Rapid estimation of in situ growth rates of Caridina nilotica (Crustacea: Decapoda) in Lake Victoria: Description and pilot application of a simple, field-compatible technique

Hart, Rob C.

Limnol. Oceanogr., 46(3), 2001, 692-698 | DOI: 10.4319/lo.2001.46.3.0692

ABSTRACT: A simple rapid approach to estimating in situ growth rates of Caridina nilotica (Roux), a small shrimp that plays a pivotal role in Lake Victoria's food web, is described. The approach, potentially applicable to many arthropods, is based on moulting intervals (MI) and per moult size increments (PMI) determined during brief experimental confinements. Physiological justification of its reliability as a measure of in situ rates is given. Caridina moults at night. Feral animals collected shortly (≤2 h) before dusk were sorted into one of five arbitrary size classes and held around 27°C overnight without food. MI, the inverse proportion of a batch moulting overnight, increased from 2 d in small shrimps (carapace length [CL] ≤1.8 mm) to >9 d in larger animals (CL ≥4.3 mm)-a value comparable to egg development time (10.3 d) at the corresponding temperature. PMI was measured from differences in CL of postecdysal shrimps (CLi-1) and corresponding cast exuviae (CLi). In absolute terms, PMI, surprisingly, was constant (0.284 ± 0.027 mm moult-1) over the size range of shrimps tested, although relative growth (PMI as a percentage of CLi) declined with size. Growth trajectories modeled with regressions fitted to the data (MI = 1.573 x CL0.999 and CLi-1 = 0.284 + 0.977 x CLi) show that C. nilotica grows significantly faster (by at least 20%) in L. Victoria than previously estimated (Ignatow et al.), with corresponding implications to other evaluations of lake productivity. Prospective refinements and future uses for this simple technique are outlined.

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