Index of Relative Importance; a new perspective
Studies on fish food rely mainly in the measure of three variables, number of prey (n), weight (w) and frequency of occurrence (f). These variables are usually combined in composite indexes, like the Index of Relative Importance, IRI = f (n% + w%). Comparisons based on this index are made frequently after calculating percentage values (IRI%) and relying on percentage similarity coefficients like Renkonen’s (1938). One setback of using composed indexes is the loss of information. However it is possible to write the IRI as a sum of two sub-indexes; IRI = fn% + fw%. After calculating percentage values for both sub-indexes it is possible to plot values in a square graphic 100x100. Each point corresponds to a prey. To compare two samples the distance between points representing the same prey (or prey-group) in each sample can be calculated with the Pitagoras theorem. The sum of these distances will give the difference between samples. To normalize values we propose to calculate basic distance values for each sample as the some of distance of each prey “x” to the origin of axes “0;0”, DA = ∑√ (fn%2 + fw%2). The normalized distance between samples A and B, will be calculated as D = DAB / DA + DB. Distance can varied between “0” for exactly the same values for both samples or “1” for complete absence of common preys. Instead of distance it can be calculate a similarity as S = 1 – D. Similarity values will varied also between “0” and “1”, corresponding to completely different samples or equal ones, respectively. The advantage of this procedure is to individualize the numeric aspect (fn%) from the weight one (fw%) of the IRI index and to maximize information.
References
Pinkas, L.; M. S. Oliphant & I. L. K. Iverson (1971). Food habits of albacore, blue fin tuna, and bonito in California waters. Fish. Bull. Calif., 152: 1-105.
Rosecchi, E. & Y. Nouaze (1985). Comparaison de cinq indices alimentaires utilisés dans l’analyse des contenus stomacaux. Rev. Trav. Inst. Pêches marit., 49(3-4): 111-123.
Keywords:
feeding,
IRI,
Comparisons,
Fishes,
Indexes
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Oral Presentation
Topic:
1. ECOLOGY, BIODIVERSITY AND VULNERABLE ECOSYSTEMS
Citation:
Cunha
PL and
Cunha
MA
(2016). Index of Relative Importance; a new perspective.
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00038
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Received:
27 Apr 2016;
Published Online:
02 Sep 2016.
*
Correspondence:
Mr. Pedro L Cunha, IPMA, Div AV, Lisbon, Portugal, pgcruzlopescunha@gmail.com