Graduation Date
Fall 2025
Document Type
Thesis
Program
Master of Science degree with a major in Natural Resources, option Fisheries
Committee Chair Name
Jose Marin Jarrin
Committee Chair Affiliation
Cal Poly Humboldt Faculty or Staff
Second Committee Member Name
Andre Buchheister
Second Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Third Committee Member Name
Eric Bjorkstedt
Third Committee Member Affiliation
Cal Poly Humboldt Faculty or Staff
Keywords
Galapagos, Fisheries, FAD, Yellowfin tuna, Aggregation, Oceanography, Statistics
Abstract
The Galapagos Marine Reserve was established in 1998 to protect biodiversity and reduce fishing pressure by only allowing artisanal fishing. Despite its small scale, artisanal fishing has impacted vulnerable, slow-growing species. As a result, managers are attempting to redirect fishing toward faster-growing, migratory species like yellowfin tuna. To support this shift, four fixed fish aggregating devices (FADs) were deployed in 2016 off Isabela, Floreana, and San Cristóbal islands. These anchored buoys were equipped with solar-powered sonars to estimate pelagic fish biomass,primarily yellowfin tuna,within the upper 100 meters of the water column. Although data was collected at four FADs, the analysis presented here focuses on the overlapping records available between January to July 2018 from three sites: Isabela, PuntaPitt, and SCNorte. Observations were summed by day at each site, yielding 415 days of biomass estimates in total ranging from 0 to 560 tons (mean 44.6 tons). Zero-biomass days were relatively rare (~10%), but were most frequent at PuntaPitt. I modeled temporal variation in biomass using a negative binomial generalized additive mixed model (GAMM) with site as a fixed effect, site-specific smooths of time, and smooth terms for sea surface temperature (SST), distance to nearest front, and frontal gradient strength (Maximum Sobel), while accounting for temporal autocorrelation using a continuous-time AR(1) structure. The model explained ~34% of the variability in daily biomass, which was substantially higher at the Isabela FAD than at PuntaPitt or SCNorte, and exhibited strong, site-specific temporal trends. Among environmental predictors, frontal gradient strength had a negative effect on biomass, whereas temperature and distance to the nearest front showed weaker or non-significant effects over the study period. The negative relationship between fish biomass and the Maximum Sobel index could indicate that when fronts are present, yellowfin tuna prefer them, but when they are not they inhabit waters below FADs, potentially because they think there is a front at that location or as an alternative gathering location A second model (negative binomial generalized linear mixed model) was run to study how biomass varied among depths at each of the three FADs. Biomass was highest at Isabela and SCNorte on the upper part of the water column (8.6-31 m), while the opposite was true at PuntaPitt with biomass being highest at the bottom of the water column (75.8 to 109.4 m). Differences among depths, may be because the fish are located above or below the thermocline, which is often at 30 m in depth in the Galapagos. Similar to previous studies, these findings indicate that under site specific conditions, fishers can expect to find high biomass of yellowfin tuna under FADs. Understanding environmental drivers of biomass around FADs can guide future management to balance environmental protection with sustainable fishing practices redirecting fishing effort in the Galapagos Marine Reserve.
Recommended Citation
Moreau, Sarah Payton, "Factors associated with the biomass of Yellowfin tuna, Thunnus albacares, at fixed fish aggregating devices (FADs) in the Galapagos Islands" (2025). Cal Poly Humboldt theses and projects. 2504.
https://digitalcommons.humboldt.edu/etd/2504