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Age and growth rates of bull shark Carcharhinus leucas [n ¼ 255; 555-2230 mm fork length (L F )] from the northern Gulf of Mexico were estimated from ring counts on vertebral sections collected from fishery-dependent and -independent surveys. Two growth models were fitted to observed data: the von Bertalanffy growth model (VBGM) with t 0 as the third parameter and a modified version of the VBGM using a fixed size-at-birth intercept as the third parameter. To address the variability in size-at-birth, a Monte Carlo simulation was incorporated into the sizeat-birth intercept. The sex-specific growth models were not significantly different, allowing a sexes combined model to be generated. The traditional VBGM predicted a theoretical maximum size (L 1 ) of 3007Á1 mm L F , a growth coefficient (K) of 0Á042 year À1 and a theoretical age at zero length (t 0 ) of -6Á844 years. The modified VBGM with a fixed size-at-birth intercept of 565 mm L F predicted an L 1 of 2289Á2 mm L F and a K value of 0Á089 year À1 . When comparing model estimates to previously published information, the traditional VBGM predicted a significantly lower theoretical maximum size and a higher growth coefficient than those produced using data collected during the 1980s. Overall, results obtained using the VBGM with a fixed size-at-birth produced more biologically realistic parameters than that of the VBGM with t 0 . The Monte-Carlo simulation incorporating variability in size-at-birth produced similar results to the VBGM using a fixed size-at-birth. This study provides the first attempt to incorporate variability at size-at-birth and provide measurements of variability around the individual parameter estimates for an elasmobranch. # 2005 The Fisheries Society of the British Isles

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Standard oxygen consumption rate (MO(2)) was determined for 19 cownose rays (Rhinoptera bonasus) using flow-through respirometry. Rays ranged in size from 0.4 to 8.25 kg (350-790 mm DW). Respirometry experiments were conducted on seasonally acclimatized rays at temperatures from 19.0 to 28.8 degrees C. Estimates of mass-dependent MO(2) ranged from 55.88 mg O(2) kg(-1) h(-1) for an 8.25 kg ray to 332.75 mg O(2) kg(-1) h(-1) for a 2.2 kg animal at 22-25 degrees C. Multiple regression analysis examining the effect of temperature, salinity, and mass on standard mass-independent MO(2) found temperature (P < 0.01), and mass (P < 0.0001) to have a significant effect on oxygen consumption, whereas salinity did not (P > 0.05). Q (10) was calculated as 2.33 (19-28 degrees C), falling between the estimates determined for two other batoid species, the bull ray (Myliobatos aquila; Q (10) = 1.87) and the bat ray (Myliobatis californica; Q (10) = 3.00). The difference in the Q (10) estimates may be attributed to the use of seasonally acclimatized as opposed to laboratory-acclimated animals.

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