Introduction
Abalone is one of the most valued and highly sought-after seafoods globally with South Africa being the largest producer of farmed abalone outside Asia. They are cultivated in flow-through raceway tanks in South Africa and fed fish and soy meal protein-based pellets or a combination of pellets and live macroalgae until they reach market size. Research is underway to substitute the
fish meal component with a less expensive and more sustainable protein sources . The seaweed Ulva is cultivated as part of
an intergrated multi-trophic aquaculture (IMTA) in the effluent water of several commercial abalone farms , where it acts as a biofilter to remove dissolved waste nutrients
and harvested as a fresh abalone feed supplement . Effluent-grown Ulva can be an excellent source of vitamins, minerals, alanine, glutamic acid, iron, soluble fibers and with protein levels of up to 40% . In the present study we evaluated
the
inclusion
effect of nutrient-enriched macroalgae (Ulva species) meal in a pelleted abalone diet.
Materials and methods
Ulva meal, experimental diets preparation
U. lactuca was cultured on a commercial abalone farm for 30 days using a 308.57 m3 (36.735 x 8.4 x 0.70 m) D-ended paddle-wheel biofilter pond which received effluent from abalone production raceway tanks upstream. The abalones supplying the effluent were daily fed a combination diet of commercial feed pellet (Abfeed™ S34, i.e., 34% protein, 4.5% lipid) and fresh macroalgae (Ulva and Gracilaria sp ). At the end of the production cycle of
the Ulva,
it was harvested, rinsed in fresh seawater, and dried at 40 °C in a locally fabricated drier. The Ulva meal was subjected to biosecurity measures before being included as a feed ingredient in pelleted diets manufactured by at a commercial abalone feed company (Marifeed Pty Ltd, South Africa).
Six isonitrogenous experimental diets containing 34% protein were prepared to contain graded levels of IMTA Ulva meal ranging from 0% – 12% of the dry mass. The Ulva meal replaced protein from fish and soya at 0% (basal diet), 0.75%, 1.5%, 3%, 6%, and 12%.
Feeding trial
The diets were fed daily to triplicate groups of grow-out (35-45g) H. midae for 244 days (December 2020–August 2021). Randomly selected abalones per treatment (n=90) were sampled for shell length and weight at stocking, after 4 and 8 months of intensive feeding. Triplicate samples of abalone for each dietary treatment were collected at the end of the trial and analysed for their proximate compositions. The daily increase in shell length, weight, specific growth rate, body weight to shell length ratio, feed conversion ratio, protein efficiency ratio and the energy efficiency ratio of cultured abalones were measured and calculated at the end of the experiment.
Results
After eight months of feeding, the weight and shell length of abalone across the various feed treatments had improved substantially as their growth and nutrient utilisation were influenced by the dietary inclusion of IMTA Ulva meal in the diet. Feed consumption significantly improved (p <0.05) with an increasing level of Ulva meal in the pellet (F (5, 12) = 10.242, p = 0.0005). The best apparent feed conversion (1.18 ± 0.11) was recorded for abalone fed 1.5% IMTA Ulva inclusion diet while the lowest (2.12 ± 0.28) recorded at 12% inclusion. There was no significant difference (p >0.05) in the final weights, shell length and weight gains of the animals up to 12% inclusion of Ulva meal
in the diet. However, the highest weight gains were recorded at 3%-6% inclusion of Ulva . The results obtained in this study suggest that the use of IMTA-produced Ulva meal in the diet for South African abalone (H. midae)
could be promising as no adverse effect on growth or body composition was observed.