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Entering from the ocean....
After about 2-3 months of development in the open ocean, wild pua (juvenile mullet) recruit into Waiāhole and Kapalaho loko iʻa through a single mākāhā that connects the fishponds to the coast. These brackish ponds support two mullet species: the native ʻamaʻama (Mugil cephalus) and the invasive kanda (Osteomugil engeli), which are hard to distinguish at sizes under ~6cm. Since their introduction to Hawaiʻi in the 1950s, kanda populations in loko iʻa and Hawaiʻi’s nearshore waters have been rising, and there are now concerns about competition between kanda and the culturally and ecologically valued ʻamaʻama for shared environmental resources.
Pua
Pua (juvenile fish) of both ʻamaʻama and kanda enter the loko iʻa between 2.2 and 4.5cm in length in schools generally between 1 and 50 individuals. ʻAmaʻama spawn once a year and we see a window of pua ʻamaʻama recruitment from November through July. Kanda spawn and recruit year-round with highest levels of pua kanda recruitment seen May through December. We see 10 times more pua kanda entering the loko iʻa than pua ʻamaʻama!
Spot the Difference
ʻAmaʻama and kanda look nearly identical as juveniles, but develop distinct characteristics as they grow past ~6cm. ʻAmaʻama are sometimes referred to as "striped mullet" and have multiple dark lines running horizontally along the body. ʻAmaʻama also develop a blue dot at the base of their pectoral fin, where as kanda remain silver as adults.
genetics...the key to identifying pua to species
To address the challenge of early identification between the two species, a collaboration between Kumuola, Kamehameha Schools Hawaiʻi, UH Hilo, and Hawaiʻi Pacific University developed species-specific primers and a genetic barcoding methodology in 2019. DNA is extracted from a small tissue sample from an unidentified pua and amplified using PCR with primers that produce different fragment lengths for each species (~400 bp for kanda, ~600 bp for ʻamaʻama) in a process that can be completed in just 3 hours! Since January 2019, kiaʻi have proportionally collected pua samples from recruiting schools of 10 or more individuals and have analyzed over 800 individual samples.
A small tissue sample is removed from a pua whose species is unknown.
DNA is extracted from the tissue and purified in a series of seven steps.
DNA is combined with custom primers and run through a polymerase chain reaction (PCR).
PCR product is stained with fluorescent dye and run through a gel electrophoresis to identify the pua as an ʻamaʻama or kanda.
what we have learned so far...
Kamehameha Schools students used the barcoding techniques to identify the pua to species, and have built the first species-specific recruitment calendars for our system. Kumuola staff and our student leaders have been fortunate enough to bring these techniques to others across the pae ʻāina who share the goals of better understanding the cycles of both our native and invasive species in the spaces they manage. Our collaboration aims to publish both the methodology and findings of our research shortly to share in detail both our research protocols as well as our findings.
On average, pua kanda recruitment is ten times greater than recruitment of pua ʻamaʻama, with variability seen year-to-year.
Pua ʻamaʻama recruitment ranges from November through July, with highest recruitment seen in the month of April followed by May. Kanda recruitment occurs year-round, with highest recruitment observed May-December. High year-to-year variability is observed as indicated by the large standard deviation on the graphs.
For more information about this research or to partner with us, please contact Trisha Olayon at trolayon@ksbe.edu.
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