FISHERY BULLETIN: VOL. 78, NO. 2 



Other natural history patterns also support the 

 relationship between community zonation and 

 substrate motion. In particular, the small 

 peracarid and ostracod crustaceans seem well 

 suited for life in the shifting substrate of the crus- 

 tacean zone. Their durable chitonous exoskeleton 

 and general activity probably ensure a greater 

 survival relative to many soft-bodied and more 

 sessile forms. Furthermore, their brooding habit 

 and the parturition of large, armoured juveniles 

 undoubtedly increased recruitment success. 

 While no general pattern of crustacean mobility 

 characterized a particular depth, almost all the 

 crustaceans were active burrowers and few main- 

 tained a permanent tube or burrow. The lack of 

 dependence on such structures is probably impor- 

 tant to the persistence of these large, shallow- 

 water populations. In addition, the most fre- 

 quently occurring polychaetes in the crustacean 

 zone were also active burrowers that did not live in 

 permanent tubes or burrows. 



Variations in zonal patterns along the two 

 sandflats also support the wave-disturbance 

 hypothesis. Wave activity and therefore bottom 

 disturbance were greater along the northern 

 sandflat, where faunal zones were wider and 

 shifted into deeper water. At least one seasonal 

 change in zonation was coincident with an in- 

 crease in wave activity. The season (late fall and 

 winter) and location (shallowest station, M-1, 6 m) 

 of the greatest wave-induced bottom currents 

 were characterized by the lowest recruitment and 

 survival of polychaetes. 



The last source of evidence supporting the 

 hypothesis involves the canyon. While the gra- 

 dient of substrate motion along the sandflat was 

 caused by waves intercepting a shoaling bottom, 

 the gradient of substrate motion along the 

 canyon ridge transect was caused by unidirec- 

 tional sediment slumping at a constant depth. De- 

 spite these very different types of sediment move- 

 ment gradients, changes in community zonation 

 along the canyon ridge and sandflat transects 

 were similar. This result negates the importance 

 of several other ecological factors that vary with 

 water depth along the sandflat, but were held con- 

 stant by the canyon contrast. These include light, 

 temperature, the deposition and resuspension of 

 fine food particles, and the zonation of bottom fish. 



The deposition and resuspension of fine parti- 

 cles, which might be used as food, depend upon 

 bottom currents. The strongest bottom currents 

 were caused by wave swell (see Environmental 



Setting). These oscillatory bottom currents are 

 highly dependent upon water depth and other fac- 

 tors that did not vary along the canyon ridge 

 transect. Thus, while there were probably sig- 

 nificant variations in the availability of suspended 

 particles to the different sandflat stations (i.e., 

 their fauna), deposition and resuspension were 

 apparently uniform along the canyon transect. 



Demersal flatfish have a zonation that coincides 

 with the zonation of bottom invertebrates. Many 

 species of fish become more abundant with in- 

 creasing water depth and are more common in the 

 polychaete zone (Table 5). The only species that 

 was numerous in <20 m was the speckled 

 sanddab, Citharichthys stigmaeus. Its peak abun- 

 dance, however, was in 14-18 m and decreased 

 markedly in shallower depths (Ford 1965; 

 Kukowski 1973). Since these flatfish are major 

 predators of sand-bottom invertebrates and they 

 primarily feed by sight (Ford 1965; VanBlaricom 

 1978; Hulberg and Oliver 1979), active, surface- 

 dwelling crustaceans might be particularly sus- 

 ceptible prey. If this is true, the depth-related in- 

 crease in bottom feeding fish might account for the 

 correlated decrease in the shallow-water crusta- 

 ceans. The changes in community zonation along 

 the canyon ridge do not support this idea. Large 

 and highly mobile flatfish can easily patrol the 

 entire length of a 40 m transect. 



In summary, trends in the natural history of the 

 animals, changes in zonal patterns along the 

 southern and northern sandflats, seasonal pat- 

 terns of polychaete recruitment and survival in 

 the shallows, and the similarity between the can- 

 yon and sandflat transects support the contention 

 that community zonation is influenced by changes 

 in wave-induced bottom disturbance. Alternate 

 explanations concerning changes in physical 

 sedimentary parameters (Table 1), the avail- 

 ability of suspended food, and the zonation of large 

 flatfish are not consistent with as many observa- 

 tions. One potentially important alternate 

 hypothesis could not be evaluated here. This is the 

 effect of active crustaceans on the settlement and 



Table 5. — Total number of species and individuals of fish and 

 abundance of the three most common demersal flatfish in otter 

 trawls taken in Monterey Bay, Calif, (from Kukowski 1973). 



'Mean number caught per 10-min tow with 20 tows at each depth. 



452 



