Population Structure of Burbot (Lota Lota L.) in the Danube - page 01

Materials and Methods

Burbot specimens (145) were obtained from commercial catches along the Bačka Palanka-Belgrade course of the Danube. Fish were caught in January using modified hoop nets (30 mm mesh size) deployed at the off shore sites of the river. Collected fish were transferred to the laboratory, where total lengths (TL) and weights were measured to the nearest centimetre and 1.0 g, respectively. From each specimen otoliths (sagittae) were extracted, cleaned, dried and stored in paper envelopes. Otolith readings were performed under a dissecting stereomicroscope in whole view with reflected light. Otoliths were read three times and only in the case of the largest fish (TL = 63 cm) ageing was not achieved. Sex was determined, but this variable was not considered in further analysis. The length frequency distribution was determined applying a 4 cm interval. To evaluate fish condition the relative weight (Wr) was calculated according to the following equation:

Wr = (W/Ws) * 100

where, W is the individual weight and Ws is a length specific standard weight for the species. The length specific standard weights (Ws) were determined using the coefficients for burbot reported by Fisher et al. (1996): log10Ws = -4.868 + 2/898 log10L, where Ws is the standard weight in grams and L is the total length in millimetres. The equation is applicable for fish ≥20 cm, the condition that was satisfied in our sample. A length-categorization was performed according to Gabelhouse (1984) and Fisher et al. (1996). According to Willis et al. (1993), proportional stock density (PSD) was calculated as:

PSD = (Number of fish ≥ quality length/Number of fish ≥ stock fish) * 100.

The value of PSD varies from 0 to 100, should be rounded to the nearest whole number, and is unitless.


Results and Discussion

Fish were caught during the second half of January, meaning that they were caught at a time of annual cycle completion. Fish ages were determined by reading whole otolithts, and only in the case of the largest specimen (TL = 63 cm) aging was not achieved. Burbot ranged from 1 to 6 years of age and ages 1-3 contributed 97.9% of aged specimens (Figure 1). Regarding individual age classes, the age class 2 highly dominated constituting 45% of the sample size. Other age classes constituted 25%, 27.1%, 0.7% and 1.4% for age class 1, 3, 5 and 6, respectively.

In the age structure the sub-adult fish predominate. Based on the recorded lengths and corresponding ages only one fish older than 6 years was found, but unfortunately, it remained unaged. Concerning age structure, the life span of species has to be considered. Although burbot is considered to be a long-lived fish, species longevity varies geographically and the general pattern is that populations at its southern range of distribution rarely contain individuals older than 7 years (McPhail and Paragamian, 2000). Magnin and Fradette (1977, according to McPhail and Paragamian, 2000) noted that individuals older than 7 years are rare in populations at 45° N, but at 55° N most adults are 8 to 12 years old. However, previous data on the life span of burbot in the Danube are unavailable and further studies in this respect are needed. Fish were caught during the peak of the reproductive season (Janković, 1986), which partly influenced the age distribution in the sample. Burbot are mass spawners that form aggregations at the spawning sites in the inshore areas (McPhail and Paragamian, 2000); therefore, matured fish were not available at the fishing sites in the mid-channel of the river. It is indicative that age 4, at which most of the individuals from the Danube populations became mature was absent (Janković, 1986), thus suggesting relevance for the assumption that age structure was at some extent influenced by the season of capture.


Figure 1: Age-frequency distribution of aged burbot.