Determination of bioecological characteristics of the rays (rajidae) in the North Aegean Sea

In this study, age composition, growth characteristics, length at first maturity, reproduction, stock densities and feeding habits of Raja clavata, Raja miraletus, Raja radula, Rostroraja alba, Dipturus oxyrinchus and Leucoraja naevus belonging the Rajidae family were studied in the Saros Bay, North Aegean Sea between February 2005-December 2007. Samples were collected monthly at depths ranging from 0-50m, 50-100m, 100-200m and 200-500m by trawling. von Bertalanffy growth constants of were calculated by using species specific growth characteristics; R. clavata; L?=99,99cm, K=0,15yr-1, t0= -1,08yr, R. miraletus; L?=62,43cm, K=0,28yr-1, t0=-0,54yr, R. radula; L?=107,75cm, K=0,09yr-1, t0= -1,40yr, R. alba; L?=311 cm, K=0,04yr-1, t0=-0,25yr ve D. oxyrinchus; L?=256,46cm, K=0,04yr-1, t0=-1,17yr. The length-weight and disc length-weight relationships were calculated as TW=0,0016*TL3,3249 and TW=0,0138*DW3,1019 for R.clavata; TW=0,0017*TL3,2666 and TW=0,0055*DW3,3757 for R. miraletus; TW=0,0021*TL3,3195 and TW=0,0091*DW3,2914 for R. radula; TW=0,0019*TL3,2679 and TW=0,0063*DW3,2161 for R. alba; TW=0,0008*TL3,3532 and TW=0,0043*DW3,2875 for D. oxyrinchus, respectively.Gonadosomatic index and condition factors of these species, indicated that the reproduction period of R. clavata was in the spring and early summer. On the other hand the reproduction periods of R. miraletus was longer and showed seasonal patterns; the first one was in the spring, followed by late summer and autumn. Similarly, the reproduction period of D. oxyrinchus was also longer including spawning periods starting from late summer to autumn and through the winter. The reproduction period of R. alba was almost year around and was considerably longer than those of others.Exponential coefficients of total mortality of R. clavata, R. miraletus, R.radula, R. alba and D.oxyrinchus were estimated as Z=0,25, 0,50, 0,23, 0,20 and 0,20yr-1, respectively. Exponential coefficients of natural mortality for these species were calculated as M= 0,12, 0,18, 0,14, 0,09, 0,13 and exponential coefficients of fishing mortality were calculated as F= 0,13, 0,32, 0,09, 0,11 and 0,07yr-1. The explotation rate of R. clavata and R.alba were 0,51 ve 0,57yr-1, respectively and indicated that slightly overfished populations. The explotation rate for R. radula and D. oxyrinchus were calculated as 0,39 and 0,37yr-1 , respectively and indicated that there is no fishing pressure for these two species. On the other hand, the explotation rate for R. miraletus was estimated as 0,64yr-1 showed that this species is currently being overfished in Saros Bay.The analysis of stomach contents revealed that R. clavata, R. miraletus, R. radula, R.alba and D. oxyrinchus feed on a wide variety of crustaceans and fishes. There were not significantly difference in composition of the diet of male and female of these species (P>0,05). In the study area the diet composition mainly of crustaceans (Goneplax rhomboides, Liocarcinus depurator, Parapenaus longirostris, Athelecyclus roduntatus) and fish (Mullus barbatus).The average catch per unit effort (kg/sa) and biomass index (kg/km2) values of the rays were 4,87kg/h and 91,18 kg/km2 respectively. On a yearly basis, average catch per unit effort and biomass index values for these species were determined as 4,41kg/h and 84,00kg/km2 in 2005, 4,74kg/h and 90,41kg/km2 in 2006, 5,29kg/h and 96,64kg/km2 in 2007.