Life history and demographic modelling of shark species caught in Indo-Pacific fisheries

Smart, Jonathan James (2016) Life history and demographic modelling of shark species caught in Indo-Pacific fisheries. PhD thesis, James Cook University.

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Abstract

Understanding a species' life history and its demographics is paramount to effective population and fisheries management. Many aspects of a species life history are reflected in how it grows – making accurate estimates of age and growth some of the most important information on a species' biology. However, the approach to modelling growth for elasmobranchs (sharks and rays) has changed considerably over time and a variety of approaches are now used in the literature. Therefore, a review of these approaches was justified along with recommendations for a best practice framework to maximise the accuracy of resulting growth estimates. This dissertation defined such a framework and applied it to species of fisheries and conservation interest in the western and central Indo-Pacific regions of Indonesia and Papua New Guinea. These life history estimates were used to perform demographic analyses designed to determine these species susceptibility to overfishing.

Elasmobranch growth modelling approaches were founded on the techniques used for teleosts (bony fishes). These approaches typically use one growth model a priori – the von Bertalanffy growth function (VBGF), which was originally based on the growth of humans but is now commonly used to estimate growth for a variety of taxa. The use of the VBGF was justified as it is based on biological processes and provides reasonable fits for a variety of aquatic taxa such as teleosts, molluscs and crustaceans. However, contemporary approaches to growth modelling include multi-model approaches – where multiple models are fit to the data and selected based on comparative fits. Previous studies on elasmobranch growth have hypothesised that different candidate growth models produce better fits for different elasmobranch taxa. Sigmoid functions (e.g. Gompertz and logistic) are a popular alternative to the commonly used VBGF as they are hypothesised to better suit certain taxa based on body shape (such as batoids) or reproductive mode (such as egg-layers). However, this hypothesis has never been tested.

This dissertation examined 74 elasmobranch multi-model growth studies by comparing the growth curves of their respective candidate models. Hypotheses regarding model performances were rejected as the VBGF was equally likely to fit best for all taxa and reproductive modes. Subsequently no individual model was suited to be used a priori. Differences between candidate model fits were greatest at age-zero with Gompertz and logistic functions providing estimates that were 15% and 23% larger on average than the VBGF, respectively. However, length-at-age estimates of the different models became negligible at older ages. Differences between candidate models were mostly small (≤ 5%) and the multi-model framework only marginally affected length-at-age estimates. However, there were cases where some candidate models provided inappropriate fits that contrasted considerably to the best fitting model. In some of these instances a single model framework could have yielded biologically unrealistic growth estimates. Therefore, no study should pre-empt whether or not it required a multi-model framework.

Based on this evaluation of elasmobranch growth modelling this dissertation determined that a basic growth modelling framework should at least include the VBGF, Gompertz function and logistic function as well as any additional functions that could reasonably fit the data. Model selection should be based on AIC results with a weighted model produced by multi-model inference if there is no outright best model (w = > 90%). An averaged L∞ and L0 should be calculated from the multi-model inference estimates and a tabulated set of length-at-age estimates presented to allow reproducibility. If data are limited from the smaller length classes, then techniques such as back-calculation should be used to prevent model constraint.

Life history information such age and growth is rarely available for tropical shark species which often confounds their management and fisheries status in developing nations. This dissertation addressed this issue for three species of whaler sharks (family Carcharhinidae) that occur in the western and central Indo-Pacific around Indonesia and Papua New Guinea (PNG). These species were the common blacktip shark (Carcharhinus limbatus), the grey reef shark (C. amblyrhynchos) and the silvertip shark (C. albimarginatus). Estimates of age and growth were produced for all three species using the framework outlined by this dissertation. Maturity estimates were also produced for C. amblyrhynchos and C. albimarginatus.

Age and growth estimates were produced for the common blacktip shark Carcharhinus limbatus sampled from Indonesia. Back-calculation techniques were used due to a low sample size (n = 30) which was dominated by large mature sharks. The VBGF provided the best fit for the separate sexes using the back-calculation data. The VBGF estimates for males were L₀ = 69 cm TL, k = 0.14 yr⁻¹ and L∞ = 230 cm TL. The VBGF estimates for females were L₀ = 62 cm TL, k = 0.11 yr⁻¹ and L∞ = 264 cm TL. These age and growth estimates were then compared to other populations of C. limbatus from the USA and South Africa using a combination of VBGF parameters and growth rates at birth (dL/dt₀) and maturity (dL/dtₘₐₜ). In comparison to populations from the USA, C. limbatus from Indonesia grow substantially larger and are more similar to South African populations. These results indicated that life history information from other regions would introduce error if used in population assessments for C. limbatus from the Indo-Pacific.

Life history estimates were produced for C. amblyrhynchos and C. albimarginatus using samples collected by fisheries observers from the National Fisheries Authority (NFA) of PNG. However, observer error may misidentify shark species in areas such as PNG where many whaler shark species have similar morphologies. Therefore, this dissertation quantified this observer error by validating species identifications using diagnostic photographs taken on board supplemented with DNA barcoding. In addition to ensuring that the life history estimates produced in this dissertation were not affected by observer error, this also provided insights into potential error rates from samples that have not had species identifications verified. This was achieved by producing life history estimates with and without the misidentified individuals for C. amblyrhynchos.

Vertebrae were retained from a total of 155 sharks identified by observers as C. amblyrhynchos. However, 22 (14%) of these were sharks were misidentified by the observers and were subsequently re-identified based on photographs and/or DNA barcoding. Of the 22 individuals misidentified as C. amblyrhynchos, 16 (73%) were detected using photographs and a further 6 via genetic validation. If misidentified individuals had been included, substantial error would have been introduced to both the length-at-age and the maturity estimates. Thus validating the species identification, increased the accuracy of estimated life history parameters for C. amblyrhynchos. From the corrected sample a multi-model inference approach was used to estimate growth for C. amblyrhynchos using three candidate models. The model averaged length-at-age parameters for C. amblyrhynchos with the sexes combined were L∞ = 159 cm TL and L₀ = 72 cm TL. Females mature at a greater length (l₅₀ = 136 cm TL) and older age (A₅₀ = 9.1 years) than males (l₅₀= 123 cm TL; A₅₀ = 5.9 years).

A total of 48 C. albimarginatus: 28 males (95 – 219 cm TL) and 20 females (116 – 250 cm TL) were collected by NFA fisheries observers without any species misidentifications occurring. Back-calculation techniques were used to account for missing juvenile length classes and supplemented the sample size. The VBGF provided the best fitting growth estimates. Parameter estimates were L₀ = 72.1 cm TL, k = 0.04 yr⁻¹ and L∞= 311.3 cm TL for males; and L₀ = 70.8 cm TL, k =0.02 yr⁻¹ and L∞= 497.9 cm TL for females. The biologically implausible L∞ occurred for females as their growth did not asymptote; a typical trait of large shark species where older age classes were unavailable. The maximum age estimated from vertebral analysis was 18 years for both sexes while the calculated longevity from the VBGF parameters were 27.4 years for males and 32.2 years for females. Males matured at 174.7 cm TL and 10.5 years old, while females matured at 208.9 cm TL and 14.8 years old.

Demographic analyses were performed using the life history estimates produced for C. albimarginatus and C. limbatus. These analyses were age-structured Leslie Matrix models which incorporated stochasticity by varying vital rates though Monte Carlo simulations. Varying levels of fishing mortality (F) were introduced to the analyses to determine how both species respond to fishing pressure. Management scenarios were further developed to determine strategies that could facilitate sustainable harvesting. The demographic estimates demonstrated that without fishing both species would have increasing populations (λ = 1.07yr⁻¹ for both species) until density dependent effects occur. However, both populations would decline when low levels of F (> 0.1 yr⁻¹) were applied to all age-classes. The matrix elasticities revealed that changes to fertility elements had little effect on λ, while changes in juvenile survival produced the largest changes. However, age-at-first-capture analysis suggests protecting the juvenile life stage of both species would be an ineffective management strategy as both species mature at old ages. An age-at-last-capture analysis suggests these species could be harvested while maintaining increasing populations through a gauntlet fishery. This required F to be restrained to individuals < 100cm TL while protecting the older age-classes to preserve the breeding stock. This strategy would allow up to 16% and 22% of this size class to be harvested for C. limbatus and C. albimarginatus, respectively, until density dependent effects begin to manifest. However, this strategy is dependent on the ability to successfully protect all other age-classes from fishing – a strategy which may not be pragmatic in developing nations where little regulation occurs.

Through the use of robust life history and demographic analyses, this dissertation has produced important information for the fisheries and conservation management of several species from the Indo-Pacific. The approaches outlined in this thesis now also provides opportunities for other species from the region to be assessed.

Item ID: 49698
Item Type: Thesis (PhD)
Keywords: age determination, age, Akaike's information criterion (AIC), back-calculation, Carcharhinidae, Chondrichthyes, elasmobranch, fisheries, gauntlet fisheries, growth, Indonesia, Indo-Pacific Region, Leslie matrix models, life cycles, life history, maturity ogives, modelling, Monte Carlo simulation, Papua New Guinea, reef sharks, sharks
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Smart, Jonathan J., Chin, Andrew, Tobin, Andrew J., and Simpfendorfer, Colin A. (2016) Multimodel approaches in shark and ray growth studies: strengths, weaknesses and the future. Fish and Fisheries, 17 (4). pp. 955-971.

Chapter 4: Smart, J.J., Chin, A., Tobin, A.J., Simpfendorfer, C.A., and White, W.T. (2015) Age and growth of the common blacktip shark Carcharhinus limbatusfrom Indonesia, incorporating an improved approach to comparing regional population growth rates. African Journal of Marine Science, 37 (2). pp. 177-188.

Chapter 5: Smart, Jonathan J., Chin, Andrew, Baje, Leontine, Green, Madeline E., Appleyard, Sharon A., Tobin, Andrew J., Simpfendorfer, Colin A., and White, William T. (2016) Effects of including misidentified sharks in life history analyses: a case study on the grey reef shark Carcharhinus amblyrhynchos from Papua New Guinea. PLoS ONE, 11 (4). pp. 1-18.

Chapter 6: Smart, Jonathan J., Chin, Andrew, Baje, Leontine, Tobin, Andrew J., Simpfendorfer, Colin A., and White, William T. (2017) Life history of the silvertip shark Carcharhinus albimarginatus from Papua New Guinea. Coral Reefs, 36 (2). pp. 577-588.

Chapter 7: Smart, Jonathan J., Chin, Andrew, Tobin, Andrew J., White, William T., Kumasi, Brian, and Simpfendorfer, Colin A. (2017) Stochastic demographic analyses of the silvertip shark (Carcharhinus albimarginatus) and the common blacktip shark (Carcharhinus limbatus) from the Indo-Pacific. Fisheries Research, 191. pp. 95-107.

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Date Deposited: 26 Jul 2017 23:48
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070402 Aquatic Ecosystem Studies and Stock Assessment @ 100%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8302 Fisheries - Wild Caught > 830204 Wild Caught Fin Fish (excl. Tuna) @ 100%
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