Age determination of minke whales (Balaenoptera acutorostrata ) using the aspartic acid racemization technique Erik Olsen & Jan Sunde Olsen E, Sunde J. 2002. Age determination of minke whales (Balaenoptera acutorostrat a) using the aspartic acid racemization technique. Sarsia 87:1–8. SARSIA Eyeballs were collected from 31 female minke whales caught in the North Sea, Norwegian Sea and Barents Sea. The eye-lens nucleus was dissected out and the relative amount of D- and L- enantiomer s of aspartic acid measured using high-performanc e liquid chromatography (HPLC). A racemization reaction changes the D/L aspartic acid relationship in the nucleus over time. The age of each whale was estimated using the racemization rate for n whales along with the measured D/L ratio. The minimum age estimated was 0 years, while the maximum was 32 years. The SE of the age estimates ranged from 4.5 to 8.7 years. Age at sexual maturity was estimated to 5.8 (§3.0) years by regressing the number of ovulations versus the age estimate. Using the von Bertalanffy growth model, age at sexual maturity was estimated to 7.8 (§6.0) years, and age at physical maturity to 13 years. These estimates are consistent with other demographic data for the species. No systematic errors in the age estimates were detected, but the racemization rate (2k ASP ) and initial ratio of D/L should be estimated specically for minke whales to ensure accurate age estimates using this technique. E. Olsen, Division of Resource Ecology, Institute of Marine Research, P.O. Box 1870 Nordnes, NO-5817 Bergen, Norway. – J. Sunde, Matre Aquaculture Research Station, NO-5984 Matredal, Norway. E-mail: eriko@imr.no – jans@imr.no Keywords: age-determination ; aspartic acid; minke whale; racemization. INTRODUCTION The minke whale (Balaenoptera acutorostrata, Lace ´- pe `de 1804) is the smallest baleen whale in the North Atlantic (Stewart & Leatherwood 1985). Because these whales are exploited by Inuit in Greenland (Anon. 1999) and by coastal whalers in Norway (Jonsga ¨rd 1992; Anon. 1999), demographic studies of the species are important. To achieve this, however, precise and accurate age estimates are vital. In contrast to Antarctic minke whales (Balaenoptera bonaerensi , Burmeister 1867) and n whales (B. physalus, Linneaeus 1758), North Atlantic minke whales do not form readable earplugs, which are commonly used to estimate the age of these species (Christensen 1981). Reading lamina- tions in the ear bone (bulla tympanica) has been the method used (Christensen 1981; Sukhovskaya & al. 1985), but it has been found that the age of whales can be severely underestimated (Olsen 1997; Larsen & Kapel 1982). In order to nd an alternative age estimation method, we attempted to determine the age of minke whales using the aspartic acid racemization (AAR) technique that George & al. (1999) used to determine the age of bowhead whales. AAR age- determination of baleen whales is done by measuring the level of the L- and D-enantiomers of aspartic acid (an amino acid) in the core of the eye lens, which is formed at the foetal stage and is metabolically inactive for the rest of the animal’s life cycle (Fagerholm & Tengroth 2000). When formed, the eye-lens proteins consist mostly of L-amino acids, but over time a racemization process takes place, altering L-enantio- mers to D-enantiomers (and vice versa) and resulting in an increasing content of D-Asp acid until 50–50 equilibrium is reached. Aspartic acid has the highest racemization rate of the amino acids (Ohtani 1997), and with knowledge of the reaction rate and initial levels of D-Asp (D/L) 0 it is possible to calculate how much time has passed before measured D/L ratio is reached. Even though this method has been successfully applied to the large arctic bowhead whale and the eyes and teeth of narwhals (Bada & al. 1983), we investigate whether the method was suitable for the small boreal minke whale. The differences in size, behaviour and life histories between these species are striking: while bowhead whales reside in arctic waters year round, minke whales carry out long seasonal migrations, exposing their eyes to temperatures ranging from 20°C in the breeding areas to 2°C in the feeding areas. The racemization rate is temperature-dependent, and thus ambient water tem- perature might play a role in altering the rate. Because ovulation rate in minke whales is well documented (Christensen 1981), we can use the post-sexual maturity age as an independent validation of the age estimate. # 2002 Taylor & Francis Published in collaboration with the University of Bergen and the Institute of Marine Research, Norway