Abstract: In this research it has been targeted to estimate fishable stocks of Karakaya Dam Lake (in Turkey) by using fishing with gillnets method that had been suggested as an alternative method for estimating fish stocks. Productivity per hectare in the dam lake and the amount of fishable stocks has been calculated to be 13.67 kg ha^-1 and 312.005 kg year^-1, respectively. Fishable stocks have also been analyzed with respect to fish species. According to this, Capoeta sp. is estimated to be 184.107 kg year^-1 (8.06 kg ha^-1), Cyprinus carpio carpio is estimated to be 56.353 kg year^-1 (2.47 kg ha^-1), Barbus sp. is estimated to be 40.580 kg year^-1 (1.78 kg ha^-1), Squalius cephalus is estimated to be 30.455 kg year ^-1 (1.34 kg ha^-1) and Onchorincus mykiss is estimated to be 510 kg year^-1 (0.02 kg ha^-1).

INTRODUCTION

In the transition to modern fishery research conducted on population size is very important along with the tools and methods used in fisheries (Yuksel, 2002). Sparre and Venema (1992) have indicated that in order to continuously benefit from natural livestock sources, which demonstrate a renewable property and have a certain production level in any area at the highest level of product possible; the status of available stock in that area should be defined and necessary precautions should be taken (Ozyurt et al., 2004). Various methods are used by the researchers for the control of the stocks and estimation of population size.

These methods are selected taking into account the water environment where the research is to be conducted and the fish species available in that water environment considering their all characteristics (Hart and Gorfine, 1997; Hart et al., 1997; Tuck et al., 1997). One of the best methods to obtain the data belonging to population size is to conduct sampling by means of fishing at certain time periods (Atay, 1989). The size of a stock is proportional to the abundance of the fish caught. In other words, if the product obtained is quantitatively high then the respective stock is accepted to be efficient (Avsar, 2005).

It is very important for fisheries management to determine the fishable stock in other words, the maximum amount of fish that could be caught within a certain time period without destroying the stock. In this research it has been targeted to estimate the annual fishable stock by using fishing with gillnets method (Avsar et al., 2001). Karakaya Dam Lake (in Turkey) has been selected as the research region.

MATERIALS AND METHODS

Study site:
Karakaya Dam Lake, which is the environment of this study was constructed for both electricity and irrigation purposes. It is located in a distance of 17 km from Malatya city centre. In terms of lake surface area, it is the third biggest dam lake in Turkey. Its above sea level is 693 m and the surface area and water storage volume of the dam are 29,800 ha and 9.58x10⁶ m³, respectively. In Karakaya Dam Lake, fishing is realized in an area of 22,828 ha (Anul, 1995; Kalkan et al., 2005; Kalkan, 2008). Karakaya Dam Lake has been divided into 10 different fishing areas and then one of these fishing areas (5th fishing area) has been cancelled. The study has been carried out in 3 sub regions containing the 9 fishing areas in Karakaya Dam Lake in 2004. The areas of these sub regions have been measured by a digital area-meter.

Field study:
Gillnets with 22, 36, 44, 55 and 70 mm mesh sizes have been used in the studies. In the investigation, the length and width of gillnets have been standardized to be 100 m and 100 meshes, respectively and catching period has been set as 12 h day^-1.

In 36 catching trials performed during the study Cyprinus carpio carpio, Squalius cephalus, Barbus esocinus, Barbus grypus, Barbus mystaceus, Capoeta trutta, Capoeta umbla and Onchorincus mykiss species have been caught. Moreover, in 2004-2005 fishing season, interviews have been conducted with fisher cooperatives operating in the 9 distinct catching areas in Karakaya Dam Lake and the amount of fish caught has also been determined.

Data analysis:
Using a probable approach, total amount of economic species in the dam lake has been determined by means of projecting the findings obtained from the sub regions on the total area. In order to do this, Fishing with Gillnets method which is a version of Field Scan method proposed by some researchers (Pauly, 1980; Bingel, 1981, 1985, 1987; Sparre et al., 1989; Avsar, 1999) and first applied to gillnets by Avsar et al. (2001) has been used.

For this method, primarily, surface area (ha) of gillnet i has been measured.

(1)

Where:

It is assumed that gillnets used for catching are able to catch all the fish in an area that has been equal to its total area (aa_i). This area has been calculated by using the Eq. 2:

(2)

The amount of product per unit area (U_i) is calculated by dividing the average product (ou_i) caught in sub region i by fishing for half a day by the area (aa_i) where all of the fish are caught.

(3)

Biomass of sub region i (B_i) is calculated by projecting the biomass calculated for unit area (U_i) upon the total area (A_i) of the sub region i.

(4)

Variation and standard deviation of the biomass is calculated using the Eq. 5:

(5)

(6)

Where:

Legal fishing size has been considered in the estimation of fishable stock. t test has been used for the statistical analysis of the difference between the values of fishable stock per unit area and the amounts of produced fish obtained by catching.

RESULTS AND DISCUSSION

The study has been carried out in 3 sub regions containing the 9 fishing areas in Karakaya Dam Lake. Fishing areas which have been included in the sub regions and their respective areas have been show in Table 1.

Total catching amounts estimated fishable stock per unit area and fishable stock in total area of the economic species of Karakaya Dam Lake that have been Cyprinus carpio carpio, Squalius cephalus, Barbus esocinus, Barbus grypus, Barbus mystaceus, Capoeta trutta and Capoeta umbla have been shown in Table 2-5. The catch per unit effort has been observed to be high generally in the period of May-July in each 3 sub region. Annual fishable stock amount of Onchorincus mykiss has been estimated to be 510 kg year^-1 only for the 10th fishing area.

When the productivity of sub regions in terms of Capoeta sp. is analyzed, it is observed that the 2nd sub region is in the first place with 9.64 kg ha^-1, it is followed by the 1st sub region with 8.49 kg ha^-1 and the 3rd sub region is in the last place with a productivity level of 4.27 kg ha^-1. When the findings have been evaluated statistically, it has been found that the difference between the 1st sub region and the 2nd sub region in terms of fishable Capoeta sp. stock per unit area has been statistically insignificant (p>0.05) and the difference between both two regions and the 3rd region has been statistically significant (p<0.05). It has been estimated that there have been 45.514 kg Capoeta sp. stock amount in the 1st sub region, 114.918 kg in the 2nd sub region and 23.675 kg in the 3rd sub region with a total amount of 184.107 kg annual fishable stock (Table 2).

Table 1:	Fishing areas in the dam lake

Table 2:	Fishable stock amounts of Capoeta sp. in the sub regions

Table 3:	Fishable stock amounts of C.c. carpio in the sub regions

The most productive sub region in terms of Cyprinus carpio carpio has been the 3rd sub region with productivity level of 3.21 kg ha^-1. This has been followed by the 2nd sub region with a productivity level of 2.69 kg ha^-1 and the 1st sub region with a productivity level of 1.21 kg ha^-1.

The difference between the 1st sub region and the other sub regions in terms of fishable stock per unit area has been found statistically significant (p<0.05), however the difference between the 2nd sub region and the 3rd sub region has been found to be statistically insignificant (p>0.05). The estimated annual fishable C.c. carpio amount in Karakaya Dam Lake has been calculated to be 6.484 kg in the 1st sub region, 32.061 kg in the 2nd sub region and 17.808 kg in the 3rd sub region where total amount has summed to 56.353 kg (Table 3).

It has been determined that the sub region with the highest productivity for Barbus sp. is the 3rd sub region with 2.32 kg ha^-1 and this has been followed by the 1st sub region (1.76 kg ha^-1) and the 2nd sub region (1.53 kg ha^-1), respectively. The difference between all three sub regions in terms of the amount of fishable Barbus sp. stock per unit area has been found to be statistically insignificant (p>0.05). The estimated annual fishable Barbus sp. amount available in the dam lake has been found to be 9.457 kg in the 1st sub region, 18.257 kg in the 2nd sub region and 12.866 kg in the 3rd sub region where the total amount has been 40.580 kg (Table 4).

When the sub regions are analyzed in terms of Squalius cephalus productivity, it is observed that the 3rd sub region is in the first place with 1.69 kg ha^-1, followed by the 2nd sub region with 1.24 kg ha^-1 and the 1st region is in the last place with 1.17 kg ha^-1 productivity level. When the results have been analyzed statistically, it has been determined that the difference between all three sub regions in terms of fishable Squalius cephalus stock per unit area has been insignificant (p>0.05). It is estimated that the annual fishable Squalius cephalus stock in the dam lake is 6.284 kg in the 1st sub region, 14.797 kg in the 2nd sub region and 9.374 kg in the 3rd sub region where the total amount is 30.455 kg (Table 5).

Table 4:	Fishable stock amounts of Barbus sp. in the sub regions

Table 5:	Fishable stock amounts of Squalius cephalus in the sub regions

Table 6:	Fishable stock amounts respect to economic fish species in the sub regions

Annual fishable stock amount of Onchorincus mykiss has been detected to be 510 kg being available only in the 10th fishing area (Table 6). Annual fishable stocks of the economic fish species available in Karakaya Dam Lake have been estimated with respect to the sub regions and they have been shown in Table 6.

As it can be seen in the Table 6, it is estimated that 67.739 kg year^-1 fish can be caught in the 1st sub region (5.360 ha), 180.033 kg year^-1 fish can be caught in the 2nd sub region (11.920 ha) and 64,233 kg year^-1 fish can be caught in the 3rd sub region (5.548 ha). When the annual productivity per hectare have been analyzed, the productivity levels of the 1st, the 2nd and the 3rd sub regions have been found to be 12.63, 15.1 and 11.51 kg ha^-1, respectively. Hence, throughout the dam lake, productivity per hectare is estimated to be 13.67 kg ha^-1 and the annual fishable stock is estimated to be 312.005 kg year^-1.

Distribution of the estimated fishable stock in Karakaya Dam Lake with respect to the fishing areas represented by the study stations has been shown in Table 7. It has been calculated that the 8th fishing area which had the largest area (5,680 ha) had the highest fishable stock (85.788 kg year^-1) at the same time. It is estimated that the lowest amount of fishable stock is in the 10th fishing area (700 ha).

Table 7:	Fishable stock amounts respect to fishing regions

Table 8:	The amounts of produced fish obtained by catching in the 2004-2005 fishing season

In this research in addition to the stock estimation studies, fishing efforts made in the dam lake in 2004-2005 fishing season and the amounts of fish caught have also been determined. There have been 238 fishers registered to 9 different fisheries cooperatives, 119 motor boats and gillnets with a total length of 271,000 m available in Karakaya Dam Lake.

Moreover, the amounts of produced fish obtained by catching in all fishing areas have been determined with respect to the species. About 188 kg Capoeta sp., 71 kg C.c. carpio, 58 kg Barbus sp. and 18 kg Squalius cephalus with a total of 335 kg fish has been caught in Karakaya Dam Lake during 2004-2005 fishing season. The region where the highest amount of fish (99 kg) has been caught has been the 8th fishing area; the region where the lowest amount of fish (8 kg) has been caught has been the 10th fishing area (Table 8).

The difference between the estimated fishable stock and the amounts of produced fish obtained by catching in the 2004-2005 fishing season is statistically insignificant (p>0.05) for Capoeta sp., C.c. carpio, Barbus sp. and Squalius cephalus. Although, it has been estimated that there has been a fishable Onchorincus mykiss stock of 510 kg year^-1 in the 10th fishing area, Onchorincus mykiss has not been produced by catching during the fishing season in question. In this study, it is observed that the amount of fish caught per unit time is higher in the warmer months. This finding could be explained with the fact that the metabolic activities of the fish belonging to Cyprinidae family are faster in relatively warmer waters and the possibility of them running into passive fishing gear is higher since they are more active.

When the unit fishing quantities of species are analyzed in terms of sub regions, Capoeta sp. is observed more in the 2nd sub region, C.c. carpio, Barbus sp., Squalius cephalus ve Onchorincus mykiss are observed more in the 3rd sub region. However, when all species are considered, it is understood that the most productive region is the 2nd sub region (15.1 kg ha^-1) and it is followed by the 1st sub region (12.63 kg ha^-1) and the 3rd sub region (11.51 kg ha^-1), respectively. In this study, it has been estimated that there are 184.107 kg (59.0%) Capoeta sp., 56.353 kg (18.1%) C.c. carpio, 40.580 kg (13.0%) Barbus sp., 30.455 kg (9.7%) Squalius cephalus and 510 kg (0.2%) Onchorincus mykiss fishable stock in Karakaya Dam Lake with a total of 312.005 kg annual fishable stock.

In a report prepared as a result of a fishable stock determination study conducted by using biomass estimation (swept area) in Kar akaya Dam Lake (Anonymous, 2001) it is stated that there are 180.573 kg (40.2%) Capoeta sp., 166.191 kg (37.0%) C.c. carpio, 47.320 kg (10.5%) Barbus sp. and 55.466 kg (12.3%) Squalius cephalus in the dam lake with a total annual fishable stock of 449.550 kg. When the percentage distribution of fish species in this study which is conducted 3 years prior to the study is analyzed, it is observed that Capoeta sp. is lower with respect to the study, C.c. carpio is higher and other species are close to each other.

It is observed that fishable Onchorincus mykiss stock is not available in the study in question. The difference between the two studies in terms of total fishable stock may be due to the high fishing efforts practiced in the past three years. In the interviews conducted with the fisheries cooperatives it is indicated that fishing productivity is significantly decreased over the years. Fish stocks are affected by natural death and death by fishery. The structure of a fish stock which has never been subject to fishing changes when the fishing starts. According to Russel (1942), the greatest losses are caused by fishery. Although, the age groups in the stock are caught at the same proportion, age composition and growth changes, smaller and younger fish are gradually caught. Under these circumstances after some time, unit fish quantity in other words density of the stock decreases. Initially, an increase in the product is observed by the increasing fishing, however a decrease in the product is observed later although the fishing efforts are increased (Bingel, 2002).

It has been detected that 335 kg fish is caught in Karakaya Dam Lake in 2004-2005 fishing season. Productivity per hectare is 14.67 kg ha^-1, annual fish quantity per boat is 2.815 kg. The difference between the estimated fishable stock quantity and the fished quantity is statistically insignificant (p>0.05) for all species other than Onchorincus mykiss.

DISCUSSION

Fishing with gillnets method that has been first used by Avsar et al. (2001) has also been used in the studies of Ozyurt et al. (2004) and Cicek et al. (2006). Demirhan (2006) has reported that one of the methods they have used in the fish stock determination and evaluation studies conducted within the body of General Directorate of State Hydraulic Works. In this study, annual fishable stock of Karakaya Dam Lake is estimated by using fishing with gillnets method which has emerged as an alternative method for population size estimation. It is assumed that this method can be used for inland waters fishery management, however the studies should be repeated at certain intervals and comparison of the amounts of produced fish obtained by catching should give better results.

CONCLUSION

In this study, the fish stock estimation method used in this study can be used for fishery management. It is supposed that better results would be achieved if the studies are repeated periodically and the amounts of produced fish obtained by catching in between the estimation periods are compared to each other.