1. Introduction

Salmo trutta is the only species from the gender Salmo, naturally distributed in the rivers of Bulgaria [1]. The species is native for almost all the Europe. It is widespread and overall Least Concern. However, anadromous part of populations (sea trout) and many lacustrine stocks have in many cases markedly declined because of pollution (and possibly from impacts from salmon farming). The phylogeographic structure is almost destroyed by stocking. Its preferred habitat is cold streams, rivers and lakes. Spawns in rivers and streams with swift water. Spawning sites are usually characterized by downward movement of water into gravel. Spawns in couples between late October and March, usually in November-December [2]. The brown trout is not a subject to commercial fishing in the rivers in Bulgaria, as there are many trout farms. However, its populations are under pressure as it is one of the preferred subjects for angling. Additionally, there is a severe pressure on brown trout’s population trough poacher activities. For this reason, it is very important periodically to check the status of brown trout’s populations.

Contemporary studies on the biology of S. trutta in Bulgaria are missing. The present study is the first since 30 years pause of studying the biology of wiled brown trout’s populations. As studying area Iliyna River was chosen. Its Valley is part of the National Park “Rila” and Natura 2000 site “Rilski monastery” in Bulgaria.

The aim of the study is to evaluate the, age-size composition, L-W relationship and condition factor of the brown trout (S. trutta) in the River Iliyna, Bulgaria. For the successful management of the species in the protected area it is crucial to evaluate the studied basic biological parameters. Classical methods for assessment of fish age, size structure and condition were applied.

2. Material and Methods

The material was collected in September 2011 by electro fishing with 700 A/50-70 Hz straight, pulsating current. Two sampling points with the following coordinates were studied: 1. N 42, 09592˚ E 023, 40512^°; 2. N 42, 10099^° E 023, 34944^°. Altogether 38 brown trout from Iliyna River were analyzed. Each specimen was measured the standard length (to the end of the scale cover) to the nearest millimeter (mm) and the total weight to the nearest gram (g). After the measurements on site, the fish were returned back alive in the river of their catchment.

The age of the fish was determined on their scales, counting the number of annuli. The diagonal radius of the scale as well as of each annulus was measured by the use of Dokumator, Lasergeret (Carl Zeiss, Jena) at magnification 17 and 5х. To assess the size structure, the fish catch was divided in size classes over 1 cm each. The length-weight relationship was calculated using the power function:

W=aLⁿ,

Where,

W is the total weight (g)

L is the standard length (cm)

a is the regression constant

n is the regression coefficient [3].

Four approaches were applied to evaluate the condition of the trout from the Iliyna River: 1. Fulton’s coefficient: CF= (W/L³) x100 [4-5]; 2. The modified Fulton’s coefficient CF_n= (W/Lⁿ) x100, where instead of the rate indicator 3 was used the rate indicator from the L-W relationship of the population; 3. The coefficient a (CF_a) from the L-W relationship of the population W = aLⁿ, magnified by 10³ to be converted as an integer for easier comparison; 4. Comparing the values of the fish weight (W) from different water bodies, calculated with the same randomly selected round values of the lengths (L) (for example 5, 10, 15 cm) using the L-W relationship (W = aLⁿ) for each of the populations. The population with the better condition is the one with higher values of W, at the same value of L [6-9].

3. Results and Discussion

Five age groups were found in the population of brown trout from Iliyna River. The maximum age, length and weight of the caught trout was 5 years, 24.7 cm and 199 g. The most abundant was the 2^nd age group - 43.2% of the population, followed by the first age group with 21.6%. The less numerous was the 5^th age group composed from 2.7% of the individuals in the population (Figure 1).

The fish catch was divided in 17 size classes. The most abundant were the size classes 15.1-16 and 16.1-17 with six individuals each (Figure 2, Table 1). The age-size structure of the catch is presented on Table 1. The first and the second age groups are with highest size range with 5 and 6 size classes respectively. The main probable reason for the low number of age groups and the prevalence of the young individuals (up to two years) in the trout’s population from Iliyna River is the unregulated fishing in the river. The authorized by the law as well as the preferred from the angler’s fish are with size 15-20 cm. This significantly decreases the possibility of the species to gain its maximal size and age in the studied river. Another probable reason for the smaller size is the unfavorable river conditions during the period of low water such as high temperature, low oxygen level, river pollution and etc.

The L-W relationship for the brown trout from Iliyna River is presented on Figure 3. Power function is describing the regression between the length and the weight of the fish (W=0.0162L^2.9444, r=0,999). The coefficient n from the equation is close to 3. This indicates isometric growth of the population of S. trutta from Iliyna River. This means that the fish increase in their length with increasing weight in cubic form [10-11].

The values for the Condition Factor, calculated with the use of the four methods are given in (Table 2). Fulton’s Condition Factor for the trout from Iliyna River is 1.84. This value, higher than 1 shows the wellbeing of the population and good habitat condition [12]. The condition of the population from Iliyina River was compared with other populations from rivers in Rila Mountain. All the four methods show higher values of the Condition for the fish from Iliyna River. Most probably the reason for this is the better food base and the autumn fish catch for the study as well as the young average age of the population. The Condition Factor for fish decreases in winter and beginning of the spring, which is connected with keeping up the active metabolism and mostly for ensuring the spawning process [13].

Figure 1: Age composition of the trout catch from the Iliyna River.

Figure 2: Size structure of the trout catch from the Iliyna River.

Figure 3: L-W relationship for S. trutta from the Iliyna River.

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