Sakuma and Larson: Distribution of Cithanchthys sordidus and C stigmaeus 



527 



The decrease in buoyancy due to increased otolith 

 size, the ossification of other bony structures, and 

 the loss of the gas bladder may account for the deeper 

 bathymetric distribution of later metamorphic stages 

 in both sanddab species (Tables 3 and 4; Fig. 6). The 

 greater water density at deeper depths may reduce 

 the energy expenditure required for less buoyant fish 

 to remain pelagic; however, beyond a given range of 

 SL and otolith size, individuals may sink more rap- 

 idly. Therefore, the decrease in buoyancy may account 

 directly for the deeper bathymetric distribution of 

 later-stage pelagic sanddabs, although fish may be- 

 haviorally seek deeper water as well. 



Lenarz et al. (1991) reported that younger, smaller 

 juveniles of the shortbelly rockfish, Sebastes jordani, 

 were found deeper in the water column than larger 

 individuals during May-June. It was suggested that 

 the smaller individuals were adapted to seeking 

 deeper water as a method of avoiding offshore ad- 

 vection due to upwelling ( Lenarz et al., 1991). In con- 

 trast, the early stages of both sanddab species showed 

 a shallower distribution than the later stages (Tables 

 3 and 4; Fig. 6). The shallow distribution of early 

 stages may explain the comparisons of abundance 

 in upwelling and non-upwelling areas for the 30-m 

 depth trawls, which suggest that early stages are 

 subject to offshore advection due to coastal upwelling 

 (Table 6; Figs. 6, 8, 9, and 10). The offshore advec- 

 tion of early stages may also explain their predomi- 

 nantly offshore distribution (Figs. 8 and 9). 



The lack of significant differences in abundance 

 between upwelling and non-upwelling areas for the 

 shallow depth trawls observed in both sanddab spe- 

 cies might be explained by the fact that the majority 

 of these trawls were conducted nearshore where 

 early stages were generally less abundant and that 

 later stages were generally less abundant at the shal- 

 low trawl depth (Tables 3, 4, and 6; Figs. 6, 8, 9, and 

 10). In contrast, the 30-m mid-depth trawls, in which 

 most metamorphic stages occurred, were more widely 

 distributed and probably indicated better the rela- 

 tion between upwelling and the distribution of meta- 

 morphic stages (Tables 3, 4, and 6; Fig. 6). Figure 10 

 shows a change in distribution of metamorphic stages 

 in the 30-m mid-depth trawls; earlier stages tended 

 to be more abundant in water that had not been up- 

 welled recently, whereas later stages were more 

 abundant in recently upwelled water. 



The reduction in abundance of early stages in 

 trawls within upwelling areas may have been due to 

 the fact that these stages were present predomi- 

 nantly offshore while upwelling events occurred 

 nearshore (Figs. 8 and 9). However, the large abun- 

 dances of stage-2 individuals of both species observed 

 nearshore north of Point Reyes during sweep 1 of 



1990 associated with an extension of oceanic waters 

 toward shore (Fig. 8) and the large abundances of 

 stage-2 individuals occurring within the transitional 

 areas between offshore oceanic waters and nearshore 

 coastal waters (Figs. 8 and 9) suggest that these early 

 stages were subject to transport by ocean currents. 

 Analysis of California Cooperative Oceanic Fisher- 

 ies Investigations (CalCOFI) data by Ahlstrom and 

 Moser (1975) and Loeb et al. (1983) indicated that 

 the larvae of both sanddab species were collected both 

 nearshore and well offshore. Therefore, sanddabs of 

 both species are probably passive drifters during 

 their early life history stages as evidenced by the pre- 

 dominantly offshore distribution of their early meta- 

 morphic stages (Figs. 8 and 9) and the somewhat dis- 

 persed distribution of their larvae (Ahlstrom and 

 Moser, 1975; Loeb et al., 1983). 



In contrast to the distributional patterns of early 

 stage metamorphic sanddabs, the tendency for later 

 stages of both species to be more abundant in up- 

 welling areas was probably due to the fact that later 

 stage individuals occurred predominantly nearshore 

 (Figs. 8 and 9). Although large numbers of stage-5 

 individuals of both species occurred within upwelling 

 plumes (Figs. 8 and 9), large abundances of stage-5 

 individuals could also be found nearshore in areas 

 outside of the upwelling plumes (Figs. 8 and 9). Be- 

 cause later-stage individuals were more abundant 

 at deeper depths, they were probably less suscep- 

 tible to offshore advection by upwelling. The deeper 

 and more shoreward distribution of later-stage 

 sanddabs parallels the observations of Barnett et al. 

 ( 1984) on later stages of northern anchovy, Engraulis 

 mordax, white croaker, Genyonemus lineatus, and 

 queenfish, Seriphus politus, off southern California. 

 Larson et al. (1994) also found that late-stage pe- 

 lagic juvenile rockfish (Sebastes spp.) off central Cali- 

 fornia had a more shoreward distribution, although 

 there was no evidence that later-stage fish were dis- 

 tributed at greater depths (Lenarz et al., 1991). 



In summary, it appears that physical changes in 

 metamorphosing sanddabs are correlated with 

 changes in their depth distributions; more developed, 

 less buoyant individuals occur deeper in the water 

 column (Fig. 6). This may influence the horizontal 

 distributions of pelagic sanddabs in the upwelling 

 regions off central California. Early-stage sanddabs 

 present within the upper mixed layer would be sub- 

 ject to offshore advection associated with coastal 

 upwelling or to onshore advection associated with 

 downwelling (Table 6; Figs. 8-10). In contrast, the 

 deeper distribution of later-stage sanddabs decreases 

 their susceptibility to upwelling-associated offshore 

 advection and could potentially lead to onshore ad- 

 vection facilitated by the shoreward movement of 



