Akademik Veri Yönetim Sistemi
2nd International Symposium on Limnology and Freshwater Fisheries, Elazığ, Türkiye, 3 - 05 Eylül 2019, cilt.FIM-074, ss.125
The Van Lake is the largest lake in the Turkey. Its alkaline waters host only one teleostean species,
(Alburnus tarichi Güldenstädt, 1814), the tarek which, constitutes about 25% of yield harvested from inland
waters of Turkey. This preliminary study presented the results of a test made to evaluate whether hydro acoustic
method can be used to monitor the stock of the species in the Lake Van.
The entire lake was surveyed over more than 300 nm transects between 23 and 27 October 2018. A
SIMRAD EY60 portable scientific echosounder with 120 kHz split beam transducer mounted on 15 m long
vessel. To ground truth the acoustical recordings, fish sampling was carried out at 3 points with a set of gill-net
composed of 11 panels ranging between 9 and 50 mm mesh size. Also, temperature profiles were measured
at 10 points representing three limnologically different basins in the lake, namely, Erciş bay, Van-Gevaş bay
and Tatvan-Ahlat-Adilcevaz. Each zone was further stratified into two zones on the horizontal, as shallow and
deeper (> 60 m) based on vertical temperature profiles and the vertical fish distribution pattern displayed on the
echograms. The collected acoustic data were analysed with EchoView 5.3 over a total of 6 strata.
The highest acoustic biomass was observed at the deeper part of Tatvan-Ahlat-Adilcevaz region (54.26
NASC), which was followed by the deep (49.41 NASC) and shallow waters (40.51 NASC) of Van-Gevaş. The
acoustic biomass observed in deep (20.40) and shallow (9.78) waters of Erciş was significantly lower than
those in the south of the lake. The lowest biomass was observed at the shallow part of Tatvan-Ahlat-Adilcevaz
region (6.84 NASC). These results indicated that the overwintering migration of the tarek in the lake was
started and a significant part of the stock in the north (Erciş) headed south for warmer waters. The results also
suggests that the fishes might follow the lower boundary of the epilimnion during the overwintering migrating.
It was observed that the mesh size series of the gillnet used for sampling was selective over the small
size groups and therefore a significant part of the stock (recruits) could not be sampled. Consequently, Target
Strength of the tarek in the lake, which is a critical function used to convert the acoustical biomass (NASC)
into absolute biomass could not be derived. However, this preliminary study indicated that the schooling
behaviour displayed by tarek and the aggregations formed in autumn can clearly be detected using acoustic
method.