MISA - Marine Innovation SA

• Seafood Product Quality & Value-adding
• Aquaculture Innovation
• Ecosystem Services
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Nutritional profiles of baitfish 3: effects of harvest and post-harvest processes on quality of local baitfish for feeding SBT.

Outcomes Achieved to Date

The research documented in this report demonstrates the effects of postharvest practices on the nutritional quality of locally-caught Australian sardines (Sardinops sagax) and of redbait (Emmelichthys nitidus nitidus) sourced from Tasmania. The research showed that there were substantial losses in key elements of the nutritional profile (i.e. vitamins and nucleotides), concomitant with increases in rancidity and loss of freshness. Greatest losses in quality and freshness occurred during post-harvest transport and freezing, and thawing of baitfish, prior to feeding to SBT. Losses during extended frozen storage can be substantial.

This information will be used by suppliers to improve post-harvest treatment of baitfish to optimise quality. The tuna industry will use the information to improve the efficiency and effectiveness of their tuna-feeding strategy to optimise SBT growth performance, health, flesh quality and return at the market.

Non-Technical Summary

Past SBT Aquaculture Subprogram-Aquafin CRC projects have sought to determine the nutritional profiles of the 23 or so different baitfish species that are used by tuna farming companies to feed to southern bluefin tuna (SBT). These studies have shown inter- and intra-species differences in nutritional parameters including amino acid and fatty acid ratios, crude protein, crude fat, ash and energy, and indicators of biochemical quality, such as free fatty acids and peroxide levels. Whilst differences in these characteristics between species are expected, differences between different batches of the same species can indicate either seasonal or regional effects, and/or the impacts of different harvest and post-harvest practices on baitfish quality. For example, data were collected from 34 different batches of local sardines supplied to the tuna farms. Whilst some proximate parameters such as protein vary only slightly (10% or less), the level of fat changes over 6-fold, and that of peroxides over 10-fold between batches.

Not only does this indicate that it is important to know when to harvest the sardines in order to maximise their nutritional value (i.e. when fat levels are high before spawning), it also shows the effects of oxidative processes (e.g. lack of antioxidants, prolonged processing and/or storage times, inadequate storage temperatures) on the quality of the nutrients (especially the fats) in the baitfish. In this regard there was a significant positive correlation (R2 = 0.61) between storage time and peroxide concentration, suggesting that the storage conditions in Port Lincoln (South Australia) can lead to high peroxide levels in the local sardines (Ellis and Rough, 2005).

Tissue integrity and health of the SBT is damaged by oxidative processes and protected by antioxidant vitamins. These protective antioxidants have to be supplied to tuna in their feed. High peroxide values are of concern because they indicate low levels of antioxidant vitamins in baitfish, and that the tuna consuming the baitfish are taking in a high burden of reactive oxygen species that can lead to further antioxidant depletion and peroxidation of the structural and functional compounds. Both mechanisms (i.e. low vitamins or high oxidative burden) could, in turn affect metabolic processes, and impact on flesh quality characteristics. It will be important to be aware of the potential deleterious effects of long-term feeding of nutritionally poor-quality baitfish on the health and flesh quality of tuna.

Postharvest deterioration in Australian sardine (Sardinops sagax) nutritional quality was followed from the jetty through short (4-6 weeks) and long (3-6 months) term storage trials (Objective 1) to subsequent thawing (Objective 2) for feeding to SBT. Samples were taken from fishing vessels at the jetty and during and after storage and thawing trials. Redbait (Emmelichthys nitidus nitidus) were sampled directly after capture onboard the fishing vessel, at the Triabunna (Tasmania) factory, then at the Port Lincoln factory/commercial freezer during and after storage and thawing trials.

The results of this study are as follows: 

• Existing harvest and post-harvest practices have a significant impact on the nutritional quality of baitfish fed to SBT in seacages. Vitamins and nucleotides decline to low levels, rancidity increases and fish freshness declines significantly, to the point where the baitfish becomes of questionable utility, particularly as a source of antioxidants.
• Freezing method affected the rate of vitamin E loss. IQF sardines stored at -20oC lost vitamin E at a higher rate than those which were block frozen, possibly because individually frozen fish are more likely to be exposed to air than those in a solid block.
• Baitfish, particularly sardines, should be thawed in seawater, not air, to minimise vitamin losses. Vitamin E losses were actually least in freshwater and greatest in air, whereas for vitamin C this was reversed in some trials. The seawater recommendation is a compromise to minimise losses of both vitamins. Air-thawing, particularly at room temperature, accelerates oxidative processes and thus irreversible breakdown of antioxidants within fish tissue.
• Block feeding at sea is recommended over land-based thawing as rapid thawing (1-2 hours vs 1-2 days) may reduce vitamin losses
•  In addition, fish should be kept on ice or refrigerated when transferred from the fishing vessel’s refrigerated seawater tanks to the factory, and storage time limited.
• Good correlations between TBARS and remaining vitamin E suggest a practical utility of this measure as an indicator of antioxidant status, with 1-1.5mg/kg TBARS equating to 50% remaining vitamin E in each baitfish species. In view of this result it is recommended that TBARS be included in as an analytical requirement for baitfish quality analysis in Port Lincoln. Analysis of TBARS is also methodologically simpler and less time-consuming than for K factor. 

Objective 3 was addressed in consultation with the ASBTIA. It was decided that the most effective means of industry extension would be the production of a laminated summary of the key results for distribution to sardine fishers, processors and tuna farm operators. This was carried out. Key results were also included in industry newsletters (Tuna Briefs; http://www.sardi.sa.gov.au/pages/aquafin/southern_bluefin_tuna_publications.htm:sectID=967&tempID=11#Newsletters) and discussed with industry at SBT Aquaculture Subprogram-Aquafin CRC meetings at Port Lincoln. Results were also presented at the Aquafin CRC conference in the Barossa Valley, May 2007.