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

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 con- founds inter-size-class comparisons and may require the further division of life-stage data into addi- tional size classes. The results of the 3 morphometric measures were comparable but were not corre- lated 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.