Population Structure of Burbot (Lota Lota L.) in the Danube - page 02

The total length of fish varied within the range of 22-63 cm and the mean length was 33.3 cm (SD = 6.042 cm). The determined length-frequency distribution indicated that ≈ 73% of the sample constituted individuals belonging to the three length classes between 28 and 39.9 cm, while each of the length classes between 44 and 65.9 cm was represented with a single individual together comprising 2.8% of examined fish (Figure 2).

Relative weight (Wr) was calculated as a measure of condition (Figure 3). The mean Wr = 92.0 ± 10.51 and ranged from 68.1 to 122.2 for all examined fish. Calculated values were highly scattered for each of the particulate length without showing any pattern in relation to length. The majority of obtained values were positioned under the line that represent the 75th percentile of the species average as reported by Fisher et al. (1996).

 

Fig02
Figure 2: Length-frequency distribution of all collected burbot.

 

Fig03
Figure 3: Relative weights of burbot in the Danube in comparison to the species average (solid line).

 

In the studied population, 3 out of 5 recognised length groups were found (Table 1). These are stock-to quality-length (S-Q); quality-to preferred-length (Q-P); preferred-to memorable-length (P-M). Population structure characterizes strong dominance of the S-Q length group. Generally, an increasing trend of mean relative weight values was observed across length groups. However, mean relative values for S-Q and Q-P length group were statistically compared, and testing showed that they were no significantly different (t = 0.646; df = 141; P > 0.05). The calculated proportional stock density index was 20.

 

Table 1: Length categorization of the studied population and means of population relative weights for obtained length groups: stock-to quality-length (S-Q); quality-to preferred-length (Q-P); preferred-to memorable-length (P-M).
Tab01

 

A length-categorization system was developed in order to describe and evaluate the structure of stocks of exploited and managed fish species. Gabelhouse (1984), provided five-cell length categories including stock (S), quality (Q), preferred (P), memorable (M), and trophy (T) lengths, and minimum lengths for each category were defined as percentage lengths of all-tackle, world-record fish. According to the author, the length categories of S, Q, P, M, and T for all fishes are defined by ranges of 20-26, 36-41, 45-55, 59-64, and 74-80% of world record lengths, respectively. Arrangement data of fish population samples according to the five length groups includes quantifiable and easily understandable measures of population size structure. Fisher et al. (1996), reported that the length of the documented longest burbot was 104.3 cm, and recommended that 20 cm, 38 cm, 53 cm, 67 cm, and 82 cm be used as the minimum stock, quality, preferred, memorable, and trophy length, respectively. Proposed length values were used to describe size structure of the studied population. Length data on burbot in Serbian waters are extremely rare and insufficient. Ristić (1977), noted that the average length of burbot in commercial landings was 45 cm; Janković (1986), reported that the burbot population from the Djerdap reservoir was constituted of specimens smaller than 40 cm; only Simonović (2001), stated that burbot in Serbian waters may rarely reach 75 cm. Presented information, as well as the fact that the two largest length categories were absent in the studied population, suggest the inability of the ecosystem to produce large individuals compared to worldwide standard. The proportional stocking density index is a valuable indicator of the quality of the fish population.

 

 It reflects the population rate functions of recruitment, growth, and mortality (Wilis et al., 1993). For example, when population density increases the PSD has a tendency to decrease, while as growth of fish increases the PSD tends to increase. Moreover, especially at low population density, low PSD values may indicate problems with poor habitat, food resources and overharvesting (Gabehouse, 1984). Therefore, determination of PSD over seasons or years and in combination with other evaluation tools could be used to assess population structure and effects of management measures. Relative weight index allows interpopulation comparisons by making the standard weight-length regression species-specific rather than population-specific or state-specific (Murphy et al. 1991). Fisher et al. (1996), reported that for burbot in North America the objective ranges of Wr = 100 ± 5, and Wr = 80 ± 5 are applicable for lake, and riverine populations, respectively. In that respect, it seems that burbot from the Danube with Wr = 92 occupies an intermediate position. The better condition of burbot in the Danube as compared to their counterparts from American rivers is probably caused by more favourable environmental conditions in the Danube. Furthermore, an increasing trend of conditions across the length groups indicates favourable conditions for fattening of larger fish.

Acknowledgments

The reported work is supported by the Ministry of Education, Science and Technological Development (project number TR37009), and the Serbian Academy of Sciences and Arts.