Studying and monitoring aggregating species

Abstract

The scientific study and monitoring of spawning aggregations requires field and laboratory work using a wide variety of physical and biological methods; but field work, no matter the technological tools available, will remain of fundamental importance in studying aggregations particularly for future comparison. Methods are becoming increasingly standardized allowing for meaningful comparison between sites, times and species. Methods can be fishery-dependent (gathering data from captured fishes) or -independent (observational, instrumental physical, interview data) within the subject areas of aggregation (1) discovery, (2) composition, (3) dynamics, (4) life history parameters and (5) physical parameters. Methods for each are summarized with selected subjects explored in more detail. Particularly problematic areas for data gathering have been fishery-independent determination of numbers and/or sizes of fishes and their distribution within aggregations and the larger environment. GPS based methods of mapping aggregation location, extent and fish density provide discrete snapshots of an aggregation, allowing visualization of dynamics over days to years and are repeatable any time in the future by others. Digital imaging (still and video) allows documentation previously impossible. Acoustic tagging, particularly alongside conventional tagging, allows delineating spatial and temporal aspects of aggregations. Hydroacoustic surveys are promising, but require validation of data on species present, fish numbers and their sizes. Aggregation sites should be instrumented to record physical data (e.g. temperature, currents, light) ideally year round, rather than just during aggregation, to allow comparison of aggregation periods with the entire year, and non-aggregation sites also evaluated for comparison Detailed bathymetric mapping of sites is important and feasible and allows visualization of geomorphology in relation to aggregations.