No abstract

A majority of the world's largest net-based fisheries target planktivorous forage fish that serve as a critical trophic link between the plankton and upper-level consumers such as large predatory fishes, seabirds, and marine mammals. Because the plankton production that drives forage fish also drives jellyfish production, these taxa often overlap in space, time, and diet in coastal ecosystems. This overlap likely leads to predatory and competitive interactions, as jellyfish are effective predators of fish early life stages and zooplankton. The trophic interplay between these groups is made more complex by the harvest of forage fish, which presumably releases jellyfish from competition and is hypothesized to lead to an increase in their production. To understand the role forage fish and jellyfish play as alternate energy transfer pathways in coastal ecosystems, we explore how functional group productivity is altered in three oceanographically distinct ecosystems when jellyfish are abundant and when fish harvest rates are reduced using ecosystem modeling. We propose that ecosystem-based fishery management approaches to forage fish stocks include the use of jellyfish as an independent, empirical "ecosystem health" indicator. DEDICATION. This paper is dedicated to our beloved friend and colleague, Hermes Mianzan, for his innumerable contributions to fisheries, zooplankton, and gelatinous zooplankton ecology.

The wide availability of ECOPATH data sets provides a valuable resource for the comparative analysis of marine ecosystems. We show how to derive a bottom-up transform from the topdown ECOPATH; couple this to a simple NPZD web with physical forcing; and use the end-toend model (E2E) for scenario construction. This steady state format also provides a framework and initial conditions for different dynamic simulations. This model can be applied to shelf ecosystems with a wide range of physical forcing, coupled benthic/pelagic food webs, and nutrient recycling. We illustrate the general application and the specific problems by transforming an ECOPATH model for the Northern Californian Current (NCC). We adapt results on the upwelling regime to provide estimates of physical fluxes and use these to show the consequences of different upwelling rates combined with variable retention mechanism for plankton, for the productivity of fish and other top predators; and for the resilience of the ecosystem. Finally we show how the effects of inter-annual to decadal variations in upwelling on fishery yields can be studied using dynamic simulations with different prey-predator relations. The general conclusion is that the nature of the physical regimes for shelf ecosystems cannot be ignored in comparing end-to-end representations of these food webs.

We describe a spatially explicit, intermediate complexity end-to-end model platform that integrates physical, trophic, and nutrient cycling processes. A two-dimensional advection and mixing model drives nitrate input into the model continental shelf domain, the transport of nutrients and plankton between sub-regions, and the export of nutrients and plankton from the model domain. Trophic relationships are defined by classical mass-balanced food web model techniques (e.g., ECOPATH). Inclusion of nitrate and ammonium nutrient pools and bacterial recycling of detritus allows consideration of alternate "new" vs "recycled" production regimes. The model platform was applied to the Northern California Current (NCC) shelf ecosystem. Seasonal upwelling of nutrients along the coast is the primary driver of NCC productivity, however the characteristics of upwelling vary considerably between years and are expected to change into the future as a result of global climate change. The model was run under alternate physical driver scenarios to examine the effects of changing upwelling characteristics on the production and spatial distribution of functional groups across all trophic levels. Productivity on the shelf had a dome-shaped relationship with upwelling strength. As the intensity of individual upwelling events increased, productivity increased throughout the food web. However, strong upwelling had a detrimental effect when the physical export of plankton exceeded the capacity of