Age, growth and reproduction of Sillago schomburgkii in south-western Australian, nearshore waters and comparisons of life history styles of a suite of Sillago species

Abstract

Samples from sheltered nearshore waters in south-western Australia, in which Sillago schomburgkii spends its entire life cycle, have been used to determine the age structure, growth rate, age and length at first sexual maturity, and spawning period of this whiting species. Several S. schomburgkii reached four to seven years in age and one 12+ fish was caught. The respective maximum and asymptotic lengths (L∞) were 350 and 333 mm for females and 348 and 325 mm for males, while the growth coefficients (K) for females and males were 0.53 and 0.49, respectively. Sexual maturity was attained by both sexes of S. schomburgkii at ca. 200 mm, a length reached at the end of the second year of life. Monthly trends exhibited by gonadosomatic indices, the proportions of mature gonads and the prevalence of advanced oocytes and post-ovulatory follicles demonstrate that S. schomburgkii spawns predominantly from December to February. The presence of yolk vesicle and yolk granule oocytes and post-ovulatory follicles in the same ovaries during the spawning period, indicate that S. schomburgkii is a multiple spawner. The patterns of growth of the five Sillago species, that occur in south-western Australian marine waters, fall into two categories. The first, which consists of S. burrus and S. robusta, has a small L∞, i.e. < 190, and a high growth coefficient (K), i.e. ≥ 1.0, whereas the second, which comprises S. schomburgkii, S. vittata and S. bassensis, attain a larger size, i.e. L∞ > 00 mm, and has a low K, i.e. ≤ 0.5. The lengths and ages at maturity of S. schomburgkii, S. bassensis, S. burrus and S. robusta, as well as of S. analis and S. flindersi found elsewhere in Australia, are linearly related to their asymptotic lengths and maximum ages, respectively. The two smallest species, S. burrus and S. robusta, attain maturity at ca. 130 mm. However, the former species, whose juveniles occupy productive nearshore waters, grows rapidly and reaches this length by the end of the first year of life, whereas the latter species, which is restricted to deeper waters, grows more slowly and thus does not attain this length until a year later. Sillagoflindersi, which is slightly larger than S. burrus and S. robusta, migrates out into deeper waters and attains maturity at ca. 170 mm and two years of age. Although S. schomburgkii, S. analis and S. bassensis attain maturity at ca. 200 mm and reach similar lengths, the first two of these species, which remain in nearshore waters and display more rapid growth, reach maturity one year earlier than the last species, which migrates out into deeper and presumably less productive waters. While S. vittata reaches a similar size and likewise migrates out into deep waters, it reaches maturity earlier, i.e. at the end of its first year of life.