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Fishery Bulletin 104(3) 



gid for which we could find information on ontogeny of 

 swimming abilities (Sakakura and Tsukamoto, 1999), 

 although the method of measuring swimming perfor- 

 mance (so-called "routine speed") differed from those 

 used in our study. Reared S. quinqueradiata larvae 

 and juveniles of 5-40 mm TL were filmed swimming 

 in small (30-cm diameter, 7-cm deep) laboratory tanks 

 lacking currents and the speed was measured over 3- 

 min intervals. Speed of S. quinqueradiata increased at 

 about 0.4 cm/s per each mm of growth until about 10 

 mm TL, after which speed remained steady at about 

 2-2.5 cm/s. This is much slower, both in terms of rate of 

 increase and absolute speed, than was found for either 

 measure of swimming speed in C. ignobilis. Possibly, 

 the slow speeds for S. quinqueradiata were due to the 

 small containers in which the fish were confined and 

 which lacked currents, as has been found for other spe- 

 cies (Theilacker and Dorsey, 1980). 



Although we did not study larvae smaller than 8 

 mm SL, it is safe to assume that smaller larvae have 

 poorer swimming abilities. Previous investigations of 

 swimming ontogeny have revealed that critical speed 

 increases steadily with size (Fisher et al., 2000; Clark 

 et al., 2005), as we did in our study; therefore it seems 

 likely that a reasonable estimate of performance of 

 smaller larvae can be obtained by extrapolating the 

 linear relationship in Figure 2. Thus, a larva of 5 mm 

 (about the size when the caudal fin is formed in caran- 

 gids; Leis and Carson-Ewart, 2004) would be expected 

 to have critical speeds of about 4-5 cm/s. We found 

 that critical speeds for C. ignobilis were about twice in 

 situ speeds, and similar relationships have been found 

 for larvae of other species at other sizes (Fisher and 

 Wilson, 2004; Leis and Fisher, in press). Therefore, a 

 5-mm C. ignobilis would be expected to be able to swim 

 at about 2 cm/s (=72 m/h) in the ocean. This speed 

 would have limited effectiveness for directly influencing 

 horizontal position, but would be sufficient to determine 

 vertical distribution at scales over which fish larvae are 

 known to migrate vertically. 



To put these swimming performances into an ecologi- 

 cal context requires knowledge of ambient currents in 

 the area of interest. If it is assumed that effective swim- 

 mers (those with swimming speed equal to or greater 

 than average current speed [Leis and Stobutzki, 1999]) 

 have the ability to control their dispersal by horizontal 

 swimming, then the size at which larvae become effec- 

 tive swimmers is of interest to dispersal modelers. For 

 example, at Lizard Island on the Great Barrier Reef, 

 average current speeds are 10-15 cm/s (Frith et al., 

 1986). At Lizard Island, average performing C. ignobilis 

 would become effective swimmers at about 7-9 mm SL 

 on the basis of critical speed, and at about 11-14 mm 

 based on in situ speed which is about half of critical 

 speed (assuming the relationship between in situ and 

 critical speeds found in the present study). The best 

 performers would be able to reach these mean Lizard 

 Island current speeds at about 4-6 mm SL, if based 

 on critical speed (assuming the relationship in Fig. 2 

 applies to smaller larvae), and at 8-12 mm if based on 



in situ speed. Larvae that are too small to be effective 

 swimmers may still readily influence their dispersal by 

 other means, particularly by vertical swimming where 

 current velocity is not uniform with depth (Sponaugle 

 et al., 2002; Paris and Cowen, 2004). Speeds of only 1 

 cm/s (36 m/h) are sufficient for vertical migration, and 

 this could be achieved by very small larvae. Indeed, 

 vertical movements by small carangid larvae are well 

 documented in studies where plankton nets were used 

 (e.g., Ahlstrom, 1959; Olivar and Sabates, 1997; Flores- 

 Coto, et al., 2001). 



In situ speeds of C. ignobilis are within the range 

 of those reported for other perciform species, although 

 most of the available values are for settlement-stage 

 larvae of demersal reef fishes (Trnski, 2002; Leis and 

 Fisher, in press). Further, the rate of increase in speed 

 at 1.6-2.6 cra/s per each mm of growth is similar to 

 that found in the three other perciform species for 

 which there are data (0.3-2.0 cm/s per each mm of 

 growth; Leis et al., 2006). Differences in swimming 

 speed among areas has been reported in a variety of 

 reef fish larvae (Leis and McCormick, 2002); therefore 

 finding a difference for C. ignobilis was not unexpected. 

 This variation in behavior among areas and the fact 

 that the rate of increase in speed with growth in the 

 field was less than the increase of U^^^^ in the laboratory 

 clearly demonstrates the complexities of applying labo- 

 ratory measures to the field, and how behavior in the 

 field can vary with location and situation. The cause for 

 differences in behavior among locations was not clear 

 in our study, but it is only through in situ observations 

 that such differences can be discovered. 



The approximately linear increase in swimming speed 

 with growth found in C. ignobilis was similar to that 

 reported in other species (Fisher et al., 2000; Clark et 

 al., 2005). The ontogeny of endurance in C. ignobilis 

 was also linear, in contrast to that of some other spe- 

 cies, which are reported to have a strongly concave 

 curvilinear increase in endurance with growth (Fisher 

 et al., 2000; Clark et al., 2005). Perhaps this linearity 

 was the result of studying C. ignobilis larger than 8 

 mm. In other species, preflexion and early postflexion 

 larvae (which we did not study) have very low endur- 

 ance, but endurance increases rapidly with growth in 

 postflexion larvae. 



All our speed measurements were made over periods 

 of 10 minutes or less, but endurance measurements 

 showed that speeds of 10 cm/s can be maintained from 

 several to many hours, enabling distances of up to 40 

 km to be traversed. The ecological relevance of labora- 

 tory measurements of swimming endurance is difficult 

 to assess because, on one hand, these are forced mea- 

 sures of performance, and it is unlikely that larval 

 fishes would swim to exhaustion. On the other hand, 

 endurance measurements are made without rest or food, 

 and are therefore conservative because larvae of other 

 species of the size studied in the present study can 

 swim at least three times farther when fed (Fisher and 

 Bellwood, 2001; Leis and Clark, 2005). In any case, it 

 is clear that C. ignobilis larvae can swim for extended 



