Age determination methods in harbour seals (<i>Phoca vitulina</i>) with a review of methods applicable to carnivores

Abstract

The development of age determination methods in marine mammals is reviewed with particular reference to the use of teeth Growth Layer Groups (GLGs) formed in the dentine and cement of carnivores. Using this background, practices for sampling, tooth extraction and collection, storage and different methods of preparation of teeth as well as reading and counting GLGs are discussed and evaluated for the harbour seal (Phoca vitulina). The paper includes comments on best practices for counting GLGs with new examples from known-age seals, and also a detailed examination of confounding factors in interpreting GLGs such as mineralization anomalies and the phenomena of accessory lines, "false annuli" and "paired laminae" which have not been discussed previously. The paper concludes with recommendations for undertaking age estimation in harbour seals from sampling through final GLG interpretation with special emphasis on standardization of methods with other researchers.Lockyer, C., Mackey, B., Read, F., Härkönen, T. and Hasselmeier, I. 2010. Age determination methods in harbour seals (Phoca vitulina) with a review of methods applicable to carnivores. NAMMCO

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

“…A commonly used modification of Mayer's haematoxylin that eliminates the need for aging of the stain is provided in Myrick et al (1983). Lockyer et al (2010) reported that GLGs in the cementum of harbour seal tooth sections are more prominent with Toluidine Blue while haematoxylins such as Mayer's or Ehrlich's were better for dentine. Murphy et al (2014) suggested that Toluidine Blue may have a better affinity for tissues decalcified using organic acids and chelating agents.…”

“…Thus, GLGs can be found in dentine and cementum. For a more detailed description of the structure of mammalian teeth, see Boyde (1980) and Lockyer et al (2010).…”

“…For a detailed explanation of extraction, cleaning and storage of tooth samples for age estimation studies see Lockyer et al (2010). After extraction, two NAMMCO Scientific Publications, Volume 10 14 methods have traditionally dominated for preparing teeth for age estimation: 1) "thick" sections not otherwise treated, that is, not decalcified or stained, and 2) decalcified and stained thin sections.…”

“…Lockyer (1993) and Lockyer et al (2010) provided detailed reviews of methods for preparing thin stained sections.…”

From the forests to teeth: Visual crossdating to refine age estimates in marine mammals

“…These could include GLGs that (a) were relatively narrow or wide (compared qualitatively to the width of the adjacent two to three GLGs on either side), or (b) following previously published descriptions of distinctive features within GLGs as either “marker lines” or “mineralization interference”; had conspicuous features such as a prominent ridge of the GLG, noticeably narrow or wide ridge or groove components of the GLG, an obvious accessory layer occurring within the GLG, or paired ridges or “double GLGs/laminae” (Myrick ; Lockyer , ; Lockyer et al . ) ( e.g ., see Fig. ).

Two sectioned teeth from the same individual female sperm whale ( ID # 2542) showing distinctive “signature” GLG s (marked by red arrows) used in visual crossdating.

…”