Length base spawning potential ratio (LBSPR) and Length Based Indicators (LBI) of Bullet tuna, i Auxis rochei /i (Risso, 1810) in the Oman Sea (Iranian southern waters)

The length-Based Spawning Potential Ratio (LB-SPR) model and Length Based Indicators (LBI) were applied to describe the stock status of Bullet tuna, Auxis rochei in the Oman Sea. From April 2021 to March 2022, a total of 254 specimens were collected from fish landing sites monthly, and their length (FL) and weight were measured. The maximum length recorded for this species in the area was 43 cm FL with mean length of 32±4cm. The relationship between length and weight was calculated as, W= 0.0095×L3.12 (R2= 0.92). The t –test showed not significant differences between estimated b and b= 3 at the level of 0.05 (P 0.05), which means Isometric growth pattern for Bullet tuna species in the northern waters of the Oman Sea. Length Based Indicators (LBI) of Bullet tuna catches were (Linf= 45cm, Lopt= 28cm, LF=M= 25cm, Lmat= 32cm). Also, length-based reference point was percentage of mature fish in the catch (Lmat= 32cm, Pmat= 0.85), percentage of fish caught at the optimum length for harvest (Lopt= 28 cm, Popt=0.19), and percentage of mega-spawners in the catch (Lmega= 40cm, Pmega= 0.26) and Pobj= 1.79, respectively. Based on ratio of the Pobj value is 1, Pmega 0.1, Lmean/Lopt and Lmean/LF=M 1 show considered desirable. The range of annual SPR for Bullet tuna stock in the Oman sea (Iranian waters), estimated to be 0.66 (0.56-0.79) which is upper than the threshold value 0.2. Based on time series analysis of SPR, it means that the study indicated that Bullet tuna stock exploitation ratio in Iranian southern waters (Oman Sea) is below sustainable levels (under exploitation/green color), suggesting the need to increase exploitation ratio and fishing effort.