Resource removal - indirect effects

The indirect effects of collecting “seed” for capture-based aquaculture include the impact on biological interactions between species in the ecosystem (i.e. competition, predation, changes in the trophic chain), mortality caused by lost gear (ghost fishing), and the environmental effects of dumping discards. The removal of fish with key characteristics and functions in a specific ecosystem may result in loss of resilience and a change from one equilibrium state to another.

Fisheries managers are becoming increasingly aware that the impacts of fishing and overfishing can spread through the entire food chain because of changes in competition and predation patterns. It is very difficult to separate natural and man-induced causes for the changes observed at different levels of the ecosystem. Evaluating impacts on fisheries and on communities is also difficult because there are normally no control sites where fishing has not occurred (Goni 1998).
Fishing for capture-based aquaculture species can alter the structure of marine communities by selective removal of some species and by changing the physical support for the communities. Biomass replacement, in which a dominant species is driven to low levels and is substituted by another species, can occur as a result of fishing and can cause ripple effects on other components of the ecosystem. While biomass flips in species abundance in a pelagic marine ecosystem appear to be caused by density-independent environmental changes (affecting nutrient entrainment, primary production, and recruitment success), dominance flips in several continental shelf marine ecosystems are attributed to density-dependent predation, which includes fishing (Goni 1998).

Hughes (1994) showed how this recovery mechanism has been hindered on Jamaican coral reefs by human activity. Since the 1950s, the Jamaican coral reefs have been chronically overfished to such an extent that sharks, snappers, jacks, triggerfish, groupers, and a number of other target species have declined markedly. The loss of herbivorous and predatory fish species has reduced total fish biomass and altered the taxonomic composition of the fish community. However, the ecological effects of this decrease in biodiversity were not realized for several decades, as the reef appeared to be healthy with large coral cover and high benthic diversity. This was largely due to the high abundance of one grazing echinoid Diadema antillarum, which held the growth of algae on the reef in check. With the decline of fish predators and competitors, the abundance of Diadema increased.
Other indirect effects have been caused by discards and offal; the large quantities that can result from the processing of fish at sea and from discards may cause changes in the structure and biodiversity of marine communities. Assessments of these effects requires knowledge about the fate of the discards and offal that, until recently, has been largely neglected in studies of fishery ecosystem interactions (Goni 1998).
Fishing procedures also pollute the environment through the accidental loss of fishing gear and/or by the dumping or abandoning of gear that may continue to capture and entangle animals. The impact of such “ghost fishing gear” is basically unknown but there are indications that the problem is increasing to significant proportions (Goni 1998).
Though there has been some work on the indirect effects of fishery activities in general, only a few specific impact studies related to the capture of wild “seed” for capture-based aquaculture
species are documented. For example, Sadovy (2000) showed that some grouper seed collection methods have significant impact on the long term status of the stock. The authors of this report consider it unlikely that these fishing methods have a greater effect on fish than other types of fisheries.