Comparison of time dependant effects in two different methods of reef fish sampling.

Gary Carlos

CRC Reef Research Centre, TOWNSVILLE, AUSTRALIA

THEME: ASFB

The fine-scale dynamics of shallow water reef fish sampling using line fishing and underwater visual surveys (UVS) are compared. Both survey methods operate in a complex, multi-species array with varying degrees of selectivity and their desired outcome is usually, directly or indirectly, an instantaneous estimate of abundance within a defined area. Practicality dictates that sampling must occur over a finite time-span and (for line catches and also most visual transect methods) within an ill-defined spatial unit. Time-dependant processes, over and above gear selectivity effects, therefore have the potential to change the relationship between the actual populations and sample data. Similar types of events comprising the sampling process are discernable within both methods, although their actual manifestations differ considerably. 'Capture' order is a species-dependant phenomena and varies between methods, while 'capture' rate in line fishing is not negligible with respect to the duration of sampling. Movements of fish over areas larger than the sampling unit effect UVS counts and have the potential interact with time-dependant events in these and line catches. Standardization of sampling time is not sufficient to eliminate time-dependant effects, the relation of capture rates relative to sampling time and fish mobility also needs to be considered. Multi species interactions can mask changes in abundance of some species whose order of capture is delayed, and this effect can be exacerbated by differential movement which can alter species composition of the sampling area. These combined effects may result in conflicting estimates of both relative and absolute abundance between methods.