Stock Assessment of the Hooghly Croaker Panna Heterolepis From the Bay of Bengal in Bangladesh Through Multi-Models

Abstract

The Hooghly croaker (Panna heterolepis Trewavas, 1977) is a very popular marine and brackish water fish species in Bangladesh under the family Sciaenidae. This species is locally known as Poa and commercially very important as a table food item in the coastal region of Bangladesh. The overall objective of this study is to assess the stock of P. heterolepis in the Bay of Bengal, Bangladesh. A total of 1223 specimens were collected monthly basis using traditional fishing gears from the Bay of Bengal, Bangladesh, during January to December, 2019. Male and female individuals were identified by gonadal observation under microscope. Individual total length (TL) and body weight (BW) were measured by measuring board and digital balance with 0.01 cm and 0.01 g accuracy. The length frequency distribution was estimated by 1.0 cm TL intervals. The length-weight relationship (LWR) was calculated by W=a*Lb. Multiple condition factors (Fulton’s, KF; allometric, KA; Relative, KR and relative weight, WR) were analyzed for assessing the well-being of this species in relation to eco-climatic factors. To assess the size at sexual maturity (TLm), spawning season and peak - spawning season; the GSI (gonadosomatic index in %), MGSI (modified gonadosomatic index in %) and DI (Dobriyal index) were considered. The von Bertalanffy growth function (VBGF) was used to determine asymptotic length (L∞), growth coefficient (K), growth performance index (Ø') and life-span (tmax). Total mortality (Z) was calculated by the length-converted catch curve method. Natural mortality (M) was assessed as log10M = -0.0066 – 0.279log10 L∞ + 0:6543log10K + 0:0463log10T; where T indicates the average temperature of the habitat (28.5oC). The fishing mortality (F) was estimated as Z-M. Besides, exploitation rate (E) was determined as E = F/Z = F / (F + M). Recruitment rate was assessed from the analysis of the total time series of LFDs and growth parameters using VBGF models. Finally, MSY (Maximum Sustainable Yield) was determined following the formula of Gulland (1983). The fin formula was: D. 43 – 55 (VIII–X + i/34–44); P. 15 17 (i/14–16); Pv. 6 (I/5); A. 7 – 10 (II/5–8); C. 17 – 19 (ii/15–17). The overall sex ratio (male: female=1.00:0.87) varied statistically from the predictable ratio of 1:1 (df =1, χ2 = 5.91, p > 0.05). The overall growth pattern was negative allometric (b < 3.0) for both sexes. Further, the allometric co-efficient (b) of LWRs was found significantly related with habitat temperature for both male and female, respectively. Total length varied between 10.7-31.4 cm for males and 10.5-34.5 cm for females. The calculated form factor (a3.0) was 0.0070 and 0.0071 for male and female P. heterolepis, respectively. The KF was found the best for assessing the well-being of this species. Moreover, KF was found significantly related with temperature for both male and female. The WR denoted that the ecosystem was in balanced condition. Based on GSI, MGSI and DI index, the Lm was documented 15.0 cm in TL. Further, the higher values of GSI, MGSI and DI denoted that the spawning season was extended from January to July, with a peak in February. In addition, GSI was recorded statistically correlated with water temperature. The von Bertalanffy growth function revealed growth coefficient (K = 0.13 year-1), growth performance index (Ø' = 2.30) and life-span (tmax = 3.85 year). Further, P. heterolepis was found to grow rapidly with an asymptotic length (L∞) of 39.08 cm. We found that the natural mortality (M = 0.44 year-1) rate is almost similar with fishing mortality (F = 0.42 year-1). Consequently, the standing stock is not quite sustainable with the existing fishing strategy. Further, the maximum allowable exploitation rate (Emax = 0.471) was found a bit lower than the recorded exploitation rate (E = 0.490). Subsequently, overfishing is the most focal threat for the wild population of P. heterolepis in the context of Bay of Bengal, Bangladesh. The recruitment pattern was almost continuous throughout sampling period with one major peak in April-May. Finally, the MSY was assessed at 10,234.47 metric tons. The findings would be very useful to introduce appropriate fishing regulations in the Bay of Bengal ecosystem.