Temporal variability of total biomass

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Indicator summary

Summary of indicator structure and function

Indicator	Attribute	Purpose	If restricted to taxa, list which ones	Ecosystem applicability	Identified capability	Biological classification level	Response variable	Drivers	Robustness
Temporal variability of total biomass	Tropic shifts; Community structure	Fisheries		Should be applicable to all ecosystems	Demonstrable	Community; Ecosystem	Species-based; Trophodynamics	Trophodynamics	Potentially medium to high

Definition and/or background

The following is from Fulton et al (2004a) -

Total biomass variability is a measure for comparing the variability of total biomass of a community (CV_t) with the biomass variability of individual species (CV_i) in that community. The ratio of CV_t to CV_i can be used as an indicator for detecting unstable communities (Blanchard and Boucher 2001). The biological basis for the index is that in an unstressed “healthy” community the coefficient of variation of the total fish community biomass should be lower than the coefficient of variation of individual species biomass. This is because energy limitations are regulated by the interactions between competing species, or between predators and prey (particularly in temperate systems where food chains are simpler). Thus, an extreme low in one species should be compensated for by a high in another species and so while the individual species are highly variable the sum of their biomasses is fairly steady. As a result, the coefficient of variation (CV_t) of the total biomass is lower than the coefficient of variation of species biomass (CV_i) in a stable system. In a system that is fished, the removal of biomass by fisheries may move the system to a point where the variation of species is more independent, as the species are no longer energy limited and the influence of interactions between species are not as strong, and as a result the overall variation can increase. Consequently, the coefficient of variation (CV_t) of the total biomass is not necessarily lower than the coefficient of variation of species biomass (CV_i) in such a perturbed state (Blanchard and Boucher 2001).

The index is calculated as follows:

where CV_i is the coefficient of variation for species i, σ_i is the standard deviation and is the mean of the measured variable (biomass landings, CPUE and biomass) for species i. CV_t is the sum of all the CV_i.

This approach (of comparing CV_t and CV_i) was initially developed for examining fish assemblages, and it has been used as a broad indicator of fisheries impact on demersal fish communities (Duplisea et al. 1997, Blanchard and Boucher 2001). However, other ecosystem components (e.g. invertebrates; plankton, seagrass, algae) could also be used, but this suggestion has yet to be tested. In addition to its role as an indicator of community structure, the tracking of the coefficients of variation through time could also be a useful indicator of stock and trophic status, as a change in the evenness of fluctuations in biomass may reflect a shift from large, stable k-selected species to more variable r-selected species.

Attribute

Tropic shifts, community structure

Purpose

fisheries, birds and mammals

Data required

The following is from Fulton et al 2004a -

Biomass (wet weight) of species caught (target and non-target)
CPUE (and effort)

Ecosystem applicability

The following if from Fulton et al (2004a) -

Should be applicable to all ecosystems, and has been applied to comparisons of fish communities in the Bay of Biscay and Gulf of Lions (Blanchard and Boucher 2001).

Robustness

The following is from Fulton et al 2004a -

Potentially medium to high: The main limitations relate to the use of CPUE data, which can be highly variable and inconsistent. As a result, accurate data on total biomass may not be recorded. Moreover, market forces, rather than just the ecological effects of fishing, can influence the catches (so catch shifts to planktivores from piscivores without a concomitant shift in the underlying system). It is also worth remembering that if a species goes extinct its CV_i will be zero and any fluctuation in overall biomass will be greater than it. Thus, to be truly robust, this indicator requires testing on data sets derived from research surveys that are not linked to effort, and/or from data collected in unfished reference areas. It could also be useful to examine time series data of target species (which tend to be large piscivores), as this could give a measure of the total biomass variability over time in established fisheries and provide a snapshot of how the higher trophic levels, which regulate the energy flow, have varied. Lastly, estimating variability can require a lot of data, especially if it is to be separated from time trends in the data sets used (Rochet and Trenkel 2003).

References

Fulton, E.A., Smith, A.D.M., Webb, H., and Slater, J. (2004a) Ecological indicators for the impacts of fishing on non-target species, communities and ecosystems: Review of potential indicators. AFMA Final Research Report, report Number R99/1546.

References that Fulton et al uses for this indicator:

Blanchard, F., and J. Boucher. 2001. Temporal variability of total biomass in harvested communities of demersal fishes. Fisheries Research 49: pp 283-93.

Duplisea, D. E., S. R. Kerr, and L. M. Dickie. 1997. Demersal fish biomass size spectra on the Scotian Shelf, Canada: species replacement at the shelfwide scale. Canadian Journal of Fisheries and Aquatic Sciences 54: pp 1725-35.

Rochet, M.-J., and V. M. Trenkel. 2003. Which community indicators can measure the impact of fishing? a review and proposals. Canadian Journal of Fisheries and Aquatic Science 60: pp 86-99.

Background reading

Fulton, E.A., Fuller,M., Smith, A.D.M., and Punt, A. (2004) Ecological indicators of the ecosystem effects of fishing: Final report. AFMA Final Research Report, report Number R99/1546.

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