Abstract
The cultivation of fish is a rapidly growing industry. Fish are raised on farms both for food production, and for release into the wild (i.e. for wild-stock enhancement). Recently, there has been a move towards more intensive culturing systems. Intensive culturing allows farmers to enhance farm production by employing practices that manipulate the biology of their fish stocks. However, fish are complex creatures, and the practices designed to manipulate their biology often have many unintended effects. Importantly, these side effects can result in detrimental impacts on fish farms, which could potentially outweigh the benefits of using some manipulation practices. Therefore, before farmers attempt to manipulate their stocks, we need to understand both what side effects could arise and how such effects could implicate fish farms. In this thesis, I describe some of the side effects of two practices commonly used to manipulate fish: the use of compensatory growth, and the use of feed restrictions.
Compensatory growth is the phenomenon where fish will grow faster than normal when recovering from periods of growth suppression. Using compensatory growth in aquaculture represents a unique opportunity to boost profit margins. For example, food and feeding costs can be reduced, which suppresses fish growth, but once fish are re-fed, compensatory growth can ensure that the fish quickly regain normal sizes. Thus, by using compensatory growth, farmers can reduce costs without reducing harvesting yields. There is a large body of research addressing the side effects of compensatory growth. In Chapter 2, I present an opinion article that addresses some of the key areas that have been neglected, areas that, if researched, will yield important results that can be applied to fish cultivation. Specifically, three areas are discussed. First, I suggest that research is hampered by the lack of unifying terminology to describe the various dimensions of compensatory growth’s side effects. I proposes specific terms that will aid future research by conveying the nature of side effects in three dimensions (where effects arise, when they arise, and how long they last). Second, I discuss the possibility of five potential side effects that could arise from inducing compensation, but which have been neglected in the literature. Farmers inducing compensatory growth may unwittingly: i) increase the occurrence of cannibalism, ii) elevate the incidence of aggression, iii) increase the rate of deformation, iv) lower resistance to infections, and v) decrease the physical fitness of the fish. If these side effects arise when using compensatory growth, there will be many negative consequences on the productivity and welfare of fish farms. Third, I highlight several methodological concerns plaguing compensatory growth research, along with a number of suggestions for improvements. By addressing these three areas, Chapter 2 makes suggestions that will greatly aid the accumulation of knowledge that can then be translated to practical applications of the use of compensatory growth on fish farms.
The second practice that was examined was the use of feed restrictions (Chapter 3). Typically, fish are fed to satiation on farms, however, farmers often employ periods of feed restriction. Feed restrictions refer to the practice of temporarily limiting the amount of food given to fish. These feed restrictions can last from weeks, to months, before fish are again re-fed to satiation. Feed restrictions are used of a number of reasons, including to reduce the costs of purchasing food, to lower the spread of disease from faecal cross contamination, to decrease the environmental impact of fish farms, and to induce compensatory growth. Despite the widespread practice of feed restrictions in aquaculture, there are very few reports regarding the effects of this practice on later fish behaviour. Chapter 3 presents the results of an experiment addressing such effects in the western mosquitofish (Gambusia affinis). It was found that an early period of feed restriction increases the plasticity of aggressive behaviour, but not the plasticity of activity, risk-taking, or neophobic behaviour. Feed restrictions also increased the rate that the fish became habituated to novel objects. Furthermore, there did not appear to be any lasting effects of feed restriction on the average level of these behaviours. Taken together, these results suggest that feed restriction does not have detrimental side effects of later behaviour. In fact, increased behavioural plasticity and rapid habituation are potentially beneficial for fish cultivation. Elevated behavioural plasticity and rapid habituation indicate greater capacities to respond and adapt behaviour to environmental changes. Such abilities should increase the survival of fish when released into the wild. Moreover, such abilities could also make fish more responsive to other practices intended to manipulate their behaviour, such as management of aggression.
In summary, both compensatory growth and feed restriction practices have the potential to manipulate fish in beneficial ways, but we must remember that both practices can also result in unintended side effects. From our current understanding of these practices’ side effects, it appears that compensatory growth produces a large number of detrimental effects, whereas the side effects resulting from feed restriction do not appear to be overly undesirable. However, data on the later effects of feed restriction (e.g. effects that persist and/or arise after re-feeding) are few and far between, and so we still have a very elementary understanding of how the practice of restriction will affect fish farms in the long term. Before these two practices are extensively used on fish farms, we need to know more about the potentially harmful side effects arising from their use, the explicit benefits of their use, as well as how these costs and benefits balance out. Once we gain such an understanding, we can use this knowledge to help farmers make informed decisions about whether, and how, to use compensatory growth and feed restriction practices.