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Catch and discard fish species of trawl fisheries in the Iskenderun Bay (North-eastern Mediterranean) with emphasis on lessepsian and chondricthyan species

Fish species in catch and discard of trawl fisheries in and around Iskenderun Bay were examined within the fishing closure period and fishing period. The sampling was performed from May 2010 to January 2011 by a commercial trawl vessel. Chondricthyan species accounted for 51 % of discard catch biomass while Gymnura altavela and Dasyatis pastinaca were dominant in hauls. 27 lessepsian fish species were captured during the study, nine of them being target species for trawl fisheries. In total, 14 of the lessepsian fish species were determined as discard species. In both sampling periods, Equulites klunzingeri and Citharus linguatula contributed to discard fish catch dissimilarity among depth ranges (deeper and shallower than 60 m). E. klunzingeri showed high abundance in discard catch. There were no significant differences in the distribution of the discard fish biomass between the sampling periods (ANOVA test, p>0.05). However, depth range highlighted significant differences in discard fish catch composition (p<0.05). Among major commercial fish species of trawl fisheries, Mullus surmuletus and Sparus aurata were not separated as discard in any haul by fishermen. Any size of these two species was included in the commercial catch (Total length ranged from 61 to 721 mm).


Introduction
World fisheries have not considered the sustainability of wild food resources until a few decades ago. Destruction of fish stocks was discovered by researchers in 1980s. Thereafter, the discard problem in marine fisheries was described (Alverson et al., 1994;Hall, 1996). Non-target or non-commercial catches are defined as discard, generally brought onto the deck of fishing vessels and then returned to the sea. Although some species are included in the commercial part, they might be determined as discard because of unmarketable consideration (Hall et al., 2000). Unmarketable sizes may change with fishing time and depth depending on fishermen (Kelleher, 2005).
The objective of this study is to detect the fish composition of trawl fisheries in Iskenderun Bay and to analyze the commercial and discard fish species with their rates in different periods with emphasis on Lessepsian and Chondrichtiyan fish species.

Study Area
The study was carried out in and around Iskenderun Bay, which is located in the Levantine Sea (North-eastern Mediterranean) (Fig. 1), an important trawling area in the Eastern Mediterranean Sea. The bay shows high productivity caused by local wind effects and movements such as upwelling and rich terrestrial nutrient inputs. Surface water temperature ranges from 16°C to 33°C, and salinity between 37 and 40 psu (Polat & Piner, 2002;Polat, 2010). Regional fish communities are exposed to fishing pressure from almost one hundred trawlers vessel every year. Marine fisheries along Turkish south coast (North of the Levantine Sea) include 6.93 % of marine fisheries productivity in Turkish seas (Anonymous, 2012a). The trawling ban period is between April 15 and September 15 in Turkish seas, in accordance with the decision of the Ministry of Agriculture and Rural Affairs, General Directorate of Protection and Control Vision of the Republic of Turkey. Trawling is permitted over two miles in the bay and one mile off Samandag coast (Anonymous, 2012b). Thus, in general, trawl fishery activities in the bay take place on bottoms deeper than 60 m.

Sampling Data
Field work was performed from May 2010 to January 2011 in Iskenderun Bay. The May and August surveys were included in the trawl fishing closure period, while the October and January surveys took place during the fishing period. Data was collected using a commercial trawl vessel (23 m in length) with engine power up to 400 HP, and the stretched codend mesh size of the trawl net was 44 mm. A total of 32 hauls were performed at depths between 31 and 110 m determined by the fishermen. Ten hauls were carried out in May, seven hauls in August, seven hauls in October and eight hauls in January. The mean haul duration was 1.5 hours at an average speed of 2.5 knots. The commercial catch in each haul was selected by the crew on board, and the remaining catch was considered as discard. Biomass and abundance data of the fish species was recorded as discard and commercial catch by researchers. We randomly applied subsampling for the most abundant species such as E. klunzingeri. The commercial and discard catches were stored at -20 ºC at the laboratory. After identification of the fish species, total length (mm) and body weight (g) were measured.

Data Analysis
The catch data of each haul was standardized with the catch per unit effort (CPUE) (D'Onghia et al., 2003). It was performed as fish weight divided by catch time, where Cw is the weight of the fish catch and Ct is the towing time.

CPUE = Cw/Ct
The number of species (S) was counted for each haul. The diversity indices were computed using the number of specimens to standardize with catch hour per haul. The estimation was made using the Simpson diversity index, Species Richness, Shannon-Wiener diversity index H´ (log 2 ) and Pielou evenness index; Simpson Diversity Index = 1-λ → λ = "λ" is the Simpson Index and Simpson diversity index is found 1-λ, where "n i " is the total number of specimens of each species and "N" is the total number of specimens of all species. Margalef Species Richness index (d) = (S-1) / ln N, where "S" is the number of species in the samples (Beisel and Moreteau, 1997 Haul depths were divided in two: hauls with depths shallower than 60 m and deeper than 60 m. Homogeneity of variance was tested with Levene's test. Variances were transformed using the log(x+1) formula. After data transformation, the factor on fish discard biomass was analyzed with the one-way ANOVA statistical test. The analysis was carried out using the statistical programme SPSS, version 17 (Levesque, 2007).
Abundance of fish discard data was transformed [log(x+1)] and then rank-ordered using the Bray Curtis similarity matrix. The Non-parametric Multidimensional Scaling test was performed using the Primer 6 statistical package programme (Clarke & Warwick, 2001). The SIM-PER test was performed to determine the level of similarity and dissimilarity according to trawl fishing sampling depth.

Results
77994 individuals were obtained in 32 hauls, belonging to 97 fish species (Table 1). 69 (n=54119) of them were determined as discard. 43 (n=52655) species were thrown back into sea regardless of their size, while 26 species (n=1464) have been considered as unmarketable size of commercial species. 993.4 kg of the total 2491.3 kg of fish catch was discard (Table 1).
Total fish biomass caught per haul was between 5.7 and 119.5 kg/h during the fishing period and between 1.1 and 40.9 kg/h of that amount was discard catch. However, total fish biomass caught per haul ranged from 3.4 to 211.2 kg/h during the fishing closure time, and 0.6 to 151.2 kg/h of that amount was discard catch. The average discard catch rate was found to be 32.2 ± 16.5 % during the study. The estimates of fish biomass have been separately studied in terms of depth ranges (Table 2).

Ecological Index
The ecological indices of total and discard fish catch per month are shown in Table 3. The evenness index (J') ranged from 0.40 to 0.63 in total catch. The Shannon -Weiner diversity index per haul was estimated to be 2.46 in May 2010, 1.91 in August 2010, 2.55 in October 2010 and 2.68 in January 2011.
Abundance of discard catches showed differences according to depth (ANOSIM. R= 0.531, p< 0.001). The SIM-PER analysis indicated that C. linguatula displayed highest average similarity among discard fish. In waters shallower than 60 m., E. klunzingeri showed highest average abundance and nine Lessepsian species contributed mainly to average similarity. Also, these two species contributed to dissimilarity in the sampling depth groups ( Table 4).

Comparison of Discard Fish Biomass
The computation of the discard fish rates indicated that 44.5 % of the catch was in the depth range of 30-60 m and 28.1% was in the depth range of 60-110 m. The average of discard rates among sampling months showed close values except of January. The highest discard CPUE was determined in August (average: 44.86 kg/h), while the lowest was in October (average: 9.04 kg/h). Although a significant relationship was found between depth and discard fish biomass (p < 0.05, p = 0.0001), there is no differences between sampling periods and discard biomass (p > 0.05, p = 0.106).

Lessepsian Species
In total, 27 Red Sea originated fish species were found in the study. 13 of them were commercial and 14 species were discard. Lessepsian fish constituted 71 % of total catch abundance. They constituted 55 % of total catch biomass and dominated the commercial catch biomass at the rate of 69.5 %. In January, commercial biomass of Lessepsian fish reached 93.5% in a haul. Furthermore, they represented between 0.7 % and 92.8 % (on average 38.5 %) of the discard catch.
Some of the other Lessepsian species such as N. randalli, U. moluccesis, U. pori and S. undosquamis were mainly commercial species for regional fisheries. Biomass values of these species in total catch were 3.4 kg/h, 0.2 kg/h, 5 kg/h and 13.6 kg/h, respectively. While U. moluccensis was found in waters deeper than 60 m, U. pori was detected at depths between 30 and 60 m.

Length Ranges of the Commercial Species
S. undosquamis (654 kg), U. pori (244 kg), P. erythrinus (219 kg), N. randalli (166 kg) and S. aurata (51 kg) constitute the major target species in commercial trawl fisheries of Iskenderun Bay. These five species consisted of 54 % of the commercial catch these species. Besides, these fishes have low discard weight rates (0 to 2.2%). S. undosquamis, the highest biomass commercial species, attained a length range of 61 to 172 mm (mean = 129 mm, SD= 45.2) in discard. The length of P. erythrinus ranged between 71 mm and 135 mm (mean = 113 mm, SD = 21.8) in discard. However, individuals that were bigger than 105 mm were evaluated to have economic value. Discard length of N. randalli was between 52 mm and 130 mm (mean = 97.2 mm, SD= 14.3), while Upeneus moluccensis attained a discard length of 66 to 115 mm (mean = 89 mm, SD= 12.2) (Fig. 5).  S. aurata and M. surmuletus were not in the discard catch. Body size of the commercial fish ranged from 69 mm to 721 mm (Fig. 5).

Discussion
The variations of abundance and biomass data demonstrate the direct effect of fisheries on fish fauna. The percentages of discard fish species showed different results due to the changeable factors such as depth and time in Mediterranean Sea trawl fisheries (Machias et al., 2001;D'Onghia et al., 2003;Sanchez et al., 2004;Tudela, 2004;Alsayes & Fatthouh, 2009;Damalas et al., 2010;Edelist et al., 2011).
The study area is highly exploited during the fishing season (September 15 -April 15). High rates of discard were accompanied by low homogeneity in August ( Table  2). The fish catch shown a more homogenous structure in fishing season. Also, the number of discard species declined toward the end of fishing season (Table 3). Sanchez et al. (2007) showed that M. barbatus had low discard rates in the Adriatic and Catalan sea. Gucu (2012) verified that discard rates increased toward shallower depths in Mersin Bay. Bothus podas and the unmarketable size of M. barbatus were included in discard composition shallower than 100 m in Mersin Bay (North-Western Levantine Sea) (Gucu, 2012). In our study, M. barbatus also had a low ratio in discard catch. Besides. S. aurata and M. surmuletus was not included in discard catch. All specimens were considered as marketable, but some were not suitable for the minimum landing size regulation (Fig. 4).
Discard biomass ratios were calculated to be 44 % for the shallower than 60 m depth, and its value was 75 % of all the discard catch. 28 % of the total catches consisted of biomass of trawl fishing area (deeper than 60 m). These ratios indicate that shallow water has higher fish productivity than the trawl fishing area. We can support this opinion with biomass of S. undosquamis. It was found to be 9.94 kg/h and 19.57 kg/h in shallower water and deeper depths, respectively. It could be followed by long-term researches on commercial species biomass such as S. undosquamis in order to get more information in the fishing closed area of Iskenderun bay.

Chondrichtyan Fishes
The biomass percentages of cartiloginous species constituted the main discard group within the discard catch (Fig.3). Carcharhinus plumbeus, Gymnura altavela and Mustelus mustelus are classified as "vulnerable", while Rhinobatos cemiculus is "endangered" on the IUCN list (IUCN, 2013). There are some studies on Carcharhinus plumbeus in the Turkish Seas. One of these studies was carried out in Boncuk Bay (South-eastern Aegean Sea), which is thought to be a spawning and hatching area for the species (Bilecenoglu, 2008;Akca, 2010). Although the marketing of this specie is banned by law in Turkey (Anonymous, 2012b). C. plumbeus is still caught and sold on the markets due to the poor control mechanism. Damalas & Vassilopoulou (2009) pointed out that Dasyatis pastinaca and Mustelus sp. are mostly marketed in the central Aegean Sea. However, Gurbet et al. (2013) mentioned that cartilaginous species were not included in the commercial catch composition in Izmir Bay (Aegean Sea). Mustelus species are rarely found, and it is evaluated as an incidental commercial catch in Iskenderun Bay. Dasyatis is not considered as a marketable species in the area.

Lessepsian fishes
The structure of the coastal fish community shows variability in the area because of new immigrant species entering from the Red Sea. In addition, the majority of the total catches consisted of these species in the regional fisheries. For example, N. randalli was recorded in 2008 (Bilecenoglu & Russell, 2008), and is now considered as a commercial species by fishermen. The biomass of N. randalli was found to be 2.42 kg/h in the study. Gucu et al. (2010) found that the percentages of Lessepsian fish species decreased in total catch when compared to the 1980s and 2000s, but the number of Lessepsian fish species increased. Gucu et al. (1994) reported 20 Lessepsian fish species in the 1980s. Recently, we captured 27 Lessepsian fish species in Iskenderun Bay. Until now, over 50 Lessepsian fish species are known in Turkish Seas (Bilecenoglu et al., 2002;Erguden et al., 2012;Dalyan et al., 2012;Bodilis et al., 2014). In the 1980s, S. undosquamis and E. klunzingeri formed the highest biomass at the rate of 23 % and 14 % of total catch in the Levantine Sea (Gucu et al., 2010). We obtained practically the same results for these species (S. undosquamis 26 % and E. klunzingeri 13 % of total fish catch). Equulites klunzingeri was a major species within the discard catch. On the continental shelf of the North Eastern Levantine Sea, E. klunzingeri was found at the rates of 9.4 %, 2.3 % and 20.9 % of the fish fauna in fall 1983, spring 1984 and fall 1984, respectively (Gucu et al., 2010). Çiçek (2006) mentioned that E. klunzingeri was 0.81 kg/h in Tasucu Bay (The west of Iskenderun Bay). The distribution of Pony fish probably decreases towards the west of Iskenderun Bay. In the sampling area, its distribution depends on the depth range (Table 3). In our study, it was observed that Citharus linguatula and Champsodon sp. replaced E. klunzingeri within the discard catch of deeper than 60 m. It was also observed that Champsodon sp. and C. linguatula constituted 11.26 and 9.13 % of total catch, respectively. In the last five years, three Champsodon species were recorded from the Mediterranean Sea (Cicek & Bilecenoglu 2009;Dalyan et al., 2012;Gokoglu & Ozvarol, 2013). Our laboratory studies showed that the obtained Champsodon species belonged to C. capensis and C. nudivittis. However, samples were classified as Champsodon sp. because it was not possible to identify the differences between species on board. In this way, incorrect values were not employed while calculating abundance and biomass values. They were found in increasing rates during sampling.
The minimum landing sizes of the commercial fishes could be consider inappropriate according to legislative regulations of fisheries management in Turkey (Table 1). The reason is that weak control mechanism of trawl fisheries in the studying area. In the last declaration of minimum landing size of commercial fish species, one Lessepsian fish species, namely U. moluccensis, was on the list. But almost 60 % of the commercial fish species biomass consisted of Lessepsians in Iskenderun Bay. Biological research on commercial species should increase not only for the conservation of commercial species but also for taking management measures for the fisheries in the area.
Studies on discard catches are important for the management of fisheries activities, especially for multispecies trawl fisheries. Additional comprehensive studies on fisheries and their problems in the area, which is characterised by a dynamic structure due to the entry of new Red Sea originating fish species.