Determinants of selective mortality during the early life history of Spratelloides delicatulus in the northern Great Barrier Reef

Villacorta Rath, Ana Cecilia (2011) Determinants of selective mortality during the early life history of Spratelloides delicatulus in the northern Great Barrier Reef. Masters (Research) thesis, James Cook University.

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Early life stages of small clupeids are characterized by high mortality rates, mainly driven by predation and starvation. This mortality is thought to be selective for traits such as size, growth and body condition. It has been demonstrated that larvae with slow growth and reduced body condition do not have sufficient energy reserves to cope with suboptimal environments and will undergo environmental stress. Environmental stress can affect larval performance, increasing their susceptibility to mortality through predation and starvation. Thus, environmental conditions underlie selective mortality of fish by hindering growth rates during the vulnerable early stages. In fish larvae, otoliths are a good tool to explore the effects of the environment on survival as a daily record of growth is imprinted within them. In addition, otolith shape asymmetry (Fluctuating Asymmetry – FA) can give an insight of environmental stress, as the latter has a direct influence on metabolic rates, which regulate otolith accretion.

In this thesis, selective mortality and the effect of the environment on larval growth, size and body condition were studied in cohorts of the tropical sprat, Spratelloides delicatulus, sampled during their larval and juvenile stages at Lizard Island, Great Barrier Reef, Australia. Otolith microstructure analysis allowed back-calculation of size-at-age and daily growth rates. In addition, data of three environmental factors, temperature, Chlorophyll-a (Chl-a) and wind-induced turbulence was collected, allowing the examination of relationships between growth and the environment, and FA and body condition.

The first part of this thesis explores size and growth selection and the influence of the environment on those traits. The results demonstrate that S. delicatulus underwent positive size-selective mortality. However, smaller individuals exhibited faster growth rates which could have had increased survival through enhanced anti-predatory response. Additionally, a 3 cohort of larvae that experienced warmer waters were larger and grew faster than the cohort that hatched earlier in the year, suggesting that temperature could be one of the main factors driving growth rates during early life history.

The second part of the thesis investigates the influence of environmental stress on growth and body condition, and the link of the latter with fluctuating otolith asymmetry. It was found that slow-growing larvae were more dependent on the environment than those with higher growth rates. As a result, slow growers exhibited poor body condition and higher otolith FA, which was maintained through ontogeny, suggesting that individuals with insufficient energy reserves were not able to compensate for otolith asymmetry. The findings highlight the importance of environmental conditions during early life stages of S. delicatulus and the link between slow growth, poor body condition and high otolith asymmetry as drivers of selective mortality.

Item ID: 29199
Item Type: Thesis (Masters (Research))
Keywords: larvae; growth; mortality; environment; Lizard Island; Great Barrier Reef; GBR; Spratelloides delicatulus; otolith microstructure analysis; environmental stress
Date Deposited: 10 Sep 2013 23:16
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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