- The role that anthropogenic and natural habitats in estuaries play in long-term population trends for Oregon’s nearshore marine fishes is poorly understood, in part due to limited temporal sampling. One important nearshore marine group is northeastern Pacific rockfishes (Sebastes spp.), which are highly diverse, with around 96 documented species, and are targeted both commercially and recreationally. Previous work suggests that Oregon estuaries function as a nursery area and may play an important role in settlement and recruitment of rockfishes. No research, however, has been conducted simultaneously on their feeding ecology and growth in estuarine habitats, which is necessary to evaluate habitat use and quality.
To begin understanding the role that Oregon estuaries play for nearshore marine fishes, Chapter 2 examined the entire juvenile demersal fish community at both anthropogenic (dock) and natural (eelgrass) habitats in a marine-dominated estuary on the northeast Pacific coast for six years and measured how community structure changed temporally and differed between the two habitat types. It was found that community structure varied among years and between habitats, with greater differences in community structure being found between habitat types than among years. In general, greater juvenile fish abundances were observed at the eelgrass habitat compared to the dock habitat and community structure was less variable over time. Greater abundances and communities with reduced temporal variability at the eelgrass habitat suggests that maintaining these habitats is potentially critical for many estuary-rearing species, and continued monitoring is important.
In Chapter 3, I established which species of juvenile rockfish utilize eelgrass and dock habitats from 2015-2017 within Alsea, Nehalem, and Yaquina Bay estuaries which are moderately river dominated, highly river dominated, and highly tide dominated estuaries, respectively. I also provided an in-depth discussion of the juvenile rockfish community at Yaquina Bay to evaluate if species composition differs from previous studies. Ten species of rockfish were found: Black, Bocaccio, Brown, Canary, China, Copper, Gopher, Grass, Quillback, and Yellowtail. The largest number of individuals was found at Yaquina (n = 801), with greater abundances found at the eelgrass habitat, followed by Nehalem (n = 111) and Alsea (n = 83). Species richness decreased as ocean influence decreased; all ten species were caught in Yaquina, four species were collected in Alsea (Black, Brown, Copper, Grass) and only two in Nehalem (Black, Copper). The overall catch was dominated by Black (79%), Copper (9%), and Quillback (8%) Rockfishes. Juveniles utilized Yaquina Bay during the entire year, as total length generally increased throughout each year for multiple rockfish species.
To fill knowledge gaps on life history traits of juvenile rockfishes in Oregon’s estuaries, Chapter 4 first examines the feeding ecology of juvenile Black Rockfish. Stomach contents and carbon and nitrogen stable isotope ratios of muscle tissue were examined for juveniles collected from May to September in 2016 and 2017 at both dock and eelgrass habitats in Yaquina Bay. I found consumption of 94 different prey items, the majority of which are estuary derived, and benthic prey were most frequently consumed. In general, it appears that fish are feeding in the habitat in which they are caught, with marine-fouling prey being consumed in greater numbers at the dock habitat, whereas algae- or eelgrass-associated species were consumed more at the eelgrass habitat. The increase in both δ15N and δ13C values of muscle tissue over time and positive correlation with upwelling in 2016 suggests that upwelled, oceanic waters were the primary source of nutrients to Yaquina Bay in 2016. Yaquina Bay appears to be an important foraging ground for juvenile Black Rockfish during summer months, providing a diversity of prey items, with special importance of benthic and eelgrass associated prey.
In Chapter 5, I use daily mean increment widths of otoliths to determine the recent growth of juvenile Black Rockfish in Yaquina Bay. Recent growth was determined for the last 30 days of each individual and was compared between years and dock and eelgrass habitat types. Recent growth decreased throughout each year, which correlates to an increase in fish size. Recent growth differed between habitats in 2017 but not in 2016 and remained consistently high between years at the eelgrass habitat. Conversely, recent growth significantly decreased at the dock habitat from 2016 to 2017. Similar growth rates between habitats in 2016 may have been due to the consistency in prey resources and source of nutrients in this year.
This research is the first of its kind to analytically evaluate temporal and habitat changes in the juvenile fish community and simultaneously examine feeding ecology and growth of juvenile rockfish in an Oregon estuary. The results reveal that community structure, feeding ecology, and growth exhibited similar interannual variations. During high upwelling years, Yaquina Bay may simply be an extension of nearshore waters, where a high diversity of prey resources and less variation in source of nutrients leads to high juvenile Black Rockfish growth rates in multiple habitat types. In years of variable ocean conditions, however, habitat type may become more important for growth, where eelgrass may provide a higher quality and more consistent habitat compared to anthropogenic structure, and therefore contribute to consistent interannual growth rates. This work provides further evidence that habitat quality should be considered in management strategies and a high priority should be placed on conservation of eelgrass habitats in estuaries.