Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Ecology and Ecosystems Science (EES)

Department

Fisheries, Animal and Veterinary Science

First Advisor

David Bengtson

Abstract

Sharks, skates, and rays are particularly sensitive to over-exploitation due to their life history traits (slow growth, late age at maturity, small litter size, and extended longevity). It is important to know the age of sharks, skates, and rays because it has implications for our ability to assess the status of populations and to manage fisheries. They are aged most often using the concentric bands that alternate in appearance: opaque and translucent in their vertebral centra. An opaque and translucent band together (a band pair) is assumed to represent one year of growth and is counted to estimate an individual’s age. However, counts of these bands are being shown to underestimate age in a growing number of instances.

An alternate explanation to annual band-pair deposition suggests that the number of band pairs may vary with body size and vertebral centrum morphology and not age. I examined centrum morphology and band-pair counts along the vertebral column within and among species. I measured the morphology of 80 centra from various-sized individuals of both sexes of five batoid species and counted the band pairs in every fifth centrum along the vertebral column from a subset of these individuals. Centrum morphology and band-pair count both varied along the vertebral column in all individuals of all species except young of the year.

This evidence that the number of opaque and translucent bands does not reflect age reinforces the need to understand the differences between these two band types. In the second study, the bulk chemical composition of opaque and translucent bands was examined using energy-dispersive X-ray spectrometry, focusing on 11 elements across 12 elasmobranch species. I found that there was no difference in chemical composition between opaque and translucent bands in the little skate, Leucoraja erinacea, (p = 0.954) or across the 12 species (p = 0.532). Vertebral centra are composed mostly of oxygen, calcium, and phosphorus. The evolutionary significance of optical differences between opaque and translucent band types requires further research.

Validation of age estimates from band-pair counts has only been successful for individuals at or prior to sexual maturity. Therefore, I investigated the rate of formation of band pairs in mature individuals. Mature male and female little skates were injected with oxytetracycline and maintained in captivity for 13 months to assess centrum growth and the frequency of band-pair deposition. Of 41 individuals analyzed, 63% did not deposit a full band pair over the 13-month period, meaning that a majority of individuals did not exhibit deposition of an annual band pair.

Such potentially inaccurate age estimates are still used in the construction of stock-assessment models that dictate how elasmobranch fisheries are managed. To reconcile the fact that the data for stock assessment models is biased I examined the effect of intentionally biased age data on stock assessment model output in the final part of the study. Length-at-age data for little skate and winter skate were biased ±10% and ±25% of the lifespan for (1) all ages and (2) mature ages only. For each species, these eight scenarios and an unbiased (normal) scenario were modeled with the von Bertalanffy growth model and applied to a statistical catch-at-age model. The effects of biased age data were subtle and had the largest effect on estimating spawning stock biomass. As age underestimation is identified in more elasmobranch species, research on the implications of biased age estimates that are incorporated into stock assessment results will be crucial until an alternate method to estimate elasmobranch age is found.

Available for download on Saturday, July 27, 2019

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