Rockfish recruitment and trophic dynamics on the west coast of Vancouver Island : fishing, ocean climate, and sea otters

Abstract

Fundamental questions in marine ecology concern processes that underlie the replenishment of marine populations and the cascading effects of large marine predators. My dissertation describes three studies in which I investigated the recruitment and trophic dynamics of nearshore Pacific rockfishes in the context of these two themes. In the first study, my collaborators and I investigated spatial variation in rockfish recruitment success in the context of the effectiveness of Rockfish Conservation Areas (RCAs). We show that although rockfish recruitment is highly variable throughout nearshore seascapes, spatial patterns of recruitment success are generally consistent from year to year. We identify characteristics of locations that contribute to spatially recruitment patterns, including fetch, distance to open coast, tidal velocity, and sea surface temperature. Because most high-value areas with respect to rockfish recruitment fall outside of the RCA considered in this study, we propose that this protected area may be vulnerable to reduced long-term effectiveness. Next, I investigated how interannual variation in the timing and magnitude of coastal upwelling affects the recruitment success of two groups of nearshore rockfishes. I used otolith microstructure analyses to compare birth and settlement dates, pelagic durations, and growth rates of rockfishes in years of contrasting recruitment success and upwelling dynamics. I found that although growth rates of both rockfish groups responded positively to ocean temperature, the highest recruitment success I observed over a six-year period corresponded to strong and prolonged upwelling that favored onshore transportation of pelagic larvae, but not pre-settlement growth and survival. In the final study, I examined how the indirect effects of sea otters on kelp forest habitat size and productivity affect rockfish recruitment dynamics and post-settlement trophic dynamics. I tested the effects of sea otters on kelp forest size, and concomitant effects on rockfish recruitment success. I used stable isotopes and otolith microstructure analyses to compare rockfish trophic position and kelp-derived carbon content, and their relative effects on juvenile growth rates, between regions with and without sea otters. I show that kelp forests are larger in the presence of sea otters and associated with higher rockfish recruitment; however, higher kelp productivity does not translate to rockfishes having higher kelp-derived carbon contents. Instead, in the presence of sea otters, adult rockfishes had higher mean trophic levels, indicating that these populations consume higher proportions of relatively high trophic level prey, i.e. fish. These studies contribute broadly to understanding marine population dynamics, and hold important implications for predicting responses of species and ecosystems to marine protected areas, climate change, and the indirect effects of predator loss and reintroduction.