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.