Quantitative analysis of production traits in saltwater crocodiles (Crocodylus porosus): IV. number of scale rows

A total of 3156 scale row records, comprising 1739 full-sibling records from 30 families from Janamba Croc Farm (NT, Australia) and 1417 parent-offspring records from 19 families from Wildlife Management International, Pty Ltd (NT, Australia), collected at each facility using a different method, were analysed using ASReml. The full-sibling heritability estimate for the Janamba data was 0.37 (SE 0.03). The animal model estimate of heritability for the Wildlife Management International (WMI) data, also based predominantly on full-sibling data, was 0.42 (SE 0.04). The counts from three counting methods were evaluated by regression analysis on 100 individuals and were found to be highly correlated. Using the regression relationship, the WMI data were transformed and pooled with the Janamba data to give an animal model heritability estimate of 0.42 (SE 0.04). A multitrait analysis revealed negligible correlations (both phenotypical and genetical) between hatchling size traits and the number of scale rows. There is ample genetic variation to incorporate this trait into a genetic improvement programme for farmed saltwater crocodiles.

Estimating the Abundance of Saltwater Crocodiles, Crocodylus-Porosus Schneider, in Tidal Wetlands of the Northern Territory - a Mark-Recapture Experiment to Correct Spotlight Counts to Absolute Numbers, and the Calibration of Helicopter and Spotlight Coun

A mark-recapture experiment was used to estimate the total population of crocodiles in four 10-km sections of the Adelaide River; a downstream and upstream section of the mainstream, and two sections of tidal side creeks. The Petersen Estimate yielded 135 � 5 (SE) crocodiles in the downstream river section, 54 � 6 in the upstream section, and 44 � 2 in the two side creeks combined (approximately 22 crocodiles per 10 km of side creek). For the two mainstream sections and the combined creek sections, respectively, 66%, 59%, and 35% of crocodiles present were sighted in a typical spotlight survey. The probability of sighting a crocodile decreased with increasing density of bank vegetation, with decreasing stream width, and with increasing stream sinuosity. Detection of crocodiles in the downstream river section was also affected by size-related wariness: the probability of detecting hatchlings was large (P = 0.69), and for crocodiles longer than 3.0 m, small (P = 0.15). Diurnal counts of crocodiles from a helicopter were calibrated to spotlight counts from a boat at low tide in a 40-km mainstream section and four tidal side creeks of the Adelaide River system. In river habitats with large exposed mudbanks, aerial observers missed 20-33% of sightable crocodiles in their field of view; and in side creeks lined with dense mangroves, 33-75%. When corrected for this observer bias, helicopter surveys in the mainstream at low spring tide produced similar counts to spotlight surveys. In the mainstream, helicopter counts at low neap tide were 37% lower than spotlight counts at low spring tide, but in side creeks were 45% higher. In comparison with spotlight counts, only 18% and 29% of crocodiles were seen . from a helicopter in two densely vegetated freshwater billabongs. A feasibility survey showed that in remote areas helicopter surveys produced counts of crocodiles similar to those by spotlight surveys from boats, and cost one-quarter as much ($7 km-1 compared with $28 km-1 of habitat surveyed).

Crocodylus johnstoni in the McKinlay Area, N.T. VIII. A Population Simulation Model

A population simulation model was constructed for the McKinlay River population of the freshwater crocodile Crocodylus johnstoni. C, johnstoni are long-lived reptiles (50 y +) which take 9-16 years to reach maturity. As a consequence, the effects of legal hunting before 1963 are still reflected in an unstable population age structure. No quantitative data on the original population densities exist, nor are there data on the densities at which equilibrium can be expected in the future. The model examines the dynamics of a population which is still expanding and does not take into account density-dependent factors which may ultimately limit the population. If the population was undisturbed for 10 years, the model predicted the mean natural rate of population increase would be 1 .5% per annum. However, the population has been disturbed during research activities, and when these disturbances and manipulations were simulated the model predicted a 4% decrease in the population between 1979 and 1983. Independent surveys in both 1979 and 1983 indicated a 5% decrease in the population. This consistency has been interpreted as indicating that the model's predictions are not grossly erroneous. Sensitivity tests were carried out in which most parameters in the model were independently varied by plus or minus their estimated error, while other parameters were held constant. The resultant changes in the estimated population size after 10 years indicated the model was most sensitive to the age-specific mortality estimates. C. johnstoni management has the conservation requirement of maintaining or even enhancing the density of wild populations. Sustained-yield harvesting can theoretically be achieved without compromising this requirement by harvesting eggs and/or hatchlings, and later returning a proportion of the harvest to the population when they are larger and have a greater probability of surviving. When released into the wild, captive-raised C. johnstoni survive as well as wild ones of equivalent sizes. The model was used to simulate egg and hatchling harvests with different collection and return rates, and different ages of returned animals. It was also used to simulate harvests of post-hatchling crocodiles, without a return of captiveraised animals.

Crocodylus johnstoni in the McKinlay River Area N. T, IV.* A Demonstration of Homing

Of 17 wild male and female C, johnstoni which were released at a site 30 km upstream of their capture site, eight were caught again, seven at the site of capture and one between the two sites; they demonstrated a distinct homing ability.

Crocodylus johnstoni in the McKinlay River Area N. T, VI.* Nesting Biology

In the McKinlay River area most female C. johnstoni mature at 74-78 cm snout-vent length (SVL), and 11-14 years of age, whereas maies are about 87 cm SYL and 16-17 years of age. Adult sex ratios in the field are lM:3-4F. There is a well defined, brief nesting period in late August and early September, and the nests are typical 'hole' nests excavated in sand or other friable substrates close to permanent water. Mean clutch size is 13.2 � 3.2 eggs (� SD) and mean egg dimensions are: length 6.64 � 0.26 cm, breadth 4.19 � 0.19 cm and weight 68.2 � 8.0 g. There is a variable but significant increase in egg size with clutch size, and large clutches of large eggs tend to be laid earlier in the nesting season than small clutches of small eggs. From 2 to 4% of eggs are infertile. Nest temperatures show considerable daily variation and baseline levels differ between shaded and exposed nest sites. Incubation times are temperaturedependent and range from 9 to 14 weeks. Females excavate nests at hatching time, but attend the nests little, ifat all, during incubation. Hatchlings are 11.2 � 0.5 cm SVL and weigh 42.0 � 6.1 g. Formulae for predicting egg and hatchling dimensions from each other are presented. A preliminary method for aging C. johnstoni embryos is described. Artificial incubation at 26�, 30� and 34�C, respectively, resulted in 0, 63% and 21% survival; at 26 and 34�C physically deformed embryos were common. Hatching success is correlated with the age at which eggs are transported to incubators. Sex determinantion is influenced by incubation temperature, and at temperatures we tested between 26 and 34�C, females predominated; males were produced at 31-32�C. Histological examination of females from high-temperature incubation (34�C) indicated slight hermaphroditic tendencies in two of 35 animals examined; their status remains to be clarified. The temperature-sensitive period for sex determination appears to be between 20 and 57 days of age (30�C equivalent ages), but this may well vary with incubation temperature, as in Alligator mississippiensis. Egg losses due to predators (particular varanid lizards) were estimated as 64%, although they may have been increased by our interference with nests. Eggs are also lost to flooding and overheating, and a 60-70% annual egg mortality may be common. In one experiment, hatchling mortality was estimated at 98% within the first year. An egg and/or hatchling harvest balanced by a proportional return to the wild of raised 1-year-olds (5% and 10% of eggs and hatchlings collected respectively) is a potential strategy through which sustained-yield harvesting could be introduced into a conservation-management program.

Crocodylus johnstoni in the McKinlay River Area N. T, III.* Growth, Movement and the Population Age Structure

Regression constants for predicting C. johnstoni body measurements from each other are presented. In the McKinlay River area, C. johnstoni grow primarily in the wet season, dry-season growth appearing negligible. Males grow faster and attain larger sizes than females, and those downstream in the study area grow faster than those upstream. Neither sex nor location (upstream v. downstream) accounts for a significant proportion of the variation in growth of C. johnstoni of <210 mm HL (head length). A single exponential curve is a poor fit to the size-age relationship, which is more adequately modelled by two exponentials. Separate mean age-size curves for C. johnstoni <200 and >200 mm HL are derived, and methods for predicting (and correcting) the age of individuals are tested. Accuracy of prediction is increased by using C. johnstoni recaught at least once (n = 240), and the age structure of this segment of the population is derived. The size range of animals recaught is not significantly different from that of those originally marked, so this age structure is considered a good estimate of the total population age structure. Between 3 and 4% of the number of eggs laid annually are represented as 2-year-olds, indicating a high mortality before this age. In general, mortality appears greater downstream than upstream, and may be independent of crocodile density; in the few years before and after the introduction of protection (1964) recruitment downstream was still minimal, even though numbers of crocodiles had been much reduced. Females mature when between 9 and 14 years of age, with a mean age of about 12 years (240-254 mm HL). For the small sample of males examined, half of those 15-19 years of age (n = 4) and all of those 20 years or older (n = 7) were mature. A 13-year-old male had two sperm cells in its smear, and maturity may be attained then, although 16-17 appears a more usual age (17 y; 287-294 mm HL). Of all C. johnstoni recaught after 1 year, 83.4% were within 1 km of the pool in which they had been marked and released. Of those which had moved, more were upstream than downstream of their capture site, and there was no difference between the proportions of males and females moving. Younger animals moved more than older ones, and those from upstream in the study area moved more than those from downstream.

Crocodylus johnstoni and C. porosus Coexisting in a Tidal River

A 52.5-km section of the Adelaide River, N.T. (12�13'S., 131�13'E.). was spotlight-surveyed 20 times between June 1979 and September 1981. C, johnstoni (15.3 � 9.2 sighted per survey) were less abundant than C. porosus (137.6 � 36.5 sighted per survey), and were mainly in the upstream 20 km of the survey route (96% of C. johnstoni sightings); here considered a zone of syntopy within the survey route. C. johnstoni congregate in the main stream during the dry season and disperse from it during the wet season, which parallels similar seasonal movements to and from dry-season refuges in non-tidal areas lacking C. porosus. As the dry season progresses, C. johnstoni are located further and further upstream, and this movement (or loss ofanimals) appears unrelated to changes in salinity. Numbers of C.johnstoni within the zone of syntopy are negatively correlated with numbers of C. porosus (r*2 = 0.50, P=0.005). and competitive exclusion may be occurring. Independent of seasonal factors, numbers of C. johnstoni within the zone of syntopy declined with consecutive month (1979-81: r*2=0.47, P= O.004), whereas numbers ofthe more recently protected C, porosus increased (r2 = 0.48, P= 0,006). The location of the syntopic zone was unchanged.

An Examination of Crocodylus porosus Nests in Two Northern Australian Freshwater Swamps, with an Analysis of Embryo Mortality

Crocodylus porosus nesting was examined in a perennial freshwater swamp (Melacca Swamp) adjacent to a tidal river, and in perennial floodplain river channels where floating mats of vegetation overlie fresh water (Finniss R. and Reynolds R.). Time of nesting was quantified. Vegetation at nest sites was identified and ranked according to importance indices, which are used as a descriptive tool. Most nests are exposed during mid-morning and afternoon, but variously shaded during early morning and late afternoon. Nests in Melacca Swamp are more shaded, and have lower nest temperatures, than those in the Finniss-Reynolds region. Nest visibility from the air is influenced by species-specific plant regrowth on nests. In Melacca Swamp the height at which clutches are deposited is related to swamp water level. Mean clutch depth, the distance between top and bottom eggs, was 22.6 � 8.3 cm (SD); the distance between the top egg and water level at the time of laying was 34.7 � 8.0 cm. Mean clutch size was 53.1 � 9.4 eggs; formulae relating egg and hatchling dimensions are presented. A highly variable but significant increase in egg size accompanied increased clutch size, but no relationship was found between clutch or egg size and time of nesting. A high correlation between egg widths of four pairs of clutches laid at the same sites strongly suggested multiple nesting by some females. Nest temperatures, embryo development rates and total incubation times were highly variable, both within and between nests. Of the 2712 eggs examined, 31.6% (856) produced live, apparently normal hatchlings. Flooding was the major cause of mortality, accounting for 36.3% of Melacca eggs and 40.6% of Finniss-Reynolds eggs. Other causes of egg failure included: infertility (M 9.4%, F-R 5.0%), high-temperature incubation (2.0%, 0.4%), low-temperature incubation (6.3%, 3.2%), mechanical damage to eggs by adult crocodiles (O.6%, 2.4%) and undetermined development failures (9.8%, 18.0%). No instance of dehydration of eggs within a nest, or predation on eggs, was recorded. A model for simulating Melacca nest flooding predicted a 33.2% loss of eggs in 1980-81 (estimated real loss was 36.3%), and indicated possible losses of O-54.9% between 1960 and 1980 if the same numbers of nests had been made; mean loss was 26.3%. Three double-yolked eggs (0.1% of eggs examined) were recorded. Developmental anomalies and possible causes (incubation temperature-genetic), have been tabulated. In attempting to analyse the siting of C, porosus nests and explain variation in nesting vegetation, nest site locations, nest mounds and embryo mortality rates, insights were gained by examining nest site selection from the point of view of the female's well-being rather than that of the nest. Resulting criteria considered important in nest site selection are listed and discussed. If the numbers of nests in freshwater swamps are an index of the total population size in such areas, there are clearly many more C. porosus in such swamps than have hitherto been estimated. Nest surveying may be the only practical method of estimating the total population. Regardless of high mortality rates, an egg-collection strategy may not prove a practical method of incorporating sustained-yield harvesting into an overall C. porosus management program.

An Interim Method for Estimating the Age of Crocodylus porosus

On the basis of a reference series incubated at 30�C, methods for assigning unknown-aged embryos an approximate '30�C age' are derived. Determining real age is confounded by a paucity of information on the nest environment and its influence on embryonic development rate. Preliminary constants relating development rate to temperature (derived from studies with C. johnstoni) were tested with C. porosus and, as an interim measure, have practical utility in both the laboratory and the field.

Crocodylus johnstoni in the McKinlay River Area N. T, V.* Abnormalities and Injuries

Abnormalities and injuries in a sample of 797 C. johnstoni are described. 56.3% of animals were scarred or freshly injured, and 81.1% of injured animals had the tail injured. Rake and bite marks were the most common injury, and these apparently result from intraspecific interactions, perhaps including a specific 'tail-biting' behaviour. Interspecific comparisons indicate the frequencies of injuries are greatest in species which congregate in high densities, namely C. johnstoni and Caiman c. crocodilus. Nematode worm trails were rarely encountered on C. johnstoni under 31 cm snout-vent length (SVL) but were on 100% of animals over 80 cm SVL. Leeches were on 59% of animals caught in one area during the wet season, but only 2.6% of those caught further upstream in the dry season. The SVLs at which mandibular teeth protrude through the premaxilla are quantified and the significance of tooth protrusion discussed.

Crocodylus johnstoni in a Controlled-Environment Chamber: a Raising Trial

Crocodylus johnstoni hatchlings were raised in a constant-environment chamber of the type used for Alligator mississippiensis by Joanen and McNease. Minced marine fish with vitamin and mineral supplements appeared an adequate diet for growth, but growth rates were highly variable. Density was an important determinant of mortality and food conversion rates. No significant variation in growth rate was attributable to sex, but mortality in low-density pens was slightly higher for males than for females; samples were too small to determine whether or not this was significant. Daily feeding resulted in higher conversion rates and a greater intake of food per pen than feeding 5 days per week, but within the period tested significant differences in linear growth were not apparent. This could reflect excessive fat deposition, which is known to result in chronic obesity. No significant difference in growth was attributable to chopped or minced food. Food intake and conversion rates dropped during the dry season, when feeding and growth in the field are also retarded. The application of constant-environment chambers for testing variables likely to influence raising success is discussed; however, C. johnstoni may not be as suited to this style of raising as are some other crocodilians. At all densities tested fighting was a problem, and even minor disturbances were associated with reduced food intake and defensive behaviour.

Crocodylus johnstoni in the McKinlay River Area N. T, II.* Dry-Season Habitat Slection and an Estimate of the Total Population Size

In July 1979, midway through the dry season, most C, johnstoni were congregated in pools greater than 1m deep and 900 m2 in area. Use of some, but not all of these large, deep pools is not dependent on whether the pools are in the mainstream or not, nor on the clarity (an indicator of siltation) of the water in them. Food availability and access to nesting banks may be related to selection of individual pools, although the former is considered unlikely. The homing ability of C. johnstoni would seem a decided advantage for relocating specific refuge sites, which tend to be used each year by the same individuals. The C. johnstoni population in the McKinlay River study area is estimated at 963 individuals (826- 1156).

Crocodylus johnstoni in the McKinlay River Area, N.T. I. Variation in the Diet, and a New Method of Assessing the Relative Importance of Prey

The stomach contents of 153 C. johnstoni were examined by a modification of the method for stomach contents removal described by Taylor et al. (1978). Prey are analysed on the basis of taxonomy, although more emphasis is placed on prey equivalents, using a concept termed 'target size'. The relative importance of different taxa and prey equivalents is determined by a number of methods, and a ranking method is preferred. The most important prey are aquatic and terrestrial insects, fish and crustaceans. The most important sized organisms are target size 5, animals presenting a maximum area of 1.0-4.0 cm2. With regard to size of prey eaten, three size groups of C. johnstoni (16-25, 26-55 and 56-129 cm snout-vent length) were homogeneous within themselves but were significantly different from each other. With increased body size there was a significant increase in the proportion of aquatic prey eaten. Secondary ingestion did not appear a major bias. C. johnstoni ate appreciably more during the wet season than during the dry season, although seasonal comparisons were restricted due to the samples not coming from the same pools. Vegetation was found in 39.9% of crocodile stomachs, and its presence varied with season but not with crocodile size. Stomach parasites were present in 43.8% of animals, and the number of infected crocodiles varied with season and site. Stones were present in 88.2% of crocodiles; however, when compared with those of C. niloticus the stone loads were relatively small. Most data indicate that C. johnstoni is very much an opportunistic predator at the water's edge, which feeds primarily on small aquatic prey, although it may also take substantial numbers of terrestrial prey organisms. During the wet season there is a major shift in the importance of different prey taxa eaten, although the importance of prey equivalents remains largely unchanged.

Wariness in Crocodylus porosus (Reptilia : Crocodilidae)

Of 1364 C. porosus sighted at night, 954 (69.9%) could be approached by a boat and spotlight to within 5 ft, and 1191 (87.3%) to within 20 ft. The percentage which could be approached to within 5 ft decreased with increasing body size and varied between rivers. Such differences appear to reflect mainly learned wariness, though some data suggest that a natural increase in wariness accompanies the transition from hatchlings to 1-year-olds. The low approach distances make the species vulnerable to over-harvesting.

Movement and Dispersal Patterns of Crocodylus porosus in some Rivers of Arnhem Land, Northern Australia

The dispersal of juvenile C. povosus hatched in the Liverpool River is described. Of those initially caught when hatchlings, 1-y-olds and 2–4-y-olds, and recaught 1 year later, 93 %, 73% and 57% respectively were within 10 km of the original capture site. Movements of up to 21 km in one direction in 1 year could be followed by a similar movement in the opposite direction the following year. Relocated C. povosus demonstrated a homing instinct. A possible explanation for the long distances travelled by some juveniles (up to 81 km in a year), is that chance wandering placed them in the mouth of another stream, and attempts to 'home' in the new stream took them further away from their original capture site. The distribution of C. povosus in other rivers can be explained by dispersal patterns similar to those found in the Liverpool River. Some rivers seem to have supported successful C. povosus breeding for a number of years, others seem to have been sporadically successful, and still others unsuccessful. Movement of juveniles from 'successful' to 'unsuccessful' rivers could account for older juveniles found in the latter type of river.

Growth Rates of Crocodylus Porosus (Reptilia: Crocodilia) From Arnhem Land, Northern Australia

In C. porosus < 80 cm SVL (snout-vent length), growth rate decreases as body size increases; males grow faster than females, and both sexes grow faster in the wet season than in the dry season. If the exponential growth curve for specimens < 80cm SVL is extrapolated, it underestimates the maximum size of C. porosus. A second growth curve for specimens > 80cm SVL has been derived separately, and the two curves intersect in the vicinity of 70 cm SVL. The size at which C. porosus cease growing seems quite variable and some data indicate a range of 3.9-6.0 m total length for males. The twocurve growth model is thought to reflect an energetic advantage in feeding on large rather than small prey.

Abnormalities and Injuries in the Estuarine Crocodile Crocodylus Porosus

Abnormalities and injuries in C, porosus are described, and their frequency in different size classes determined. There is no significant increase in injury frequency in size classes up to 70cm snoutvent length (SVL), which suggests that juvenile injuries are obtained soon after hatching. In size classes above 70cm SVL, injury frequency is high; 80% of individuals between 151 and 210cm SVL had scars.

Two New Locality Records, a New Habitat and a Nest Description for Xeromys myoides Thomas (Rodentia : Muridae)

One X. myoides was captured on the bank of the Tomkinson River, Arnhem Land, and three on the banks of Andranangoo Creek, Melville I. The latter specimens, an adult and two juveniles, were in a mud mound containing a nest. The remains of a fifth specimen were recovered from the stomach of a Crocodylus porosus caught in the Tomkinson River. All specimens were associated with mangrove forests. Behavioural observations are presented and discussed. X. myoides could be more widespread in the mangroves which border northern rivers.