PRE-ONGROWING OF ATLANTIC BLUEFIN TUNA Thunnus thynnus WITH COMMERCIALLY AVAILABLE DIETS IN EUROPE

Introduction

The development of artificial diets for rearing Atlantic Bluefin Tuna (ABT) is crucial for several reasons. Overall, it will support sustainability, economic viability, and reduced environmental impact in tuna aquaculture.

Availability of a commercial feed program for ABT on the market is mandatory for successful ABT aquaculture. The commercial feed should be at least as good performing as feeding based on bait fish, sustain the fish health and performance under aquaculture conditions, and open the opportunity to reduce the amount of marine ingredients in ABT feeding and reduce the trophic level of ABT aquaculture.

A full feeding program should offer solutions for all development and age stages in the aquaculture lifecycle of ABT, starting from weaning over grow-out to brood-stock diets.

The trial presented here aimed to continue the very successful weaning trials with readily available commercial weaning diets, performed in 2023 and presented at WAS conference in 2024, where fish were successfully fed a commercial diet up to 150g. The target of the trial presented here, was to rear juvenile ABT from about 150g onwards to the end of the growth season in winter with a readily available Skretting diet, developed for Tuna.

Material & Methods

Over 16.5 weeks, an ABT feeding trial was conducted at the Instituto Oceanográfico de Murcia (IEO). A group of 1175 ABT juveniles with a mean weight of 20g at 60 days post hatch (dph) was split into two groups. One group of 520 fish was fed artificial diets from Skretting (Comm. feed group), the other group of 625 fish was re-weaned from artificial diets to bait and continued fed bait fish (sardine and squid) (Bait group). The detailed feeding regime is listed in Table 1.

For the first three weeks of the trial both groups were fed Gemma Silk (GS) a commercial starter diet for marine fish.

Then from about 85dph onwards the Comm. feed group was transferred to Magro diet (M). The Bait group was transferred at the same time (85dph) to natural diets consisting of pieces of sardine and small squid.

For the Comm. feed group, the size of the pellet was successively increased. At 96 dph the fish were changed from 3mm to 6mm Magro, at 123 dph to 7mm Magro. For every size change, there was a period of at least 3 days of co-feeding with both feed sizes.

At 153 dph (after 3 month) the Comm. feed group was changed to a wet diet produced on site, consisting of hydrated Vitalis feed, enriched with Ori-N3 DHA enrichment.

Results & Discussion

Both groups showed unusual and very high mortality rates. By the end of the trials (mid-December) both groups were left with less then 25 fish per group and a mortality rate of 95.9 and 96.7% for the Comm feed and bait fish group respectively. These abnormal mortalities were attributed to a previous bacterial infection, not fully overcome.

What became apparent already in the first and second feeding period from 60 and 85 dph onwards is the lower growth performance of the Comm. feed group. While the Bait group reached already 125g at 91 dph, the Comm. feed group only had 68g. The lower performance of the Comm feed group continued until 153dph. During all this time the specific growth rates (SGR) of the Comm. feed was 0.9% lower compared to the bait group.

Only during the last feeding period from 153-175 dph the Comm. feed group had a 0.4% higher SGR. However, by the end of the trial the size difference between the two groups was huge. While the Comm. feed group reached a mean weight of 452g by day 175 post hatch, the Bait group was more then double the size with 987g.

This preliminary trial delivered two main results:

1. The group offered the commercial feed Magro was much worth performing compared to the bait fish reference. In addition, the fish showed also a lower performance then experienced in the previous trials, where the fish were fed the Magro diet up to 150g. The Magro feed was optimized for high growth rate with corresponding high protein levels in the feed.
2. When the fish from the Comm. feed group were offered a different diet, the performance increased.

Additional data evaluation on the whole-body composition of the fish from both groups is planned, however, the results are not available yet. It is expected that these analysis may reveal a specific nutrient deficiency in the Comm feed group.

The preliminary results lead to the following conclusions:

• Offering ABT a high protein diet during the critical growth period of 50g onwards, seems to the wrong strategy.
• Based on the assumption that starting from 50g onwards, ABT is developing its internal temperature regime, a different feeding strategy, less focusing on growth and rather on key physiological changes in the fish might be more promising.
• Additional trials and more in-depth research are needed to understand the specific requirements of ABT in this vulnerable life stage, to develop an adequate feeding strategy for Atlantic Bluefin Tuna.