Stock structure analysis of Japanese threadfin bream, Nemipterus japonicus (Bloch, 1791) along the Indian coast based on truss network analysis

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1 Indian Journal of Geo Marine Sciences Vol. 46 (09), September 2017, pp Stock structure analysis of Japanese threadfin bream, Nemipterus japonicus (Bloch, 1791) along the Indian coast based on truss network analysis * Sreekanth G.B. 1, S. K. Chakraborty 2 & A.K. Jaiswar 2 1 ICAR-Central Coastal Agricultural Research Institute, Old Goa, Goa, , India 2 ICAR-Central Institute of Fisheries Education, Mumbai, Maharashtra, , India * [ gbsree@gmail.com] Received 01 February 2016 ; revised 18 February 2016 Fish samples were collected from four locations, two each from west and east coast of India. Thirty six morphometric distances were measured from each individual using a nine point truss network system and the canonical discriminant analysis showed that the distances along the mid body region are useful for differentiating these fish stocks along the Indian coast. Discriminant function analysis revealed % and 69.15% classification for coasts and locations with Kakinada stock turned out to be the well-defined group. The study indicated more than one stock of this species along east and west coasts. [Keywords: Japanese threadfin bream; Nemipterus japonicus; stock structure; truss network system; canonical discriminant analysis; discriminant analysis] Introduction A fish stock is defined as a subset of species which contains an intraspecific group of randomly mating individuals with temporal and spatial integrity 1. To manage fish stocks, it is very important to assess the stock structure of the species 2. Fisheries management exclusive of the stock structure will lead to increase in fishing effort, habitat degradation, overexploitation and depletion of fishery resources 3. The commercially important fisheries along Indian coast are now expected to be in a stage of transition from the current, fully exploited phase to the overexploited phase which may further leads to collapses 4. For effective management of a fishery resource and implementation of worthwhile stock rebuilding programmes, it is important to know the identity of the stock structure of that species, as each stock must be managed separately to optimize their yield 2. Fisheries management programmes without the identity of stocks can lead to unfavorable changes in the biology and productivity rate of a species 3, 5, 6. Morphometry of fishes could give insights on phenotypic stocks and groups of a species with similar growth and mortality traits 7. Japanese threadfin bream, Nemipterus japonicus 8 is the most dominant sought species of the family Nemipteridae along all the coastal stretches of India. As a group, threadfin breams contribute about 4% of total marine fish landings of India 9. N. japonicus contributes about 40 % of the total production of threadfin breams. Demersal fisheries contribute to about 26% of the total all India marine landings with threadfin breams shares 14.6% of demersal fisheries. This tropical species live in schools, generally close to bottom normally around depth zones of m with high concentration in m. The species is mainly landed in India at Cochin, Mumbai, Chennai, Visakhapatnam, Mangalore and Kakinada as a bycatch of shrimp trawls. It has a continuous distribution along the Northwest, Southwest and Southeast regions along the Indian coast 10. Being a

2 Latitude INDIAN J. MAR. SCI., VOL. 46, NO. 09, SEPTEMBER demersal and non-migratory species, N. japonicus requires in-depth stock structure analysis for developing better fisheries management practices. This will also have huge implications in the management of different genetic populations of the species in the long run. INDIA Table 1-Percentage of fish from each location (in rows) classified by discriminant analysis to their respective groups (in columns), `n indicates the total number of samples collected from the respective locations (CH-Chennai, KA-Kakinada, CO- Cochin, M-Mumbai), SL indicate the mean standard length and the means bearing the same superscript within a subgroup are not significantly different from each other (p 0.01). Arabian Sea Bay of Bengal Mumbai Cochin Chennai Kakinada Site n SL Site CH KA CO M Chennai a Kakinada b Cochin c Mumbai d No extensive study has been carried out to identify the number of stocks of this species along the Indian coast and so the stocks of this species are assessed disregard of the stock structure 4. This paper aims to identify the stock structure of N. japonicus populations at four sites along Indian coast using truss morphometric analysis so that the management strategies can be formulated for the species in future. Material and methods Samples of N. japonicus from the Indian waters were collected two locations from west coast (Mumbai and Kochi) and two locations (Kakinada and Chennai) from the east coasts of India (Fig. 1). The sampling sites were chosen from centres which are apart in latitudinal aspect to avoid mixing up of specimens from the same regions. A total of 389 fish samples were collected from commercial fish landing centres between October 2010 and December 2010 (Table 1) Longitude Fig. 1- Locations selected for sampling of, N. japonicus The intact fish samples were cleaned and placed on a flat platform with graph paper as a background (vertical and horizontal grids) which was used for calibrating the co-ordinates of digital images. The fins were erected and placed on a platform in such position that it makes the origin and insertion points visible. Each individual was labeled with a specific code to identify it in the image. For digitizing images of individual fishes, a cyber shot DSC-W300 digital camera (Sony, Japan) was used. Digital records of specimens provide a complete archive of the body shape and helps in making repeated measurements if required 11. Truss network followed in the present study was based on nine landmarks and the truss network was constructed by interconnecting them to form a total of 36 truss distances (Fig. 2). The extraction of truss distances from the digital images of specimens was conducted using a linear combination of two software platforms, tpsdig2 v and PAleontological STatistics (PAST) 13.The truss morphometric distances measured were first tested for outliers and 15 observations were removed based on Cook s distance estimates using PROC ROBUSTREG procedure of SAS software. Significant correlations were observed between the body size and the morphometric variables (p<0.01). Therefore, the absolute morphometric variables were first transformed into shape variables (sizeindependent) using an allometric approach by the following the formula 1, 14.

3 1838 SREEKANTH et al.: STOCK STRUCTURE ANALYSIS OF JAPANESE THREADFIN BREAM M trans = M Х (SL mean / SL) ß Where, M trans M SL SL mean : transformed truss measurement : original truss measurement : standard length of fish : mean standard length There was a significant morphometric variation observed for the samples from different locations along west and east coasts of India (Fig. 4). First and second canonical axes explained 58.8% and 20% of the total morphometric variation. Truss distances with meaningful loading on the first canonical axis belonged to oblique mid-body depth measurements (Fig. 3a). The truss distances with meaningful loading on the second canonical axis belonged to mid-ventral body (Fig. 3b). ß : within group slope of the geometric mean regressions of the log M against log SL. A Fig. 2. Truss network of N. japonicus The transformed truss measurements were subjected to canonical discriminant analysis (CDA) using the CANDISC procedure of SAS 15 to reveal the identity of samples from different sampling locations along west and east coasts. The truss distances loaded on canonical axes were selected based on the Hatcher s scratching procedure. The transformed distances loaded heavily on the first and second canonical axes in CDA were used to carry out the discriminant function analysis (DFA). The DFA develops a discriminant criterion to classify each specimen into one of the groups which can be locations or coasts, based on the context of analysis. Results The CDA on transformed truss distances displayed a significant morphometric variation between individuals obtained from all sampling locations. Truss morphometric distances with high loadings on the first two canonical axes were found useful in distinguishing these samples. B Fig. 3- Truss variables of N. japonicus with meaningful loadings on first (A) and second (B) canonical axes in CDA. There was a significant morphometric variation observed for the samples from west and east coasts of India (Fig. 5). First and second canonical axes explained 65% and 25% of the total morphometric variation. The truss distances from mid-body region have significantly loaded on the first canonical axis (Fig. 3a) and truss distances from mid-ventral body have significantly loaded on the second canonical axis (Fig. 3b). In CDA, the truss distances which loaded on the first canonical variable have been taken to find out the classification rates of individuals with respect to coasts and locations using DFA (Table 1). The truss morphometric distances have correctly classified 69 % of the fish samples to their respective locations and 79% of the fish samples to their respective coasts. The location-wise analysis

4 INDIAN J. MAR. SCI., VOL. 46, NO. 09, SEPTEMBER has given overall 69% of classification accuracy i.e., 86.61%, 72.09%, 60.78% and 53.93% for Kakinada, Cochin, Chennai and Mumbai samples respectively. Fig. 4- Location wise canonical discriminant plot between scores of first and second canonical axes extracted from 9 point truss measurements of N. japonicus. Fig. 5. Coast wise canonical discriminant plot between scores of first and second canonical axes extracted from 9 point truss measurements of N. japonicus. Discussion The truss morphometric analysis has revealed significant difference among the populations of N. japonicus along Indian coast. The vertical distances related to mid-body region are useful for classifying populations between the different sampling locations. The results pointed separate stocks of this species may exist along the east and west coasts. In the following paragraphs, we discuss the reasons which could have contributed to the separation of these populations. N. japonicus is a non-migratory demersal fish with limited mixing in their populations. Hence, the geographical separation of these populations may result in formation of different fish stocks due to limited genetic exchange 16. Hence, this might have resulted in the establishment of different fish stocks on east and west coast of India. Moreover, many research reports revealed a significant variation in the northern and southern parts of Arabian Sea and the Bay of Bengal 17, 18. Using multivariate morphometric analysis, studies reported that the morphometry of populations of this species differs in Arabian Sea and Bay of Bengal 19. However, studies on the meristic traits did not reveal any significant difference in these populations 20. Thus, the difference in ecology and the geographical separation may have resulted in the development of significantly different morphological features between fish populations. However, more focused studies are required to establish the influence of ecological factors to the formation of distinct stock within the east and west coast of India. The development of singular morphometric characters are mediated by the specific environmental conditions 21. Hence, the organisms of the same species may develop different morphological features in different environments. The vertical measurements in the mid-body region were found to be significant in separation of fish samples from west and east coasts of India and also to differentiate stocks from various locations. Further, the fishes from west coast were found to be bigger in size in comparison with the east coast (Table 1). The presence of upwelling on the west coast of India results in huge availability of food resources for the fishes 22. This could be considered as a reason for the availability of bigger individuals along west coast. On the other hand, the east coast of India is characterized by peculiar environmental features such as lower salinity, high turbulence and strong water currents 23. Previous studies from India underlined that such environmental features could lead to the formation of phenotypically distinct stocks 19, 24, 25. Moreover, significant difference in food and feeding and length-weight for this species is reported between west and east coasts of India 26, 27. In this study, the morphometric characters belonging to the mid-body measurements were found to be significant in differentiating the samples from east and west coasts. Thus, it is inferred that the environmental and ecological

5 1840 SREEKANTH et al.: STOCK STRUCTURE ANALYSIS OF JAPANESE THREADFIN BREAM factors in east and west coasts of India might have resulted in the development of morphometric characters in this species. Acknowledgements This study was carried out with financial and technical support from the Central Institute of Fisheries Education (Indian Council of Agricultural Research), India. Authors extend our sincere gratitude towards the Director and faculty of CIFE for the same. References 1. 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