MEPS 380:199-212 (2009)  -  doi:10.3354/meps07918

The condition conundrum: application of multiple condition indices to the dusky shark Carcharhinus obscurus

Nigel E. Hussey1,*, David T. Cocks1, Sheldon F. J. Dudley2,3, Ian D. McCarthy1, Sabine P. Wintner2,3

1Nuffield Fish Laboratory, School of Ocean Sciences, College of Natural Sciences, Bangor University, Menai Bridge, Anglesey LL59 5AB, UK
2KwaZulu-Natal Sharks Board, Private Bag 2, Umhlanga Rocks 4320, South Africa
3Biomedical Resource Unit, University of KwaZulu-Natal, PO Box X54001, Durban 4000, South Africa

ABSTRACT: Measuring fish condition has become a standard practice in the management of fishes, both at the individual and population level. The comparative application of several condition indices to sharks, however, has not yet received a rigorous evaluation. Data for a total of 2120 dusky sharks Carcharhinus obscurus (Lesueur, 1818), ranging in size from newborns (≤79 cm precaudal length [PCL]) to mature adults (≥210 cm PCL), were used to calculate seasonal trends in condition and to facilitate index comparisons. Four commonly used condition measures were selected, including a somatic measure, hepatosomatic index (HSI), and 3 morphometric measures, condition factor (CF), relative condition (Kn) and residual condition (RrPCL). The effect of month was significant for most condition analyses by size class, sex and reproductive state. HSI was found to be the most sensitive index and rapid indicator of condition, but its appropriate use requires the disaggregation of data by clearly defined life stages and reproductive states. The relatively large liver size of neonates and the relatively small liver size of pregnant and postpartum females may otherwise bias interpretations of seasonal variations in condition. HSI was also affected by increasing size of the animal, which confounds inter-size-class comparisons and may require the further division of life-stage data into additional size classes. The results of the 3 morphometric measures were comparable but were not correlated with HSI. CF, Kn and RrPCL lagged behind HSI, were unable to differentiate between neonate and juvenile animals and were insensitive to short-term variations. The effect of increasing size did not affect calculated CF and RrPCL, but Kn demonstrated a negative correlation. The fact that large predators may consume large volumes of food in a single feeding event was identified as a possible complicating factor in interpreting condition indices.

KEY WORDS: Hepatosomatic index · Condition factor · Morphometric condition indices · Carcharhinus obscurus · Ontogenetic variation

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Cite this article as: Hussey NE, Cocks DT, Dudley SFJ, McCarthy ID, Wintner SP (2009) The condition conundrum: application of multiple condition indices to the dusky shark Carcharhinus obscurus. Mar Ecol Prog Ser 380:199-212

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