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Age, growth, reproductive biology, movements and diets of six species of whiting (Teleostei: Sillaginidae), and their implications for resource partitioning

Hyndes, Glenn Andrew (1996) Age, growth, reproductive biology, movements and diets of six species of whiting (Teleostei: Sillaginidae), and their implications for resource partitioning. PhD thesis, Murdoch University.

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Whiting (Sillaginidae) were sampled at regular intervals in the coastal marine waters of the lower west coast of Australia, using a fine-meshed seine net in nearshore waters (<1.5 m) and a trawl net in shallow (5-15 m) and deep (20-35 m) waters of the inner continental shelf. Shallow nearshore waters are shown to act as nursery habitats for five of the six whiting species found in the region. The juveniles of Sillago burrus, Sillago vittata, Sillago schomburgkii and Sillaginodes punctata occur mainly over bare sand in protected areas, while those of Sillago bassensis are found predominantly in areas that are exposed to substantial wave and swell activity. Although S. schomburgkii remains in these nearshore areas for the rest of its life cycle, the other four species move offshore into shallow inner-shelf waters as they increase in length. Sillago burrus and S. vittata then remain in these waters, whereas S. bassensis eventually migrates out into deeper waters. Sillago bassensis thus then co-occurs with Sillago robusta, which spends its entire life cycle in those deeper waters. Large S. punctata congregate in areas near reefs in the inner-shelf waters, where they are consistently caught by hand-line but are out of the range of trawlers.

The age and size compositions, growth rates and ages and sizes at first maturity, spawning periods and diets of the above six whiting species have been determined and compared. Sillago burrus rarely exceeded two years of age and was never found beyond four years of age, whereas S. robusta, S. vittata and S. schomburgkii reached maximum ages of six to seven years, and S. bassensis and S. punctata attained ages of 10 and 13 years, respectively. The patterns of growth of these species fell into three broad categories. The first category, which contains S. burrus and S. robusta, are characterised by relatively small asymptotic lengths (L∞), i.e. <190 mm, and high growth coefficients (K), i.e. >1.0, whereas the second, comprising S. schomburgkii, S. vittata and S. bassensis, attain moderate sizes, i.e. L∞ = ca 320 mm, and have low growth coefficients, i.e. <0.5. Although S. punctata has a similar growth coefficient to the last three of these species, i.e. ca 0.5, it has a far greater asymptotic length, i.e. ca 520 mm. The two smallest species, namely S. burrus and S. robusta, attain sexual maturity at ca 130 mm. However, the former species, whose juveniles occupy the productive nearshore waters, grows rapidly and thus reaches this length by the end of the first year, whereas S. robust a, a species that remains in deeper waters and exhibits slower initial growth, takes a further year to attain that size. Unlike S. bassensis, S. schomburgkii spends its entire life cycle in the productive environment of sheltered nearshore waters and grows more rapidly, reaching maturity one year earlier, i.e. at the end of the second year of life. Sillago vittata, which reaches similar lengths and ages as these latter two species, attains maturity at a far smaller length, i.e. 140 mm, and an earlier age, i.e. by the end of its first year of life. The largest and longest-living species, S. punctata, attains maturity at ca 400 mm, a length reached at the end of its fourth year of life. The lengths and ages at maturity of sillaginid species in south-western Australia have been compared with those of Sillago analis, Sillago ciliata and Sillago flindersi, which are found in marine waters elsewhere in Australia.

All of the five Sillago species spawn predominantly in summer, i.e. December to February. However, the spawning of S. bassensis commences as early as September and extends into mid-autumn. In contrast to the Sillago species, S. punctata spawns in the winter to early spring, i.e. June to September. The frequent co-occurrence of yolk vesicle and yolk granule oocytes and post-ovulatory follicles in the mature ovaries of the six species of whiting found in south-western Australia, provides strong circumstantial evidence that all of these species spawn on more than one occasion in each breeding season.

The broad similarities in the diets of the six whiting species presumably reflects the fact that they are all benthic and, particularly with the Sillago species, have a similar mouth morphology. In protected nearshore waters, small S. burr us, S. vittata, S. schomburgkii and S. punctata feed on substantial volumes of amphipods, copepods and errant polychaetes, while the larger representatives of the last two species ingest mainly amphipods, decapods and errant and sedentary polychaetes. Small S. bassensis, which occupy the more exposed nearshore waters, feed almost exclusively on amphipods. The diets of S. burrus, S. vittata and S. bassensis change as these species increase in size and move out into the shallow inner-shelf waters. However, although all three species feed on large volumes of polychaetes in these waters, S. burrus and S. vittata also consume large amounts of bivalves and echinoderms, whereas S. bassensis ingests a considerable volume of amphipods. The largest S. bassensis and S. robusta, a species with which it co-occurs in deeper waters, have different diets. The former has a diet similar to that of S. burrus and S. vittata in the shallow inner-shelf waters, whereas the latter species feeds on large volumes of amphipods and other small crustaceans. Thus, the diets of the whiting species, which comprise mainly benthic and epibenthic fauna, vary both with size and with depth. Furthermore, differences exhibited by the diets of co-occurring species, were often reflected by differences in the size compositions of those species in each region. From comparisons of the diets of the various size groups of the different whiting species, it is evident that habitat partitioning by these species plays a major role in reducing any potential competition for food resources by the abundant species in southwestern Australian waters.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian
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