Abstract— When stocks are depleted, 

 direct assessments of population levels 

 are not only difficult but may be 

 highly inaccurate. The work reported 

 here was motivated by the need to 

 manage the collapse of the far eastern 

 sardine {Swdinops melanostictus) pop- 

 ulation. Spawning area was chosen as 

 the first indicator of population size 

 because the spatial spread of the stock 

 increases when the spawning popula- 

 tion increases. Our objective was to 

 clarify the relation between spawning 

 area and the spawning biomass of this 

 species off the Pacific coast of Japan 

 in order to estimate biomass with the 

 spawning area data. 



The pilchard spawning area (A,) in 

 a given year was calculated by sum- 

 ming the areas of 1° longitude x 1° 

 latitude squares where early develop- 

 mental stage eggs were present. The 

 optimal relationship between A, and 

 the spawning biomass (B I was A, =2. 5 18 

 go 4610 ^ fiQt a simple linear relationship. 

 One cause of the nonlinear relationship 

 seemed to be that pilchard egg aggre- 

 gations were distributed over space in 

 a patchy manner 



Finally, we introduced an approxi- 

 mate method for estimating the pilchard 

 spawning biomass by using the inverted 

 equation of the optimal relationship 

 between A, and B. tS=0.135A, - '™' i. 



The relation between spawning area and biomass 

 of Japanese pilchard, Sardinops melanostictus, 

 along the Pacific coast of Japan 



Hiromu Zenitani 



National Research Institute of Fisheries and Environment of Inland Sea 

 2-17-5, Maruishi, Ohno-cho Saeki-gun, 

 Hiroshima, 739-0452 Japan 

 E-mail address zenilanim'nnfaffrcgoip 



Sakutaro Yamada 



Tokyo University of Fisheries 

 4-5-7, Konan, Minato-ku, 

 Tokyo, 108-0075 Japan 



Manuscript accepted 26 June 2000. 

 Fish. Bull. 98:842-848 (2000). 



Japanese pilchard, Sardinops melanost- 

 ictus (known as "Japanese pilchard" in 

 Japan) is found off the Pacific coast 

 of Japan and is an important com- 

 mercial species. The pilchard stock has 

 fluctuated widely over a period of sev- 

 eral decades iKondo et al., 1976). The 

 annual catch in Japan peaked at 1.5 

 X 10« metric tons (t) in the 1930s, but 

 decreased to only 3.0 x lO"* t in the 

 1960s. It recovered to 4.6 x 10^^ t in the 

 1970s and reached the second highest 

 peak for this century in 1988. However, 

 the catch in 1995 fell to one-tenth of 

 that in 1988 (Fig. 1). 



When stocks are depleted, direct 

 assessments of population levels are 

 not only difficult but may be highly 

 inaccurate (Hewitt et al.M. In such situ- 

 ations, an alternative method of popu- 

 lation assessment is to sui-vey egg or 

 larval populations as a means of esti- 

 mating biomass (e.g. the egg produc- 

 tion method: Lasker, 1981). However, 

 the egg production method (EPMi is 

 e.xpensive and time consuming ( Mangel 

 and Smith, 1990). 



A biological characteristic of the 

 Japanese pilchard is that its distri- 

 bution range changes in accordance 

 with stock size. During pilchard stock 

 size increases, the distributional range 

 expands widely (Wada and Kashiwai, 

 1991). In contrast, when stock size is 

 declining, the distributional range may 

 became quite limited (Hiramoto, 1981). 

 The spawning grounds of Japanese pil- 

 chard are known to expand with egg 

 abundance increases and contract with 



abundance declines (Watanabe et al., 

 1996; Zenitani et al, 1998). In general, 

 as the spawning population increases, 

 the spatial spread of the stock also 

 increases (Rosenzweig, 1981; MacCall, 

 1988). Therefore, the area of spawning 

 grounds (spawning area) was chosen as 

 the first indicator of pilchard popula- 

 tion recovery (Smith, 1973; Smith and 

 Hewitt, 1985). 



Our objective was to clarify the rela- 

 tionship between pilchard spawning 

 area and spawning biomass off the 

 Pacific coast of Japan, to estimate bio- 

 mass with spawning area data. More- 

 over, to explain why the relationship 

 between pilchard spawning area and 

 spawning biomass formed, we used a 

 model in which a patchy egg distribu- 

 tion was assumed. 



Materials and methods 



Spawning area and spawning 

 biomass 



Intensive egg sui-veys of coastal pelagic 

 fish have been conducted every year 

 since 1978 by the Fisheries Agency of 

 Japan (Mori et al, 1988; Kikuchi and 

 Konishi, 1990; Ishida and Kikuchi, 1992; 



' Hewitt, R., A. Bindmann. and N. C. Lo. 

 1984. Procedures for calculating the egg 

 production estimate of spawning biomass. 

 Administrative Report LJ-84-19. South- 

 west Fisheries Center, La Jolla. OA 92038- 

 0271. 



