Discard rate

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 -

In traditional stock assessment methods, catch-per-unit-effort (CPUE) is often used as an indicator of stock health. Unfortunately, recorded catch is most often landed catch and so it has been found that the assumption that CPUE is proportional to stock size is often far from true (Jennings et al. 2001). To get a more realistic idea of what is caught (and thus what true CPUE is) discards must be recorded. Apart from its role in determining the true values of CPUE, discard rate can give insight into the pressures and effects on the entire community exposed to fishing. For example, the discard rate (or more explicitly the ratio of discards to landings) can give a qualitative indication of the amount of detritus added to a system (by discarding) and the breadth of taxa suffering additional mortality (as they are taken as discards). For vulnerable species, the discard rate can be a crucial part of any attempt to assess the degree of vulnerability and their likelihood of recovery. This is particularly true for species, such as the razorbills Alca torda of Newfoundland, where incidental capture by fisheries represents the greatest source of mortality for the species (Jennings et al. 2001). Discard rates (or the ratio of discards to landings) are also useful for identifying gear types that may have larger ecological effects. For example, the average ratio of discards:landings for shrimp fisheries is 5.2:1. This represents the discarding of billions of fish (some of which are commercially important in their own right in other fisheries) each year and is why shrimp fisheries are considered to be some of the least environmentally acceptable fisheries (Jennings et al. 2001).

Attribute

Discard availability or depletion, community structure, trophic structure, population structure

Purpose

Fisheries

Taxa

Data required

The following is from Fulton et al 2004a -

• Biomass or numbers discarded
• Effort
• Taxonomic make up of the bycatch

Ecosystem applicability

The following is from Fulton et al 2004a -

Should be applicable in all ecosystems.

Identified capability

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

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Biological classification level

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Response variable

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Drivers

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

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Robustness

The following is from Fulton et al 2004a -

Low to medium: while discard rate is a vital part of determining true levels of mortality and total catch, it has only limited use as an ecological indicator. The prime limitation is that it is dependent on fisheries data and so is easily confounded by changes in gear, targeting, and effort. Furthermore, a decline in the discard rate of a species does not necessarily equate to less of an impact on the species if the population is actually declining. At a gross level, overall rates of discarding can be useful for determining the breadth of the ecosystem directly or indirectly impacted by fishing, but at a finer scale it is not as robust. Its usefulness is increased if reliable effort data is available and there are fisheries independent measures of population size for the groups of interest (to ensure interpretation of declines or increases is correct).

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:

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

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 can be used to update this page

Citation

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