# Effects of Feeding Newly Hatched Larvae on the Growth, Survival, and Growth Patterns of Kawakawa (Euthynnus affinis) Larvae and Juveniles

## TL;DR

Feeding newly hatched larvae improves growth in kawakawa fish but lowers survival, highlighting the need for better alternative diets and insights into their growth patterns.

## Contribution

The study introduces the use of striped beakfish larvae as a biomimetic feed for kawakawa and identifies unique early growth traits in kawakawa compared to other tuna species.

## Key findings

- Kawakawa larvae fed newly hatched striped beakfish larvae showed 36% greater growth but 34% lower survival.
- Kawakawa exhibit early accelerated upper jaw growth, aiding in piscivory under captivity.
- Growth patterns varied at specific body lengths, showing morphological traits typical of scombrid fish.

## Abstract

The results of this study demonstrate that feeding live hatched larvae is essential for rearing larval and juvenile kawakawa (Euthynnus affinis), where there are no alternative feeds hatched larvae at present. Using hatched larvae of other species, such as Oplegnathus fasciatus, as feed for Kawakawa larvae and juveniles is a form of biomimicry, simulating their natural feeding habits. However, supplying large quantities of hatched larvae is labor-intensive and costly. Therefore, it is necessary to develop a formulated diet that can serve as an alternative to hatched larvae, which will require detailed nutritional studies. Furthermore, the analysis of relative growth in Kawakawa larvae and juveniles obtained in this study, along with comparisons to other tuna species, is expected to contribute significantly to advancing full-cycle aquaculture techniques for tuna. It also offers valuable insights into the ecological understanding of these species.

This study investigated the effects of feeding striped beakfish (Oplegnathus fasciatus) newly hatched larvae on the survival and growth of kawakawa (Euthynnus affinis) larvae and juveniles, as well as their relative growth patterns. Fertilized eggs of kawakawa were reared in 1 m3 experimental tanks until 13 days post-hatch (dph). From 3 to 8 dph, larvae were fed enriched rotifers, and from 9 to 12 dph, they were assigned to two experimental groups: one receiving only a formulated diet and the other receiving a combination of the formulated diet and 10,000 striped beakfish newly hatched larvae every day in each tank. The group fed newly hatched larvae exhibited approximately 36% greater growth than the group fed only the formulated diet. However, survival at 13 dph was approximately 34% lower, suggesting that further investigation is needed to determine the optimal feeding quantity of newly hatched larvae. The relative growth patterns of larvae and juveniles reared in a commercial 30 m3 tank showed morphological traits characteristic of scombrid fish, such as a longer preanal length and upper jaw length. Growth patterns varied at three key body lengths (notochord length or standard length; 3, 8–10, and 30 mm), as well as at their corresponding standard lengths. In kawakawa, the upper jaw exhibited early accelerated growth compared to other scombrid species. This characteristic is believed to facilitate the early onset of piscivory under captive conditions.