Growth and ageing in Antarctic krill : growth, ageing and mortality in Euphausia superba Dana : new information using extractable pigments to estimate the age of krill, with implications for population modeling and the management of the fishery
Creators: name: McGaffin, AF
Publisher: name: University Of Tasmania
Year: 2023

description: Antarctic krill, Euphausia superba, are an integral component of the Southern Ocean ecosystem and the target of a substantial fishery. Demographic information used to regulate the krill fishery is presently deficient, mainly due to difficulties in measuring growth and ageing using conventional approaches. This study investigates alternative techniques used to age krill, in conjunction with studies of growth and mortality. Age-pigments were extracted from both wild caught krill and krill hatched from eggs and reared for four years in the laboratory. Wild krill collected from Antarctica were used for growth and mortality studies, and to investigate the effects of environmental variability on the accumulation of age-pigments. Growth and mortality of krill was observed in the laboratory under food and temperature conditions that reflected the natural environment. Carapace length and eye diameter of live krill were regularly measured, and population mortality rates calculated. Control experiments were used to establish the effects of handling. Growth curves for live krill at different temperature and food regimes were constructed over 230 days. Short-term growth (&lt;90 days) was comparable with published data, but in the longer term (90-230 days) growth patterns reflected temperature and nutritional differences. In all conditions krill initially shrank, followed by positive growth, recovering quickly when well-fed. Growth was significantly higher and occurred earlier in krill maintained at 3¬¨‚àûC compared with those at 0¬¨‚àûC. Under control conditions the average mortality rate in krill was 1 in 1000 per day. Mortality rates significantly increased when krill were either food-limited, fed phytoplankton monoculture, or fed a mixed diet, compared to the mortality rate for the control group. Mortality was not significantly influenced by temperature or handling in the long-term. The krill populations were periodically sampled and analysed for biochemical evidence of ageing using extractable pigments. The population reared from hatching in the laboratory provided known-age krill for age-pigment calibration. Extractable pigments from the eyestalk ganglia and eyes of krill were quantified using fluorescence intensity, and standardized against a measure of protein in each sample. Quantities of pigment at two fluorescence peak intensities of ˜í¬™\\(_{ex}\\)=280nm, ˜í¬™\\(_{em}\\)=625nm and ˜í¬™\\(_{ex}\\)=463nm, ˜í¬™\\(_{em}\\)=620nm correlated significantly with chronological age, and the accumulation rate of these pigments was dependent on sex. A model was developed to use pigment to predict age, which was tested against the real data. Quantity of extracted pigment predicted age better than length or eye size, which have both previously been used as proxies for age. For mature adult hill, this method can discriminate between krill of a similar size aged 2 or more years apart. Manipulation of environmental variables showed that temperature, diet and stress have significant effects on accumulation of agepigment, which increases variance in pigment in older aged krill. This study contributes improved understanding of growth, ageing and mortality in Antarctic hill. It has shown that age-pigments can be used to estimate age in hill, particularly if used in conjunction with other demographic information.