Introduction

A number of authors have attempted to compare survival rates of captive and wild cetaceans in recent years. Some of these studies have used flawed methodologies and are of little value. The generally accepted method, comparisons of annual survival rates, was described by DeMaster and Drevenak (1988) and is the best available approach, given present knowledge of survival rates in the wild. The interpretation of any comparison using this method depends on two additional points, however. One is the variability in survival rates among captive populations and among wild populations. The second is whether survival rates are stable through time.

DeMaster and Drevenak (1988) reported significant differences in survival rates of dolphins among public display facilities, and variation among survival rates of populations of wild cetaceans have also been described. Therefore, rather than compare survival rates between captivity and the wild in general, we wanted to ask if survival rates in a captive setting could be as good as in a "typical" stable, stationary population (that is, a population that is neither increasing nor decreasing in size). A positive answer to this question would indicate that captivity per se does not cause shortened life spans.

To address this question, we examined survival patterns of bottlenose dolphins at Marine World Africa USA and compared them to those of wild dolphins.

Methods

We examined survivorship data as recorded in the NMFS Marine Mammal Inventory Report (see DeMaster and Drevenak 1988 and Bain 1988 for a description of this data base). Individuals for which key data (e.g., date of acquisition) were missing were excluded. In order to produce a data set comparable to the study by Wells and Scott (1990) of wild dolphins over one year of age, we also excluded data for calves born at Marine World until they reached their first birthday. Since our copy of the NMFS MMIR was not current, we supplemented it with institutional records. To allow comparison of survival rates through time, data were pooled for five year periods beginning in 1968, 1973, 1978, 1983, 1988, and for the two year period of 1993-1994. We followed the methodology of Demaster and Drevenak (1988). The daily survival rate was calculated as 1 -(the number of deaths /the number of days survived). The DSR was raised to the 365.25 power to produce an annual survival rate.

Each five year block consisted of at least 15,000 days survived, and the total data set covers over 100,000 days survived. We had useful data for an average of 10.2 dolphins throughout the 27 year period.

To compare for trends through time, we tested the probability that the number of deaths in the last complete five year interval (1988-1992) was the number expected based on the survival rate during the first five year interval (1968-1972).

We pooled data over recent years to determine for how long a period survival rates were better in captivity than in the wild. During the period since the last death at Marine World, the ASR would be 1.0 by definition, which, of course, is greater than the average ASR in the wild. Including data from the time of the last death and preceding years would drop the ASR. Maintaining a high ASR for a short period of time would indicate that recent rates are unlikely to be typical, while maintaining high rates for an extended period would indicate that survival rates similar to or better than those in the wild are likely to be sustainable.

To test whether Marine World could achieve survival rates as high as for wild dolphins, we compared data from Wells and Scott s (1990) study of Sarasota dolphins. This population appears to be stable and stationary, and thus is likely to be representative of dolphins generally. Other studies could have been chosen, such as Scott et al.'s (1988) study of Atlantic Tursions in which over 50% of the individuals died in a two year period. However, this was during a period of population decline, and hence did not reflect "typical" conditions.

Results and Discussion

Figure 1 shows the data for the six time periods. The comparison line is the ASR from Wells and Scott (1990). It can be seen that initially Marine World's rate was below that for the wild, but gradually increased to levels above that observed in the wild.

Click on the chart to see a larger view.

No deaths occurred during the last complete five-year interval or the subsequent two-year period. The probability of no deaths occurring based on the ASR of 0.829 observed in 1968-1972 is less than 0.001, indicating this improvement is statistically significant. Thus survival rates at Marine World have improved through time.

This increase in survival rate is likely to be due to a number of factors. The marine mammal display community has taken great strides forward in animal husbandry and veterinary care. The experience and training of Marine World's animal care staff has increased through time. In addition, the animals have been conditioned to facilitate a number of diagnostic and treatment procedures. These two improvements result in prevention of health problems, earlier and more accurate diagnosis of disease, and minimize the stress involved in treating individuals when their conditions are most fragile. Practice with techniques during routine procedures on healthy individuals and better medical equipment, tests, and medications allow more proficient treatment. Facility designs have been improved by taking husbandry concerns into consideration, also improving diagnosis and treatment. In addition, better understanding of the emotional needs of our animals, combined with improved facility design, allows us to improve physical and psychological wellbeing by managing the social structure of our collection. Improvements in nutrition and water quality also are likely to have contributed to increased survival.

The survival rate of Marine World's bottlenose dolphins over the last 15 years has been the same as in Sarasota (0.962 at Marine World versus .961 in Sarasota). This indicates that similar survival rates are likely to be sustainable.

We did not formally compare survival rates of calves due to the small sample size at Marine World, the paucity of published data from the wild, and difficulty in determining a point in the developmental cycle for comparison (e.g., conception, 6 months fetal development, near term development, live births, and average 6 months post-natal development). Wells and Scott (1990) reported approximately 81% of the calves they observed survived to age one. However, they did not estimate the number of calves that died prior to being observed. Richards (1993) reported that 57% of Tursiops calves survived the first year, based on observations of females which visibly appeared pregnant and their calves. This is similar to the value (57-58%) reported for killer whales (Bain 1990, Olesiuk et al. 1990). At Marine World, 50% of calves born survived to age 1.

Although the sample is small, we were also curious about how Marine World's killer whales compared to killer whales in the wild. In contrast to the situation for Tursiops, no data are available for a stationary, stable population of Orcinus. Reliable data are available for an increasing population, the resident communities of British Columbia and Washington, however (see Olesiuk et al. 1990). It is unknown how wild killer whales in general would compare to this population. Potentially, a reduction in the pregnancy rate would result in a stationary population without a change in survivorship patterns. Reduced pregnancy rates seem unlikely to account completely for the difference between "typical" and resident killer whales, however, because the pregnancy rate in residents is similar to that reported for Orcinus in the North Atlantic and Antarctic (Bain 1990). Thus "typical" killer whale populations are likely to have higher mortality rates than reported by Olesiuk et al. (1990). (Note that if higher mortality rates occur primarily among juveniles, then the effect on average survival rates would be greater than if higher mortality occurred among adult females. Changes in mortality rates of males would have little effect on population growth rates.) Regardless of what the ASR is in a "typical" population, Marine World's ASR for killer whales over the last 25 years has been higher than in the expanding population (.983 at Marine World versus .974 in British Columbia and Washington).

Summary and Conclusions

Survival rates of bottlenose dolphins increased through time. Survival rates of bottlenose dolphins over 1 year of age and killer whales at Marine World were similar to or better than those of their wild counterparts in the last 15 to 25 years, respectively. This suggests that reports indicating shortened life-spans for cetaceans in captivity, if valid, reflect correlations rather than causation, and those decisions based on survival patterns need to take future improvements in husbandry as well as historical data into account.

References

1.  Bain, D. E. 1988. A journey through the NMFS marine mammal inventory. Proc. 1987 IMATA Conf. 103-130.

2.  Bain, D. E. 1990. Examining the validity of inferences drawn from photo-identification data, with special reference to studies of the killer whale (Orcinus orca) in British Columbia. Rep. IWC Special Issue 12:93-100. DeMaster, D. P. and J. K. Drevenak. 1988. Survivorship patterns in three species of captive cetaceans. Mar. Mamm. Sci. 4: 291-3 11.

3.  Olesiuk, P., M. Bigg and G. Ellis. 1990. Life history and population dynamics of killer whales (Orcinus orca) in the coastal waters of British Columbia and Washington State. Rep. IWC Special Issue 12:209-243.

4.  Richards, A. 1993. Reproductive parameters of bottlenose dolphins in Shark Bay, Western Australia. Paper presented to the Society for Marine Mammalogy Conference. Galveston, TX.

5.  Scott, G. P., P. M. Burn and L. J. Hansen. 1988. The dolphin die off: long-term effects and recovery of the population. Proc. Oceans 88 Conf. Baltimore, MD. 819-823.

6.  Wells, R. S. and M. D. Scott. 1990. Estimating bottlenose dolphin population parameters from individual identification and capture-release techniques. Rep. IWC Special Issue 12:407-415.