Documenting Aggregations

 

Once an aggregation is known or located, we need to learn something about its physical and biological characteristics. The uses of this information are diverse. It may have fisheries and conservation value, to preserve aggregations for both exploitation and to serve as a source population for future generations of fish. Scientists may be interested in aggregations for the same reasons, but also have the intention of trying to understand them as a biological phenomenon. A Marine Protected Area (MPA) may be centered on a large aggregation such that its physical location is critical to locating and sizing the MPA properly.

Aggregations can be documented in many ways. These can be as simple as using our eyes to count things and make observations, which are then recorded, or as complicated as instrumentation and equipment that provide a wealth of data on an aggregation and its environment. The ultimate value of work on aggregations depends on the skillful employment of the tools and resources that are available. Like most things, it is useful to learn the skills necessary and to practice them before needing them, so that when you are faced with a short-lived aggregation, you can gain maximum information without a major learning curve at a critical time.

At one level, we want to be able to document the biological and physical parameters of an aggregation. This includes documenting basics such as how many fish are there, their sizes, sexes, how large an area they occupy, their density throughout the aggregation, when they arrive, how long they stay aggregated, and how much they move around in that area. This may seem straightforward at first. However, once you encounter an aggregation of hundreds to thousands of fish interacting with one another, constantly moving in and out of shelter, and affected by every movement and breath of a diver watching or counting them, these theoretically simple tasks quickly become daunting. All of this in an environment that often has limited visibility (which may mean you can't see the entire aggregation area at one time), currents, or occurs at depths where bottom time is limited. Getting an accurate census of any spawning aggregation is a difficult task and our best methods still leave much to be improved upon.

The documentation of the status of aggregations is perhaps the most important aspect of any scientific work we might do on aggregations. Aggregation demographics are essential to understanding the effects of exploitation, management, natural mortality and natural variation over time. Because this information underpins most of what we are concerned about, the methods used to document aggregations are important. It is critical to remember that the methods presently used provide us in most cases with nothing more than estimates of numbers of fish in an aggregation. Since they are only estimates, we must be cautious in projecting these data beyond their limits. Outside of destructive sampling, it will be some time before we have methods that provide a highly accurate census of fish. Until then, we have to use the best methods we can and remember, and record, the limits of accuracy and precision.

No single method will fit all aggregations. Keep in mind, though, that our intention is to collect qualitative and quantitative data that are comparable between aggregations and years, no matter what methods are used. When fish are too numerous to accurately count visually or by other methods, it is necessary to subsample a portion of the aggregation in some manner. Such samples need to be placed into an area or volume context. The surveys need to be repeatable, particularly by others who might not have been part of the original surveys. Ideally there should be some sort of permanent record, such as video recordings, still photographs, maps or other permanent media indicating the structure and size of the aggregation. The methods used, areas examined and other aspects of how the data were gathered need to be accurately described, with enough detail so that others can duplicate them. The bottom line is to make surveys “repeatable with precision”.