8 March 2009 Demonstrations and laboratory exercises in aquaculture VI. Fish pond inventory and feed budgets Matthew Landau1 and John Scarpa2 Introduction to Animal Inventory One of the recurring problems in pond aquaculture is estimating the number of fish in a pond. You can’t rely on stocking data, since fish populations decrease over time because of predation, cannibalism and disease. The reasons a farmer would want to determine the number of fish in a pond (i.e., the inventory) are obvious. If the farmer underestimates the number of fish, and puts too little feed in the pond, growth will be suboptimal. However, if the farmer thinks that there are more fish in the pond than there really are, and overfeeds, this not only means that money is wasted on uneaten feed, but water quality will also likely be reduced. So how do you know how many fish are in the pond? Some techniques that have been suggested by researchers involve acoustical measurements or optical fish counters. Other techniques, much more commonly used, require an historical knowledge of how the fish populations change over time in the ponds that are used. That is, if you know what’s happened in the past, that’s a good place to start when trying to figure out what’s going on at the moment. This is certainly important to know, but we’ll take a different approach. In this exercise, we’ll be using tagging/marking. Currently, these are used more in fisheries management than aquaculture, but still can be useful for farmers. Tagging is the term used when an internal or external tag (Figure 1) is applied that can identify a particular individual fish. Tagging is useful for fisheries biologists, but it requires a lot of manpower to tag fish, as well as for record keeping. Marking refers to fin clipping, branding or the use of dyes and stains. Marking is easier to do when you have a large number of animals, or the animals are too small to be easily tagged. Marking allows you to identify a fish as part of the batch that was manipulated, but not the individual fish. Regardless of which method is used, it should not significantly affect the fish in terms of its behavior, growth or survival. The tags/marks that work well for one type of fish will not necessarily work well for another. Ecologists have developed a number of ways to estimate population size using marks and tags. The methods depend on knowing if the ecosystem is open” or closed (a pond would certainly be closed) and if there is replacement after sampling and future samplings. We will use a relatively straight forward procedure called a Petersen-Lincoln estimator: Fig. 1. Examples of tags that are used to identify fish. Photo by M. Landau. In practice, some fish (a) are collected and marked, then released back into the pond to mix with the unmarked fish. After a day or two, a small seine net is dragged through the pond and a sample of fish (n) is collected and counted. While examining the fish in the sample, the farmer counts how many of those are marked (r). Finally (N), the estimate of the number of the fish in the pond is computed.
RkJQdWJsaXNoZXIy MjExNDY=