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Fish and fishers are affected by the environmental conditions they experience throughout their lives, from daily, annual to decadal time scales. Currently, the oceans are changing fast, as global warming increases the temperature of the water and reduces oxygen levels within it. However, there is still an important knowledge gap about how these shifting conditions influence wild populations of fish, especially in the early life stages of tropical species inhabiting mangrove lagoons or for adult fishes dwelling in the deep ocean. In this dissertation, we use the chronological and chemical properties of otoliths – calcified structures within the inner ear of fish – to investigate how temperature correlates with fish growth, to improve our understanding of their populations, and to develop proxies for hypoxia exposure in deep-sea fishes. Chapter 1 asks how the water temperature inside mangrove lagoons regulates the first year of growth for yellow snappers in the Gulf of California. We found that these animals grow faster in warmer waters until they experience a thermal threshold (~ 32° C) beyond which their growth rate is reduced. Chapter 2 tests the effects of extrinsic (water chemistry and temperature) and intrinsic (growth rate and taxonomy) factors on otolith chemistry. Using distinct species from Galápagos (yellow snapper and sailfin grouper) and the same species (yellow snapper) between Galápagos and the Gulf of California, we observed that extrinsic factors seem to be more important than intrinsic factors as influences on otolith microchemistry. Chapter 3 examines the population structure of yellow snappers in the Gulf of California and Galápagos mangroves by using otolith microchemistry and genetic analyses in tandem. These methodologies were complementary and helped to elucidate a source-sink metapopulation structure for Galápagos snappers, and a self-recruitment scenario for the Gulf snappers, with important implications for the mangrove management at these ecosystems. Chapter 4 explores the use of fish as mobile monitors of hypoxic conditions in Oxygen Minimum Zones (OMZs). Surprisingly, fishes with distinct life-history traits (longevity and thermal history) and from different OMZs (NE Pacific and SE Atlantic), but exposed to comparable low oxygen conditions, exhibited high similarity in their otolith chemistry. These findings may provide a baseline for tracking the ongoing expansion of OMZs. Lastly, Chapter 5 inquires how fishers’ local ecological knowledge (LEK) in the Galápagos Archipelago can help to elucidate the effects of climate variability on fish. We observed that LEK is in line with the scientific literature regarding distributional shifts in marine species and anomalous weather conditions during strong El Niño years.

Spatial management is a popular tool for resource managers to protect and conserve natural resources. However, a number of emerging threats are testing the ability of these tools to address management needs. Marine protected areas and slow speed zones are popular tools employed by resource managers to mitigate anthropogenic threats; however climate change and whale ship strikes represent new threats that may complicate the benefits of these tools. This dissertation examines the efficacy of incentivizing slow vessel transits to reduce cetacean mortality risk and the application of MPAs to mitigate climate change. A trial program to monetarily incentivize slow transits through the Santa Barbara Channel showed high compliance compared to a similar voluntary program. During incentivized transits, the large majority of ships maintained a 12 knot transit speed as determined by the program guidelines. An incentivized program may be key in reducing risk to whale mortality and reducing ships speeds; however scaling up this program may face financial difficulty.Marine Protected Areas have been claimed to offer additional protection to areas affected by climate change. However, a recent warm water marine heatwave changed the fish community’s abundance, biodiversity, and recruitment around the Channel Islands. While the ecological community changes across strong longitudinal biogeographic patterns, forecasts built from GLMs with environmental conditions predict shifts in species abundance. Upwelling and cool waters coming to the surface may mitigate warming ocean conditions in the region but marine protected areas showed no increased resilience to acute climate affects like marine heatwaves.

Richly productive polar marine ecosystems are hypothesized to have evolved within the last ~30 million years through the rise of diatoms to ecological dominance and diversification of distinctive polar fish, sea birds, seals and whales. Today, short diatom-based food chains support substantial fish biomass, but whether polar fish production is high enough to sustain current industrial fishing is unknown. To this end, we compared ichthyolith accumulation rates (IAR), a proxy for fish production, across ocean ecosystems to trace the development of global fisheries stocks over the past 1.8 million years. We find that the magnitude of polar fish production, based on the flux of fish teeth to deep-sea sediments, is an order of magnitude lower than seen in subtropical and tropical sites. We suggest that polar fish production is systematically suppressed by extreme seasonality, phenological mismatch, low functional redundancy, and extreme glacial-interglacial climatic variability in the high latitude oceans. Comparisons of our Pleistocene data to similar records from the Eocene and Oligocene oceans (~42-28 Ma) show that fish production in high latitudes has been consistently low for the last 30-40 Ma relative to most of the tropical and subtropical locations. We conclude that the stock crashes observed in the polar regions over the past several decades reflect overexploitation of ecosystems that have had low fish production for tens of millions of years.