MARINE MAMMAL SCIENCE, 15(4):1115-1123 (October 1999) 0 1999 by the Society for Marine Mammalogy

POPULATION ANALYSES OF INDO-PACIFIC HUMPBACK DOLPHINS SOUSA CHINENSIS IN ALGOA BAY, EASTERN CAPE, SOUTH AFRICA'

LESZEK KARCZMARSKI~ Centre for Dolphin Studies,

Port Elizabeth Museum, P. 0. Box 13147, Humewood 6013, South Africa

and Department of Zoology,

University of Port Elizabeth, P. 0. Box 1600, Port Elizabeth 6000, South Africa

E-mail: karczmal@tamug.tamu.edu

PAUL E. D. WINTER Department of Zoology,

University of Port Elizabeth, P. 0. Box 1600, Port Elizabeth 6000, South Africa

VICTOR G. cOCKCROFT3

Centre for Dolphin Studies, Port Elizabeth Museum,

P. 0. Box 13147, Humewood 6013, South Africa

ANTON MCLACHLAN4

Department of Zoology, University of Port Elizabeth,

P. 0. Box 1600, Port Elizabeth 6000, South Africa

This work is dedicated to the memory of Kenneth Norris, whose lifelong desire to under- stand the natural world and that of cetaceans in particular, inspired a continuing scientific curiosity and a sense of wonder in many, myself included.-kzek Karczmurski

Ken, your recognition, enthusiasm, and encouragement gave enormous impetus to work in Africa. I hope that we can foster and nurture the next generation of interest the way you did ours.-Vic Cockcro#

Present address: Marine Mammal Research Program, Texas A&M University at Galveston, 4700 Ave. U, Bldg. 303, Galveston, TX 77551, U.S.A. and Oceanic Society, Midway Spinner Dolphin Research Project, Midway Island Station # 2, P. 0. Box 29460, Honolulu, HI 96820- 1860, U.S.A.

Present address: Centre for Dolphin Studies, P. 0. Box 1856, Plettenberg Bay 6600, South Africa.

* Present address: College of Science, SQU, P. 0. Box 36, SQU 123, Oman.

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1116 MARINE MAMMAL SCIENCE, VOL. 15, NO. 4, 1999

ABSTRACT

Mark-recapture analyses were performed on photo-identification data for Indo-Pacific humpback dolphins (Soma chinensir) inhabiting Algoa Bay, on the Eastern Cape coast of South Africa. Boat-based photo-ID surveys were undertaken between May 1991 and May 1994. The rate of discovery of newly identified dolphins, distribution of sightings and frequency of resightings of known individuals indicate a high level of seasonal immigration of humpback dolphins into, and emigration from, the Algoa Bay region in summer. Con- sequently, humpback dolphins from Algoa Bay appear to be part of a sub- stantially larger population that uses a considerable length of the coastal zone. The minimum population site is estimated to be about 466 dolphins.

Key words: Indo-Pacific humpback dolphin, Soma chinemis, mark-recapture analyses, seasonal immigration/emigration, population estimate, Algoa Bay- Eastern Cape-South Africa.

Indo-Pacific humpback dolphins (Soma chinensis) inhabit coastal waters of Indian and western Pacific oceans (Ross et al. 1994) and are known to occur along the southeast coast of southern Africa (Ross 1984). There have been very few studies of humpback dolphins and, in general, the species remains poorly known. In South Africa, where humpback dolphins fall victim to shark nets (gillnets set to deplete the number of large sharks in an attempt to protect the main bathing beaches along the KwaZulu-Natal coast), there is concern regarding their continued survival. Cockcroft (1990) suggested that mortality from net entanglement and other sources may be close to, or exceed, the likely replacement rate of this species in the KwaZulu-Natal region.

In 1991, a field study in the Algoa Bay region on the southern Eastern Cape coast was initiated in order to provide base-line data on an unexploited humpback dolphin population (Karczmarski 1996), for comparison with data collected in areas heavily impacted by human activities. This study made extensive use of the individual photo-identification technique, an approach which has been used in field studies of a large number of other cetaceans and, in conjunction with mark-recapture, proven to be an effective tool in the estimation of a variety of population parameters (Hammond et al. 1990).

In Algoa Bay humpback dolphins inhabit a narrow strip of coastal zone and remain mostly in water <15 m deep (Karczmarski et al. 2000). Their groups are seldom larger than 13 animals, with no obvious diurnal, tidal, or lunar variability in size. There is, however, a seasonal variation in occurrencelabun- dance in Algoa Bay, with two annual peaks (in summer and, to a lesser degree, late winter) probably related to the apparent seasonal changes in the abundance and distribution of inshore prey (Karczmarski et al., in press). The dolphins display varying, but generally weak, degrees of site fidelity to Algoa Bay. Their social structure is highly fluid, with only casual and short-lasting affiliations. Lack of consistency in the group membership appears to be the general pattern (Karczmarski, in press).

The present paper reports part of the results of a three-year study (Karcz- marski 1996), the base-line mark-recapture analyses for humpback dolphins

KARCZMARSKI ETAL.: HUMPBACK DOLPHINS 1117

Figure 1. Algoa Bay study area on Eastern Cape coast of South Africa.

in the Algoa Bay region. This work contributes to our overall knowledge of this little-studied species and is important in determining long-term man- agement options.

METHODS

Study Area and Survey Procedure

Algoa Bay, flanked by Cape Recife (34"02'S, 25'42'E) on the western side and Cape Padrone (33"46'S, 26'28'E) on the eastern side, is located along a generally exposed coastline of the Eastern Cape Province of South Africa. Over most of the bay the depth is (50 m (Fig. 1). Both land- and sea-based surveys were undertaken over approximately 55 km of coastline of the southwestern part of Algoa Bay (Fig. 1) between May 1991 and May 1994. Land-based

1118 MARINE MAMMAL SCIENCE, VOL. 15, NO. 4, 1999

surveys were done daily, weather permitting, and observations of inshore wa- ters, to approximately 1 km offshore, were carried out from several vantage points. In many cases the individual sites were visually overlapping.

Sea-based surveys were opportunistic and limited by both the presence of dolphins and weather conditions. Once a group of dolphins was sighted from land, a photo-identification boat survey was launched. Sea work was conducted using a 3.5-m inflatable boat powered by a 30-HP outboard engine. The boat initially progressed approximately 1 km past the dolphin group, to ensure that all members of the group were located. Dolphins were photographed using a motordrive camera equipped with a variable focal length (70-2 10 mm) lens and 100 ASA color slide film. Individuals were subsequently identified, fol- lowing the procedure described by Karczmarski and Cockcroft (1998). Esti- mates of group size and composition were repeated several times during a survey and subsequently confirmed with the photographic material, following the procedure described by Ballance (1990). As the sea conditions in the area usually deteriorated during the course of the day, the extent of coastline cov- ered and time spent at sea varied considerably between surveys and ranged between 12 and 35 km and 1.5 and 7.0 h, respectively. Sea-based surveys were never performed more than once a day. Several additional boat surveys, which followed the same procedure, were performed in the southwestern part of St. Francis Bay, along about 20 km of coastline (Fig. 1).

Definitions and Analysej Three age classes were distinguished: calf, juvenile, and adult. Calves are

defined as animals % or less the length of an adult, regularly accompanying a larger animal presumed to be the mother. Dolphins of approximately 2 m in length and visibly less robust than adults, which often swam independently, were classified as juveniles. Adult humpback dolphins are robust in form, approximately 2.5 m in length and have a well-pronounced dorsal hump.

The term “population” is used here to describe the dolphins recorded in the Algoa Bay study area. It does not refer to a condition of reproductive isolation, unless specifically stated.

Two seasons, summer and winter, are distinguished here. “Summer” (late October through early May) is defined as the period when the mean temper- ature of the inshore surface water is higher than the annual mean (18°C). The period when the surface water temperature drops below this annual mean is referred to as “winter” (early May through late October). This corresponds with the general climatological pattern of the region (for details see Karcz- marski 1996).

An estimate of size of the humpback dolphin population was obtained from mark-recapture analyses, using the program Popan-4, version of 30 November 1995 (Arnason et a/. 1995).

RESULTS Humpback dolphins were seen 113 times; the majority (104 sightings) were

in Algoa Bay. The sighting rate (individuals per hour) in Algoa Bay (3.78)

KARCZMARSKI ET AL.: HUMPBACK DOLPHINS 1119

Sighting frequency

Figwe ghting frequencies of individual humpback dolphins (0 = a,. indi- viduals, = adults only) identified photographically during boat surveys in Algoa Bay between May 1991 and May 1994.

was similar to that recorded in St. Francis Bay (3.33). Over 3,000 photographs were taken during 60 boat surveys (58 surveys in Algoa Bay and two in St. Francis Bay). A total of 70 individuals were identified and cataloged. All but one dolphin identified in St. Francis Bay were also seen in Algoa Bay.

The majority of identified dolphins (81.5%) were adults (n = 57). Generally, all adult humpback dolphins seen and photographed were individually iden- tified. In contrast, only one calf and 12 juveniles displayed individually rec- ognizable marks. During five surveys, however, a fully grown adult dolphin which could not be photographically identified was seen. Assuming that this was a different individual each time, identified and cataloged adults would represent a minimum of 92% of all adult humpback dolphins seen during the study period. This figure is referred to as the “Ratio of Identified Adults”. As adults generally represent almost 63% of group members (Karczmarski, in press), it can be assumed that about 100 different individuals were seen during the study period.

Sighting frequencies of individual humpback dolphins identified in Algoa Bay ranged between 1 and 25 (Fig. 2). Only a small number of these indi- viduals (10%) were seen frequently throughout the duration of the study, while a considerable proportion (3 5 %) were seen only once. Seventeen non-calf humpback dolphins (25%) were seen at least once in each of the three con- secutive years of the study.

The cumulative number of identified and cataloged animals (rate of discov- ery) increased throughout the study period, though it is characterized by al- ternate increases and plateaus (Fig. 3). The increases in the discovery rate occurred predominantly during the summer months. This pattern was similar

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75

December'93

QT' I I

" f

lo 1

203

15 i 5

3 0

Figure 3 . Rate of discovery of newly identified adult (.) and all non-calf (0) humpback dolphins in Algoa Bay between May 1991 and May 1994. Frequency of resightings of known individuals also shown.

for all three years and was accompanied by an increase in the number of resightings of known individuals (Fig. 3).

In order to avoid biases caused by the initial rapid increase in the number of newly identified dolphins at the onset of the study, the interannual com- parisons were based on the last two years of the study. The total number of animals seen in summer was significantly greater (by almost three times) than in winter (Yates corrected x2 = 80.57, df = 1, P < 0.01). The number of resightings of known individuals varied seasonally with the resighting rate significantly greater in summer (Yates corrected x2 = 57.31, df = 1, P < 0.02) and no significant difference between the years (Yates corrected x2 = 0.02, df = 1, P > 0.75). Furthermore, the overall number of sightings of humpback dolphins per unit effort in Algoa Bay (sightings per hour of search- ing effort during land-based surveys) increased significantly in summer and, to a lesser degree, late winter (Kruskal-Wallis ANOVA, KW = 26.68, n = 104, P < 0.01; for details see Karczmarski et al., in press).

Considering the noted seasonal variation, an open population model (Jolly 1965, Seber 1965) was applied in the estimation of size of the population in the Algoa Bay region. In order to minimize the error caused by possible misidentification, the estimate was based on the sighting data of the identified adult dolphins only. The final population estimate (Table 1) took into account the "Ratio of Identified Adults" and the mean proportion of adults in a group of humpback dolphins (see earlier) and was calculated using the formula:

KARCZMARSKI ETAL.: HUMPBACK DOLPHINS 1121

population estimate = ( X / Y ) / Z

where

X = estimate of the number of adults

Y = Ratio of Identified Adults (0.92)

Z = mean proportion of adults in a group of humpback dolphins (0.63).

Table 1 . Population estimate for humpback dolphins inhabiting Algoa Bay region obtained from mark-recapture analyses for open population golly-Seber model).

Estimated Estimated number population Standard 95 % confidence of adults size error interval

270 466 9.73 447-485

DISCUSSION

The shape of the discovery curve and sighting frequencies of humpback dolphins identified in Algoa Bay indicate an open population. The initial rapid increase in the number of newly identified dolphins is an obvious attribute of the onset of the study. However, the alternating steep increases and plateaus of the discovery curve, accompanied by the seasonally varying frequency of resightings of known individuals, seem to indicate seasonal movement into and out of the study area. Furthermore, the seasonal immigration corresponds with the observed increase in abundance of humpback dolphins in the region (Karczmarski et al., in press). All this evidence indicates that humpback dol- phins occurring in Algoa Bay are likely to be a part of a substantially larger population.

The present study covered a relatively small stretch of coastline. Conse- quently, the extent of humpback dolphin range, or the limits (if any) of a particular stock, are unknown. Although the dolphins in Algoa Bay displayed a generally low level of site fidelity (Karczmarski, in press) and long-range movement to the west from Algoa Bay is possible (Karczmarski 1996), move- ment northeastwards over distances approximating 1,000 km seems unlikely. Despite examination of over 2,500 humpback dolphin ID photographs taken in KwaZulu-Natal (some 900-1,000 km north of Algoa Bay; photographs by B. Durham, University of Natal, Durban), none of the dolphins recorded in Algoa Bay was seen off the KwaZulu-Natal coast.

The results of genetic analyses indicate that humpback dolphins inhabiting the over 2,000-km-long east coast of South Africa and southern Mozambique seem to form at least three maternally distinct biological units. Although these units are probably not reproductively fully isolated, the level of genetic ex- change between them appears to be fairly limited (Goodwin 1997). Conse- quently, humpback dolphins from Algoa Bay are likely to be a part of one of

1122 MARINE MAMMAL SCIENCE, VOL. 1 5 , NO. 4, 1999

these larger units (populations) which uses a considerable length of the south and southeast coast of South Africa. Therefore, the high level of seasonal im- migration into Algoa Bay may possibly represent a seasonal shift in the dis- tribution of members of the larger population; while the weak site fidelity and apparent lack of social fidelity (Karczmarski, in press) may reflect a larger- scale fidelity on a population level. The non-asymptotic nature of the discovery curve suggests that photographic effort has so far not been sufficient to achieve the “saturation level” of the population. The estimate produced (Table 1) is subject to downward bias caused by heterogeneity (Hammond 1986, 1990) and represents the minimum population estimate for humpback dolphins in- habiting Eastern Cape waters. This estimate is considerably higher (factor of two) than previously suggested (Karczmarski 1996). A more precise definition of the population genetic identity and its geographical range appear to be the most important aspects still to be investigated.

ACKNOWLEDGMENTS

This project would not have been possible without the financial assistance of the Foundation for Research Development (FRD), WWF South Africa, and the University of Port Elizabeth, which is gratefully acknowledged. The senior author wishes to thank Meredith Thornton, Jimmy Henderson, and Shane Norris for their indispensable help during many hours of fieldwork and An De Ruyck, Mary Wilmot, Andrzej Bona- Wysocki, and Pierre and Cindy Botha for their invaluable support throughout the study.

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Received: 23 January 1997 Accepted: 12 January 1999