AbstractAs anthropogenic pressures including over-fishing and climate change intensify, high-order predators are being threatened within coastal communities. Predicting consequences hinges on our understanding of how species similarities and differences shape ecosystem function and resilience. I compared ecological similarity between two morphologically similar carcharhinids, bull (Carcharhinus leucas) and pigeye (C. amboinensis) sharks in northern Australia. Ecological similarity was measured by comparing a) age structure and growth dynamics, b) habitat use and c) genetic population structure and phylogeography.
Age structure and growth dynamics were quantified by analysing vertebrae of 199 pigeye and 94 bull sharks. Model averaging results indicated female pigeye sharks matured at 13 years, lived > 30 years and with a growth coefficient (k) of 0.145 year –1. Male pigeye sharks matured slightly earlier (12 years), survived > 26 years and grew faster (k = 0.161 year –1) than females. Bull sharks matured at 9.5 years and displayed similar growth coefficient (k = 0.158 year –1) and survival (> 27 years) as pigeye sharks.
Analysis of vertebral microchemistry identified unique elemental signatures in each of the freshwater bull shark nurseries, but these signatures did not occur in adult vertebrae. Furthermore, age-specific changes in microchemistry synonymous with known ontogenetic changes in patterns of habitat use were also evident. However, vertebral microchemistry could not discriminate among natal habitats of pigeye sharks and age-specific changes in vertebral microchemistry were absent. Inter-specific differences in vertebral microchemistry suggest species display different patterns of habitat use.
Population genetic structure and phylogeography of pigeye sharks defined two populations within northern Australia on the basis of mitochondrial DNA evidence, but this result was not supported by nuclear microsatellite or RAG 1 markers. Phylogeography identified three distinct clades present in differing frequencies across northern Australia. Bull sharks displayed starkly different patterns. High genetic diversity among juveniles sampled from different rivers supported the hypothesis of female reproductive philopatry. Furthermore, phylogeography identified only one clade across northern Australia.
Despite similar age and growth structure, different patterns of habitat use and genetic evolution confirm that morphological likeliness does not equate to similar ecologies in these large carcharhinid sharks.
|Date of Award
|Michael Douglas (Supervisor)