Size or age structure

 Indicator summary

Summary of indicator structure and function

Examples of how the indicators is used for ecosystem management and ecosystem status and trends

Definition and/or background

The following is from Fulton et al 2004a -

Since the inception of fisheries science, analysis of the size structure of populations has been one of the main foci. Beyond simple total catch data, size data is often the cheapest, easiest and most readily obtainable data from a fishery and it is collected to try and gain an understanding of variation in growth (and thus productivity), longevity, mortality and recruitment (Hilborn and Walters 1992). All of these can give insight into the “health” of a stock and the sustainability of any fishery targeting it and therefore these measures are widely used in existing stock assessment approaches around the world. Experience with sized based data and development of aging techniques has seen the expansion into age-based indices and assessment methods (Hilborn and Walters 1992). The biological basis for the use of size and age structure as ecological indicators is that most forms of fishing selectively remove older and larger individuals and so the structure of the population is shifted, with the mean size and age decreasing as fishing increases (Jennings et al. 2001). While continued use of single species management may not ensure that entire ecosystems are managed sustainably, judicious choice of indicator species and the use of these well-known (and understood) population based indices may produce efficient and robust indicators of the broader effects of fishing. Mean length of the catch is a good example, as it has been found to be a powerful indicator in study of the Celtic sea (Trenkel and Rochet 2003).

Attribute

Population structure, Trophic structure

Purpose

Fisheries

Taxa

Data required

The following is from Fulton et al 2004a -

• Length frequency data from catches (and preferably also from fisheries independent sampling).
• Age data (where possible)

Ecosystem applicability

The following is from Fulton et al 2004a -

Size based methods should be appropriate in all habitats and ecosystems. Theoretically age based methods could be used anywhere, but until aging methods are developed the validity of age based methods may be more restricted.

Identified capability

Is there any additional information that would be of interest in regards to the identified capability?

Otherwise can leave this section blank and just fill in the table instead.

Biological classification level

Is there any additional information that would be of interest in regards to the biological classification?

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Response variable

Is there any additional information that would be of interest in regards to the response variable?

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Drivers

Is there any additional information that would be of interest in regards to ecological drivers?

If not can leave this section blank and just fill in the table instead.

Robustness

The following is from Fulton et al 2004a -

Medium to high: these methods are well known and understood for single species assessment and this should make their use as more general ecological indicators more reliable. At a population level they have been found to be powerful indicators (Trenkel and Rochet 2003). However, the true value of these measures as ecological indicators will rely on the appropriate choice of indicator species, as existing evidence indicates that using the size and age structures of target species in isolation is not a robust means of tracking the state of the ecosystem. Moreover, the same limitations that apply to the usage of these methods in a single species context (e.g. sampling is biased if only fisheries data is used) will apply to their use as more general ecological indicators. Thus, reference points and fisheries independent sampling will be required.

Current status and trends

what was it like in an undisturbed/unexploited system?

how would it be expected to change?

which way is the indicator showing a population is going in?  decreasing or increasing ??

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

Stakeholder/public acceptability

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.

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:

Hilborn. R., and C.J. Walters. 1992. Quantitative fisheries stock assessment: choice, dynamics and uncertainty, New York: Chapman & Hall.

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

Trenkel, V.M., and M.-J. Rochet. 2003. Performance of indicators derived from abundance estimates for detecting the impact of fishing on a fish community. Canadian Journal of Fisheries and Aquatic Sciences 60: pp 67-85.

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.

Other references that may be used to update this page

Hall, S. J., et al. (2006). "A length-based multispecies model for evaluating community responses to fishing " Canadian Journal of Fisheries and Aquatic Sciences 63: 1344-1359.

Jennings, S., and Dulvy, N.K. (2005). "Reference points and reference directions for size-based indicators of community structure." ICES Journal of Marine Science 62: 397-404.

Nicholson, M. D., and Jennings, S.. (2004). "Testing candidate indicators to support ecosystem-based management: the power of monitoring surveys to detect temporal trends in fish community metrics." Ices Journal of Marine Science 61(1): 35-42.

Shin, Y-J., Rochet, M-J., Jennings, S., Field, J.G., and Gislason, H. (2005) Using size-based indicators to evaluate the ecosystem effects of fishing.  ICES Journal of Marine Science. 62, 384-396.

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