Ocean-estuary coupling in the Oregon upwelling region: abundance and transport of juvenile fish and of crab megalopae

Numerous species of fish and invertebrates move between the continental shelf and estuaries during their early life-history. The physical mechanisms that regulate such movement and the extent of coupling between the near-shore ocean and estuaries are poorly understood. It is unclear how, or whether, similar physical mechanisms regulate transport to the outer coast and between the outer coast and estuarine areas. We used high frequency light trap collections at 2 sites, outer coastal and estuarine, to compare the timing and magnitude of the relative abundance of juvenile fish and crab megalopae. The time series of juvenile fish and crab megalopae were statistically compared with physical variables indicative of wind and tidal conditions to identify potential transport mechanisms. Species examined included juvenileEngraulis mordax(northern anchovy),Sardinops sagax(Pacific sardine),Sebastes melanopsandS. caurinus(black and copper rockfish), and megalopae ofCancer magister(Dungeness crab),C. oregonensisandC. productuscombined (pygmy and red rock crabs), andPagurusspp. (hermit crabs). The abundances of juvenileE. mordaxandS. sagaxandC. magistermegalopae in the estuary were significantly cross-correlated with abundances at the outer coast (at 0 to –4 d lags). These data support the idea that estuarine ingress may be a 2-stage process with initial arrival in the near-shore as the first stage and subsequent entrance into nearby estuaries as the second stage. Significant cross correlations between species abundance and physical variables indicative of wind-driven transport, both upwelling and downwelling-related, and tidal transport, specifically the spring-neap tidal cycle, were found at both the outer coast and estuarine site. These data indicate that both wind-driven and tidal transport may contribute to the cross-shelf transport and estuarine ingress of organisms.