Age estimation, growth and age-related mortality of Mediterranean monk seals Monachus monachus

Abstract

Mediterranean monk seals Monachus monachus are classified as Critically Endangered on the IUCN Red List, with < 600 individuals split into 3 isolated sub-populations, the largest in the eastern Mediterranean Sea. Canine teeth collected during the last 2 decades from 45 dead monk seals inhabiting Greek waters were processed for age estimation. Ages were best estimated by counting growth layer groups (GLGs) in the cementum adjacent to the root tip using unprocessed longitudinal or transverse sections (360 µm thickness) observed under polarized light. Decalcified and stained thin sections (8 to 23 µm) of both cementum and dentine were inferior to unprocessed sections. From analysing patterns of deposition in the cementum of known agematurity class individuals, one GLG was found to be deposited annually in M. monachus. Ages ranged from 0.5 to 36 yr for females, 0.5 to 21 yr for males and 0.5 to 25.5 yr for individuals of unknown sex. The majority of seals (65%) were considered adults (≥4 yr), followed by juveniles (20%, <1 yr) and sub-adults (15%, 1−3.9 yr). Thirty percent of the aged sample had died from human-related causes, such as accidental entanglement in fishing gear and direct killings. A single-Gompertz growth curve was generated for both sexes using standard length data, resulting in asymptotic values of 212.3 cm for females and 221.8 cm for males. This study represents the first quantitative glimpse of sex-specific growth in monk seals and the age structure of dead individuals in this rare species' core range.KEY WORDS: Mediterranean monk seal · Monachus monachus · Teeth · Age estimation · Growth layer groups · Mortality · Endangered species · Conservation Resale or republication not permitted without written consent of the publisher 157 Editorial responsibility:

Deposition of growth layer groups in dentine tissue of captive common dolphins <i>Delphinus delphis</i>

“…However, X-rays are a relatively expensive option, and within the current study chemical endpoint testing was not found to be consistently reliable as some calcium deposits remained present upon sectioning, despite the ammonium oxalate test indicating all calcium had been removed. This was also observed using a larger sample of teeth from stranded NZ common dolphins, and in a previous study ageing monk seal (Monachus monachus) dental tissues (Murphy et al 2012). Determining pliability through bending the specimen is less accurate than chemical testing, and can potentially damage samples.…”

“…It delineates the pre-and post-natal dentine tissue, and after which the first incremental layer or Growth Layer Group (GLG) begins to be deposited. GLGs may be recognised by their cyclic repetition and must involve at least one change that can be defined as a countable unit representing, for most species, a 1-year period (Klevezal 1980, Scheffer and Myrick 1980, Myrick 1998, Murphy et al 2012. In decalcified and stained sections of small delphinid dental tissue, GLGs are composed of two components in post-natal dentine: (1) a broad layer of intermediate optical density and (2) a thinner more translucent layer.…”

From coral reefs to whale teeth: estimating mortality from natural accumulations of skeletal materials

“…7; Murphy et al 2012). Growth rings form as the rate of cement accretion is not constant over the entire year and depends primarily on seasonal variations in diet; it is more intense during the growing period when there is plenty of food (Lieberman 1993(Lieberman , 1994.…”

“…Because of this, authors often apply inferential statistics or just a description (Murphy et al 2012). Analysis of data on AD and cause of death of a threatened Mediterranean monk seal Monachus monachus revealed that sub-adults are particularly prone to accidental fishery-related death, as they may be less cautious and less experienced.…”

A review of age estimation methods in marine mammals with special reference to monodontids

“…Cementum is also used to age most species of beaked whales (Perrin and Myrick 1980) and the Franciscana (Pinedo and Hohn 2000), and is equally as valuable as dentine in South Asian river dolphin (Platanista gangetica) (Lockyer and Braulik, 2014). For pinnipeds, the use of dentine for young animals and cementum for older animals has been suggested by several authors (e.g., Smith 1973, Bowen et al 1983, Murphy et al 2012. Stewart et al (1996) found a good correlation between the two tissues in ringed seal up to the age of about 10 years.…”