The Team: Trika Gerard  (Lead Investigator, NOAA Southeast Fisheries Science Center (SEFSC), trika.gerard@noaa.gov),  John Lamkin (NOAA), Angela N. Knapp (Florida State University), Michael R. Landry (Scripps Inst. of Oceanography, University of California San Diego), Karen E. Selph (University of Hawaii at Manoa), and Michael R. Stukel (Florida State University)

Technical Monitor: Barbara Muhling (barbara.muhling@noaa.gov)

Federal Program Officer/Point of Contact: Frank Parker (frank.parker@noaa.gov)

This project began in June 2017 and ended in May 2022.

Award Amount: $1,613,288

Why it matters: The largest recreational fisheries in the U.S. occur in the Gulf of Mexico, including economically important species such as bluefin, skipjack, and yellowfin tuna. Atlantic bluefin tuna are highly migratory and spawn in the Gulf of Mexico, but are distributed as adults throughout the Atlantic Ocean. This migratory behavior, as well as year to year changes in environmental conditions at sites where Atlantic bluefin tuna spawn, makes management of the species complex. Being able to track how changes in nutrient availability impact tuna food webs and larval survival is essential to managing open ocean ecosystems.

What the team did: This project investigated the impact of variability in a key nutrient, nitrogen, on lower food webs and the resulting availability of zooplanktonic food resources for Atlantic bluefin tuna larvae in the Gulf of Mexico ecosystem. This information allows for improvement in Atlantic bluefin tuna stock assessments by making it possible to forecast the feeding, growth, and survival of Atlantic bluefin tuna larvae based on ocean conditions. To accomplish their objectives, the investigators determined the sources of nitrogen to the ecosystem and measured plankton biomass and rates of primary productivity. They also assessed the abundance, stomach contents, feeding selectivity, age and growth rates of Atlantic bluefin tuna larvae. This project sought to improve Atlantic bluefin tuna stock assessments and ultimately improve understanding of open-water ecosystems and how variations in productivity impact bluefin tuna.

Summary of Outcome: The objective of this project was to improve western ABT stock assessment by elucidating the mechanisms that link variability in nitrogen sources and food-web dynamics in the GoM to habitat quality, feeding, growth and survival for ABT larvae. The primary accomplishment of the past year has been the analyses, writing and publication of a volume of papers on the project results in the Journal of Plankton Research. In addition, our key accomplishments are:

1. Establishment of a high precision method of Compound Specific Isotopic Analysis of Amino Acid (CSIA-AA) Nitrogen and the effect of chemical preservation of fish tissue amino acid nitrogen isotopes (Swalethorp et al., 2020).
2. Mesozooplankton biomass in the oceanic GoM is similar to the long-term average for the subtropical Pacific at Hawaii Ocean Time Series (HOT) Stn. A Long-term Oligotrophic Habitat Assessment (ALOHA) but higher than measured at Bermuda Atlantic Time-series Study (BATS) or in the subtropical Indian Ocean. Interestingly, the ratio of zooplankton carbon standing stock to mean daily carbon primary production (1.08 in the GoM) is substantially higher than either HOT (0.68) or BATS (0.48) (Landry and Swalethorp, 2021).
3. The oceanic GoM is similar to the subtropical North Pacific in terms of a dominant role (≥50%) of Prochlorococcus and falls intermediate between the equatorial Pacific and Costa Rica Dome in the relative contribution (7–16%) of Synechococcus. The GoM is further distinguished by relatively high contributions from prymnesiophytes (14 to ≥40%) and low contributions from diatoms and dinoflagellates (Landry et al., 2021).
4. Deep water Gulf of Mexico sites are dominated by ≤2 μm phytoplankton, similar to communities in open ocean oligotrophic gyres, with distinct assemblages found in surface mixed layer and deep chlorophyll maximum habitats (Selph, et al., 2021).
5. Dietary composition shifts from smaller (copepod nauplii, appendicularians and ciliates) to larger prey (calanoid copepodids and especially cladocerans) during larval development (Shiroza et al., 2021). Quantitative importance of ciliates (up to 9% of ingested C) is documented for the first time, and postflexion larvae are shown to be highly selective for cladocerans (up to 70% of ingested C). Diet and prey selection is broader (generalist feeding) when preferred taxa (notably cladocerans) are rare, but narrows sharply, implying active prey selection, when preferred prey are more abundant.
6. Despite lower primary production, export rates at the base of the euphotic zone are slightly higher than measured for HOT and BATS, leading to higher export ratios of 10–20% in the GoM. Similar to BATS and HOT, subsurface nitrate fuels the majority of export in the GoM (Knapp et al., 2021), although it is likely transported to deepwater regions via lateral currents (Kelly et al. 2021). Transfer efficiencies through mesopelagic depths are similar in the three regions giving a slightly higher proportion of NPP sequestered in the deep ocean of the GoM (Stukel et al., 2021).
7. Larval ABT likely benefit from horizontal transport, which advects prey-rich water from coastal regions thus supporting first-feeding larvae in generally prey-deficient regions (Shropshire et al. 2021) and from distinct foodweb pathways that minimize energy loss from phytoplankton to larvae (Stukel et al. 2021b).