Details

Autor(en) / Beteiligte

• Thomson, R. B.
• Butterworth, D. S.
• Kato, H.

Titel

HAS THE AGE AT TRANSITION OF SOUTHERN HEMISPHERE MINKE WHALES DECLINED OVER RECENT DECADES?

Ist Teil von

• Marine mammal science, 1999-07, Vol.15 (3), p.661-682

Ort / Verlag

Oxford, UK: Blackwell Publishing Ltd

Erscheinungsjahr

1999

Quelle

Wiley Online Library

Beschreibungen/Notizen

• The earplugs of several baleen whale stocks exhibit seasonal growth layers which have been shown for some species to indicate the total age of the animals. A transition from early, irregular layers, to later, more regular layers can be seen in these earplugs, and this is thought to indicate the age at maturity of the whale. The earplugs of Southern Hemisphere minke whales are relatively difficult to read, particularly for the age at transition, and it has been suggested that these readings reflect no more than random allocations by the reader, rather than any real effect. Plots of average transition phase against year of birth (cohort) show a decline in the average age at transition. However, certain factors can result in a downward bias in this trend, particularly when only a short time‐series is available for analysis, as was the case when this trend was first estimated some 15 yr ago. Data collected over 25 yr are now available and are reanalyzed here. A plot of mean age at transition against year of sampling for animals of similar age shows a decline, as does the conventional plot against cohort, suggesting that the decline is real. A model that simulates random allocation of the transition phase by earplug readers yields predictions that show systematic deviations from the data. In contrast, a second model that allows for a real trend in the average age at transition and that takes account of two potentially biasing effects (termed truncation and the fringe effect) fits the data well. A tendency is evident for readers to recognize a transition phase in a greater proportion of the earplugs that they read as they gain experience over time. This is taken into account in the model. This model shows a decline in the average age at transition from roughly 11 for the cohorts of the 1950s to roughly 7 for those of the 1970s. These results suggest that minke whale transition‐phase readings are related to a real signal in the data and are not simply an artefact, and further that there has been a real decline in the age to which this signal corresponds.