Natural growth rates in Antarctic krill (Euphausia superba): I. Improving methodology and predicting intermolt period

Tarling, Geraint A., Rachael S. Shreeve, Andrew G. Hirst, Angus Atkinson, Dave W. Pond, Eugene J. Murphy, Jon L. Watkins

Limnol. Oceanogr., 51(2), 2006, 959-972 | DOI: 10.4319/lo.2006.51.2.0959

ABSTRACT: The growth rates of postlarval krill (Euphausia superba) were measured across a wide range of environments in the Scotia Sea and around South Georgia using the Instantaneous Growth Rate (IGR) method. Each IGR experiment determined the intermolt period (IMP) and growth increment at molt (GI) of an average of 120 individuals incubated for 5 d in through-flowing ambient, filtered seawater. We examined the results from 51 IGR experiments involving 5,927 animals ranging between 25 mm and 62 mm. Animals were collected from an area that covered a latitudinal range of 108 and surface temperatures of between -0.85°C and 4.75°C. The measurement of IMP has rarely been achieved in IGR experiments because synchronous molting biases estimates. We overcame this by applying a binary logistic regression model to our data. This related IMP to temperature, body length, and maturity stage. Food did not influence IMP. Our model estimated that krill within our experiments had IMPs ranging from 9 d to 57 d. Temperature affected the IMP of females more than that of males. The IMPs of females were shortest around 2°C and increased at lower and higher temperatures. IMP increased with body size and altered according to gender, with male IMPs being 50% longer than those of equivalently sized females. One of the main assumptions of the IGR method is that the GI measured in the first few days reflects the in situ conditions experienced by krill in the previous intermolt period. However, we found that the GIs declined immediately and rapidly after capture, particularly when growth was initially high. Thus, conditions at time of molt also influence GI. We developed a method of correcting measured GIs to natural growth in field conditions. These refinements to IGR methodology (IMP and GI estimation) enable more accurate and precise predictions of krill growth rates in summer to be made.

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