Title: Potentials for sustainable, cost-effective biofertiliser from yellowfin tuna by-products

Abstract

Sudeepa Ranasinghe's final project will be published in due time. Until then we share the abstract and a link to a poster.

This study explored the potential for producing sustainable, cost-effective biofertilisers from yellowfin tuna by-products, contributing to environmental sustainability by adapting organic agriculture and waste reduction in fish processing. Three production methods were evaluated: acid fish silage, microbial fermentation, and enzymatic hydrolysis, using minced redfish (Sebastes marinus) by-products as a nutritional proxy for yellowfin tuna. Acid silage involved formic acid treatment, while fermentation utilised Lactobacillus plantarum with barley malt extract. This was carried out over 42 days, while monitoring the pH and sensory attributes. The enzymatic method employed the Alcalase enzyme under controlled temperature and agitation, followed by filtration to yield liquid fertiliser. Nutrient profiles varied by method. The initial sample contained 2.55% nitrogen (N), 4.83 ± 0.97 g/kg phosphorus (P), and 1.18 ± 0.24 g/kg potassium (K). Among the processing methods examined, fermentation yielded the highest nutrient levels, with 2.64% N, 5.39 ± 1.08 g/kg P, and 2.68 ± 0.54 g/kg K. The acid fish silage method demonstrated moderate nutrient enhancement, resulting in 2.59% N, 2.82 ± 0.57 g/kg P, and 1.69 ± 0.34 g/kg K. The enzymatic hydrolysate method produced nutrient levels similar to those of the initial sample, recording 2.52% N, 2.72 ± 0.54 g/kg P, and 1.24 ± 0.25 g/kg K. Overall, fermentation emerged as the most effective method for enhancing the nutrient content of the final product. However, the potassium (K) levels were insufficient for rice cultivation in Sri Lanka´s wet zone. Standard fertiliser recommendations for rice include 140 kg of urea, 35 kg of TSP, and 50 kg of MOP per hectare, which equates to a demand of approximately 64.4 kg of N, 7.0 kg of P, and 25.0 kg of K. With a nutrient profile of 2.64% N, 5.39 g/kg P, and 2.68 g/kg K, fermented fish fertiliser can satisfy the N and P needs with approximately 2.4 and 1.3 tons per hectare, respectively. However, fulfilling the K requirement would necessitate over 9 tons per hectare, which is logistically and economically impractical. This study recommends supplementing with organic compounds, such as animal manure and wood ash to enrich K content of the fermented fertiliser. Further research is needed to determine the best organic matter for K enrichment and its appropriate quantity.