THE COMMERCIAL AND DISCARD CATCH RATES OF THE TRAWL FISHERY IN THE İSKENDERUN BAY (NORTHEASTERN LEVANTINE SEA)

: The management of fishery is significant due to the sustainability of marine resources. Therefore, the fishing areas should be constantly monitored. In this study, the fishery data were collected from the Iskenderun Bay (Northeastern Levantine Sea) with a rented commercial trawl vessel. The fishery-dependent data was recorded during the 2012-2013 fishing season with the help of the crew. 33 tows were achieved using a trawl net (codend diamond mesh size of 44 mm) for 26 hours. Each tow was limited with 70 min and the towing speed varied between 2.5 to 3.0 knots. The depth contour ranged from 39 to 69 m. While 32 species were evaluated as discard, 35 species were included in the landed catch. The total catch consisted of 67.2% the landed and 32.8% of the discarded fish in terms of CPUE W (catch per unit effort by weight).


Introduction
Discard is a part of the catch that is unwanted by fishers due to being too small, damaged, inedible, having a little or no economic value, or not being able to be retained with management or quota restrictions (Zeller et al. 2018).Data on discard rates plays a key role for understanding the energy flow of the marine ecosystem (Machias et al. 2001).
The bottom trawl makes the highest contribution to the 9.1 million tonnes annual discard levels worldwide with 4.2 million tonnes (Pérez Roda et al. 2019).Mitigation of discard catches is extremely decisive on biodiversity and ecosystem health.There are some ways like mesh size regulations, catch quotas or effort limitations, minimum landing sizes to reduce the amount of discard catch caught by trawls (Weissenberger 2014).In Turkish waters, the mesh size and minimum landing size regulations are preferred by the Ministry of Agriculture and Forestry.
Researchers performed so far in Turkish Seas have focused on discard catch in the last few decades and most of them used beam trawls (Bayhan et al. 2006, Demirci 2003, Gökçe & Metin 2006, Kınacıgil 1999a, b, Soykan et al. 2006, Yazıcı et al. 2006).However, conducting a long-term monitoring study to determine the discarded yield takes too much effort and it is financially compelling as well.
The present study was conducted in the Iskenderun Bay (Easternmost Levantine Sea) where a high biodiversity is present.Erythrean invasion is one of the main causes of this situation.Invasion is also the most important ecological process affecting fisheries in northeastern Mediterranean Sea.The Red Sea immigrants are evaluated in trawl catch from the Northeastern Levantine Sea (Gücü et al. 1994, Gücü et al. 2010, Özyurt et al. 2018, Yemişken et al. 2014).Apart from this, the bay hosts many cartilaginous fishes and these fishes were previously investigated in terms of bycatch and discard.However, the majority of Elasmobranch species are threatened with extinction in the area (Yağlıoğlu et al. 2015, Yemişken et al. 2014).
This study aimed to define the catch composition and catch/discard rates of trawl fisheries in Iskenderun Bay.

Study Area
The Northeastern Levantine Sea has a wide continental shelf including the Iskenderun Bay (Fig. 1).The bay is preferred as a study area due to being an important fishing area in the Eastern Mediterranean (Yemisken et al. 2014).Its surface area is about 2275 km 2 with an average depth of 70 m, and its bottom is mainly covered with sand and mud.The range of sea surface water temperature was recorded between 16 and 30°C during 2012-2013 (Karpuz & Sakalli 2019) while salinity varied from 37.0 to 39.4 psu (Terbıyık Kurt 2018).
The regulations and restrictions of trawl fishery are determined by the Ministry of Agriculture and Forestry.The fishing season in the Iskenderun Bay is between September 15 and April 15 every year.Fishing in the bay is prohibited within the first 2 miles off the coast and 44 mm minimum mesh size for the diamond mesh can be used in the Turkish coasts of the Mediterranean Sea.

Sampling
Fishery dependent data were collected by the commercial trawler (Faik Baba, 22 m, 400 HP), in December 2012, February and April 2013, during the legal fishing season.33 tows with trawl net had achieved with a codend diamond mesh size of 44 mm.Each towing duration ranged from 30 to 70 min depending on topology, bottom type and vessel speed and the towing speed varied between 2.5 to 3.0 knots.Fieldwork included recording the characteristics of the haul, the estimation of the total catch and landed catch.When the catch reached to the board, it was sorted out as commercial and discard by the crew.After the catch was sorted by the crew, the discard portion was put in boxes.Then the number of individuals of the discard catch was counted and the total weights of each species were noted.Weight was determined to the nearest 1 g.Sub-sampling was performed for the species with a high number of individuals.Species that are difficult to identify were identified in the laboratory.

Statistical Analysis
The most reliable estimation used to determine negative or positive changes in stocks, is the CPUE (Catch Per Unit Effort) calculation (Bordalo-Machado 2006).The CPUE of the trawl was calculated and expressed as biomass/towing time or abundance/towing time (Eq.

Results and Discussion
The tows duration lasted 26 hours in 33 tows (Table 1).The tows were performed at 39-69 m (average depths) depth contour, which is the most visited by fishermen in fishing seasons.Among the tows, the highest abundance and biomass values belonged to the haul at 39 m, which is the shallowest depth.While the discard catch per haul was estimated between 0.25 and 25.5 kg/h, total fish catch varied between 2.3 and 41 kg/h during the fishing period (Table 1).
In 2012-2013 fishing season, a total of 67 species were evaluated by the crew, of which 32 were in discard and 35 were in the landed catch.Glaucostegus cemiculus (Geoffroy St. Hilaire, 1817), Mustelus mustelus (Linnaeus, 1758) and Zeus faber Linnaeus, 1758 were placed in discard catch due to avoided sizes of the individuals.
A total of 332 kg of the catch was obtained during the study.The total abundance of fishes was 7738.Data refers to 10506 individuals when the number of individuals in each tow is standardized by the swept area (CPUEA), 60.5% of which are discarded (Table 2).In terms of CPUEW the trawl catches mainly consisted of 67.2% landed and 32.8% discarded fish.Five species were identified as cartilaginous fishes, among those G. cemiculus and M. mustelus are of limited economic value.These fishes take part in IUCN Red List as CR and VU, respectively.All chondrichthyes was sorted as discard and they covered 11% of the total catch.Cartilaginous fish were estimated to 33% of the total discard catch biomass.Dasyatis pastinaca covered the largest part of the total discard catch biomass but was represented by low numbers in the total abundance.Among Erythrean species, Saurida lessepsianus and Nemipterus randalli were the most important species that were obtained in the trawl fishery (Fig. 2).Also, these species constitute 41% of the landed catch.Gücü et al. (1994) estimated that about 30% of the Teleost catch from the bay belonged to S. lessepsianus.Dasyatis pastinaca, Equulites klunzingeri and N. randalli constituted 58% of the discard catch biomass while E. klunzingeri, Champsodon nudivittis and N. randalli were included in the abundance of discard catch with a percentage of 68%.

Conger conger, Diplodus annularis, Epinephelus aeneus, E. costae, Etrumeus teres, Chelon auratus,
Pomatomus saltatrix, Sardina pilchardus, Scomber colias, Siganus rivulatus, Sparus aurata, Sphyraena chrysotaenia, Synodus saurus, Trachurus mediterraneus and Upeneus pori were not found in the discard catch.The local fishermen tend to evaluate all sizes of E. aeneus, E. costae and S. aurata in the landed catch.The intense trawl fisheries activity is the main threat for these fishes in the bay (Fig. 3).In the Northeastern Levantine Sea, the catch and discard of the trawl fishery have a trend to decrease towards the end of the fishing season.The biomass value of seasonal catch declines sharply in the January-February period (Yemişken et al. 2014, Gökçe et al. 2016).The total catch biomass estimated in April was about 2 times higher than the February (Fig. 4).The depth range differences between the studies may have caused this mismatch.

Species
Considering the values of trawl fishery of the bay, it appeared that most of the discard biomass consisted of chondrichthyes species that feed on carcasses formed as a result of intense fishing activity.Besides, two of the three species (S. lessepsianus and N. randalli) occupying the majority of the landed biomass belong to the Red Sea immigrants, indicating that the existence of these species in the region has irreversible effects.The dispersal success of the Red Sea immigrants plays a significant role in the trawl fisheries and these species should be monitored with long-running studies.
Studies on discard catches are important in terms of fisheries management, especially for multispecies trawl fisheries.Additional comprehensive studies and long-term monitoring are needed in order to understand the population dynamics and to detect ultimate changes in the target area.Moreover, there is no study to reduce fishing pressure on the fish stocks, except Gücü (2012) who revealed that regulations of depth and fishing period had a positive effect to mitigate discard rates in the north-eastern Levantine Sea.Advanced studies should be encouraged in the area which has significant demersal resources that are under the pressure of intense fisheries activities.

Fig. 2 .
Fig. 2. The standardized values of the catch in the Iskenderun Bay; A) CPUEA of the discard catch, B) CPUEW of the discard catch, C) CPUEA of the landed catch, D) CPUEW of the landed catch.

Fig. 3 .
Fig. 3.The discard rates of fishes in the landed catch in the Iskenderun Bay at 2012-2013 fishing period.

The
Commercial and Discard Catch Rates of the Trawl Fishery in the İskenderun Bay 129 Trakya Univ J Nat Sci, 21(2): 123-129, 2020

Table 1 .
The informative data of the tows achieved in the Iskenderun Bay at 2012-2013 fishing season (Tow numbers 1-12 were carried out in Dec-2012, 13-25 in Feb-2013 and 26-33 in Apr-2013).

Table 2 .
Yemişken et al. (2014)mentioned that the species is among the vast majority of the discard catch biomass with Gymnura altavela in the area.Also, Yaglioglu et al.The standardized values of the landed and discard catch as CPUEA (ind./h) and CPUEW (g/h) in the Iskenderun Bay.Status of the listed fishes were determined as D (Discard) and L (Landed) (Red Sea immigrants*; Cartilaginous ++ ).The Commercial and Discard Catch Rates of the Trawl Fishery in the İskenderun Bay 127 Trakya Univ J Nat Sci, 21(2): 123-129, 2020