Indicator summary

 Summary of indicator structure and function

Definition and/or background

The following is from Fulton et al 2004a -

Fish communities can be relatively hard to study (Jennings et al. 2001) and summary statistics for general community characteristics are very useful. One such statistic is mean trophic level, which can also be a good indicator of the effects of fishing (due to the preferential targeting of fish from higher trophic levels by fisheries). ... While this is an attractive indicator, the best means of calculating it has yet to be decided upon and it is likely to require a lot of data (e.g. diet information) and so may only be useful only in well-studied ecosystems (Rochet and Trenkel 2003).

Mean trophic level of the catch

Mean trophic level of the catch has also been used as a summary statistic for the consideration of broad-scale, coarsely resolved datasets such as the FAO dataset. The index is also the basis for other indicators, such as the “Fishery is balanced” (or FIB) index (see separate page). While these catch-based statistics can be informative, great care must be taken that they are not being confounded by economic forces guiding targeting or technological advances (Caddy and Garibaldi 2000).

Attribute

Trophic shifts, Trophic structure

Purpose

Fisheries

Data required

The following is from Fulton et al 2004a -

• Time series of biomass or abundance of species (or group) in the system
• Time series of catch data by species (or group)
• Diet composition (or at least basic understanding of usual feeding habits) of species (or groups) in the system or catch

Ecosystem applicability

The following is from Fulton et al 2004a -

Applicable to all systems.

Robustness

The following is from Fulton et al 2004a -

Medium to High: if based on fisheries independent data the trends in this statistic give a good indication of changes in community structure that result from the effects of fishing. The robustness is weakened if based solely on catch data (as that can be confounded by the usual concerns about catch data such as targeting and reporting), but it is still relatively sound given the simplicity and ease with which the indicator can be calculated. The indicator can be effected by other changes in the environment that also effect community composition, such as eutrophication, but that is true of all indicators considering community structure.

Current status and trends

The following is from Fulton et al 2004a -

A drop in the mean trophic level has been seen in both temperate (e.g. North sea, Greenstreet and Hall 1996) and tropical systems (e.g. Islands of the Philippines, Russ and Alcala 1996) and indicates a substantial shift in community structure as smaller, faster growing species which feed lower in the food web come to dominate the system.

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:

Caddy, J. F., and L. Garibaldi. 2000. Apparent changes in the trophic composition of world marine harvests: the perspective from the FAO capture database. Ocean & Coastal Management 43: pp 615-55.

Greenstreet, S. P. R., and S. J. Hall. 1996. Fishing and the ground-fish assemblage structure in the north-western North Sea: an analysis of long-term and spatial trends. Journal of Animal Ecology 65: pp 577-98.

Jennings, S., M.J. Kaiser, and J.D. Reynolds. 2001.  Marine fisheries ecology.,. 417 p . London: Blackwell Science.

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.

Russ, G. R., and A. C. Alcala. 1996. Marine reserves: rates and patterns of recovery and decline of large predatory fish. Ecological Applications 6 , no. 3: pp 947-61.

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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