Date of Award

Fall 12-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

Advisor

Walter Golet

Second Committee Member

Gayle B. Zydlewski

Third Committee Member

John Logan

Abstract

Atlantic bluefin tuna (Thunnus thynnus, ABFT) have been a commercially and recreationally valuable species in the Gulf of Maine (GOM) since the early 1950s. Over the past few decades, abundance, spatial distribution, and physical condition of ABFT have shifted, possibly as a result of trophic changes including the composition, distribution, and/or condition of available prey. Historically, ABFT forage has most commonly consisted of Atlantic herring (Clupea harengus), Atlantic mackerel (Scomber scombrus), squids (Cephalopoda), bluefish (Pomatomus saltatrix), and sand lance (Ammodytes spp.). The current stock assessment for Atlantic herring, the dominant prey item for ABFT, suggests a reduction in spawning stock biomass and an overall decline in the population, which, given their contribution to historical ABFT diet and high lipid content, may impact ABFT somatic condition and distribution. Conversely, Atlantic menhaden (Brevoortia tyrannus), another lipid-rich clupeid, have experienced a resurgence over the past few years within the GOM. ABFT are known to consume Atlantic menhaden regularly along the Mid-Atlantic Bight, but similar diet trends have yet to be documented in the GOM. Due to the similarities between the two clupeids, menhaden were evaluated to determine if they could be an energetically viable diet “alternative” in lieu of the historically dominant Atlantic herring. ABFT diet has been evaluated in the GOM only twice before (Chase, 2002 and Logan et al., 2015). To determine if dietary changes have occurred, the current study results were compared to stomach content analysis (SCA), energetic content (EC) analysis, and stable isotope analysis (SIA), with those of the previous GOM studies. Results of the SCA suggest that Atlantic herring are no longer the dominant prey among ABFT foraging in the GOM. Shortfin squid (Illex illicebrosus) were the dominant prey in percent presence (61.7%, 49.4%) and weight (25.3%, 19.0%) for both 2018 and 2019, respectively. Over the last three decades, percent presence of consumed Atlantic herring (29.4%, 50.0%, 23.5%) and mackerel (19.0%, 8.2%, 35.7%) have fluctuated across Chase (2002; study period 1988-1992), Logan et al. (2015; 2004-2008), and the current study, respectively. Conversely, menhaden have had little to no presence among ABFT diet in the GOM until now (1.2%, 0%, 11.7%). River herring were documented in ABFT diet for the first time. While Illex were the most common prey item within the diet, they were on average the least energetically valuable (22.78 megajoules per kilogram) within this study. Average EC for Atlantic herring (24.09 MJ/Kg) and mackerel (24.79 MJ/Kg) were energetically similar, but menhaden were the most energetically rich prey overall (26.07 MJ/Kg). Average ABFT somatic condition was significantly higher in 2018 (28.67 MJ/Kg) than 2019 (27.42 MJ/Kg). This interannual trend was also significant for tissue C:N (lipid proxy). Similar to SCA results, SIA mixing model results evaluating δ13C and δ15N of select prey also showed a high contribution of Illex (median 2018 = 11.0%, 2019 = 19.5%), but instead identified silver hake (Merluccius bilinearis) as the main prey source overall (median 2018 = 50.2%, 2019 = 45.9%). Collectively, these results suggest that changes among ABFT forage have occurred potentially influencing the energetics and distribution of one of the world’s most highly migratory species.

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