Date of Award:

8-2023

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Trisha B. Atwood

Committee

Trisha B. Atwood

Committee

Edward Hammill

Committee

Timothy Walsworth

Abstract

Marine fisheries are one of the most impactful human activities on the planet. Since the 1950s, marine fisheries have removed billions of metric tons of marine biomass leading to substantial declines in many fish populations. Among their impacts, we have begun to investigate the role of fisheries in disrupting marine nutrient cycles. Specifically, removing biomass can change nutrient cycles by reducing the amount of nutrients stored within animal biomass.

No studies have investigated the large-scale geographical and ecological contexts of nutrient removal by fisheries over a large timescale. For my thesis, we compiled data on fishery-targeted organisms' carbon, nitrogen, and phosphorus content and quantified the amount of these nutrients removed via fisheries. Furthermore, we investigated the removal of nutrient extractions in an ecological context by quantifying nutrient loss across trophic levels and functional groups.

We estimated that between 1950 and 2018, industrial fisheries removed around 452 million metric tons (MMT) of carbon, 115 MMT of nitrogen, and 24 MMT of phosphorus. Nutrient extraction occurred most strongly over continental shelves where waters tend to be more productive and closer to land. Even in these productive regions, nutrient extractions can still threaten nutrient cycling mediated by fishery-targeted species. Selective targeting of specific species could lead to inequalities in the extraction of nutrients from different trophic and functional groups. In our case, ~62% of all nutrient extractions occurred through the targeted removal of mid-level predators, particularly pelagic species. Additionally, we saw that over time, nutrient extractions via fisheries expanded further offshore to more nutrient-poor ecosystems, such as the open ocean. Presently, researchers remain unclear on the consequences nutrient loss through biomass removal can have on marine ecosystems, especially those that have evolved to tightly recycle nutrients within food webs. As these consequences remain unclear, future studies must begin considering how nutrient losses caused by fisheries can alter marine ecosystems. Furthermore, with current initiatives to rebuild marine systems in the 21st century, managers may want to consider the benefits of restoring animal-mediated nutrient cycling.

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