7 ENVIRONMENTAL IMPACTS

Introduction

As noted in Chapter 1, capture-based aquaculture is an overlap between fisheries and aquaculture, since it is based on the removal of “seed” from the wild stocks for subsequent on growing in captivity using aquacultural techniques. This practice may have an impact on both the environment and the ecosystem. Although these impacts may be negligible, and the benefits to the local communities considerable, it is still necessary to understand and evaluate the potential effect on the overall ecosystem.

There are many examples of ecologically-unsustainable development and the need to prevent their repetition on a global basis has been generally accepted. The problem has to be balanced with the requirement for economic development and increased food production in developing countries. Finding the balance between rational use, conservation and preservation is the logical course to optimize man’s use of natural resources on a long-term basis. To assess the sustainability of capture-based aquaculture, all environmental impacts need to be considered, bearing in mind the complexity of the ecosystems involved. This chapter considers environmental impact under two general headings or steps.
The first step, which relates to fisheries, consists of the collection of the “seed” from wild resources. The effects of “seed” collection are direct or indirect and are considered in the next two sections of this chapter, on “resource removal”.

The direct effects consist of the fishing mortality exerted on target populations (overfishing), the impact of removing immature fish from the genetic stock, the fishing mortality sustained by non-target populations that are caught or killed along with the target species (bycatch and discards), and the physical impacts on benthic organisms and habitats (by detrimental fishing methods). Indirect effects include the impacts caused by biological interactions between species in the ecosystem (i.e. competition, predation, changes in the trophic chain), the mortality caused by lost gear (ghost fishing), and the environmental effects of dumping discards (Goni 1998). Indirect effects have the potential to create greater impacts on aquatic ecosystem structure and function than fish removal (Hammer, Jansson and Jansson 1993; Botsford, Castilla and Peterson 1997; Helmlund and Hammer 1999). It is more difficult to characterize indirect effects because they are complex, respond to poorly understood feedback mechanisms and may occur over large areas and at a variety of time scales (Goni 1998).
The second step, concerns the aquaculture systems used for capture-based aquaculture. These share most of the environmental aspects related to “classical” aquaculture methods. The culture of fish in cages has the potential to cause both onshore and offshore impacts on the surrounding environment, with a severity scaled to the size and the intensity of the farming operation. Such impacts include distortion of the local ecosystem; short- and long-term near-field and far-field eutrophication, chemical pollution (i.e. by xenobiotics), cross-transmission of parasites and pathogens, aesthetic deterioration in coastal areas, organic enrichment, and habitat modification, etc.
Other impacts concerning the aquaculture side of the operation are specific to capture-based aquaculture practices and include both environmental and social effects. One example of this is the towing of the cages used for the collection of live tuna at sea, which can take several weeks or months, depending on the distance between the catching location and the aquaculture site. This operation may result in conflicts with other fisheries and with shipping navigation. The effect of farming operations are considered in the section of this chapter entitled “effects of farming operations (grow-out)”.
It is very clear that there are many potential environmental impacts of capture-based aquaculture. The problems associated with the removal of “seed” from wild stocks are very difficult to quantify or accurately assess, and the concerns of the fisheries sector must be addressed. However, the benefits and potential value-added inputs to a fishery need to be balanced against the negative effects.