Trophic effects of fishing on temperate coastal food webs and ecosystem dynamics

I examined the relative importance of top-down (consumer-driven) versus bottom-up (resource-driven) control of two temperate, rocky reef food webs. In Alaska, I quantified the direct and indirect effects of a keystone consumer removal by a subsistence fishery, while in New Zealand I examined the ecosystem-level consequences of previously-fished consumer recovery in four marine reserves.; In Alaska, localized reductions of the black leather chiton ( Katharina tunicata) is a direct consequence of shoreline collection by humans and sea otter (Enhydra lutris) predation. Experimental removals at sites varying in ambient chiton densities, revealed that at high densities, K. tunicata can reduce ribbon kelp (Alaria marginata) production by 98% and intertidal species richness by 38%. Stronger population-level impacts at higher K. tunicata density arise because its per capita effects were constant as a function of ambient chiton densities. A. marginata biomass was 7 times greater at exploited versus unexploited sites, providing evidence of a trophic cascade and revealing the extent to which fishing and natural predation, via the reduction of a shared keystone prey, can indirectly alter ecosystem dynamics. At a regional scale, herbivore removal experiments conducted within 3 oceanographic areas varying in nutrient status, revealed that algal-herbivore interactions were affected by nutrient supply and indirectly mediated by barnacle recruitment. At this scale, nearshore community dynamics were driven by interactions between small-scale benthic processes from the top-down and large-scale pelagic forcing from the bottom-up.; In New Zealand, the recovery of previously fished predators, snapper (Pagrus auratus) and lobster (Jasus edwardsii), within 2 marine reserves have resulted in localized declines of sea urchins (Evechinus chloroticus), and an order of magnitude more kelp biomass compared to fished sites. Kelp-derived organic carbon assimilated by filter feeders, oysters (Crassostrea gigas) and mussels (Perna canaliculus), transplanted within these reserves was 1.6 times greater than those outside. Kelp detrital enrichment was not observed in two younger reserves, where protection status had no effect on urchin density or kelp biomass. Growth rates of filter feeders did not differ with protection status. Consequently, the recovery of top-down regulation and the cascading effects on benthic primary production, had bottom-up consequences on carbon flux.