Draft
These pages are still under construction.
Classification
where does it fit in the classification tree ?
this will be a gliffy diagram only
| Indicator | Ecological group that uses this indicator | Certainty | Level of indicator (or Classification?) | Category of indicator | Direction of change | Ecosystem applicability | Robustness or variability | Management objective/direction | Stakeholder/Public acceptability | Reference(s) | wiki page status |
|---|---|---|---|---|---|---|---|---|---|---|---|
| slope and intercepts of the size-spectra slope for fish assemblages | Fisheries | D | Community, Ecosystem | ||||||||
| what is it? | Pick one or more of the following:
| Demonstrable (D) or Aspirational (A) based on Observations, speculation, or association | Pick one of the following:
| Pick one of the following:
| Pick one of the following:
or should it be deteriorating, improving, stable, unclear
| where can this indicator be used? all over or just locally ie:
| what error level or variability is there? robustness rating of low, medium or high for each indicator | Pick one of the following:
| Pick one of the following:
| Pick one of the following:
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Attribute
Community structure, trophic shifts and predator-prey balance
Definition
copy-paste from Fulton et al 2004
Size-spectra is a community property that can be effectively measured by analysing the slope and intercepts of the size-spectra slope for fish assemblages, where the spectra are based on either biomass or numbers per size interval (Jennings et al. 2001). Wide ranging empirical studies have shown that this is usually a robust measure for detecting trends in overall size composition of a community that has been subject to fishing pressure (Rijnsdorp et al. 1996, Bianchi et al. 2000, Rice 2000), though it is not equally strong in all systems (Macpherson and Gordoa 1996, Bianchi et al. 2000). The biological basis for the sensitivity of the slope to changes in size composition resulting from fishing is that fishing selectively harvests larger individuals first, while simultaneously increasing the mortality rate for all sizes taken by fishing gear (Rice 2000). As a result, communities shift to smaller sized fish, and this occurs within and across species and therefore the intercept of the spectrum increases, as does the steepness of the slope of the spectrum. In studies to test the sensitivity of size-spectra analysis to community change, Bianchi et al. (2000) analysed community level data sets of bony fish and elasmobranch bycatch from a range of marine ecosystems (from tropical to temperate), and found that the slope of the size-spectra responded consistently to different exploitation levels and showed a decreasing trend (i.e. decline in larger fish) in communities subject to overfishing. Results were more robust for temperate regions and less conclusive for tropical regions. This is possibly due to less consistent time series data in tropical areas, but may also be due to higher growth rates of tropical fish making the slope less sensitive to fishing induced changes. In a recent evaluation of metrics of community structure for evaluating fishery impacts, Rice (2000) concluded that the slope and intercept of the size spectrum are useful measures to use as indicators for impacts of fishing on fish assemblages. Moreover, it appears that there is a strong relationship between body size and trophic level, which means that size-spectra parameterised with these relationships could also describe the trophic structure of marine ecosystems (Jennings et al. 2002a, 2002b). This is a very important finding as size-based analyses can be far easier and cheaper to use and parameterise than explicit foodweb models. The primary disadvantages with the size-spectra approach is that the slope and intercept are not independent (so a time-series of either is difficult to interpret), the minimum size class included is often an arbitrary choice (ICES 2001A) and the size-spectra may actually be non-linear (Rochet and Trenkel 2003).
Data required
copy-paste from Fulton et al 2004
- Length frequency or weight frequency by spp. Note that weight-based size-spectra will differ from size-spectra constructed from length measurements (Bianchi et al. 2000) and there is less bias when using length-based data.
- Catch in numbers.
- Long time series.
- Baseline data on species composition and size structure from unfished reference communities - e.g. no-take marine reserves or areas recovering from fishing. Though if sufficiently long time series exist (ideally pre-dating fishing in the area) then that can be used (with caution) in place of baselines from unfished areas.
Category of indicator
size-based, species-based, trophodynamics or environmental
others????/
environmental drivers are are most likely related to total system biomass and biomass of specific biological groups variables, (Fu et al 2012).
trophodynamic drivers are most relevant to the mean trophic level of community and the demersal-to-pelagic biomass ratio (Fu et al 2012).
Direction of change (Reference points, trajectories or directions for indicator)
Slope of ln (numbers) size-spectra curves should not exceed moderately fished area (Bianchi et al 2000). Historical data or data from unfished reference areas needed.
Certainty and Robustness
copy-paste from Fulton et al 2004
High potential: It has been demonstrated as most effective for detecting changes in fish assemblages in temperate regions. However the usefulness of size spectrum analysis is limited where there is inadequate empirical and theoretical ecological or baseline data (e.g. from reference areas) for interpreting observed values of slopes and intercepts of the regression analysis. As with many indicators of system or community change, this method will detect changes that are not due to the effects of fishing (e.g. increasing nutrients can increase system productivity, which will increase the intercept of size-spectra of that system), therefore reference points or background knowledge of the system is crucial. Moreover, high latitude areas produced more conclusive size-spectra results, probably because of the slower growth rates of the fish relative to tropical species. It is recommended that size-spectra analysis be used in conjunction with substrate/sediment indicators for examining community structure.
Management strategies and/or objectives
define a standard set of management objectives?? ie from Indiseas
- Conservations biodiversity
- Ecosystem stability and resistance to perturbations
- ecosystem structure and functioning
- resource potential
has it been used in a management strategy? if so how?
relationship to management strategies/ objectives
Acceptability with stakeholders
- by all stakeholder
- by the public
- understandable to the stakeholders
Associated links
Hyperlinks to organisations, databases, webportals, and ID books, that are associated with this indicator, if appropriate.
People
Not sure if I want this here yet...who would I actually link to?? scientist? mangers? both?
References
Fulton et al 2004.
Background reading
Other references that would be useful to read in regard to the indicator referred to on this page.
Citation
Please cite this page as:
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Page created by:Shavawn Donoghue
Last modified on: Aug 30, 2012 16:10
Versions: 8, 7, 6, 5, 4, 3, 2, 1
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