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The Mystery Fish: Shedd Aquarium Invests in Larval Marine Fish Science and Sustainability

Wild Reef’s shark habitat, mirroring a thriving coral reef, is literally teeming with life. When Shedd’s animal care experts found new eggs in the habitat that they at first couldn’t identify, they embraced the surprise and spent months solving the case of the mystery fish.

Hear an account from Shedd’s Manager of Sustainability Rachel Zak about how the unexpected turned into a learning opportunity, a growing sustainability program, a flourishing population of this fish species among the Association of Zoos and Aquariums (AZA) community and a promising future for declining wild fisheries across the globe.

Fishes with silver bodies and yellow tails swim around the large shark habitat in Wild Reef.

Shedd Aquarium has long been a pioneer in aquarium science and the care of fishes. As an organization that cares about and advocates for ocean health, we are one of many institutions across AZA to join together to share best practices and years of learnings in animal care to establish sustainable fish collections in-house.

When AZA created the Aquatic Collection Sustainability Special Committee (ACSC) in 2019, Shedd answered the call to lend expertise to the effort. A working group within the committee quickly advanced methods of sharing techniques and equipment for the collection of eggs from our established fish populations to then document the rearing of species most important to the community.

At that time, Shedd’s Larval Fish team was already working on breeding a species often called a scaled herring or scaled sardine (Harengula jaguana) found in the Wild Reef shark habitat to economically sustain that population of schooling fish.

It was soon realized that a second type of egg was in the egg collector. Photographing the eggs under high-powered microscopes, we could see they were much smaller than the pilchard egg, but we could not yet identify them.

There were less than 10 options of pelagic species, or fish that inhabit the water column, that could spawn in the habitat, so we decided to attempt to hatch the eggs to determine the species and see if we could successfully rear them.

For the next two months we all tried to crack the puzzle through literature searches on all possible species to reference in comparison to the larval morph stages we were observing.

A tiny larval fish shown through a microscope.

A day-old "mystery fish" pictured under the microscope.

A woman in a Shedd jacket peers through a microscope.

Aquarist Jenny Richards, and member of the Larval Fish team, analyzes a day-old "mystery fish" through a microscope.

In March 2020, we had our first fish transition to its juvenile stage, and we were able to identify it as a yellowtail fusilier (Caesio cuning)! We also confirmed this knowledge with our colleagues in Shedd’s Molecular and Microbial Ecology Lab through DNA sequencing. Only four individuals fully developed in our first batch of eggs, but we had begun to document a protocol for rearing the species.

Once we figured out what species we were raising, we started to collect eggs daily from Wild Reef to track the breeding habits of the school of yellowtail fusilier. How often were they breeding? Did they require an adjusted diet for egg production? Were there ideal times throughout the year that were better for collecting eggs? We quickly found out they were spawning continuously all while dazzling our guests in Wild Reef!

Over the next two years, our team focused on improving our methods to raise yellowtail fusilier. Raising this fish is difficult because when they hatch, they are completely helpless. They don’t have a brain or eyes, and the width of their mouth is about two tenths of a millimeter. In the wild, they would free float in the water column and eat zooplankton. We must recreate that environment to the best of our ability. With practice and experience, we went from only four to 10 fish reaching a juvenile stage per batch to a whopping 30 to 50 fish per batch!

A woman behind the scenes in Wild Reef looks into a round basin lit by a blue light.

Aquarist Jenny Richards tends to larval yellowtail fusilier behind the scenes.

Tiny translucent fishes swim in a basin, lit from above by a bright white light.

Yellowtail fusilier, about a month old, grow behind the scenes.

In May 2021, we moved our first group of eight Shedd-raised yellowtail fusilier into the Wild Reef shark habitat. For the first month we observed those individuals, keeping detailed notes on their behavior and found that they acclimated incredibly well. We continue to add Shedd aquaculture yellowtail fusilier to the habitat. Now, over 70% of the population have come through our larval program efforts.

Our next milestone was to achieve the successful transport of our juvenile yellowtail fusiliers to an AZA partner aquarium. In November 2022, 60 yellowtail fusiliers arrived at Seattle Aquarium where they are now thriving in their new home.

A silver fish with a yellow tail swims among corals in Shedd's Wild Reef.

Yellowtail fusiliers swim in the shark habitat in Wild Reef.

The ACSC piloted a Larval Production and Distribution Cooperative Program in 2022 across AZA with five species offered from aquarium aquaculture programs. Shedd is now offering 2023 co-op members yellowtail fusiliers from our newly formed Sustainability Team. Co-op requests came from 11 AZA member aquariums from across the country, for whom the Sustainability Team will be raising and distributing over 1,000 yellowtail fusilier this year.

Also, the Washington Sea Grant from the National Oceanic and Atmospheric Association (NOAA) was awarded in 2022 to ACSC-involved aquariums to further invest in this aquaculture effort over the next three years, helping to heighten the importance of this work to address the concerns of declining fisheries globally. As we embark on our journey toward Shedd’s Centennial Commitment, the Sustainability Team will be building on this success in the years to come.

- Rachel Zak, Manager of Sustainability