Effects of stocking density on production and behaviour of farmed grower Nile crocodiles ( Crocodylus niloticus )

Abstract

The exploitation of crocodilians in the 1950s-1970s resulted in their being listed under the CITES protection appendices. Farming of crocodilians began in South Africa in the 1970s, and has since become an international enterprise, whilst simultaneously playing a role in alleviating the pressure of harvesting from wild populations. A lack of housing standardization for intensive Nile crocodile farming has resulted in commercial farms operating with considerable differences in housing and density. Although this study will not answer the question of a preferred stocking density for every farm trading with the Nile crocodile, the hope is to establish a foundation for standardization in the future. This study assessed current commercial stocking densities for crocodilians, applied to grower-phase Nile crocodiles at a commercial crocodile farm in Gauteng, South Africa. The pens utilized in the trial were similar in size, shape and design; density was assessed by altering the number of crocodiles assigned to each pen. Three densities (0.43m2 per crocodile, 1.24m2 per crocodile and 2.60m2 per crocodile) were tested in this completely randomised control study, and multiple physiological and behavioural measures were recorded to assess the production and biological responses of crocodiles at these densities. Growth (in the form of morphometric measures), skin quality, stress and behaviours were all monitored over a six-month growth period for 261 Nile crocodiles housed at the above-mentioned densities in a commercial crocodile production system. The present study found that stocking density significantly affected the growth of crocodiles in the trial. The results suggested lower weight gains and condition scores for crocodiles in the lowest density pens compared to those in medium and high-density pens. Skin quality was analysed for every crocodile at the beginning and end of the trial. No during-trial skin analyses were performed for fear of causing the animals unnecessary stress which could yield more antagonistic behaviours, potentially altering skin qualities. The findings indicated no significant differences in pre- and post-trial skin qualities. Possible explanations include adaption to the pens and pen mates, or antagonistic interactions did not yield skin damage to the extent expected. Faecal samples were collected from all pens daily, and a random set of samples selected for analysis. Faecal stress hormone concentrations did not show any significant (P<0.05) differences between the different density groups and appeared to be correlated more to changes in climate over the six-month period. Crocodile behaviour in captivity is not well understood (as in their wild counterparts) but could have many effects and interactions resulting in stress reactions with associated skin damage and reduced growth of some crocodiles in the pens. Time-lapse data were collected for all pens and a random selection of samples used for analysis. The percentages of crocodiles on land or in the water, whether separate from their pen mates or involved in contact and piling behaviours, were studied. Density had significant effects on almost all behaviours observed. This was expected as crocodiles stocked at higher densities had less space to express certain activities, and so behaviours such as piling were seen more regularly. The findings of this study suggest that the densities assessed did not have the expected effects on all aspects of the young crocodiles’ production. There were no significant differences between the examined skin qualities or stress levels of the crocodiles stocked at varying densities in this trial. However, the growth and behavioural recordings differed significantly between the varying densities tested.

Die benutting van krokodille sedert 1950s-1970 het veroorsaak dat hulle bygeroeg is onder ‘n bylae in die CITES-beskermingswet. Boerdery met krokodille in Suid-Afrika het begin in die 1970s en het sedertien ‘n internasionale onderneming geword, veral omdat krokodil boedery in oplossing bied vir die oes van natuurlike populasies. ‘n Gebrek aan standardisering van die behuising van intensiewe Nyl krokodil boerdery toon groot verskille in behuising en digtheid op kommersiële plase. Alhoewel hierdie studie nie die vraagstuk oor die huidige behuisingsdigtheid vir elke plaas wat met Nyl krokodille boer kan beantwoord nie, word daar gehoop om riglyne vir standardisering te maak vir die toekoms. Hierdie studie se doel was om die huidige kommersiële behuisings raaddigtheid vir krokodille te evalueer en spesifiek vir produsente van Nyl krokodille by ‘n kommersiële plaas in Gauteng, Suid-Afrika. Die hokke wat gebruik is in die ondersoek, was soortgelyk in grootte, vorm en ontwerp; digtheid is ge-evalueer deur die getal krokodille toegeken per hok, te verander. Drie digthede is getoets in hierdie heeltemaal ewekansige beheerstudie en veelvuldige fisiologiese- en gedragsmaatreëls was aangeteken om die produksie en biologiese reaksies van die krokodille te evalueer by hierdie drie digthede (0.43m2 per krokodil, 1.24m2 per krokodil en 2.60m2 per krokodil). Oor ‘n ses-maande groeiperiode, is groei (in die vorm van verskeie morfometriese maatreëls), velkwaliteit, stres en gedrag van tweehonderd-een-en-sestig Nyl krokodille, gehuisves in die voorafbepaalde digthede, in ‘n kommersiële krokodil produksie sisteem, gemonitor. Dit is gevind in die ondersoek, dat behuisings raaddigtheid ‘n beduidende invloed op die groei van krokodille gehad het. Die resultate dui op ‘n kleiner gewigstoename en swakker kondisie vir krokodille in die laagste digtheid hokke in vergelyking met dié in die medium- en hoë-digtheid hokke. Die velkwaliteit van elke krokodil is ontleed aan die begin en aan die einde van die ondersoek. Gedurende die ondersoek is geen velontleding gedoen nie, omdat gevrees is dat dit die diere onnodig sal stres en antagonistiese gedrag sal ontlok, wat weer moontlike veranderde velkwaliteite kon veroorsaak. Die bevindinge het geen beduidende velkwaliteitsverskille tussen voor en ná die ondersoek uitgewys nie. Moontlike verklarings sluit in aanpassing in die hok en aanpassings met mede-krokodille in die hok, of antagonistiese interaksies het nie soveel skade aan die vel veroorsaak as wat verwag is nie. Fekale monsters is daagliks versamel van alle hokke en die analise in die studie is gedoen op ‘n ewekansige wyse met ‘n verteenwoordigende porsie van die monsters. Fekale stress hormoon konsentrasies, het ook geen beduidende (P<0.05) verskille tussen die verskillende digtheidsgroepe gewys nie en het eerder gekorreleer met die veranderinge in die klimaat oor die 6 maande periode. Die gedrag van krokodille in aanhouding word nie goed verstaan nie (net soos in die geval van hulle natuurlike eweknieë), maar kan baie potensiële nagevolge hê en interaksies wat stresreaksies tot gevolg het met geassosieerde velskade en verminderde groei van sommige krokodille in die hok. Met verloop van tyd is data versamel van alle hokke en met ewekansigheid is ‘n hanteerbare deel van die data gebruik vir analise in die studie. Die persentasie van krokodille op land of in die water, of hulle apart was of in kontak met ander krokodille en of hulle opgestapel was, is bestudeer. Die bevindinge is, dat digtheid ‘n beduidende invloed gehad het op omtrent al die gedrag wat bestudeer is. Dit is te verstane, want krokodille wat aangehou word in hoër digthede, sou minder spasie hê om sekere aktiwiteite te doen en het meer opstapeling gedrag getoon. Die bevindinge van hierdie studie toon, dat die getoetste digthede nie die verwagte effek gehad het op al die aspekte van die jong krokodille se produksie nie. Daar was geen noemenswaardige verskil tussen die vel kwaliteite wat ondersoek is, of die stresvlakke van die krokodille wat aangehou is, teen verskillende digthede nie. Daar was wel ‘n groot verskil opgemerk in die groei en gedrag van die verskillende digthede van krokodille wat getoets is.