John C. Field

Combined climate‐ and prey‐mediated range expansion of Humboldt squid (Dosidicus gigas), a large marine predator in the California Current System

Climate-driven range shifts are ongoing in pelagic marine environments, and ecosystems must respond to combined effects of altered species distributions and environmental drivers. Hypoxic oxygen minimum zones (OMZs) in midwater environments are shoaling globally; this can affect distributions of species both geographically and vertically along with predator-prey dynamics. Humboldt (jumbo) squid (Dosidicus gigas) are highly migratory predators adapted to hypoxic conditions that may be deleterious to their competitors and predators. Consequently, OMZ shoaling may preferentially facilitate foraging opportunities for Humboldt squid. With two separate modeling approaches using unique, long-term data based on in situ observations of predator, prey, and environmental variables, our analyses suggest that Humboldt squid are indirectly affected by OMZ shoaling through effects on a primary food source, myctophid fishes. Our results suggest that this indirect linkage between hypoxia and foraging is an important driver of the ongoing range expansion of Humboldt squid in the northeastern Pacific Ocean.

Application of a data‐assimilative regional ocean modeling system for assessing California Current System ocean conditions, krill, and juvenile rockfish interannual variability

To be robust and informative, marine ecosystem models and assessments require parameterized biophysical relationships that rely on realistic water column characteristics at appropriate spatial and temporal scales. We examine how hydrographic properties off California from 1990 through 2010 during late winter and spring correspond to krill and juvenile rockfish (Sebastes spp.) abundances. We evaluated coherence among temperature, salinity, depth of 26.0 potential density isopycnal, and stratification strength at regionally and monthly time scales derived from shipboard and mooring observations, and a data-assimilative Regional Ocean Model System reanalysis. The reanalysis captures spatiotemporal physical variability of coastal ocean conditions in winter and spring months and elucidates mechanisms connecting the spatial and temporal upwelling and transport dynamics on observed krill and rockfish abundances in spring. This provides evidence for a mechanistic connection between the phenology of upwelling in the California Current System and seasonal development of the shelf ecosystem.

Variability in rockfish (Sebastes spp.) fecundity: species contrasts, maternal size effects, and spatial differences

Over 60 species of rockfish (Sebastes spp.) reside off the coast of California, many of which are economically important to both recreational and commercial fisheries. Rockfish are live-bearers with a diverse array of reproductive strategies. Understanding the reproductive potential of an exploited stock is critical to assessing the health and status of a fishery. We investigated the reproductive ecology of four rockfish species to examine species contrasts and to determine spatial and maternal-size effects on reproductive potential. Females were sampled during the winter parturition season (November through March) of 2009 through 2012. Maternal length and somatic weight were positively correlated with relative fecundity (larvae per g somatic weight) in all four species, indicating a disproportionately greater reproductive output by larger, older females. Fecundity estimates in Chilipepper, S. goodei, and Yellowtail rockfish, S. flavidus, varied regionally, but did not significantly differ over time within the years sampled (sample sizes for Speckled, S. ovalis, and Blackgill rockfish, S. melanostomus, were too small to allow spatiotemporal comparisons). Two reproductive strategies were evident as Yellowtail and Blackgill rockfish produced a relatively highly fecund, single brood of smaller-sized larvae annually, in contrast to Chilipepper and Speckled rockfish, which produced larger-sized larvae with lower fecundity. In some regions multiple broods were common, complicating estimates of annual fecundity for these two species. There was some evidence that egg production was positively correlated with female condition, indicating that environmental variability in oceanographic conditions and productivity may drive changes in fecundity and reproductive strategy (i.e., single versus multiple broods).

Reproductive Complexity in a Long-Lived Deepwater Fish, the Blackgill Rockfish

AbstractFish populations are regulated by complex biological processes and ecosystem interactions. To inform stock assessments and management models, a clear understanding of a species’ reproductive biology is necessary, particularly in long-lived species, for which estimates of sustainable yield are sensitive to such parameters. Deviations from traditional views of iteroparity in marine fishes (e.g., prolonged adolescent periods and skipped spawning) complicate estimates of maturity and, in turn, spawning output. The Blackgill Rockfish Sebastes melanostomus, an important component of Californias commercial rockfish fishery, is a slow-growing deepwater species whose population is currently thought to be increasing after being fished to below conservation target levels. The objectives of this study were to describe the annual reproductive cycle, update maturity estimates, and detect unusual patterns of ovarian development by using both macroscopic and histological methods. Females were collected between 2...