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A study of Dugongs (Dugong dugong) in Northern Queensland, Australia
A Study of Dugongs (Dugong dugong) in Northern Queensland, Australia GEORGE E. HEINSOHN, Ph.D. (Berkeley, California) Department of Zoology, James Cook University of North Queensland, PO Box 999, Townsville, Queensland 4810, Australia
ABSTRACT
population density, an affluent society with an abundance of high-quality protein from livestock, and laws The Dugong population in the vicinity of Townsville, protecting Dugongs from commercial exploitation. Queensland, has been severely decimated as a result of a Very little is known about relative population sizes government-sponsored shark-netting programme begun in or the sex and age structures of populations, about 1964. Only 7 out of a total of 158 Dugongs netted from 1964 to 1971 survived drowning and were released. Eighty-two Dugongs were caught during the first year (1964-65), after which the yearly catch has fluctuated between 16 and 6. Females constituted 48 (69"6 per cent) of 69 sexed Dugongs caught. Births appear to be seasonal, occurring mainly in August and September. A growth-curve has been constructed, making it possible to estimate ages on the basis of bodylength. Young accompany their mothers for more than a year after birth. Sexual maturity is attained at about 2 years of age by both sexes. Mature Dugongs have a total bodylength of 2.4 metres or longer. The majority of Dugongs caught after the first 14 months of shark-netting were young animals estimated to be from 1 to 3 years old. There were no size and sex data available for the first 14 months of netting. Dugongs prefer extensive protected shallow-water areas where they feed selectively on four genera of 'seagrasses' occurring near Townsville. Aspects of Dugong conservation and the need for extensive ecological work are discussed.
INTRODUCTION
The Dugong (Dugong dugong Erxleben), shown in Fig. 1, is an extremely interesting animal to both scientists and naturalists, since it is the only existing species of herbivorous mammal that lives exclusively in the sea. It is a large primary consumer and has considerable potential as a source of protein. The Dugong ranges along the coast of east Africa into the Red Sea, along the coast of southern Asia to as far east as the Solomon Islands, and along the northern coasts of Australia from southern Queensland to subtropical Western Australia. It is considered to be rare over most of its range (Bertram & Bertram, 1968b). However, Dugongs are still abundant in the shallow Fig. 1. Immature female Dugong (total body-length, seas around tropical Australia, and it is likely that 1"9 metres) accidentally caught and drowned in the Picnic Bay shark net, Magnetic Island, Queensland (May 1970). Australia may contain the main reservoir of Dugongs Total body-length is measured from the most anteriorin the world. The favourable situation in Australia dorsal point on the snout to the medial notch in the tail is due to a number of factors, including a low human fluke. 205 Biological Conservation, Vol. 4, No. 3, April 1972-- 0 Applied Science Publishers Ltd, England--Printed in Great Britain
206
Biological Conservation
reproduction and growth, or about the feeding habits In August 1964, shark-control methods were begun of the Dugong. The purposes of this paper are (1) by means of nearly permanently-set nets and drumto report on information obtained from Dugongs lines placed off bathing beaches on Magnetic Island. accidentally caught and drowned in shark nets set The net specifications are as follows: average length, at bathing beaches in the vicinity of Townsville, 300 yards (274 m); average depth, 16 ft (4.9 m); and Queensland, and (2) to bring about an awareness of mesh, 18in (45.7cm). The locations (Fig. 2) and the potential threat to Dugong populations that number of nets are: Horseshoe Bay with three nets would result from any major increase in shark-netting and Alma Bay, Nelly Bay, and Picnic Bay, with one net each. In August 1970, netting began at two locations in northern Australia. additional locations on the mainland--Kissing Point and Pallarenda--with a single net used alternatively STUDY AREA AND METHODS between the two sites. During the six-year period, August 1964 to August 1970, 1162 sharks representing Townsville (latitude 19°15' south, longitude 146°15') at least 12 species were caught. The six main species is located on the tropical coast of Queensland which is of sharks and numbers netted were 525 Hammer-head protected by the Great Barrier Reef. The protected Sharks (Sphyrna lewini), 197 Black-tip Sharks (Eulamia shallow sea between the Barrier Reef and the mainland spallanzani), 175 Saw-fish (Pristis zijsron), 148 Whaler contains extensive areas of 'sea-grasses' upon which Sharks (Eulamia macrura), 40 Tawny Sharks (Nebrius Dugongs graze. Fig. 2 shows the extent of these concolor), and 37 Tiger Sharks (Galeocerdo cuvier). shallow-water habitats in the vicinity of Townsville These ranged in size from a 1 ft 9 in (0.53 m) Black-tip and Magnetic Island. Sea-water temperatures off Shark to a 16 ft 0 in (4.9 m) Saw-fish. In addition to Townsville vary from 70°F (21.1°C) in winter (August) sharks, large species of fish, marine turtles, Dugongs, to 86°F (30"0°C) in summer (December and January) and a few porpoises were taken. Drum-lines consisting of 8 in (20.3 cm) hooks on chain traces were also used. These did not catch any marine mammals. During the six-year period, 1964 to 1970, 311 sharks representing at least 9 species were caught with 24 drum-lines. The four main species of sharks and numbers caught in this manner were 120 Tiger Sharks, 45 Black-tip Sharks, 26 Whaler MAGNETIC Sharks, and 9 Hammer-head Sharks. These ranged in size from a 1 ft 11 in (0.58 m) Black-tip Shark to a 14 ft 1 in (4.29 m) Tiger Shark. All nets and drumNellyBay13(2) lines were checked every second day. Data and research material were obtained from the I~r (E V E L A N D G A Y shark netter who kept records of all animals caught in his nets. From 4 August 1964 to 2 October 1965, only the date and location were recorded for each lilting PI.(Ill LowWater Mark Dugong caught. But from the latter date to 6 June Fathom Line 1970 the shark netter recorded the date of capture, Sand location, sex, total body-length, whether or not t ~ oOCOral T Reef pregnant, and the general stomach contents, of each dead Dugong. Pregnancy was only recorded for those animals having very large foetuses, and stomach contents were only recorded by a very cursory visual examination. Dugongs found alive in the nets were immediately released by the shark netter. The only data recorded for the live Dugongs caught were the Fig. 2. Map of Townsville, Magnetic Island, and the date and location of capture and, in one instance, adjacent sea, showing the netting locations, numbers of the sex. Dugongs caught, and water depths. The number immediately Beginning in 1969, efforts were made to obtain all following each Magnetic Island location shows the number the dead Dugongs removed from the nets. The shark caught from August 1964 through July 1970. The bracketed netter began reporting to the University when Dugongs number following each of the 4 island and 2 mainland locations shows the number caught from August 1970 through were caught, leaving the carcases at a convenient location to be examined and measured. Length July 1971. One fathom equals 1.8 metres. "
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207
Heinsohn: Dugongs in Northern Queensland, Australia
measurements were made, and reproductive notes recorded. The skull and other skeletal material were saved, along with stomachs and stomach contents, urogenital tracts, and other organs. This work is still continuing. Not all Dugongs were reported at the time of removal from nets, and thus, from May 1969 to May 1971, only 16 out of 27 Dugongs that drowned in nets were examined and salvaged. NETTINGRESULTS Numbers' Caught and Locations The total catch of Dugongs from August 1964 through July 1971 was 158. Of these only 7 survived in the nets and were released; the others drowned. The distribution of the Dugong catch by location over the seven-year period of 1964 to 1971 is shown in Fig. 2. Of 146 Dugongs taken from 1964 to July 1970, 94 were caught at Picnic Bay, 34 at Horseshoe Bay, 13 at Nelly Bay, and only 5 at Alma Bay. From August 1970 through July 1971, the first year in which two mainland netting stations were added to those originally established on Magnetic Island, 12 Dugongs were caught--3 at Picnic Bay, 4 at Pallarenda, 2 at
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Nelly Bay, 2 at Alma Bay, 1 at Horseshoe Bay, and none at Kissing Point. These results indicate a preference by Dugongs for the shallow protected area between Magnetic Island and the mainland. The sea-bed at all locations consists of sand with varying amounts of silt or mud. Picnic Bay is within a very large area of continuous sea-bed under 18 feet (5.4 m) in depth (see Fig. 2), and the area extending from Picnic Bay northwards is known to have extensive 'sea-grass' beds on which Dugongs feed. Of the original netting locations, the one with the fewest captures of Dugongs (Alma Bay) is on the most exposed portion of Magnetic Island, which receives extensive southeasterly winds. It also drops very quickly into deeper water to a depth below 18 ft (5.4 m). Figure 3 shows the number of Dugongs caught-including sex ratios, where available--during each netting year (August through July) from 1964 to 1971. Eighty-two Dugongs were caught during the first year (1964-65). After that the yearly catch fluctuated between 16 and 6, indicating that the local Townsville and Magnetic Island Dugong population had been severely decimated during the first year of netting. These results indicate that the Dugongs caught off the mainland beach at Pallarenda are probably part of the same local population as those caught off Magnetic Island, since there was no increase in the total Dugong catch with the addition of two new netting sites on the mainland in 1970. One question that needs to be answered through movement studies is how far-ranging are the effects of local netting such as occurs in the vicinity of Townsville. Figure 4 shows the number of Dugongs caught per month from the time when netting began in August 1964 through July 1971. No seasonal patterns of capture are indicated by these results. There is a generally reduced catch in June and July of most years, since there is an annual six-weeks period during this two-months period when the shark nets are not set. Multiple Captures The number of Dugongs caught per Dugong netting (see below) is shown by the following:
30
20-
15
16
14
10-
1964 _
1965
1985_ 1986
1986_
1907
1987__ 1990
1989__ 1060
1959__ 1970
1970_
1971
Fig. 3. Numbers and sexes of Dugongs caught by shark nets near Townsville during each netting year (August through July) from 1964 to 1971.
No. caught per netting
No. of nettings
1 Dugong 2 Dugongs 3 Dugongs 4 Dugongs 5 Dugongs
93 18 5 1 2 Total 119
208
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Biological Conservation T o t a I - 12
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frequently occur in groups--especially under conditions of high population density. The occurrence of Dugongs in groups is reported by Hughes & OxleyOxland (1971) and by Bertram & Bertram (1968b). There are no data on the age and sex composition of most of the above multiple captures of Dugongs, since the majority of such captures occurred during the first year of netting (see above). The composition of 7 pairs netted after 30 October 1965 is known, as lengths and sexes were recorded after that date. Of these 7 pairs, there were 5 instances of an adult female being caught with an immature young, one instance of a mature male and female being captured together, and one instance of two immature males being captured together.
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F M A M J 1965 of Dugongs T a k e n / M o n t h , Townsville 1964 --1971
A netting is here defined as an instance in which one or more Dugongs are found in a single shark-net at the time it is checked every second day. Single captures are the most frequent, accounting for 93 out of the 158 Dugongs caught. Eleven out of the 18 pairs of Dugongs caught were netted in the first year. Following that there was only one pair taken per year except in 1966-67, when two pairs were caught. The largest number of Dugongs caught at a single time in a single net was five (2 instances), and aggregations of three and four were also caught. All of these larger aggregations were netted during the first year of the shark-netting programme. It would appear from these results that Dugongs
AND SIZE
Size at Reproductive Maturity All available material was used to determine the sizes reached by females at reproductive maturity. The reproductive tracts of l0 females, ranging in total body-length from 1.25 to 2.90 metres, were examined. Evidence as to whether or not these female Dugongs had reached reproductive maturity was determined by simply examining the ovaries to see whether or not corpora lutea and/or follicles were present. In addition, one record of a pregnant Dugong and 4 records of Dugongs netted with calves were also used. The ovaries of these 5 Dugongs were not preserved. These results, summarized in Table I, indicate that
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Fig. 4. Numbers of Dugongs caught per month by shark nets near Townsville from the time netting began in August 1964 through July 1971.
GROWTH,
TABLE I
Size and Reproductive Status of 15 Female Dugongs Caught off Townsville, Queensland
Body-length (metres)
1 '25
1"98 2"20 2"34 2.39 2.42 2.44 2.45 2"54 2"69 2"70 2.70 2.82 2.90 3.05
Cycling (Presence of corpora lutea and]or follicles)
Presence of foetus or of calf
no no no no -yes -yes -yes yes yes -yes --
no no no no with calf no with foetus no with calf with calf no no with calf with foetus with calf
Month of capture
NOV.
Apr. Apr. Oct. May Dec. July Dec. Sept. Nov. Aug. Sept. Oct. Dec. Feb.
209
Heinsohn : Dugongs in Northern Queensland, Australia
female Dugongs become reproductively mature by the time they reach such a length as 2.39 metres (the length of the smallest female showing ovarian activity). The largest immature female Dugong had a total body-length of 2-34 metres. These results are not biased by possible seasonal lack of reproductive activity as reproductively-mature Dugongs were caught throughout most of the year. A large mammal such as the female Dugong would certainly show signs of reproductive activity after reaching maturity even though she might not be in breeding condition. Four males ranging in body-length from 1.50 to 2-70 metres were obtained and their reproductive organs saved. The results are shown in Table II, which
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II
Size and Reproductive Status of 4 Male Dugongs Caught off Townsville, Queensland Bodylength (metres)
Testis length (mm)
Testis width (ram)
1.50
50.2 30.5 92-0 88'0 mcomplete 105'8 108.5
22-6 21.8 31.9 33-0 42.0
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May
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gives total b o d y length, testis size, presence or absence of sperm, and month of capture. The two largest S'O'N 'D' J ' F'M' A U M ~ J ' J UA z males, 2.70 and 2.41 metres in length, were reproducMonths tively mature, as was indicated by sperm presence in sections made of the testes, while in the largest male, Fig. 5. Total body-lengths of Dugongs netted off Townsville plotted against month of capture. All measured Dugongs are sperm was present in both testis and epididymis. included from the time measuring began in October 1965 Sections of testes and epididymes taken from the two through July 1971. Solid circles represent females and smaller male Dugongs, 1.50 and 2-27 metres in length, crosses represent males. The letter A indicates a calf netted with its mother, and the letter F a foetal Dugong. contained no sperm, indicating that they were immature. It can thus be concluded that male Dugongs with a body-length greater than 2-4 metres are reproduc- Lengths and sexes were not recorded before October tively-mature, and that reproductive maturity is 1965. It appears that most births occur in August attained when a body-length of between 2.27 and 2.41 and September, with additional occasional births metres is reached. This is the same size-range in which occurring through the following spring months into females reach maturity. December. The evidence for this is based on the observation that the smallest Dugongs, ranging in Season of Birth, and Growth, Development, and Size size from 1.09 to 1-50 metres, were all caught from In Fig. 5 the total body-lengths of Dugongs have September to January. With one exception, there is been plotted against the months of capture. The sex an orderly increase in size of the five smallest Dugongs of each individual is shown. Five immature Dugongs caught, with the smallest netted in September and still accompanying their mothers and one foetus are the largest in early January. included in the figure. It contains the combined data New-born Dugongs are about 1.1 metres in length. of all sexed Dugongs of known length netted off A 1.14 metre-long foetal Dugong was recorded on Townsville from October 1965 through July 1971. 31 July 1966, and a 1-09 metre-long young, apparently
Biological Conservation
210
newborn, on 10 September 1967. Another foetal Dugong was collected on 1 December 1970, but an exact total body-length could not be established because it had been fixed in formalin in the curled foetal position. The total length for this foetus lies between 0.76 metres (the crown-rump length) and 1"40 metres (a length taken from the snout to the fluke notch along the curvature of the head and back). It would appear that this foetus was close to term, thus definitely extending the season of birth from September into December. Additional evidence that the majority of births occur annually during a relatively short period of time in August and September is indicated by Fig. 5, which shows three distinct imaginary cohorts as follows: birth to one year old (body-length, 1.09 to
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can be assumed that almost all births occur in August and September, it would appear that Dugongs reach a body-length of about 1.8 metres in one year, 2.3 to 2.4 metres in two years, and 2.5 to 2.7 metres in three years. Young apparently accompany their mothers for more than a year after birth. The largest young Dugong caught in the same net with its mother was a 1.83 metre-long female caught in February and estimated to be slightly more than 12 months old. Young Dugongs begin grazing within the first three months after birth. All of the juveniles examined had an abundance of 'sea-grasses' in their stomachs. The stomach contents of the smallest Dugong examined, a female with a total body-length of 1.25 metres and an estimated age of 2 to 3 months, consisted of a large amount of 'sea-grasses' of two genera plus milk. As has been shown above, that spermatogenesis and ovarian activity occur only in Dugongs of about 2.4 metres in length or more, it is apparent that both male and female Dugongs reach reproductive maturity at about two years of age. The largest Dugong measured was a female with a total body-length of 3.05 metres, caught on 16 February 1967. All of the other Dugongs were less than 3 metres long. The ranges and means of total body-length plus numbers of all measured adult and immature Dugongs (68 netted plus 1 stranded on beach) are given as follows for each sex:
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Fig. 6. lncrease of total body-length with age of the Dugong. Solid circles represent females and crosses males. The letter A indicates a calf netted with its mother.
1.80 metres); one to 2 years old (body-length, approximately 1.80 to 2.40 metres); 2 to 3 years old (bodylength approximately 2.40 to 2.70 metres). In Fig. 6 the total body-length is plotted against the month of capture to form what appears to be a growth-curve. This is similar to what was recorded in Fig. 5. However, in Fig. 6 the horizontal axis has been extended over a period of three years and three months instead of only 12 months. The 12-monthly intervals within each yearly interval correspond to the 12 months from September (when most births occur) through August. The resulting hypothetical growth-curve shows increase in body-length with age. A few points which do not clearly fit into any one year-class very likely represent animals born outside the main August-September birth season. Since environmental conditions that are likely to affect growth could be expected to be quite uniform in the warm tropical seas around TownsviUe, little variation in growth-rate would be expected from year to year. Thus, if on the basis of the limited data available, it
Male
Female
Range: 1.09 to 2.70 m Mean: 2.24 _ 0.10m No. -----21
Range: 1.21 to 3.05 m Mean: 2.28 _ 0.06m No. ----48
Since Dugongs with a total length of 2.39 m or more are reproductively mature, the mean total length and sample size for adults of each sex are as follows: Male
Female
Mean: 2.53 + 0.03 m No. ---- 12
Mean: 2.58 _ 0.03 m No. = 25
Thus males and females seem to be very similar in size, with no distinct sexual dimorphism in bodylength. SEX RATIOS
Out of a total of 69 sexed Dugongs netted off Townsville, 21 (30.4 per cent) were males and 48 (69.6 per cent) were females. An approximation of the
211
Heinsohn : Dugongs in Northern Queensland, Australia
secondary sex-ratio (ratio of males to females at birth) was obtained by determining the ratio of sexes of young up to a total body-length of 1.83 metres, which is the largest size at which young are known still to accompany their mothers. It is assumed that mortality between birth and this size and age (approximately one year old) is equal for the two sexes. For eleven young Dugongs up to about one year of age, 5 (45-5 per cent) were males and 6 (54.5 per cent) were females. Two near-term foetuses, one male and one female, have been recorded. Thus it would appear that there is an equal or near equal sex ratio at birth. Out of a total of 37 reproductively-mature Dugongs netted, 12 (32.4 per cent) were males and 25 (67.6 per cent) were females. It does not seem likely that there should naturally be such an unbalanced tertiary sex-ratio, especially when the secondary ratio is close to 1 : 1. Such a change in sex ratios would be due to a great difference in mortality between males and females, which also seems unlikely. The great difference in numbers of males and females netted is probably due to differences in distribution and habitat utilization between adult males and females. Since females are caught more frequently than males, they may utilize shallow waters, including the bays and other inshore areas where nets are set, more frequently than do males.
the young animals within the one- to three-years-old size- and age-range move about more than the older, established, Dugongs. The older, more mature, Dugong may have a well-established home range whereas the younger individual may wander about seeking new feeding and living areas. If, during the first year of netting, most of the established breeding population of Dugongs were killed, the majority of Dugongs subsequently taken would be individuals that had immigrated into the areas where the nets
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SIZE A N D AGE D I S T R I B U T I O N
Body-lengths were obtained for 68 out of the 69 sexed Dugongs netted off Townsville. One sexed Dugong was alive and released without being measured. The 68 measured Dugongs have been divided into four ageclasses based on the hypothetical growth-curve (Fig. 6) obtained above. These age-classes are 0 to 1 year (1.00 to 1-80 metres), 1 to 2 years (1.81 to 2.40 metres), 2 to 3 years (2.41 to 2.70 metres), and 3 years or older (more than 2.70 metres). Figure 7 shows the age and sex distribution of all sexed and measured Dugongs netted from 1965 to 1971. Out of a total of 68 Dugongs, 9 (13.2 per cent) were one year or less of age, 28 (41.2 per cent) were from one to two years old, another 28 (41.2 per cent) were from two to three years old, and 3 (4.4 per cent) were more than three years old. In each netting year from 1965 to 1971, it was consistently the one- to three-years-old individuals that were most frequently caught. Thus the majority of Dugongs netted are young animals that are independent of their mothers--including ones that are not reproductively-mature as well as ones that have recently become mature. One explanation as to why there is such a high proportion of young Dugongs caught would be that
9 T
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2.7 +
Size Classes (Total Body Length-Metres) O
1 yr Estimated
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3yr +
Age
Fig. 7. Age and sex distribution o f aH sexed and measured Dugongs netted near Townsville from 1965 to 1971.
are set. The data shown in Fig. 7 indicate that these are mainly young females. The larger proportion of females caught could be due to a preference of females for inshore areas, as suggested above. Another, but less likely, explanation of the above results is that young individuals may be caught more frequently because they have not learned of the presence and dangers of set nets. Older, experienced, Dugongs may somehow learn that the nets are potentially dangerous and thus avoid them. Such learning could take place if Dugongs came to associate the presence of nets with the distress signals of a drowning Dugong.
Biological Conservation
212 FOODS
Heinsohn & Birch (in press) have shown that Dugongs caught off Townsville feed exclusively on 'sea-grasses' (Potamogetonaeeae and Hydrocharitaceae). At least six species of these, representing four out of seven available genera, are eaten. Of these, two genera (Cymodoeea and Diplanthera) comprise the main source of food. The species of 'sea-grasses' occurring near Townsville vary considerably in size, but the smaller ones seem to be used the most-especially those occurring in shallow subtidal and lower intertidal areas.
DISCUSSION
The results of this paper indicate that Dugongs are still abundant in northern Queensland. This agrees with the opinion of Bertram & Bertram (1968a) that fair residual stocks of Dugongs still occur in northern Australian waters. The Dugong has been almost totally protected by law in Queensland since 1969, the only exception being that Torres Strait islanders and aboriginal people living on reserves, missions, and other settlements established specifically for them, are allowed to hunt Dugongs for food. The main threat to Dugongs in Australia would appear to be due to the adverse environmental effects to their habitat resulting from economic, industrial, and agricultural, development and increased human population growth. Pollution of the vulnerable coastal waters inside the Great Barrier Reef is a potential threat to the very productive shallow-water 'sea-grass' ecosystems which support Dugongs. The large number of Dugongs that have been killed as a result of the shark-netting programme at Townsville is an example of the conflict between wildlife and human needs--in this case the recreational needs of a local population and an expanding tourist industry. The vulnerability of Dugong populations to continuous local netting is quite clear, since it is evident that the Magnetic Island Dugong population was almost completely destroyed in a single year. Shark-netting, as done near Townsville, should not be extended to new areas, especially where large numbers of Dugongs are known to occur. Alternative methods of protecting swimmers from possible shark attacks should be devised. There are two methods of shark control in use at present--drum-lines and netting. Drum-lines, which are not a threat to marine mammals, unfortunately are not as effective a means of controlling potentially dangerous shark species as are nets. One important exception is the Tiger Shark.
The area over which a Dugong population is decimated by fixed nets is not known, but would depend on the distances and patterns of movement. The location of nets with respect to possible migratory or movement routes of Dugongs would be very important. After the first year of netting, a high percentage of the Dugongs caught have been young that are independent of their mothers. This suggests that many of the Dugongs have been immigrants to the local Magnetic Island-Townsville population. Therefore it is essential that a Dugong tagging pro. gramme be established, so that movement and possible home ranges can be determined. It is unfortunate that body-lengths and sexes as minimal items of data were not recorded for the first 82 Dugongs removed from the nets (1964-65). These data would have given a good picture of the population structure before the accidental high mortality. It is likely that the age and sex distributions of Dugongs killed from the second year on would be considerably different from that of the first year. Although the importance of the Dugong is generally recognized, and its status as an endangered species is known (Fisher et al., 1969), there has not yet been any extensive research on its ecology and management. In order to manage and conserve Dugongs, it is necessary to determine population characteristics such as natality, recruitment, mortality, longevity, age and sex structures, and movements. More detailed information should also be obtained on reproduction and growth. This paper indicates that Dugongs could be netted for purposes of examination and tagging. Hughes & Oxley-Oxland (1971) showed that concentrations of Dugongs can be effectively located for study from aircraft. Thus extensive research on live populations, although expensive to conduct, would be feasible.
ACKNOWLEDGEMENTS
The author would like to thank the following: the Queensland Department of Harbours and Marine and the shark-netting contractors, Mr G. P. W. Hopper and Mr E. H. Brown, without whose cooperation this project could not have been carried out; Mr W. R. Dowd, Mr M. Downes, and Mr M. J. Devereux, technical staff members of James Cook University, who have assisted with the handling of Dugongs; and Miss J. E. Joiner, who prepared Figs. 2 to 7. Professor A. Starker Leopold, University of California at Berkeley, kindly read the manuscript and made some useful suggestions. The project was partly supported by a University Research Grant from James Cook University.
Heinsohn: Dugongs in Northern Queensland, Australia References BERTRAM,C. K. RICARDO• BERTRAM,G. C. L. (1968a). The Sirenia as aquatic meat-producing herbivores. Syrup. Zool. Soc. Lond., No. 21, pp. 385-91. BERTRAM,G. C. L. & BERTRAM,C. K. RICARDO(1968b). Bionomicsof dugongs and manatees. Nature (London), 218(5140), pp. 423-6. FISHER, JAMES, SIMON, NOEL & VINCENT, JACK (1969).
213
The Red Book. Wildlife in Danger. Collins, St James Place, London: 368 pp., illustr. HEINSOHN, GEORGEE. 8~; BIRCH,W. R. (in press). Foods and feeding habits of the Dugong, Dugong dugong (Erxleben), in northern Queensland, Australia. Mammalia. HUGHES, GEORGE R. & OXLEY-OXLAND, R. (1971). A survey of Dugong (Dugong dugong) in and around Antonio Enes, Northern Mo9ambique. Biol. Conserv., 3(4), pp. 299-301, illustr.
Institute of African Research and Studies, Cairo University
Courses were started, in October 1971, for the students in the new Institute of African Research and Studies, Cairo University. This Institute was first established in 1950 as a postgraduate Institute of Sudanese Studies which comprised only two departments, those of Geography and of History. The growing interest in African affairs during the 'sixties caused these two departments to widen their area of activities to include the whole of the African Continent until the Cairo University Board approved, in June 1971, the decree creating a well-balanced and all-embracing Institute of African Research and Studies as part of Cairo University. The new Institute includes six departments: Geography, History, Political and Economic Systems, Languages and Dialects, Anthropology, and Natural Resources. The Department of Natural Resources accepts students who have graduated from science-based Faculties, e.g. Science, Agriculture, Veterinary Medicine, Medicine, and Engineering. Students are registered for three years for the Master's degree. They attend courses during the first two years and then prepare a thesis in the third year. After obtaining the Master's degree, the student can apply to register for the Ph.D. degree, which is based totally on research work lasting, in all, at least three years. The aims of the Institute are to train specialists for working in the various fields related to Africa, to hold conferences and meetings on African affairs, and to create a documentation centre for research and studies dealing with Africa. Professor Mohamed E1-Sayed Ghallab, who has wide experience of various aspects of African geography, was appointed Dean of the Institute. He was previously Professor of Geography in the Faculty of Arts, Cairo University, and is Secretary-General of the Egyptian Geographical Society. He has written several textbooks on human geography, anthropology, and urbanization. The institute has accepted 154 students for the Academic Year 1971-72, among whom 28 are registered
in the Department of Natural Resources. The Institute welcomes students from outside Egypt, especially from other African countries. The courses in the Department of Natural Resources are, in the First Year for two hours weekly each: Geology of the African Continent, Climatology and Meteorology, Hydrology of African Rivers, The Vegetative Cover in the African Continent, Animal Life in the African Continent, Soil Types in the African Continent, Diseases of Warm Lands, and General Geography of Africa. In the Second Year the courses are: Mineral Resources in the African Continent, Schemes for the Control of African Rivers, Land Use in Africa: (a) Main Agricultural Crops, (b) Animal Resources and Pastures, (c) Conservation of Natural Resources, (d) Dry Farming in Northern and Southern Africa, (e) Utilization of Ground-water Basins, Hydrobiology in the African Continent, and Seminars (6 hours). It may be noted that this is a multidisciplinary set of courses of training given as formal postgraduate teaching. The course on Conservation (ll 3(c)) is noteworthy because, whereas the other courses are given in some form or other within the traditional Faculties, the course on Conservation is a completely new concept introduced for the first time in Egyptian Universities. SAMIR I. GHABBOUR, Zoology Department, Faculty of Seience, Cairo University, Giza, Egypt.
ABSTRACT
population density, an affluent society with an abundance of high-quality protein from livestock, and laws The Dugong population in the vicinity of Townsville, protecting Dugongs from commercial exploitation. Queensland, has been severely decimated as a result of a Very little is known about relative population sizes government-sponsored shark-netting programme begun in or the sex and age structures of populations, about 1964. Only 7 out of a total of 158 Dugongs netted from 1964 to 1971 survived drowning and were released. Eighty-two Dugongs were caught during the first year (1964-65), after which the yearly catch has fluctuated between 16 and 6. Females constituted 48 (69"6 per cent) of 69 sexed Dugongs caught. Births appear to be seasonal, occurring mainly in August and September. A growth-curve has been constructed, making it possible to estimate ages on the basis of bodylength. Young accompany their mothers for more than a year after birth. Sexual maturity is attained at about 2 years of age by both sexes. Mature Dugongs have a total bodylength of 2.4 metres or longer. The majority of Dugongs caught after the first 14 months of shark-netting were young animals estimated to be from 1 to 3 years old. There were no size and sex data available for the first 14 months of netting. Dugongs prefer extensive protected shallow-water areas where they feed selectively on four genera of 'seagrasses' occurring near Townsville. Aspects of Dugong conservation and the need for extensive ecological work are discussed.
INTRODUCTION
The Dugong (Dugong dugong Erxleben), shown in Fig. 1, is an extremely interesting animal to both scientists and naturalists, since it is the only existing species of herbivorous mammal that lives exclusively in the sea. It is a large primary consumer and has considerable potential as a source of protein. The Dugong ranges along the coast of east Africa into the Red Sea, along the coast of southern Asia to as far east as the Solomon Islands, and along the northern coasts of Australia from southern Queensland to subtropical Western Australia. It is considered to be rare over most of its range (Bertram & Bertram, 1968b). However, Dugongs are still abundant in the shallow Fig. 1. Immature female Dugong (total body-length, seas around tropical Australia, and it is likely that 1"9 metres) accidentally caught and drowned in the Picnic Bay shark net, Magnetic Island, Queensland (May 1970). Australia may contain the main reservoir of Dugongs Total body-length is measured from the most anteriorin the world. The favourable situation in Australia dorsal point on the snout to the medial notch in the tail is due to a number of factors, including a low human fluke. 205 Biological Conservation, Vol. 4, No. 3, April 1972-- 0 Applied Science Publishers Ltd, England--Printed in Great Britain
206
Biological Conservation
reproduction and growth, or about the feeding habits In August 1964, shark-control methods were begun of the Dugong. The purposes of this paper are (1) by means of nearly permanently-set nets and drumto report on information obtained from Dugongs lines placed off bathing beaches on Magnetic Island. accidentally caught and drowned in shark nets set The net specifications are as follows: average length, at bathing beaches in the vicinity of Townsville, 300 yards (274 m); average depth, 16 ft (4.9 m); and Queensland, and (2) to bring about an awareness of mesh, 18in (45.7cm). The locations (Fig. 2) and the potential threat to Dugong populations that number of nets are: Horseshoe Bay with three nets would result from any major increase in shark-netting and Alma Bay, Nelly Bay, and Picnic Bay, with one net each. In August 1970, netting began at two locations in northern Australia. additional locations on the mainland--Kissing Point and Pallarenda--with a single net used alternatively STUDY AREA AND METHODS between the two sites. During the six-year period, August 1964 to August 1970, 1162 sharks representing Townsville (latitude 19°15' south, longitude 146°15') at least 12 species were caught. The six main species is located on the tropical coast of Queensland which is of sharks and numbers netted were 525 Hammer-head protected by the Great Barrier Reef. The protected Sharks (Sphyrna lewini), 197 Black-tip Sharks (Eulamia shallow sea between the Barrier Reef and the mainland spallanzani), 175 Saw-fish (Pristis zijsron), 148 Whaler contains extensive areas of 'sea-grasses' upon which Sharks (Eulamia macrura), 40 Tawny Sharks (Nebrius Dugongs graze. Fig. 2 shows the extent of these concolor), and 37 Tiger Sharks (Galeocerdo cuvier). shallow-water habitats in the vicinity of Townsville These ranged in size from a 1 ft 9 in (0.53 m) Black-tip and Magnetic Island. Sea-water temperatures off Shark to a 16 ft 0 in (4.9 m) Saw-fish. In addition to Townsville vary from 70°F (21.1°C) in winter (August) sharks, large species of fish, marine turtles, Dugongs, to 86°F (30"0°C) in summer (December and January) and a few porpoises were taken. Drum-lines consisting of 8 in (20.3 cm) hooks on chain traces were also used. These did not catch any marine mammals. During the six-year period, 1964 to 1970, 311 sharks representing at least 9 species were caught with 24 drum-lines. The four main species of sharks and numbers caught in this manner were 120 Tiger Sharks, 45 Black-tip Sharks, 26 Whaler MAGNETIC Sharks, and 9 Hammer-head Sharks. These ranged in size from a 1 ft 11 in (0.58 m) Black-tip Shark to a 14 ft 1 in (4.29 m) Tiger Shark. All nets and drumNellyBay13(2) lines were checked every second day. Data and research material were obtained from the I~r (E V E L A N D G A Y shark netter who kept records of all animals caught in his nets. From 4 August 1964 to 2 October 1965, only the date and location were recorded for each lilting PI.(Ill LowWater Mark Dugong caught. But from the latter date to 6 June Fathom Line 1970 the shark netter recorded the date of capture, Sand location, sex, total body-length, whether or not t ~ oOCOral T Reef pregnant, and the general stomach contents, of each dead Dugong. Pregnancy was only recorded for those animals having very large foetuses, and stomach contents were only recorded by a very cursory visual examination. Dugongs found alive in the nets were immediately released by the shark netter. The only data recorded for the live Dugongs caught were the Fig. 2. Map of Townsville, Magnetic Island, and the date and location of capture and, in one instance, adjacent sea, showing the netting locations, numbers of the sex. Dugongs caught, and water depths. The number immediately Beginning in 1969, efforts were made to obtain all following each Magnetic Island location shows the number the dead Dugongs removed from the nets. The shark caught from August 1964 through July 1970. The bracketed netter began reporting to the University when Dugongs number following each of the 4 island and 2 mainland locations shows the number caught from August 1970 through were caught, leaving the carcases at a convenient location to be examined and measured. Length July 1971. One fathom equals 1.8 metres. "
o\
~
207
Heinsohn: Dugongs in Northern Queensland, Australia
measurements were made, and reproductive notes recorded. The skull and other skeletal material were saved, along with stomachs and stomach contents, urogenital tracts, and other organs. This work is still continuing. Not all Dugongs were reported at the time of removal from nets, and thus, from May 1969 to May 1971, only 16 out of 27 Dugongs that drowned in nets were examined and salvaged. NETTINGRESULTS Numbers' Caught and Locations The total catch of Dugongs from August 1964 through July 1971 was 158. Of these only 7 survived in the nets and were released; the others drowned. The distribution of the Dugong catch by location over the seven-year period of 1964 to 1971 is shown in Fig. 2. Of 146 Dugongs taken from 1964 to July 1970, 94 were caught at Picnic Bay, 34 at Horseshoe Bay, 13 at Nelly Bay, and only 5 at Alma Bay. From August 1970 through July 1971, the first year in which two mainland netting stations were added to those originally established on Magnetic Island, 12 Dugongs were caught--3 at Picnic Bay, 4 at Pallarenda, 2 at
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Nelly Bay, 2 at Alma Bay, 1 at Horseshoe Bay, and none at Kissing Point. These results indicate a preference by Dugongs for the shallow protected area between Magnetic Island and the mainland. The sea-bed at all locations consists of sand with varying amounts of silt or mud. Picnic Bay is within a very large area of continuous sea-bed under 18 feet (5.4 m) in depth (see Fig. 2), and the area extending from Picnic Bay northwards is known to have extensive 'sea-grass' beds on which Dugongs feed. Of the original netting locations, the one with the fewest captures of Dugongs (Alma Bay) is on the most exposed portion of Magnetic Island, which receives extensive southeasterly winds. It also drops very quickly into deeper water to a depth below 18 ft (5.4 m). Figure 3 shows the number of Dugongs caught-including sex ratios, where available--during each netting year (August through July) from 1964 to 1971. Eighty-two Dugongs were caught during the first year (1964-65). After that the yearly catch fluctuated between 16 and 6, indicating that the local Townsville and Magnetic Island Dugong population had been severely decimated during the first year of netting. These results indicate that the Dugongs caught off the mainland beach at Pallarenda are probably part of the same local population as those caught off Magnetic Island, since there was no increase in the total Dugong catch with the addition of two new netting sites on the mainland in 1970. One question that needs to be answered through movement studies is how far-ranging are the effects of local netting such as occurs in the vicinity of Townsville. Figure 4 shows the number of Dugongs caught per month from the time when netting began in August 1964 through July 1971. No seasonal patterns of capture are indicated by these results. There is a generally reduced catch in June and July of most years, since there is an annual six-weeks period during this two-months period when the shark nets are not set. Multiple Captures The number of Dugongs caught per Dugong netting (see below) is shown by the following:
30
20-
15
16
14
10-
1964 _
1965
1985_ 1986
1986_
1907
1987__ 1990
1989__ 1060
1959__ 1970
1970_
1971
Fig. 3. Numbers and sexes of Dugongs caught by shark nets near Townsville during each netting year (August through July) from 1964 to 1971.
No. caught per netting
No. of nettings
1 Dugong 2 Dugongs 3 Dugongs 4 Dugongs 5 Dugongs
93 18 5 1 2 Total 119
208
':I
Biological Conservation T o t a I - 12
ATS 1970
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frequently occur in groups--especially under conditions of high population density. The occurrence of Dugongs in groups is reported by Hughes & OxleyOxland (1971) and by Bertram & Bertram (1968b). There are no data on the age and sex composition of most of the above multiple captures of Dugongs, since the majority of such captures occurred during the first year of netting (see above). The composition of 7 pairs netted after 30 October 1965 is known, as lengths and sexes were recorded after that date. Of these 7 pairs, there were 5 instances of an adult female being caught with an immature young, one instance of a mature male and female being captured together, and one instance of two immature males being captured together.
i
J REPRODUCTION,
Total-I 4 ,
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F M A M J 1965 of Dugongs T a k e n / M o n t h , Townsville 1964 --1971
A netting is here defined as an instance in which one or more Dugongs are found in a single shark-net at the time it is checked every second day. Single captures are the most frequent, accounting for 93 out of the 158 Dugongs caught. Eleven out of the 18 pairs of Dugongs caught were netted in the first year. Following that there was only one pair taken per year except in 1966-67, when two pairs were caught. The largest number of Dugongs caught at a single time in a single net was five (2 instances), and aggregations of three and four were also caught. All of these larger aggregations were netted during the first year of the shark-netting programme. It would appear from these results that Dugongs
AND SIZE
Size at Reproductive Maturity All available material was used to determine the sizes reached by females at reproductive maturity. The reproductive tracts of l0 females, ranging in total body-length from 1.25 to 2.90 metres, were examined. Evidence as to whether or not these female Dugongs had reached reproductive maturity was determined by simply examining the ovaries to see whether or not corpora lutea and/or follicles were present. In addition, one record of a pregnant Dugong and 4 records of Dugongs netted with calves were also used. The ovaries of these 5 Dugongs were not preserved. These results, summarized in Table I, indicate that
J
Fig. 4. Numbers of Dugongs caught per month by shark nets near Townsville from the time netting began in August 1964 through July 1971.
GROWTH,
TABLE I
Size and Reproductive Status of 15 Female Dugongs Caught off Townsville, Queensland
Body-length (metres)
1 '25
1"98 2"20 2"34 2.39 2.42 2.44 2.45 2"54 2"69 2"70 2.70 2.82 2.90 3.05
Cycling (Presence of corpora lutea and]or follicles)
Presence of foetus or of calf
no no no no -yes -yes -yes yes yes -yes --
no no no no with calf no with foetus no with calf with calf no no with calf with foetus with calf
Month of capture
NOV.
Apr. Apr. Oct. May Dec. July Dec. Sept. Nov. Aug. Sept. Oct. Dec. Feb.
209
Heinsohn : Dugongs in Northern Queensland, Australia
female Dugongs become reproductively mature by the time they reach such a length as 2.39 metres (the length of the smallest female showing ovarian activity). The largest immature female Dugong had a total body-length of 2-34 metres. These results are not biased by possible seasonal lack of reproductive activity as reproductively-mature Dugongs were caught throughout most of the year. A large mammal such as the female Dugong would certainly show signs of reproductive activity after reaching maturity even though she might not be in breeding condition. Four males ranging in body-length from 1.50 to 2-70 metres were obtained and their reproductive organs saved. The results are shown in Table II, which
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TABLE
II
Size and Reproductive Status of 4 Male Dugongs Caught off Townsville, Queensland Bodylength (metres)
Testis length (mm)
Testis width (ram)
1.50
50.2 30.5 92-0 88'0 mcomplete 105'8 108.5
22-6 21.8 31.9 33-0 42.0
yes; in testis
Aug.
64.1 65.1
yes; in testis and epididymis
May
Presence of sperm
Month of capture
no
Oct.
no
Jan.
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0 b.
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2.27 2.41 2.70
•A 1.2
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gives total b o d y length, testis size, presence or absence of sperm, and month of capture. The two largest S'O'N 'D' J ' F'M' A U M ~ J ' J UA z males, 2.70 and 2.41 metres in length, were reproducMonths tively mature, as was indicated by sperm presence in sections made of the testes, while in the largest male, Fig. 5. Total body-lengths of Dugongs netted off Townsville plotted against month of capture. All measured Dugongs are sperm was present in both testis and epididymis. included from the time measuring began in October 1965 Sections of testes and epididymes taken from the two through July 1971. Solid circles represent females and smaller male Dugongs, 1.50 and 2-27 metres in length, crosses represent males. The letter A indicates a calf netted with its mother, and the letter F a foetal Dugong. contained no sperm, indicating that they were immature. It can thus be concluded that male Dugongs with a body-length greater than 2-4 metres are reproduc- Lengths and sexes were not recorded before October tively-mature, and that reproductive maturity is 1965. It appears that most births occur in August attained when a body-length of between 2.27 and 2.41 and September, with additional occasional births metres is reached. This is the same size-range in which occurring through the following spring months into females reach maturity. December. The evidence for this is based on the observation that the smallest Dugongs, ranging in Season of Birth, and Growth, Development, and Size size from 1.09 to 1-50 metres, were all caught from In Fig. 5 the total body-lengths of Dugongs have September to January. With one exception, there is been plotted against the months of capture. The sex an orderly increase in size of the five smallest Dugongs of each individual is shown. Five immature Dugongs caught, with the smallest netted in September and still accompanying their mothers and one foetus are the largest in early January. included in the figure. It contains the combined data New-born Dugongs are about 1.1 metres in length. of all sexed Dugongs of known length netted off A 1.14 metre-long foetal Dugong was recorded on Townsville from October 1965 through July 1971. 31 July 1966, and a 1-09 metre-long young, apparently
Biological Conservation
210
newborn, on 10 September 1967. Another foetal Dugong was collected on 1 December 1970, but an exact total body-length could not be established because it had been fixed in formalin in the curled foetal position. The total length for this foetus lies between 0.76 metres (the crown-rump length) and 1"40 metres (a length taken from the snout to the fluke notch along the curvature of the head and back). It would appear that this foetus was close to term, thus definitely extending the season of birth from September into December. Additional evidence that the majority of births occur annually during a relatively short period of time in August and September is indicated by Fig. 5, which shows three distinct imaginary cohorts as follows: birth to one year old (body-length, 1.09 to
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can be assumed that almost all births occur in August and September, it would appear that Dugongs reach a body-length of about 1.8 metres in one year, 2.3 to 2.4 metres in two years, and 2.5 to 2.7 metres in three years. Young apparently accompany their mothers for more than a year after birth. The largest young Dugong caught in the same net with its mother was a 1.83 metre-long female caught in February and estimated to be slightly more than 12 months old. Young Dugongs begin grazing within the first three months after birth. All of the juveniles examined had an abundance of 'sea-grasses' in their stomachs. The stomach contents of the smallest Dugong examined, a female with a total body-length of 1.25 metres and an estimated age of 2 to 3 months, consisted of a large amount of 'sea-grasses' of two genera plus milk. As has been shown above, that spermatogenesis and ovarian activity occur only in Dugongs of about 2.4 metres in length or more, it is apparent that both male and female Dugongs reach reproductive maturity at about two years of age. The largest Dugong measured was a female with a total body-length of 3.05 metres, caught on 16 February 1967. All of the other Dugongs were less than 3 metres long. The ranges and means of total body-length plus numbers of all measured adult and immature Dugongs (68 netted plus 1 stranded on beach) are given as follows for each sex:
m.kh,
Fig. 6. lncrease of total body-length with age of the Dugong. Solid circles represent females and crosses males. The letter A indicates a calf netted with its mother.
1.80 metres); one to 2 years old (body-length, approximately 1.80 to 2.40 metres); 2 to 3 years old (bodylength approximately 2.40 to 2.70 metres). In Fig. 6 the total body-length is plotted against the month of capture to form what appears to be a growth-curve. This is similar to what was recorded in Fig. 5. However, in Fig. 6 the horizontal axis has been extended over a period of three years and three months instead of only 12 months. The 12-monthly intervals within each yearly interval correspond to the 12 months from September (when most births occur) through August. The resulting hypothetical growth-curve shows increase in body-length with age. A few points which do not clearly fit into any one year-class very likely represent animals born outside the main August-September birth season. Since environmental conditions that are likely to affect growth could be expected to be quite uniform in the warm tropical seas around TownsviUe, little variation in growth-rate would be expected from year to year. Thus, if on the basis of the limited data available, it
Male
Female
Range: 1.09 to 2.70 m Mean: 2.24 _ 0.10m No. -----21
Range: 1.21 to 3.05 m Mean: 2.28 _ 0.06m No. ----48
Since Dugongs with a total length of 2.39 m or more are reproductively mature, the mean total length and sample size for adults of each sex are as follows: Male
Female
Mean: 2.53 + 0.03 m No. ---- 12
Mean: 2.58 _ 0.03 m No. = 25
Thus males and females seem to be very similar in size, with no distinct sexual dimorphism in bodylength. SEX RATIOS
Out of a total of 69 sexed Dugongs netted off Townsville, 21 (30.4 per cent) were males and 48 (69.6 per cent) were females. An approximation of the
211
Heinsohn : Dugongs in Northern Queensland, Australia
secondary sex-ratio (ratio of males to females at birth) was obtained by determining the ratio of sexes of young up to a total body-length of 1.83 metres, which is the largest size at which young are known still to accompany their mothers. It is assumed that mortality between birth and this size and age (approximately one year old) is equal for the two sexes. For eleven young Dugongs up to about one year of age, 5 (45-5 per cent) were males and 6 (54.5 per cent) were females. Two near-term foetuses, one male and one female, have been recorded. Thus it would appear that there is an equal or near equal sex ratio at birth. Out of a total of 37 reproductively-mature Dugongs netted, 12 (32.4 per cent) were males and 25 (67.6 per cent) were females. It does not seem likely that there should naturally be such an unbalanced tertiary sex-ratio, especially when the secondary ratio is close to 1 : 1. Such a change in sex ratios would be due to a great difference in mortality between males and females, which also seems unlikely. The great difference in numbers of males and females netted is probably due to differences in distribution and habitat utilization between adult males and females. Since females are caught more frequently than males, they may utilize shallow waters, including the bays and other inshore areas where nets are set, more frequently than do males.
the young animals within the one- to three-years-old size- and age-range move about more than the older, established, Dugongs. The older, more mature, Dugong may have a well-established home range whereas the younger individual may wander about seeking new feeding and living areas. If, during the first year of netting, most of the established breeding population of Dugongs were killed, the majority of Dugongs subsequently taken would be individuals that had immigrated into the areas where the nets
50
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d d
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9
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SIZE A N D AGE D I S T R I B U T I O N
Body-lengths were obtained for 68 out of the 69 sexed Dugongs netted off Townsville. One sexed Dugong was alive and released without being measured. The 68 measured Dugongs have been divided into four ageclasses based on the hypothetical growth-curve (Fig. 6) obtained above. These age-classes are 0 to 1 year (1.00 to 1-80 metres), 1 to 2 years (1.81 to 2.40 metres), 2 to 3 years (2.41 to 2.70 metres), and 3 years or older (more than 2.70 metres). Figure 7 shows the age and sex distribution of all sexed and measured Dugongs netted from 1965 to 1971. Out of a total of 68 Dugongs, 9 (13.2 per cent) were one year or less of age, 28 (41.2 per cent) were from one to two years old, another 28 (41.2 per cent) were from two to three years old, and 3 (4.4 per cent) were more than three years old. In each netting year from 1965 to 1971, it was consistently the one- to three-years-old individuals that were most frequently caught. Thus the majority of Dugongs netted are young animals that are independent of their mothers--including ones that are not reproductively-mature as well as ones that have recently become mature. One explanation as to why there is such a high proportion of young Dugongs caught would be that
9 T
0 1-0
1" 8
2-4
2.7 +
Size Classes (Total Body Length-Metres) O
1 yr Estimated
2yr
3yr +
Age
Fig. 7. Age and sex distribution o f aH sexed and measured Dugongs netted near Townsville from 1965 to 1971.
are set. The data shown in Fig. 7 indicate that these are mainly young females. The larger proportion of females caught could be due to a preference of females for inshore areas, as suggested above. Another, but less likely, explanation of the above results is that young individuals may be caught more frequently because they have not learned of the presence and dangers of set nets. Older, experienced, Dugongs may somehow learn that the nets are potentially dangerous and thus avoid them. Such learning could take place if Dugongs came to associate the presence of nets with the distress signals of a drowning Dugong.
Biological Conservation
212 FOODS
Heinsohn & Birch (in press) have shown that Dugongs caught off Townsville feed exclusively on 'sea-grasses' (Potamogetonaeeae and Hydrocharitaceae). At least six species of these, representing four out of seven available genera, are eaten. Of these, two genera (Cymodoeea and Diplanthera) comprise the main source of food. The species of 'sea-grasses' occurring near Townsville vary considerably in size, but the smaller ones seem to be used the most-especially those occurring in shallow subtidal and lower intertidal areas.
DISCUSSION
The results of this paper indicate that Dugongs are still abundant in northern Queensland. This agrees with the opinion of Bertram & Bertram (1968a) that fair residual stocks of Dugongs still occur in northern Australian waters. The Dugong has been almost totally protected by law in Queensland since 1969, the only exception being that Torres Strait islanders and aboriginal people living on reserves, missions, and other settlements established specifically for them, are allowed to hunt Dugongs for food. The main threat to Dugongs in Australia would appear to be due to the adverse environmental effects to their habitat resulting from economic, industrial, and agricultural, development and increased human population growth. Pollution of the vulnerable coastal waters inside the Great Barrier Reef is a potential threat to the very productive shallow-water 'sea-grass' ecosystems which support Dugongs. The large number of Dugongs that have been killed as a result of the shark-netting programme at Townsville is an example of the conflict between wildlife and human needs--in this case the recreational needs of a local population and an expanding tourist industry. The vulnerability of Dugong populations to continuous local netting is quite clear, since it is evident that the Magnetic Island Dugong population was almost completely destroyed in a single year. Shark-netting, as done near Townsville, should not be extended to new areas, especially where large numbers of Dugongs are known to occur. Alternative methods of protecting swimmers from possible shark attacks should be devised. There are two methods of shark control in use at present--drum-lines and netting. Drum-lines, which are not a threat to marine mammals, unfortunately are not as effective a means of controlling potentially dangerous shark species as are nets. One important exception is the Tiger Shark.
The area over which a Dugong population is decimated by fixed nets is not known, but would depend on the distances and patterns of movement. The location of nets with respect to possible migratory or movement routes of Dugongs would be very important. After the first year of netting, a high percentage of the Dugongs caught have been young that are independent of their mothers. This suggests that many of the Dugongs have been immigrants to the local Magnetic Island-Townsville population. Therefore it is essential that a Dugong tagging pro. gramme be established, so that movement and possible home ranges can be determined. It is unfortunate that body-lengths and sexes as minimal items of data were not recorded for the first 82 Dugongs removed from the nets (1964-65). These data would have given a good picture of the population structure before the accidental high mortality. It is likely that the age and sex distributions of Dugongs killed from the second year on would be considerably different from that of the first year. Although the importance of the Dugong is generally recognized, and its status as an endangered species is known (Fisher et al., 1969), there has not yet been any extensive research on its ecology and management. In order to manage and conserve Dugongs, it is necessary to determine population characteristics such as natality, recruitment, mortality, longevity, age and sex structures, and movements. More detailed information should also be obtained on reproduction and growth. This paper indicates that Dugongs could be netted for purposes of examination and tagging. Hughes & Oxley-Oxland (1971) showed that concentrations of Dugongs can be effectively located for study from aircraft. Thus extensive research on live populations, although expensive to conduct, would be feasible.
ACKNOWLEDGEMENTS
The author would like to thank the following: the Queensland Department of Harbours and Marine and the shark-netting contractors, Mr G. P. W. Hopper and Mr E. H. Brown, without whose cooperation this project could not have been carried out; Mr W. R. Dowd, Mr M. Downes, and Mr M. J. Devereux, technical staff members of James Cook University, who have assisted with the handling of Dugongs; and Miss J. E. Joiner, who prepared Figs. 2 to 7. Professor A. Starker Leopold, University of California at Berkeley, kindly read the manuscript and made some useful suggestions. The project was partly supported by a University Research Grant from James Cook University.
Heinsohn: Dugongs in Northern Queensland, Australia References BERTRAM,C. K. RICARDO• BERTRAM,G. C. L. (1968a). The Sirenia as aquatic meat-producing herbivores. Syrup. Zool. Soc. Lond., No. 21, pp. 385-91. BERTRAM,G. C. L. & BERTRAM,C. K. RICARDO(1968b). Bionomicsof dugongs and manatees. Nature (London), 218(5140), pp. 423-6. FISHER, JAMES, SIMON, NOEL & VINCENT, JACK (1969).
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The Red Book. Wildlife in Danger. Collins, St James Place, London: 368 pp., illustr. HEINSOHN, GEORGEE. 8~; BIRCH,W. R. (in press). Foods and feeding habits of the Dugong, Dugong dugong (Erxleben), in northern Queensland, Australia. Mammalia. HUGHES, GEORGE R. & OXLEY-OXLAND, R. (1971). A survey of Dugong (Dugong dugong) in and around Antonio Enes, Northern Mo9ambique. Biol. Conserv., 3(4), pp. 299-301, illustr.
Institute of African Research and Studies, Cairo University
Courses were started, in October 1971, for the students in the new Institute of African Research and Studies, Cairo University. This Institute was first established in 1950 as a postgraduate Institute of Sudanese Studies which comprised only two departments, those of Geography and of History. The growing interest in African affairs during the 'sixties caused these two departments to widen their area of activities to include the whole of the African Continent until the Cairo University Board approved, in June 1971, the decree creating a well-balanced and all-embracing Institute of African Research and Studies as part of Cairo University. The new Institute includes six departments: Geography, History, Political and Economic Systems, Languages and Dialects, Anthropology, and Natural Resources. The Department of Natural Resources accepts students who have graduated from science-based Faculties, e.g. Science, Agriculture, Veterinary Medicine, Medicine, and Engineering. Students are registered for three years for the Master's degree. They attend courses during the first two years and then prepare a thesis in the third year. After obtaining the Master's degree, the student can apply to register for the Ph.D. degree, which is based totally on research work lasting, in all, at least three years. The aims of the Institute are to train specialists for working in the various fields related to Africa, to hold conferences and meetings on African affairs, and to create a documentation centre for research and studies dealing with Africa. Professor Mohamed E1-Sayed Ghallab, who has wide experience of various aspects of African geography, was appointed Dean of the Institute. He was previously Professor of Geography in the Faculty of Arts, Cairo University, and is Secretary-General of the Egyptian Geographical Society. He has written several textbooks on human geography, anthropology, and urbanization. The institute has accepted 154 students for the Academic Year 1971-72, among whom 28 are registered
in the Department of Natural Resources. The Institute welcomes students from outside Egypt, especially from other African countries. The courses in the Department of Natural Resources are, in the First Year for two hours weekly each: Geology of the African Continent, Climatology and Meteorology, Hydrology of African Rivers, The Vegetative Cover in the African Continent, Animal Life in the African Continent, Soil Types in the African Continent, Diseases of Warm Lands, and General Geography of Africa. In the Second Year the courses are: Mineral Resources in the African Continent, Schemes for the Control of African Rivers, Land Use in Africa: (a) Main Agricultural Crops, (b) Animal Resources and Pastures, (c) Conservation of Natural Resources, (d) Dry Farming in Northern and Southern Africa, (e) Utilization of Ground-water Basins, Hydrobiology in the African Continent, and Seminars (6 hours). It may be noted that this is a multidisciplinary set of courses of training given as formal postgraduate teaching. The course on Conservation (ll 3(c)) is noteworthy because, whereas the other courses are given in some form or other within the traditional Faculties, the course on Conservation is a completely new concept introduced for the first time in Egyptian Universities. SAMIR I. GHABBOUR, Zoology Department, Faculty of Seience, Cairo University, Giza, Egypt.