| dc.description.abstract | Elasmobranchs are particularly vulnerable to population decline due to their life history traits, typically being long lived, slow growing, late maturing animals with low fecundity. With bycatch, overexploitation and entanglement depleting shark populations, it is thought that oceanic shark and ray species have experienced a 71% decline in abundance since 1970 (Pacoureau et al., 2021). With prevalence of marine diseases increasing due to global climate change, disease now presents a further threat to vulnerable elasmobranch populations. Knowledge gaps persist in the prevalence and relative importance of marine diseases as a threat to shark and ray species, as well as the ecology and morphology, and hence, vulnerability, of many elasmobranch species. Therefore, this thesis comprises two chapters with a shared objective: to better inform conservation strategies for vulnerable elasmobranch species. Chapter one provides contextual insight into elasmobranch pathogens and prevalence of diseases across 60 species via analyses of 100 reports of disease in published literature and Tweets. We analyse the 100 reports gathered to investigate the relationships between pathogen types, disease signs, host species, and environmental stressors. Outbreaks of bacterial origin were most prevalent (28%), and climate change associated affects and water temperature was the most invoked disease outbreak correlate (47%). Most disease reports in wild elasmobranchs were in species inhabiting coastal zones (64%), with the families Dasyatidae (19%), Carcharhinidae (11%), and Sphyrnidae (8%) disproportionally affected overall. Chapter two presents analyses of morphological traits of the vulnerable ocellated eagle ray Aetobatus ocellatus sampled across four sites around San Cristobal Island in the GMR. I investigated the effect of site of sampling and sex on four key morphological traits: pre-sting length, disk width, snout length and tail length. Ocellated eagle rays were significantly longer and had wider disks in La Seca (all P values < 0.001), with the difference being primarily driven by females that were twice as long as males in the same site (P = 0.0036) and as females in the other sites (all P values < 0.04). The identification of La Seca as a hotspot for large females is an important insight which can inform conservation of the vulnerable species. It is vital that La Seca is monitored and managed appropriately to adequately protect large female ocellated eagle rays to increase the potential for population rebound in the GMR. | en_GB |
