The relation between spawning area and biomass of Japanese pilchard, Sardinops melanostictus, along the Pacifi c coast of Japan

Japanese pilchard, Sardinops melanostictus (known as “Japanese pilchard” in Japan) is found off the Pacifi c coast of Japan and is an important commercial species. The pilchard stock has fl uctuated widely over a period of several decades (Kondo et al., 1976). The annual catch in Japan peaked at 1.5 × 106 metric tons (t) in the 1930s, but decreased to only 3.0 × 104 t in the 1960s. It recovered to 4.6 × 106 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 diffi cult but may be highly inaccurate (Hewitt et al.1). In such situations, an alternative method of population assessment is to survey egg or larval populations as a means of estimating biomass (e.g. the egg production method; Lasker, 1981). However, the egg production method (EPM) is expensive and time consuming (Mangel and Smith, 1990). A biological characteristic of the Japanese pilchard is that its distribution 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 pilchard 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 fi rst indicator of pilchard population recovery (Smith, 1973; Smith and Hewitt, 1985). Our objective was to clarify the relationship between pilchard spawning area and spawning biomass off the Pacifi c coast of Japan, to estimate biomass with spawning area data. Moreover, to explain why the relationship between pilchard spawning area and spawning biomass formed, we used a model in which a patchy egg distribution was assumed.