Lloyd, Julie Anne (2006) Demography of Pristipomoides multidens in northern Australia and a comparison within the Family Lutjanidae with respect to depth. PhD thesis, James Cook University.

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Abstract

Lutjanids are an important group of perciform reef fish distributed over a wide depth range throughout the world’s tropical and sub tropical oceans. It has been noted by Newman and Williams (1996) that within this complex there is a separation of genera with respect to depth. Shallower waters <100 m are dominated by the genera Lutjanus, Aprion, Macolor, Symphorichthys, and Symphorus, while in the intermediate depths (100-200 m) Pristipomoides, and Paracaesio dominate, and in the deeper waters >200 m Etelis are more prevalent. Therefore the lutjanid complex offers a unique opportunity to observe whether there is a distinction in life history characteristics of genera within the same family with respect to different depth zones and consequently different environmental influences.

While many shallow water species have been well studied, there is a paucity of information on species in the intermediate and deeper waters where research is expensive and logistically difficult. The lack of information is a significant problem for many developing countries for which lutjanids are an important source of export revenue as well as food. In developed countries, attracting funding for small-scale "boutique" fisheries is very difficult, despite the recognised need to manage all fisheries in a sustainable manner. Therefore, in the absence of research data, managers and scientists are forced to use proxy values, but how valid is this approach?

To examine this question, I undertook a comparison of the demography of genera within the Family Lutjanidae within different depth ranges; shallow (<100 m), intermediate (100-200 m) and deep (>200 m). I conducted a detailed study of Pristipomoides multidens (intermediate depth range) and compared findings from this study with information published for lutjanids from other depth ranges. This provided a framework within which to assess whether it is acceptable to use proxy values, and whether proxy values should preferentially be from lutjanids inhabiting the same locality possibly from different depth ranges, or the same depth range even if the information is from a different region.

The key findings were: Significant differences are evident in age, growth and reproductive parameters for genera from the different depth ranges. Small shallow water lutjanids appear to show greater variability in growth across smaller scales compared species in intermediate and with deeper waters. A possible explanation for this observation may relate to the tendency for small lutjanids to be more reef-associated and hence their growth more strongly influenced by localised conditions compared with lutjanids in intermediate and deep waters.

The lutajnids (Pristipomoides multidens, Lutjanus malabaricus and L. erythropterus) in my study showed a general trend for a greater mean length at age for the Timor Sea when compared with the Arafura Sea. Likewise, my results showed, longevity (defined by the upper 20% of ages for a species) was also greater for fish from the Timor Sea when compared with those from the Arafura Sea. This is possibly due to the differences in depth as the Arafura Sea is considerably shallower than the Timor Sea. Samples taken from the Arafura Sea were caught at the 50-60 m depth range, while samples from the Timor Sea were taken in the 100-150 m depth range. However, fishing gear selectivity may confound these observed differences. Samples from the Arafura Sea were taken by trawling, and it is possible that larger, stronger fish may be able to swim faster than the trawled nets and therefore avoid capture. Timor Sea samples were taken by trap and dropline methods.

Lutjanids appear to exhibit different spawning patterns with respect to depth zonation. Shallow water lutjanids favour a restricted summer peak in spawning activity, compared with lutjanids inhabiting intermediate and deeper waters, which appear to show a more protracted spawning season with a peak in activity during summer months and in some regions two spawning peaks are observed. In my study, I observed a protracted summer spawning period for P. mutidens, which was consistent with findings for this species by Brouard and Grandperrin (1985), and P. filamentosus (Kikkawa 1984).

By comparing the results from my detailed study of P. multidens with lutjanids from other depth ranges I explored the validity of using proxy values for fisheries management for data-poor situations, and concluded that caution is needed in using proxy values from other lutjanids in stock assessment modelling. Similar species within the same genera or the same location should not automatically be chosen as a proxy. However, underlying patterns exist within populations and by exploring these relationships, information can be utilised from data-rich fisheries to assist in developing appropriate management strategies for data-poor fisheries.

In conclusion, my study has shown that within the lutjanid complex there are different life-history strategies for lutjanids inhabiting different depth ranges, which are influenced by the environmental conditions within these depth ranges. However there are also confounding effects such as fishing and genetics which may also contribute to observed demographic patterns.

Item ID:	31598
Item Type:	Thesis (PhD)
Keywords:	lutjanids; lutjanid complex; validity of proxy values; variable life-history strategies; northern Australia; Timor Sea; Arafura Sea
Date Deposited:	06 May 2014 02:05
FoR Codes:	06 BIOLOGICAL SCIENCES > 0603 Evolutionary Biology > 060308 Life Histories @ 50% 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070405 Fish Physiology and Genetics @ 50%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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