Length-weight relation in the salp Thalia democratica and potential of salps as a source of food

Abstract

Improved methods of analyzing length-weight relations in the fast-growing salp Thaha dernocratica enable future calculation of productivity from population data and show that carbon density in relation to wet weight for sizes of salps commonly available as food is similar to that of Crustacea. A power relation was developed: Carbon Weight (@gC) = 1.62 X Length (r2 = 0.87, n = 39). Measurements were taken over a wider range of sizes than before (including 0.4 mm embryos). No significant difference between solitary and aggregate stages could be detected. Dry weight was 8.04 % of wet weight (SE = 1.01), ash-free dry weight was 46.5 O/o of dry weight (SE = 1.1), and carbon weight was 38.7 % of ash-free dry weight (SE = 2.8). Carbon was found to be a better reference measurement than either wet or dry weight. The C: N ratio was 3.81 : 1 (SE = 0.06). Tissue shrinkage after preservation was significantly different in buffered formalin than in Steedman's solution. Formulae are given for calculating live length from preserved length up to 2 yr after preservation. Previous estimates of wet weight, dry weight and ash in salps were improved by removlng internal seawater and salt and driving off most of the 'bound' water; these improvements are also applicable to other gelatinous organisms. Salp weight increases as length squared rather than cubed, because tissue density declines with growth between 3 and 10 mm. Salp tissue density is much closer to that of other marine invertebrates, such as molluscs and crustaceans, than was previously reported in the literature. Contrary to expectations, the ratio of dry and ash-free dry weight to wet weight is high during the most rapid growth period, from 2 to 7 mm. Whlle the average density of T. democratica in relahon to wet weight is now seen to be comparable to that of molluscs and chaetognaths (not, as previously thought, to that of medusae and siphonophores), the density of 4 mm individuals reaches that of the arthropods. Coupled with high growth rates, this result makes the salp likely to be a much more important source of food for fish than was previously realized.

Biomass of zooplankton and micronekton in the southern bluefin tuna fishing grounds off eastern Tasmania, Australia

“…Dry weights were used in all analyses. Our values for dry weights of the main prey taxa corresponded to values established by other researchers (see for example Wissing et al 1973, Wiebe et al 1975, Omori & Ikeda 1984, Heron et al 1988.…”

“…Salps may therefore provide one of the initial links between primary and secondary production in the early stages of offshore (and inshore) phytoplankton blooms (Deibel 1985). Furthermore, in areas where nutrient and hydrographic requirements sustain high biomasses of salps for extended periods, salps themselves can play a significant and sometimes leading role in fish nutrition (Kashkina 1986, Heron et al 1988. The presence of gelatinous zooplankton in the stomachs of SBT and their prey during the study period (Young et al 1994) adds support to this idea.…”

Distribution of life-history stages of the salp Thalia democratica in shelf waters during a spring bloom

“…However, enumerating gelatinous zooplankton as individuals to compare with other zooplankton may be misrepresentative due to high water content and size differences. Although it has been found that T. democratica has a tissue density more similar to crustaceans than other gelatinous zooplankton (Heron et al 1988), more specialised sampling methods are necessary to provide more insight into the ecological impacts caused by salp swarms.…”

“…However, enumerating gelatinous zooplankton as individuals to compare with other zooplankton may be misrepresentative due to high water content and size differences. Although it has been found that T. democratica has a tissue density more similar to crustaceans than other gelatinous zooplankton (Heron et al 1988), more specialised sampling methods are necessary to provide more insight into the ecological impacts caused by salp swarms.The results of the present study outline the prevalence of Thalia democratica during spring in the western Tasman Sea. It is often difficult to determine accurate abundances of salps, as abundance and biomass can fluctuate depending on time of day, growth rate, or presence of mating aggregations (Heron 1972b).…”

Composition and potential grazing impact of salp assemblages off Baja California during the 1997–1999 El Niño and La Niña

“…For an additional 122 samples from cruises 0001, 0004 and 0007, total numbers of salps were counted using 1/8 of each sample. The primary sources for the identification of salps were Yount (1954), Van Soest (1972) and Godeaux (1998 No correction for changes in length of specimens in preservative was applied, though some shrinkage of 10% or less exists for Thalia democratica (Heron et al 1988, Nishikawa & Terazaki 1996 and of 20% for Salpa fusiformis (Madin et al 1981) al. 1982) was removed from the results for each IME-COCAL cruise.…”