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Various ways of treating biomass data can be used by researchers in investigating impacts of fishing on community structure. Aside from using simple measures (e.g. biomass expressed as mean percent change by weight for aggregated species or individuals of species over time), examples of more specific treatments give insight into how measures of biomass may be used as potential indicators. In field studies, trends in biomass can give some indication of the health of the system. For example, when biomass of one or more important species or assemblages falls below a defined minimum acceptable limit for successful recruitment, recovery time, or for a species threatened with extinction, the ecosystem can be regarded as overfished (Murawski 2000). Measures of biomass are also used in a wide range of ecological models. For example, to measure dominance changes over time of a keystone species (sea urchin), McClanahan (1995) used biomass in an energy-based coral reef simulation model (Figure 5.31) to examine the impact of fishing on community structure. The model indicated that the intensity and selectivity of fishing can affect reef structure and processes, as the abundance and interactions of the coral and algae are controlled by the herbivores, which are in turn controlled by carnivores (often the targets of fishing). The changes in system state predicted by the model are obvious in the biomass trends it predicts, an example of which is given in Figure 51.3.
In traditional single species management measures of biomass (frequently trends over time in biomass / pristine biomass) have been used to judge the health of the stock, and thus the fishery. Thus, measures in biomass can be informative at a population level, but by extending the number of species tracked or choosing species carefully (e.g. using keystone or vulnerable species) more system-level information maybe encapsulated in these simple measures.
Figure 5.31: The model output of biomass (wet weight) when all fish components are fished after 39 years of simulation. It shows the response of the coral ecosystem to overfishing, in particular, the increased biomass of sea urchins, which are keystone species in this system (after McClanahan 1995). (Figure provided by Fulton et al 2004a).
Abundance of keystone species
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The biomass of certain groups can also be used to consider aspects of the system such as community composition. Predator-prey responses to fishing impacts can provide important and potentially robust indicators of biomass changes in an ecosystem. This is true not only of field data, but also of the output of simulation models. There have been numerous simulation modelling studies recently investigating the impacts of removing key predators from an ecosystem (e.g. for chondrichthyans, Stevens et al. 2000), the over-harvesting of prey species on seabird species (Furness 1999), and the over-harvesting of prey species in Port Phillip Bay, Victoria (Fulton and Smith in press). An example of the time-series output of these kinds of models is given in Figure 52.4.
Figure 5.42: ECOSIM simulation of the functional response to an aggressive sand lance fishery showing biomass declines in seabirds (which prey on sand lance) and other species (after Okey and Pauly 1999). (Figure provided by Fulton et al 2004a).
Attribute
Community structure; trophic structure; population structure; predator-prey balance
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