Hydroacoustic monitoring modelling and stock assessment of tarek (Alburnus tarichi Güldenstädt, 1814) in lake Van

In this study, fisheries-dependent and independent data were collected from Lake Van in fishing season from 2017-2021. In addition, a comprehensive hydroacoustic survey was carried out in October 2020, which was applied for the first time in the Lake Van. A Simrad EK60 portable echosounder with 120 kHz frequency equipped with a splitbeam transducer (Simrad ES 120-7C) was used to collect acoustic data. The survey pattern is a total of 25 transects with a 2.28 nmi inter transects distance. A total of 540 nmi acoustic data was collected during the 10-days survey. Biological data were collected by using gill-nets (by rigged with EN 14757 standard), synchronously with the hydroacoustic survey. The total length of the sampled fish were between 7.5 – 25.5 cm, and the target sizes (TS) were in the range of -60.54 -46.92 dB. Average total length, average body weight and average TS were calculated as 18.12 cm, 50.93 g and -49.99 dB respectively. It has been determined that the fish have almost completely left the Erciş region in the autumn season, and they were concentrated in the Van Bay, Tatvan Basin and Adilcevaz-Ahlat regions. It has been observed that the vertical distribution of fish is directly affected by the temperature stratification in the lake, and accordingly, the tarek schools are just above the thermocline (18-25 m) and do not go deeper. Fish biomass was calculated from the acoustic data that collected in the lake with two different approaches, "bathymetric" and "geostatistical". The biomass of tarek in Lake Van was calculated 17513 tons by bathymetric approach; It was calculated as 16200 tons with the geostatistical approach. In addition to the hydroacoustic method, "Age-based cohort analysis method" was used to stock assessment. Total fish amount in the lake was estimated as 579 millions and the biomass as 21919 tons. For the tarek population, fishing mortality rate (F); 0.38, total mortality (Z); 0.58 and the exploitation rate (E); 0.657 were calculated. The tarek population of Lake Van was consisted of 54.6% female and 45.4% male individuals and the ♀:♂ ratio was 1.20:1. The age distribution of population was faund 0-7 range. Average total length of the age classes were calculated as, 7.95, 14.37, 18.92, 21.96, 23.14, 23.97, 24.65 cm, average weights were calculated as, 3.38, 22.37, 54.98, 89.07, 105.87, 120.42, 132.02, 154.75 g. The length-weight relationship of population (for all individuals) was described as W=0.0033TL3.3044. The growth type of the fish was determined as positive allometric. For the all individuals fulton condition factor; 0.790, L∞; 25.9937 cm, Kbrody; 0.4674 year-1, t0; -0.7713 year, W∞; 156.1708 g; Von Bertalanffy growth equation for length Lt=25.9937[1-e(-0.4674(t+0.7713))]; the growth equation for weight was calculated as Wt=156.244[1-e(-0.4674(t+0.7713))]3.3044. It has been determined that 118 boats are actively fishing in Lake Van and each boat is used an average of 3500-4500 m of gillnet. It was determined that 45.8 tons/season of fish were caught per boat and 5400 tons of fish were caught each seasons as the average of the last 4 seasons (2017-2021) in total. In the analysis made using the "Thompson and Bell prediction model", although it was seen that it was possible to obtain the highest yield in the lake by reducing the catch power by 40%; it was concluded that the stock was not overfished because the 0, I and partially II age cohorts were not exploited with the existing fleet and fishing gears.