New feed formulations for turbot: sustainability of discards

One of the challenges in making fish farming more environmentally sustainable is to replace fishmeal and/or plant ingredients with alternative ingredients. Fish protein hydrolysates (FPH) have the potential to meet the nutritional needs of cultivated species. Moderate inclusion of FPH in aquaculture feeds improves fish growth and feed conversion ratio due to their content of amino acids and bioactive peptides. In Galicia (Northwest Spain), there is substantial acces to a large volume of raw materials for the development of new ingredients from FPH. For instance, extractive fishing is a very important sector from which many discards are obtained. A promising opportunity is to utilize FPH from fishing discards (blue whiting, Micromesistius poutassou), as a new ingredient for turbot feed, which will help promote a circular economy initiative, thereby revaluing these materials and fostering more efficient and sustainable aquaculture.

The objective of this study is to evaluate various levels of inclusion of a FPH of low molecular weight (1-2 kDa) of blue whiting from fishing discards in a diet for turbot in the growth phase, in which fishmeal will be replaced by the FPH at inclusion levels of 5, 10 and 15%. Thus, juvenile turbots (Scophthalmus maximus) were obtained from a local hatchery (Insuíña S.L., Oia, Pontevedra, Galicia, Spain) and transported to the facilities of the Oceanographic Center of Vigo. Experimental turbots (12.8 ± 0.09 g) were distributed into 12 tanks of 100 L (n = 60 fish/tank). They were kept under natural photoperiod and a constant temperature of 16ºC. After an acclimation period (two weeks), the feeding trial began and lasted for 3 months. Fish were fed ad libitum twice a day (9 am and 4 pm) up to a maximum of 3% daily. Four different diets were used and tested in triplicate: i) Control (BL0), ii) 5% inclusion (BL5) iii) 10% inclusion (BL10) and iv) 15% inclusion (BL15) of FPH of Blue whiting Low molecular weight 1-2 kDa (LB). The following zootechnical parameters were evaluated: daily feed intake, initial and final weight and total length, and liver weight. From these data, specific growth rate (SGR), feed conversion ratio (FCR), condition factor, and hepatosomatic index (HSI) were calculated. Additionally, at the end of the trial, muscle samples (n = 6 fish/diet) were taken to evaluate their proximal composition. Faeces were also collected to determine the yttrium content to obtain the apparent digestibility coefficient (ADC).

Based on the results obtained so far, it appears that the inclusion of LB FPH offers comparable results to the control feed regardless of the inclusion level (5%, 10%, or 15%). Further studies would be required to evaluate the overall response of the fish to this FPH to establish them as ingredients for aquaculture feeds. This would help to revalue products that were previously discarded and enhance aquaculture sustainability.