Age and Growth of the Shortfin Mako, Isurus oxyrinchus, Using Four Methods

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Abstract
Age and growth rate of shortfin mako, I. oxyrinchus, captured by hook and line between 1961 and 1981 were determined using 4 methods: temporal analysis of length-month information, results of tagging data, length-frequency analysis, and ring counts on vertebrae. A temporal analysis of length-month information relating size to age for individuals < 150 cm fork length (FL) was used to determine size (60-70 cm FL), time of birth (late spring) and early growth rate (50 cm/yr for ages 0-1, 32 cm/yr for ages I-II). This growth rate was used as a basis for interpreting the accuracy of other methods. Annual growth rates were calculated from 32 tag-recaptured mako sharks and resolved graphically into a growth curve. Length-frequency modes extended age estimates to intermediate-sized makos. Counts of growth rings on vertebral centra stained with AgNo3 were employed to back-calculate lengths at ages for the full range (69-328 cm). The interpretation was based on the hypothesis that 2 rings are formed on the centrum each year; these age estimates agreed well with results from other methods. Males and females had a similar growth rate even though females grow much larger than males. The oldest female in the sample was 11.5 yr at 328 cm. The oldest male was 4.5 yr at 225 cm. The von Bertalanffy asymptotic growth function adequately described shortfin mako growth: male L.infin. = 302 cm, K = 0.266, t0 = -1; female L.infin. = 345 cm, K = 0.203, t0 = -1 [L = fork length, K = constant (the rate at which the growth parameters approach L.infin.), t = estimated age].

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