Habitat utilization of Cape clawless otters Aonyx capensis

Somers, Michael John (2001-12)

Thesis (PhD)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: This study tested several hypotheses regarding various aspects of habitat use by Cape clawless otters Aonyx capensis, in various habitats. The hypotheses, namely that A. capensis do not select any prey types, prey sizes or habitat type were tested. Habitat type was investigated at a scale that enabled separating the effects of types of riparian vegetation, geomorphology and anthropogenic influences. Aspects of the resource dispersion hypothesis (RDH) and optimal foraging theory were tested. This was done in both the marine and freshwater environments. The annual and seasonal diets of A. capensis in the Olifants and Eerste Rivers, Western Cape Province, are described. Crabs were found to be the main prey and fish the second most important prey of A. capensis in both the rivers during all seasons. The percentage occurrence of prey of A. capensis in both rivers, showed an increase in the amount of crab in summer and a corresponding decrease in the number of fish eaten. The seasonal fluctuation in crabs and fish found in the spraints (faeces), corresponded with the expected frequencies as determined from trapping. Seven A. capensis were caught in the two rivers and radiotracked between 1993 and 1995. Total home range length varied from 4.9 to 54.1 km and core home range length from 0.2 to 9.8 km. Total area of water used ranged from 4.9 to 1062.5 ha and core area used from 1.1 to 138.9 ha. As predicted using the RDH total home range length was correlated to mean reed bed nearest neighbour distance. The otters were typically active from dawn for 2.6 h and moved a mean of 0.9 km. They were also typically active from sunset for a mean of 2.3 h moving a mean of 1.3 km. They were found to select for areas with boulders and/or reed beds. Aonyx capensis were found to be mainly solitary in all the habitats. The pattern of female home ranges was suggestive of territoriality. The males, however, had overlapping home ranges, both with other males and females. From spraint analysis and direct observations, the seasonal diet and foraging behaviour of A. capensis feeding in the surf zone in False Bay, Western Cape Province, were determined. The most common prey species during all seasons was Cape rock crab Plagusia chabrus. Diversity of prey was lowest in winter, increasing through the seasons from spring to autumn. Foraging behaviour data support the optimal breathing hypothesis, which predicts that both surface and dive times should increase for dives of greater depths. However, diving efficiency did not decrease with increasing depth, nor did percentage time at the surface increase with increasing depth. These results are contrary to the optimal breathing hypothesis. How A. capensis use their habitat in the presence of potential competitors spotted-necked otters Lutra maculicollis and water mongooses Atilax paludinosus was also determined by investigating their trophic overlap during enforced cohabitation in a relatively small range during drought conditions. The analysis of trophic overlap showed that even during enforced cohabitation in a relatively small range during drought conditions, there was separation of diets of A. capensis, L. maculicollis and A. paludinosus. These results and their implications for conservation management are discussed.

AFRIKAANSE OPSOMMING: Hierdie studie het 'n aantal hipoteses oor verskeie aspekte van habitat gebruik deur die Groototter Aonyx capensis, in verskillende habitatte, getoets. Die nul hipotese, nl. dat A. capensis nie vir enige prooitipe, prooigrootte of habitat-tipe selekteer nie, is getoets. Habitat tipe is ondersoek teen 'n skaalgrootte wat dit moontlik gemaak het om die effekte van tipes oewer plantegroei, geomorfologie en antropogeniese invloede van mekaar te skei. Aspekte van die hulpbron verspreiding hipotese (HVH) en optimale kossoek gedrag, is in beide die varswater en seewater omgewing getoets. Die jaarlikse en seiseonale diëte van A. capens~s in die Olifants and Eerste Riviere in die Wes-Kaap Provinsie, word beskryf. Daar is gevind dat krappe die hoof prooi en vis die tweede mees belangrike prooi van A. capensis in beide riviere, gedurende al die seisoene, was. Die persentasie A. capensis prooi teenwoordig in beide riviere het 'n toename in krap getalle in die somer en 'n ooreenstemmende afname in die hoeveelheid vis wat geeet word getoon. Die seisoenale verandering van krap en vis wat in die keutels (mis) voorkom, het ooreengestem met die verwagte frekwensies soos bepaal deur fangste in fuike. Sewe A. capensis is in die twee riviere gevang en tussen 1993 en 1995 d.m.v. radiosenders gevolg. Die totale tuisgebied lengte het van 4.9 tot 54.1 km, en die kern tuisgebied lengte van 0.2 tot 9.8 km, gewissel. Die totale area water wat gebruik is het van 4.9 tot 1062.5 ha, en kerngebiede van 1.1 tot 138.9 ha gewissel. Soos voorspel deur gebruik van die HVH, was die totale tuisgebied lengte gekorreleer met die gemiddelde rietbedding naaste-buurman analise afstand. Die otters was tipies aktief vanaf sonsopkoms vir 2.6 ure en het dan 'n gemiddeld van 0.9 km beweeg. Hulle was ook tipies aktief vanaf sonsondergang vir 'n gemiddeld van 2.3 ure en het dan gemiddeld 1.3 km beweeg. Daar is gevind dat hulle selekteer vir areas met rotse en areas met rietbeddings. Dit is ook gevind dat A. capens~s in alle habitatte hoofsaaklik alleenlopend is. Die wyfies se tuisgebied-patroon het op territorialiteit gedui, terwyl die manlike diere se tuisgebiede oorvleuel het met beide dié van wyfies, en met dié van ander mannetjies. Die seisoenale dieet en kossoekgedrag van A. capensis wat in die brander sone van Valsbaai, Wes-Kaap Provinsie, voed, is deur keutel analise en direkte waarneming bepaal. Die mees algemene prooi spesie gedurende al die seisoene was die Kaapse rotskrap Plagusia chabrus. Prooi diversiteit was die laagste in winter en het toegeneem vanaf lente tot herfs. Kossoekgedrag gegewens ondersteun die optimale asemhalings hipotese, wat voorspel dat beide oppervlak- en duik tye toeneem met dieper duike. Duik effektiwiteit het egter nie afgeneem met toenemende diepte nie, en persentasie tyd op die oppervlakte het ook nie toegeneem met toenemende diepte nie. Dit is dus in teenstelling met die optimale asemhalings hipotese. Hoe A. capensis hul habitat in die teenwoordigheid van potesieele kompiteerders nl. Klein otters Lutra maculicollis, en Water muishonde Atilax paludinosus gebruik, is ook bepaal. Dit is gedoen deur hul trofiese oorvleueling gedurende noodgedwonge saamleef, veroorsaak deur droogte kondisies in 'n relatiewe klein tuisgebied, te ondersoek. Die resultate van die trofiese oorvleueling analise het getoon dat selfs gedurende noodgedwonge saamleef gedurende droogte toestande in 'n relatiewe klein gebied, daar 'n verskil was in die diëte van A. capensis, L. maculicollis and A. paludinosus. Hierdie resultate en hul implikasies vir bewaringsbestuur word bespreek.

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