Determining Population Status and the Use of Biological Indices in the Management of Marine Mammals

Abstract

Conservation of marine mammals as currently practiced protects populations roughly in proportion to the extent they are reduced. The status of a population is expressed in terms of the population level as a fraction of the mean of natural (undisturbed ecosystem) equilibrium level. This approach requires estimates of: (1) current population level, and (2) carrying capacity of the undisturbed habitat. Implementation of this approach is fraught with practical problems: population levels are difficult and costly to estimate and current carrying capacity is essentially impossible to measure through environmental assessment. The historic or natural carrying capacity of disturbed ecosystems can not be directly assessed. When possible, estimates of historic population levels are used as surrogates for the natural equilibrium population levels. Despite difficulties with implementation, no alternatives have arisen to replace this paradigm for marine mammal conservation. Following a review of the history, utility, and potential of indirect methods for population assessment, we suggest that characteristics of populations, and the individuals within, give a better assessment of ecosystem conditions and relationships than can be achieved through direct measurement. This approach assumes that animals in the population are the best integrated expression of their environment. In this regard, some species serve as indicators of trophic relationships and ecosystem state. In wildlife management, various indices of population status have been proposed or used. In particular, measures of biological features that experience density-dependent change provide indirect means of determining population status. We review literature on the use of physiological condition, measurements of size and growth rates of body parts, and the components of population dynamics (e.g., reproductive rates, survival, age at maturation) as indicators of the status of populations. Care must be taken to choose indices that reflect integration over sufficient time to avoid variation due to momentary conditions, such as annual weather cycles. Knowledge of the relationship between population status and the magnitude of the biological feature is necessary. Although this approach involves inherent problems, there also are advantages. Often data can be collected easily, and they usually reflect current environmental conditions for assessing human-induced changes in the environment.