Graduate Thesis Or Dissertation
 

Grow fast or die young: influence of ocean conditions and larval growth on patterns of selective mortality and settlement variability of rockfishes

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  • Most benthic marine fishes have a biphasic life cycle with a dispersive pelagic larval stage that spends weeks to months in the plankton before metamorphosing into juveniles and settling to benthic habitats. The magnitude of mortality during the early life stages of marine fishes typically drives variability in year-class strength. Large size, fast growth, and rapid development during early life are predicted to increase survival through this treacherous period. As climate induced ocean warming progresses, both growth and mortality rates of larval fishes are expected to increase with potential consequences for fish populations. We examined the influence of oceanographic conditions on size, early growth, development, and survival across seven annual cohorts (2013-2019) of black rockfish, Sebastes melanops, settlers to nearshore rocky reefs along the Oregon coast. This period included the large marine heatwave (2014-2016) that impacted the physical and biological oceanography of the California Current, providing a glimpse into how black rockfish may respond to future ocean conditions. For three of these seven cohorts (2016-2018), we also tested for patterns of selective mortality across the pelagic juvenile, settlement, and recruitment stages. Finally, we compared growth rates of pelagic juvenile quillback rockfish, Sebastes maliger, consumed by, and simultaneously collected with, juvenile coho salmon Oncorhynchus kisutch. Otolith microstructure analysis revealed that size did not affect survival, that development was negatively related to larval growth, and that variation in water temperature explained most of the variability in both black and quillback rockfish growth. However, black rockfish survival was not directly related to ocean conditions and not positively related to larval growth. Instead, the relationship between growth and survival was dome-shaped, with settlement magnitude peaking at an intermediate larval growth suggesting an optimal growth window. We hypothesize that low survival in years with the most rapid black rockfish growth is likely due to the absence of suitable prey necessary to sustain this high growth. Black rockfish experienced strong selective mortality favoring rapid larval and pre-settlement growth in a year with warm water temperatures and high settlement magnitude but there was no evidence of selective mortality when water was cooler. Larval quillback rockfish grew faster in warmer water, but their susceptibility to predation by juvenile coho salmon was driven by growth immediately prior to the encounter. Our findings demonstrate the importance of temperature and temperature-mediated patterns of selective mortality in regulating early growth and survival of black and quillback rockfishes. A more mechanistic understanding of the processes underlying growth and survival will enhance our ability to predict how future changes to ocean conditions will impact early growth, survival, and, ultimately, population dynamics of nearshore fish populations.
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