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Defining breeding objectives for saltwater crocodile genetic improvement programs

Authors:
Sally R Isberg at Centre for Crocodile Research, Noonamah, Northern Territory, Australia
Sally R Isberg
  • Centre for Crocodile Research, Noonamah, Northern Territory, Australia
Frank W Nicholas at The University of Sydney
Frank W Nicholas
Peter C Thomson at The University of Sydney
Peter C Thomson
Stuart Barker at Wild Science
Stuart Barker
  • Wild Science
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Abstract

Genetic improvement of crocodilians is a novel concept. Here, breeding objectives for such a program are proposed and discussed in relation to the biological constraints inherent within the production system. The main source of income is the sale of skins, with meat as the major byproduct. Skins are sold according to a grading system. Objectives to increase the production efficiency of crocodile farms include: number of hatchlings per female per year, survival rate, food conversion efficiency, and age to slaughter (growth rate). In the future, skin “quality” (scale row number and regularity, shape and thickness) could also become important.
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Emerging Industries
166
DEFINING BREEDING OBJECTIVES FOR SALTWATER CROCODILE
GENETIC IMPROVEMENT PROGRAMS
S.R. Isberg1, F.W. Nicholas1, P.C. Thomson1, S.G. Barker2, S.C. Manolis3 and C. Moran1
1 Centre for Advanced Technologies in Animal Genetics and Reproduction (ReproGen),
Faculty of Veterinary Science, University of Sydney, NSW 2006
2Janamba Croc Farm, PO Box 496, Humpty Doo, NT 0836
3 Wildlife Management International Pty Ltd., PO Box 530, Sanderson, NT 0812
SUMMARY
Genetic improvement of crocodilians is a novel concept. Here, breeding objectives for such a
program are proposed and discussed in relation to the biological constraints inherent within the
production system. The main source of income is the sale of skins, with meat as the major by-
product. Skins are sold according to a grading system. Objectives to increase the production
efficiency of crocodile farms include: number of hatchlings per female per year, survival rate, food
conversion efficiency, and age to slaughter (growth rate). In the future, skin “quality” (scale row
number and regularity, shape and thickness) could also become important.
Keywords: Breeding objectives, Crocodylus porosus, crocodile
INTRODUCTION
Crocodile production for the lucrative skin trade is an emerging industry. To date, research in the
industry has concentrated on improved management and husbandry techniques, such as nutrition and
housing, to improve production efficiency. No research has yet been conducted into the possibility of
genetic improvement in captive breeding populations. This paper represents a first attempt to identify
breeding objectives. It also aims to sustain the enthusiasm for such an initiative indicated in a recent
(unpublished) industry survey.
MacNamara et al. (2003) recently reviewed the Australian crocodile industry. Briefly, it is based on
the production of skins from the saltwater crocodile (Crocodylus porosus), with meat produced as a
by-product. Stubbs (1998) estimated the total value of production at A$5 million per annum, with
skins contributing A$4 million of this total. Producers are paid per cm belly width with the industry-
accepted optimum belly width ranging between 32-42cm (at 1.3-2.0m total length) (MacNamara et
al. 2003). It takes between two and five years for crocodiles to reach this size. Skin prices are
determined through a grading system: skins with no blemishes are classified as 1st grade, while skins
with imperfections such as bite marks, abrasions and knife holes are classified as 2
nd, 3
rd and 4th
grade, depending on the extent of imperfections. Currently, the Australian industry is small in relation
to world trade (MacNamara et al. 2003). Webb (1989) stressed the importance of improving the
efficiency of the industry.
DISCUSSION
Here we attempt to identify breeding objectives as a prelude to estimation of relevant genetic and
phenotypic parameters, as well as economic values for the breeding objectives. As part of a larger
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study to evaluate the potential of implementing genetic improvement programs on saltwater crocodile
farms, the following breeding objectives were identified in relation to gross returns and costs.
Gross Returns. Gross returns are determined primarily by skin grade, as assessed by commercial
buyers. Skin grade mainly reflects physical damage to the skin (bites and abrasions). The quantity of
meat produced per animal is also a breeding objective, although it contributes substantially less return
per animal than skins.
Skin Grade. The largest concern of buyers of crocodile skins is the high proportion of skins failing
the requirements for 1st grade classification (Manolis et al. 2000). Typically, only around 30% meet
this grade, although MacNamara et al (2003) reported up to 50%. It is essential to improve the
average grade to meet market demand.
The issue of skin grades is complex. Since the industry is still developing, much of the downgrading
of skins is likely to be caused by inappropriate husbandry/management techniques, such as
inappropriate stocking densities and pen designs. It is possible that crocodile behaviour, especially
fighting, could also affect skin grade. Anecdotal evidence suggests that some clutches are more
“aggressive” than others, even at the time of hatch, implying a familial and possibly genetic basis for
the differences in aggressive behaviour.
Quantity of Meat Produced. A 1.5m crocodile will yield around 4-5 kg of boneless flesh. Treadwell
et al. (1991) reported a yield of 7-10 kg, but this is probably based on whole (bone-in) carcases, the
form in which meat was marketed at that time. The majority of fat is contained in the inner portion of
the tail and as a “fat body” in the abdominal cavity (Richardson et al. 2002). Fat levels are difficult to
measure on live animals, although measuring tail girth has been proposed as a possible selection
criterion for future investigation. The quantity of meat produced is also perceived as antagonistic to
skin production. However, selection indexes are designed to accommodate such antagonisms.
Costs. Breeding objectives aimed at minimising the costs of production are: high offspring output
from breeder stock, high survival, food conversion efficiency and decreased age to slaughter. Since
the industry is still at a developmental stage, research has concentrated on improving nutrition (for
example, pelletised food) and improved environmental raising conditions (for example, appropriate
water temperatures), again with no investigation into genetic sources of variation.
Breeder Output. The maintenance of breeding stock is a substantial cost in crocodile production. Pen
construction is a major cost item, particularly where breeding pairs are maintained to optimise
hatchling production per female. Females can produce between 25 and 30 hatchlings per year
(Treadwell et al. 1991). Of course, the actual clutch size laid is greater than the number of live
hatchlings produced, due to egg infertility and embryonic mortality. If higher hatch rate could be
realised, this would allow a reduction in breeder overheads.
Again, no investigation has been completed into selection criteria for annual breeder output of
finished animals. This is a complex trait requiring evaluation of fertility, age of female and male at
breeding, environmental conditions prior to hatch and survival of the embryo to hatching.
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Survival. Webb (1989) recommended that survival should be at least 95% in the first year (between
hatching and one year) and at least 95% between one year old and slaughter (~3.5 years). In reality,
very few farms achieve such high survival rates, and there is large variation in this trait. This large
variation is definitely affected by management regimes and husbandry factors, but genetic effects
may also impact.
Food Conversion Efficiency. It has been estimated that 42-45% of the operating costs of crocodile
production is accounted for by feed (Treadwell et al. 1991). Therefore, an increase in food conversion
efficiency should be a major breeding objective. Treadwell et al. (1991) also reported that on average,
a crocodile harvested at 1.5m should have consumed a total of 120kg of food. Feed costs themselves
are extremely variable between farms mainly due to the location and source of cheap protein.
Although food conversion efficiency has been estimated in biological and production-based studies, it
has not been genetically analysed in relation to growth rate.
Age to Slaughter. The majority of juveniles reach harvest size in captivity within 3.5 years (Treadwell
et al. 1991), and most of the variation in age to slaughter has been reported to occur through clutch-
specific effects (for example Manolis et al. 1989). Treadwell et al. (1991) determined that a decrease
in the average age of slaughter by one year from 3.5 to 2.5 years would increase the internal rates of
return by 250% using a small breeding farm model. Although there are no genetic parameter
estimates yet available, there does appear to be sufficient genetic variation among animals to begin
selecting for faster growth rates (unpublished data).
Skin quality- a trait of future importance? Skin quality, reflecting inherent properties of the hide
such as shape and thickness, is subjectively determined and is not rewarded under the current
marketing system. Saltwater crocodile skins, with their relatively small scales, evenly distributed
belly-scale pattern, lack of bone deposits in the belly scales, attract a premium price in comparison to
other crocodilian skins. This implies that improvements in these characteristics would be desirable
within saltwater crocodiles. Manolis et al. (2000) conducted a survey of persons involved in all
aspects of the crocodile skin industry in an effort to understand the issues that face producers of
saltwater crocodiles to enhance their product. Four “quality” concerns that could be influenced by
genetic selection were raised: skin shape, skin thickness, regularity of scale pattern and number of
scale rows between the vent and the neck.
Wild-harvested crocodiles are considered by buyers to have a superior skin shape and skin thickness
compared to their captive counterparts (Manolis et al. 2000). They are narrower per unit length and
have less wastage during product manufacture (Manolis et al. 2000). By comparison, captive-raised
crocodiles can be obese and show signs of stretching between the ventral scales. Extensive
experience in other animal industries has shown that obesity is very amenable to selective
improvement. Captive crocodiles grow faster than those in the wild, which may affect the thickness
of the skin.
At present, a premium price is not offered for skins with superior scale pattern regularity or number
of scale rows. Manolis et al. (2000) investigated the inheritance of the number of scale rows but did
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not report any genetic parameter estimates. It is likely that scale row number and regularity could be
considered as future selection objectives although there is currently no market reward for these traits.
CONCLUDING REMARKS
Feedback from the industry so far has been extremely positive (unpublished survey results) with most
producers welcoming the concept of a genetic improvement program and its potential benefits. Some
producers have expressed concerns that improving efficiency through genetic selection is premature
and that the research focus should remain on husbandry-related issues. However, optimal
improvement of productivity within the industry can best be achieved by simultaneous improvement
of the genetic value of the stock and their husbandry. This paper represents the first attempt anywhere
to comprehensively define breeding objectives for the crocodile industry and is thus an important first
step towards improving the basic resources and productivity of this industry.
ACKNOWLEDGEMENTS
This research is being supported by RIRDC (Project No. US-109A). Ms Isberg is supported by a
University of Sydney Postgraduate Award (co-funded). This document is based on discussions held
at Wildlife Management International Pty. Ltd., N.T. in February, 2002.
REFERENCES
MacNamara, K., Nicholas, P., Murphy, D., Riedel, E., Goulding, B., Horsburgh, C., Whiting, T. and
Warfield, B. (2003) RIRDC – Publ. No 02/142, Canberra.
Manolis, S.C., Webb, G.J.W., Barker, S.G., and Lippai, C. (1989) Pp. 291-312. Proc. Intensive Trop.
Animal Prod. Seminar, Townsville, Qld.
Manolis, S.C., Webb, G. and Richardson, K. (2000) RIRDC- Publ. No 00/21, Canberra.
Richardson, K.C., Webb, G.J.W. and Manolis, S.C. (2002) “Crocodiles: Inside Out”. Surrey Beatty
and Sons, Chipping Norton, Australia.
Stubbs, A. (1998) RIRDC- Publ. No 98/139, Canberra.
Treadwell, R., McElvie, L. and Maguire, G.B. 1991. “Profitability of selected aquaculture species.”
ABARE: Canberra. Pp. 63-70.
Webb, G. (1989) Proc. Intensive Trop. Animal Prod. Seminar, Townsville, Qld.
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Crocodiles: Inside Out
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