrŸ (% 9,21), Nemoura sp. (% 8,98) ve Isoperla sp. (%
6,90) dir. Benzerlik Ündekslerine (OI) gšre, Aksu, SšÛŸtlŸ ve Hurman aylarÝndaki populasyonlarÝn besin kompozisyonlarÝ diÛer
derelerden farklÝlÝk gšstermißtir (OI < 0,7). Besinsel organizmalar balÝk bŸyŸklŸÛŸne gšre farklÝ olup 40-80 mm boya sahip
alabalÝklarda Rhithrogena sp., 81-120 mm boy gruplarÝnda Nemoura sp., 121-280 mm boy gruplarÝnda Gammarus sp. ve 320
mmÕden bŸyŸk alabalÝklarda ise en šnemli besin grubu balÝktÝr. Benzerlik Ündekslerinde 80 mmÕden kŸŸk ve 240 mmÕden bŸyŸk
alabalÝklarÝn besin kompozisyonlarÝ diÛer boydaki balÝklardan farklÝdÝr (OI < 0,7). Dißi ve erkek bireylerin besin ierikleri bŸyŸk šlŸde
birbirine benzerken (OI = 0,989), ergin olmayan bireylerin besin ierikleri erginlerden farklÝ bulunmußtur (OI = 0,465, OI = 0,472).
Anahtar SšzcŸkler: Besin kompozisyonu, alabalÝk, Salmo trutta, Ceyhan nehri, FÝrat nehri
Research Article
Introduction
Three potential food groups for brown trout are
available: substrate-associated preys, surface drift and
suspended drift (1). It is generally believed that brown
trout feed chiefly on drifting invertebrates (2-5);
however, several reports have noted that dwelling
salmonids can use benthic prey (6-10). Trout diet is
mainly determined by the habitat (11,12), season (1,12-
14), prey availability (10), ontogeny (12) and sex of the
fish (15).
Salmo trutta forms resident populations in the upper
streams of rivers and occurs in North Africa, Europe,
West Asia and Anatolia (16,17) and it is an important
potential species for recreational fishery. However, in
most parts of these areas, river systems have undergone
great changes in their ecology and morphology in recent
years and river damming and degradation of spawning
habitats have caused a decline in the stocks of S. trutta.
Scarce information is available on feeding activity of S.
trutta populations in Turkey. In addition, natural food
organisms used as feed by the brown trout have received
little attention, except a recent study in which stomach
contents of 24 S. trutta from the Stream ‚atak of the
River Tigris were examined for two months (18).
In this paper, we describe diet composition of the
native resident brown trout in eight streams of River
Ceyhan and River Euphrates in Turkey by analysis of
stomach contents, with comparison by locality, season,
size classes of fish, sex of fish and food diversity.
Materials and Methods
The study was conducted in the streams of FÝrnÝz,
TerbŸzek, KšmŸr, Hurman, SšÛŸtlŸ, Nergele and Aksu of
the River Ceyhan, which runs into the eastern
Mediterranean Sea, and in the Stream Gšksu of the River
Euphrates, which runs into the Persian Gulf (Figure 1).
Some information about sampling stations is given in
Table 1.
A total of 611 individuals of the brown trout was
caught monthly at three selected sampling sites, 50 m
apart from each other in each stream, between May 2000
and April 2001 by electrofishing. All the captured fish
specimens were immediately preserved in a plastic barrel
containing 4% formalin solution for later analysis. For
each fish, total weight (g), fork length (mm) and sex
were recorded. For each fish following the removal of
digestive tract, stomach was opened, its content was
flushed into a Petri dish and contents were weighed (g).
Stomach content flooded with distilled water were
examined under a stereoscopic microscope. Contents
were sorted and prey items were identified to the lowest
feasible taxonomic units using the identification keys of
Edmondson (19), Demirsoy (20) and Geldiay and BalÝk
(17). Food items were damp dried on paper towels and
the number of individuals and total weight of each prey
category were recorded. Tract contents having no food
items were also recorded as empty stomachs.
The Fullness Index (FI) was calculated to investigate
the variations in feeding intensity, using the equation: FI
=(Weight of stomach contents/Total weight of
fish)*10000 (21). One way analysis of variance and
TukeyÕs test were used to test for deviations in feeding
intensity among the different habitats, months and fish
sizes.
Dietary importance of food categories was
determined using the modified index of relative
importance; IRI= (N% +W%) *FO% (22), where FO%:
percentage frequency occurrence of stomachs in which a
food item occurred relative to the total number of
stomachs containing food items; N% is the numeric
percentage of individuals of a food item relative to the
total number of food items in the stomach and W% is the
percentage weight of a food item relative to the weight
of the total stomach contents. Percentage of weight
(W%) was used instead of volumetric percentage (23)
Using the IRI% to compare diet composition between
the pairs of locations, months, length groups or sex, an
overlap index (OI) was estimated (24);
where Pij and Pik are the proportional use of prey type i
at locations, months, length groups or sex j and k,
respectively. This index ranges from 0 (no overlap) to 1
(complete overlap). Overlap index values >0.7 are usually
considered to indicate significant overlap (25).
OI =
S PijPik
Pij
S 2Pik
2
Feeding Habits and Diet Composition of Brown Trout (Salmo trutta) in the Upper Streams of River Ceyhan and River Euphrates in Turkey
418
C. KARA, A. ALP
419
Figure 1. The map of the working area and streams locations.
Results
Feeding intensity
Resident brown trout ranged from 57.5 mm to
395.0 mm in fork length (FL) with a mean value of 143.4
mm, and their total weight ranged from 2.8 g to 842.0
g, with a mean value of 65.64 g. The number of
specimens, mean fork length and mean weights from
different stream populations in the study are presented in
Table 1. Of the 611 resident brown trout, 259 (42.39%)
were females, 230 (37.64%) males and 122 (19.97%)
immatures.
Of the total stomach analysed, 8.70% were empty. In
the summer months most of the stomachs of resident
brown trout were full, while 24.24% of the stomachs
were empty in December during the spawning season
. The most empty stomachs were seen in
the Stream FÝrnÝz, Hurman and SšÛŸtlŸ .
The empty stomachs between 40 mm and 240 mm in
length varied from 4.88% to 10.26% .
Whereas, it was 18.82%, 25.0% and 50.0% in the
length groups 240 Ð 280; 280 - 320 and 320 Ð 360 mm,
respectively .
Significant monthly variation was found in
FI and it fluctuated throughout the year .
Maximum fullness index were observed in February,
April, May and June, while the index showed a decline
from September to January. The resident brown trout
fed most intensively during spring and early summer
(FebruaryÐJune). The results indicated that FI was also
influenced by habitat and length groups .
The trouts captured in the Stream FÝrnÝz had the highest
FI . The fish between the size of 40 mm and
320 mm fed most intensively, whilst the intensity
declined above 320 mm length .
Diet composition
A total of 42 prey was identified in the diets of the
fish and they are presented in Table 2. Ephemeroptera
were present in 214 (35.03%), Plecoptera in 208
(34.04%), Malacostraca in 192 (31.42%) stomachs,
Feeding Habits and Diet Composition of Brown Trout (Salmo trutta) in the Upper Streams of River Ceyhan and River Euphrates in Turkey
420
Table 1. Geographic locations of the streams and number of the fish caught during the study, their fork length and total weight. (FL ± CI: The
mean fork length ± Confidence interval (95%), W ± CI: Total weight ± Confidence interval (95%)).
Stream River Latitude Longitude Altitude Number FL ± CI W ± CI
system (N) (E) (m) of fish (Min-Max) (Min-Max)
(mm) (g)
FÝrnÝz Ceyhan 37¡ 45« 36¡ 39« 720 186 162 ± 8.60 95.5 ± 9.66
(80.1-395.0) (11.4-842.0)
Aksu Ceyhan 37¡ 46« 37¡ 21« 1125 79 120 ± 7.70 33.8 ± 9.64
(81.7-264.4) (8.4-256.8)
Nergele Ceyhan 38¡ 00« 37¡ 13« 1213 32 115 ± 11.00 30.3 ± 8.86
(76.8-179.8) (6.7-97.5)
Hurman Ceyhan 38¡ 26« 36¡ 54« 1258 110 165 ± 8.80 80.2 ± 12.39
(73.3-296.0) (5.7-344.7)
Gšksu Euphrates 37¡ 52« 37¡ 18« 1280 31 129 ± 16.40 46.1 ± 15.33
(63.2-217.1) (3.6-152.0)
TerbŸzek Ceyhan 38¡ 04« 36¡ 27« 1390 92 120 ± 9.30 38.7 ± 8.87
(57.5-260.1) (2.8-247.9)
KšmŸr Ceyhan 38¡ 08« 36¡ 33« 1417 64 122 ± 12.10 41.8 ± 12.63
(64.4-365.0) (3.9-549.0)
SšÛŸtlŸ Ceyhan 38¡ 09« 37¡ 36« 1474 17 193 ± 37.10 131.3±70.60
(67.0-369.0) (3.7-548.0)
while Trichoptera, Diptera and Coleoptera were present
in 155 (25.37%), 152 (24 88%) and 55 (9.00%)
stomachs, respectively. In addition, fish, Araneidae,
Odonata, Gastropoda, Acridae, Acarii, Heteroptera, fish
eggs, plant seeds and stones were rarely present in the
stomach contents (Table 2).
C. KARA, A. ALP
421
0
20
40
60
80
100
120
140
160
180
200
J F A M J J S N D
Month
0
5
10
15
20
25
30
35
Empty stomach ( % )
49
27
33
37
12
123
182
115 33
0
20
40
60
80
100
120
140
160
180
FÝrnÝz Komur SšÛŸtlŸ Aksu
Habitat
0
2
4
6
8
10
12
14
16
Empty stomach ( % )
0
20
40
60
80
100
120
140
40-80 80-120
Fork length ( mm )
0
10
20
30
40
50
60
70
Empty stomach ( % )
Fullness index Empty stomach
a
b
c
Fullness index Fullness index Fullness index
120-160160-200 200-240 240-280 280-320 320-360 360-400
Gšksu
186
92 64 110
17
32
79
31
41 216
152
126
39
22
8
4
3
Terbuzek Hurman Nergele
Figure 2. Variations in feeding intensity, fullness index and empty stomach, of brown trout:
a) Monthly variations b) Variations in the habitat c) Variations in the length groups.
Numbers above bars are sample size (n).
Feeding Habits and Diet Composition of Brown Trout (Salmo trutta) in the Upper Streams of River Ceyhan and River Euphrates in Turkey
422
Table 2. Food items and their relative importance index in the diet composition of resident brown trout from the streams of River Ceyhan and River
Euphrates.
Prey FO %FO N %N W %W IRI %IRI
Coleoptera 55 9.00 90 1.581 2.931 2.256
Pyrochroa spp. 4 0.655 4 0.070 0.183 0.141 0.138 0.004
Hydrobius spp. 15 2.455 27 0.474 0.930 0.716 2.922 0.087
Enochrus spp. 3 0.491 6 0.105 0.457 0.352 0.224 0.007
Acilius spp. 34 5.565 50 0.878 1.228 0.945 10.145 0.302
Elmidae 2 0.327 2 0.035 0.089 0.069 0.034 0.001
Agabus spp. 1 0.164 1 0.018 0.044 0.034 0.009 0.000
Trichoptera 155 25.37 507 8.907 13.733 10.569
Hydropsychidae 166 27.169 476 8.363 12.612 9.707 490.944 14.610
Leptocerus spp. 16 2.619 26 0.457 0.795 0.612 2.800 0.083
Sericostoma spp. 3 0.491 5 0.088 0.326 0.251 0.166 0.005
Ephemeroptera 214 35.03 1654 29.058 20.574 15.833
Rhithrogena spp. 55 9.002 214 3.760 2.412 1.857 50.564 1.505
Ephemerella spp. 47 7.692 176 3.092 4.626 3.561 51.175 1.523
Caenis spp. 70 11.457 401 7.045 3.251 2.502 109.380 3.255
Isonychia spp. 7 1.146 17 0.299 0.188 0.145 0.509 0.015
Ecdyonurus spp. 21 3.437 564 9.909 7.563 5.821 54.064 1.609
Heptagenia spp. 7 1.146 19 0.334 0.385 0.296 0.722 0.021
Epeorus spp. 15 2.455 48 0.843 0.559 0.430 3.125 0.093
Ephemera spp. 12 1.964 16 0.281 0.686 0.528 1.589 0.047
Baetis spp. 7 1.146 199 3.496 0.904 0.696 4.804 0.143
Plecoptera 208 34.04 822 14.441 33.380 25.689
Nemoura spp. 120 19.640 496 8.714 8.632 6.644 301.631 8.976
Perla spp. 38 6.219 94 1.651 4.110 3.163 29.938 0.891
Isoperla spp. 71 11.620 232 4.076 20.638 15.884 231.935 6.903
Malacostraca 191 31.42 1594 28.004 33.125 25.493
Gammarus spp. 191 31.260 1593 27.987 33.077 25.458 1670.691 49.719
Mysid spp. 1 0.164 1 0.018 0.048 0.037 0.009 0.000
Diptera 152 24.88 930 16.339 9.918 7.633
Unidentif Dipter 113 18.494 594 10.436 8.193 6.306 309.627 9.214
Corethra spp. 23 3.764 105 1.845 0.418 0.322 8.157 0.243
Chaoborus spp. 3 0.491 59 1.037 0.193 0.149 0.582 0.017
Chironomidae 25 4.092 172 3.022 1.114 0.857 15.873 0.472
Araneidae 1 0.164 1 0.018 0.012 0.009
Aranedi 1 0.164 1 0.018 0.012 0.009 0.004 0.000
Odonata 2 0.327 2 0.035 0.076 0.059
Calpoteryx spp. 2 0.327 2 0.035 0.076 0.059 0.031 0.001
Gastropoda 11 1.80 22 0.387 0.854 0.657
Radix spp. 4 0.655 5 0.088 0.046 0.035 0.081 0.002
Lymnea spp. 6 0.982 16 0.281 0.791 0.609 0.874 0.026
Dressiana spp. 1 0.164 1 0.018 0.017 0.013 0.005 0.000
Acrididae 2 0.327 3 0.053 0.054 0.416
Oedipoda spp. 2 0.327 3 0.053 0.540 0.416 0.153 0.005
Acarii 6 0.982 6 0.105 0.027 0.021
Hydroacari 6 0.982 6 0.105 0.027 0.021 0.124 0.004
Heteroptera 3 0.491 4 0.070 0.210 0.162
Notonecta spp. 3 0.491 4 0.070 0.210 0.162 0.114 0.003
Pisces 8 1.31 12 0.211 13.239 10.189
Capoeta capoeta 3 0.491 3 0.053 1.048 0.807 0.422 0.013
Blennius spp. 1 0.164 3 0.053 0.597 0.459 0.084 0.003
Salmo trutta 1 0.164 3 0.053 1.479 1.138 0.195 0.006
Phoxinellus spp. 3 0.491 3 0.053 10.115 7.785 3.849 0.115
Other items
Trout egg 4 0.655 7 0.123 0.280 0.216 0.223 0.007
Plant seed 14 2.291 19 0.334 0.369 0.284 1.416 0.042
Stone 7 1.146 19 0.334 0.666 0.513 0.971 0.029
Total 5692 100.000 129.94 100.000 3360.30 100.000
5692 individual preys were counted from 611 trout
examined and their total wet weight was 129.94 g. The
most representative prey were Gammarus sp. (27.99%),
an unidentified dipteran species (10.44%), Ecdyonurus
sp. (9.91%), Hydropsychidae (8.36%) and Nemoura sp.
(8.71%). By weight, Gammarus sp. (25.46%)
represented the largest proportion of the diet, followed
by Isoperla sp. (15.88%), Hydropsychidae (9.70%) and
Nemoura sp. (6.64%). The most frequent prey in the
stomachs were Gammarus sp. (31.26%),
Hydropsychidae (27.17%), Nemoura sp. (19.64%) and
unidentified dipteran species (18.49%). According to
IRI%, five food items represented more than 90% of the
diet, with the most important being Gammarus sp.
(49.72%), Hydropsychidae (14.61%), unidentified
dipteran species (9.21%), Nemoura sp. (8.98%) and
Isoperla sp. (6.90%) (Table 2).
IRI% indicate that Gammarus sp. was significantly
more important in the streams FÝrnÝz, TerbŸzek, Nergele
and Gšksu, where it made up 43.99%, 59.15%,
33.67% and 94.32% of the diets, respectively (Table 3).
Ephemeroptera were also consumed significantly more
often in the streams KšmŸr (50.06%), Hurman
(33.67%), SšÛŸtlŸ (90.10%) and Aksu (76.60%).
According to OI values, the resident brown trout in the
Stream Aksu, SšÛŸtlŸ and Hurman had different diet
compositions from the others because of the low values
of OI (OI < 0.7) (Table 4).
Temporal variation in the diet
IRI% of different food items in the stomachs of the
resident brown trout varied by season (Figure 3).
Resident brown trout had a similar feeding strategy in
January, February and December because of high overlap
C. KARA, A. ALP
423
Table 3. Major food items (% IRI) of the brown trout from different stream populations during the study.
FÝrnÝz TerbŸzek KšmŸr Hurman SšÛŸtlŸ Nergele Gšksu Aksu
Coleoptera 0.00 1.12 1.57 1.05 0.00 0.00 0.00 1.59
Hydrobius spp. 1.05 1.59
Acilius spp. 1.12 1.57
Trichoptera 13.46 13.49 9.56 17.80 2.09 31.25 1.02 0.00
Hydropsychidae 13.46 13.49 9.56 16.73 2.09 31.25 1.02
Leptocerus spp. 1.07
Ephemeroptera 0.00 7.22 50.06 33.67 90.10 22.60 0.00 76.60
Rhithrogena spp. 5.92 19.85 3.63 3.80 14.33
Ephemerella spp 1.30 16.37 15.64
Caenis spp. 11.14 2.39 66.87
Ecdyonurus spp. 1.21 12.01 86.30
Epeorus spp. 1.49 8.27
Baetis spp. 9.73
Plecoptera 31.80 13.58 1.80 8.16 0.00 0.00 0.00 0.00
Nemoura spp. 18.85 3.28
Perla spp. 13.58 1.80 3.01
Isoperla spp. 12.95 1.87
Malacostraca 43.99 59.15 28.45 14.04 0.00 33.67 94.32 9.43
Gammarus spp. 43.99 59.15 28.45 14.04 33.67 94.32 9.43
Diptera 10.23 1.81 3.50 22.40 0.00 10.64 3.50 9.31
Unidentif Dipter 10.23 17.47 2.15 9.31
Corethra spp. 3.50 4.93
Chironomidae 1.81 8.49 3.50
Total 99.48 96.37 94.94 97.12 92.19 98.16 98.84 96.92
index (OI > 0.7) (Table 5). Five food items,
Hydropsychidae., Nemoura sp., Isoperla sp., Gammarus
sp. and unidentified dipteran species, were present at
every month.
According to IRI, Gammarus sp. were mainly eaten in
February (76.25%), July (21.15%), September
(69.38%) and December (42.75%), whereas Nemoura
sp. was found to be most important in January
(41.87%), unidentified dipteran species in April
(42.33%), Ecdyonurus sp. in May (31.43%), Isoperla sp.
in June (15.06%) and Caenis sp. was the most important
food in November (36.96%). Hydropsychidae were
present in all months, but were never found to be most
important item in the diet (Figure 3).
Variation in the diet by size of the fish
Figure 4 shows a size-dependent variation in IRI% of
the most important food items in the stomachs.
Gammarus sp. was mainly eaten by the individuals in the
Feeding Habits and Diet Composition of Brown Trout (Salmo trutta) in the Upper Streams of River Ceyhan and River Euphrates in Turkey
424
Table 4. Dietary overlap index for the resident brown trouts caught in different streams during the study.
OI (% IRI) FÝrnÝz Terbuzek Komur Hurman SšÛŸtlŸ Nergele Goksu
Terbuzek 0.856*
Komur 0.640 0.763*
Hurman 0.601 0.520 0.652
SšÛŸtlŸ 0.007 0.011 0.059 0.358
Nergele 0.740* 0.811* 0.758* 0.624 0.028
Goksu 0.843* 0.949* 0.689 0.407 0.001 0.912*
Aksu 0.143 0.132 0.356 0.190 0.011 0.007 0.375
* Significant overlap
0
20
40
60
80
100
120
J F A M J J S N D
Month
% IRI
1 2 3 4 5 6 7 8 9 10
Figure 3. Monthly variations in the index of relative importance (% IRI) of the major food items of brown trout from the streams of River Ceyhan
and River Euphrates. 1) Gammarus sp., 2) Hydropsychidae, 3) Unidentified dipteran species, 4) Isoperla sp., 5) Nemoura sp., 6)
Rhithrogena sp., 7) Ephemeralla sp., 8) Caenis sp., 9) Corethra sp., 10) Ecdynourus sp.
length groups between 120 and 280 mm, while
Rhithrogena sp. was dominant food item in the length
group of 40-80 mm (76.21%), Nemoura sp. in the
length group of 80-120 mm (22.75%). The first uptake
of fish by resident brown trout was observed in
individuals of 160Ð200 mm in length groups but fish was
the most important food for the resident brown trout
larger than 320 mm in length.
Diet composition until 80 mm in length was different
from the other length groups as OI was smaller than 0.7
(Table 6). Similarly, diet composition of the fish larger
than 240 mm in length were different from the others.
Variation in the diet by sex of the fish
Of the 611 resident brown trout, 259 (42.39%)
were females, 230 (37.64%) males and 122 (19.97%)
immatures. According to OI, high value index was
observed between females and males (OI = 0.989), while
no significant dietary overlap was evident between
immatures and females (OI = 0.465) and males (OI =
0.472). Gammarus sp. was the most important food item
C. KARA, A. ALP
425
Table 5. Dietary overlap index for the resident brown trouts sampled in different months during the study.
OI (% IRI) January February April May June July September November
February 0.693*
April 0.162 0.166
May 0.259 0.159 0.353
June 0.123 0.072 0.846* 0.308
July 0.400 0.568 0.157 0.695* 0.025
September 0.638 0.987* 0.188 0.175 0.085 0.611
November 0.501 0.420 0.391 0.219 0.384 0.395 0.407
December 0.855* 0.906* 0.372 0.274 0.300 0.559 0.894* 0.557
* Significant overlap
0
20
40
60
80
100
120
40-80 80-120 120-160 160-200 200-240 240-280 280-320 320-360
Fork length (mm)
% IRI
1 2 3 4 5 6 7 8 9 10
Figure 4. Variations in the index of relative importance (% IRI) of major food items of brown trout in relation to length
groups. 1) Gammarus sp., 2) Hydropsychidae, 3) Unidentified dipteran species, 4) Isoperla sp., 5) Nemoura sp.,
6) Rhithrogena sp., 7) Ephemeralla sp., 8) Caenis sp., 9) Fish, 10) Lymnea sp.
in female and male individuals, whereas Rhithrogena sp.
was mainly eaten by immature individuals (Figure 5).
Discussion
The brown trout examined in the present study fed
most intensively during the period between February and
June . Daily ratios of stream dwelling brown
trout were reported to be highest between June and early
August, declining sharply in September (14,26,27).
Fullness index of the brown trout were also reported to
be the lowest in autumn, then increased from winter to
summer (11,28). Knutsen et al. (12), studying
anadromous brown trout at sea, observed that sea trout
fed most intensively in spring and early summer, and
feeding declined from a high in MayÐJune to a low in
July. Lyse et al. (29) reported that sea trout fed
intensively during MayÐJune. Similarly, in our study,
fullness index of the stomach was high between February
and June, started to decline in July to the lowest in
September. These variations in the feeding activity are
related to water temperature, which influence fish
Feeding Habits and Diet Composition of Brown Trout (Salmo trutta) in the Upper Streams of River Ceyhan and River Euphrates in Turkey
426
Table 6. Dietary overlap index for the resident brown trouts with different size classes as fork length (mm).
OI (% IRI) 40-80 80-120 120-160 160-200 200-240 240-280 280-320 320-360
80-120 0.140
120-160 0.060 0.793*
160-200 0.055 0.718* 0.976*
200-240 0.029 0.609 0.955* 0.959*
240-280 0.077 0.582 0.952* 0.964* 0.979*
280-320 0.160 0.343 0.466 0.468 0.378 0.499
320-360 0.003 0.106 0.182 0.186 0.201 0.191 0.585
360-400 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.232
* Significant overlap
Figure 5. Most important food items (%IRI) in immature, female and male brown trout from the streams of River
Ceyhan and River Euphrates. 1) Gammarus sp., 2) Unidentified dipteran species, 3) Hydropsychidae, 4) Isoperla
sp., 5) Nemoura sp., 6) Caenis sp., 7) Perla sp., 8) Rhithrogena sp.
0
20
40
60
80
100
120
Immature Female Male
% IRI
1 2 3 4 5 6 7 8
metabolism (30) and prey availability (11,28). In winter,
fish adopt a cost-minimizing strategy, while in summer,
when the energy requirements for metabolism and
growth are highest, they maximize their net energy
intake (14). Elliot (15) reported that there was a marked
significant difference in feeding between male and female
sea trout. In our study, there were not significant
differences in feeding between male and female
throughout the study.
Resident brown trout in the present study fed on a
variety of prey items, and the diet changed with season,
habitat and fish size. Most of the prey were found to be
benthic organisms. A majority of the researchers suggest
that brown trout feed chiefly on drifting invertebrates
(3,5), although stream-dwelling salmonids can adjust
their feeding behavior in response to changes in the
abundance of prey (6,7) and can also use benthic prey (8-
10). Lehane et al. (1) reported that Trichoptera species
(30.4%) represented the largest proportion of the diet of
brown trout in an afforested catchment in Ireland,
followed by Ephemeroptera (26%), Plecoptera (13.9%),
Coleoptera (7.3%), Gammarus sp. (6.9%), Mollusca
(4%), Diptera (3.9%) and others (8%). Ecdyonurus sp.,
Hydropsychid sp., Baetis sp., Protonemura sp. and
Gammarus sp. were the most important individual prey
species and represented the most dependable food
sources for trout (1). They also observed that not all of
these were drift-prone species. The most frequent prey
items of the brown trout in Stream ‚atak in Turkey were
reported to be Trichoptera (in 17 stomachs, 70.83%),
Ephemeroptera (in 14 stomachs, 58.33%) and
Gammarus sp. (in 11 stomachs, 45.83%) (18). In our
study, Ephemeroptera (37.971%), Plecoptera
(37.480%) and Gammarus sp. (31.424%) were the
most frequent prey items and Trichoptera has never been
dominant food item in the examined populations. It is
suggested that there may be some variations in the
feeding patterns of salmonids in the wild, and brown
trout chiefly feed on the most available prey items (10).
The variation in trout diet composition and feeding
strategy between the two macrohabitat types may be a
result of the differences in food availability related to
macro-invertebrate vulnerability (5). So the differences
between our results and Stream ‚atak brown trout
population may have resulted from prey availability.
In the present study the fish had similar feeding habit
in January, February and December. Gammarus sp. were
most frequently found food item in February (76.25%),
July (21.15%), September (69.38%) and December
(42.75%), Nemoura sp. were found to be dominant in
January (41.87%), unidentified dipteran species in April
(42.33%), Ecdyonurus sp. in May (31.43%), Isoperla sp.
in June (15.06%), while Caenis sp. were the most
important food in November (36.96%). Heggenes et al.
(13) suggested that trout follow different behavioral
rules at different times of the year. In accordance with
this statement, feeding behavior of the brown trout in the
present study changed with season.
Elliot and Hurley (30) suggested that the diet of sea
trout changed with ontogeny because young fish eat
smaller food items than older ones. This finding is in
agreement with our results because in the present study
the fish <80 mm, 80-120 mm, 120-280 mm and >280
mm fed chiefly on Rhithrogena sp., Nemoura sp.,
Gammarus sp. and fish, respectively.
In the present study, the difference in the diet
composition between female and male was not
significant, while it was significant between immature and
male or female individuals. Elliott (15) reported that the
probability of feeding was significantly lower for male
than female sea trout in rivers. The differences between
ElliottÕs result and our findings may have resulted from
the migration of the sea trouts. According to Elliott (15),
after leaving salt water sea trout feed more intermittently
and with a less robust appetite than brown trout. So
probability of feeding is lower for male than female sea
trout and decrease significantly for both sexes with
increasing distance upstream from the normal tidal limit.
In conclusion, this study shows that stream dwelling
resident brown trout, S. trutta, feed on a variety of prey
items, and the diet and feeding behavior changes by
season, habitat and fish size while does not differ by sex.
C. KARA, A. ALP
427
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2. Bachmann, R.A.: Foraging behaviour of free ranging wild and
hatchery brown trout in a stream. Trans. Am. Fisher. Soc., 1984;
113: 1-32.
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