Implications of demographic diversity for forage fish, their fisheries, and ecosystems

Abstract

Forage fish play a key role in marine ecosystems and fisheries worldwide. They are highly productive, and undergo dramatic fluctuations in productivity, which interact with fishing pressure and spatial dynamics to generate population variability. The mechanisms and demographic processes behind this variability are poorly understood, creating a challenge for fishery managers. In this dissertation, I address the implications of demographic diversity for forage fish ecology and management. I investigate demographic diversity in space (Chapter 1) in space and time (Chapter 2), and among different forage species (Chapters 3 and 4). In the first chapter, I demonstrate that spatial diversity in a largely unfished forage fish population (Puget Sound Pacific herring; Clupea pallasii) can generate portfolio effects, which stabilize their availability as prey. In the second chapter, I build a hierarchical model to address the potential processes behind these portfolio effects, distinguishing temporal variation in mortality from spatial differences, and demonstrating that adult mortality has led to age truncation and recruitment-dominated dynamics. In the third chapter, I develop an age-structured population model and use a management strategy evaluation framework to show that demographic diversity among forage fish species (specifically, for sardine, anchovy, and menhaden-like forage species) affects management outcomes and tradeoffs. In the final chapter, I use a time series approach to investigate asynchronous dynamics in sardine (Sardinops spp.) and anchovy (Engraulis spp.), and show that although these two species are often viewed as alternating members of the forage fish guild, sardine and anchovy alone do not compose a forage portfolio that buffers fisheries and food webs from dramatic shifts in productivity. These results demonstrate the rarity of asynchronous dynamics and the importance of considering, but not relying upon, demographic diversity in forage fish populations.