Biochemical Determination of Population Age Structure in Euphausiids

Abstract

Euphausiids play a key role in the marine ecosystem as a link between primary producers and top predators. Understanding their precise demographic structure is needed to assess growth and recruitment as well as determine how changes in environmental conditions alter their condition and distribution. Since age determination of crustaceans cannot be accomplished using traditional approaches, we evaluated the potential of age pigments (collectively termed lipofuscin) to determine the age structure of Euphausia pacifica in field collections. The rate of lipofuscin accumulation was determined using krill of known age reared from larvae in the laboratory for over 1 yr. Lipofuscin was extracted from neural tissues (eye and eye-stalk), quantified, and normalized to protein content to allow comparisons across animal sizes. Multiple fluorescent components from krill were observed, with the major product having a maximum fluorescence at excitation of 355nm and emission of 510nm. While the growth rate from known-age krill was log-linear (regression coefficient=0.85), the lipofuscin accumulation rate of the same individuals was linear (regression coefficient=0.76). Field collections as part of the NEP GLOBEC program were sorted immediately after tows into representative size classes for evaluation. Populations of krill contained variable levels of lipofuscin dependant on size and age. Most of field-collected sub-adults and adults were found to be older than 100 days and younger than 1yr based on lipofuscin age estimation. Based on biochemical measures of age, the lifespan of E. pacifica in the northeast Pacific is no longer than 2 yrs. Our results suggest that biochemical indices allow a practical approach to estimate population age structure and together with other measures can provide estimates of vital rates (i.e. longevity, mortality, growth) for krill populations in dynamic environments.