THE USE OF PLANT PROTEIN AS A MEANS OF IMPROVING FISH GROWTH PERFORMANCE THROUGH ITS LARVAL STAGE

Author

Connor W. Schwepe, Southern Illinois University Carbondale

Date of Award

5-2022

Degree Name

Master of Science

Department

Zoology

First Advisor

Kwasek, Karolina

Second Advisor

Lydy, Michael

Third Advisor

Garvey, Jim

Abstract

Aquaculture is a significant contributor to the global food supply and continues to grow each year. However, the continued growth of the industry will be limited by its large reliance on marine resources, such as fishmeal (FM), which are needed to make formulated diets to feed farmed fish. Fishmeal is made from processed fish caught from the wild, and the growth of the industry and overfishing has put considerable pressure on wild populations that make the current level of use unsustainable. As an alternative to FM, several sources of plant protein (PP) are used to replace FM in diet formulations. One of the most common PP sources used in aquaculture diet formulation is soybean meal (SBM) which is widely available and of a lower cost. However, complete replacement of FM with SBM is currently limited, SBM (as well as many other PP sources) is less digestible than FM and has anti-nutritional factors that result in reduced growth in fish fed diets with high level of SBM. The purpose of this thesis was to improve the utilization of PP in larval fish in order to promote growth and development through the larval stage. In order to accomplish this, two experiments were performed. The first experiment (Chapter 2) was a feeding trial that examined how PP and other protein sources could be used as an enrichment that would improve the nutrient composition of live food and improve the development of larval zebrafish Danio rerio. Zebrafish larvae were provided with live food rotifers Brachionus plicatilis and Artemia spp. nauplii enriched with either 1) Spirulina spp. algae; 2) SBM; 3) Soy protein concentrate (SPC); 4) FM hydrolysate; 5) intact FM; or 6) an unenriched control. The enrichments represented marine and plant resources of varying quality. The first feeding larvae were fed ad libitum until they fully metamorphosed into a juvenile stage at 22 days post hatch (dph). Rotifers enriched with SBM had increased protein content compared to unenriched rotifers and the larvae fed the SBM-enriched live food showed longer body length than all other groups except SPC at the end of the trial. Additionally, the larvae fed SBM-enriched live food showed an upregulation of digestive enzyme chymotrypsin indicating a more developed digestive tract. This study indicates the effectiveness of SBM as a live food enrichment source that can improve larval development and growth. The second experiment (Chapter 3) was a nutritional programming (NP) study using Largemouth bass Micropterus salmoides. Nutritional programming is a method where fish are fed PP in early life - considered a nutritional stimulus that allows them to better utilize PP in later life. The optimal time in larval fish development to induce NP is not known, so larval bass were programmed at different ages to determine at what stage NP is most effective. Three groups were programmed either from 6-15 dph with SBM-enriched Artemia nauplii (NPL), from 16-25 dph with formulated SBM-diet (NPD1) or from 26-35 dph with formulated SBM-diet (NPD2). A positive control received FM-based diet throughout the whole trial. When re-introduced to SBM later in life (100 dph) the NPL group showed highly improved growth compared to NPD1 and similar growth to the positive control. The growth of NPD2 achieved similar growth to NPL, but did not achieve similar growth to PC. Nutritional programming was not effective in NPD1 as growth was significantly reduced. This study showed that Largemouth bass can be successfully programmed to accept dietary SBM later in life right after mouth opening using SBM enriched live food in the larval stage. This timing of NP is preferable to programming with formulated diets at a later stage. The overall findings of this thesis show that SBM is an effective source of live food enrichment that can improve larval growth and induce effective NP. While high levels of SBM inclusion in formulated diets are not currently feasible for some fish throughout their entire life, the larval stage offers opportunities to improve the utilization of this raw material. In both zebrafish and Largemouth bass SBM was effectively utilized in the larval stage to support development. This was reflected in improved health status of the intestine and in improved growth of zebrafish who received enriched live food. Additionally, the Largemouth bass programming results give valuable insight into how NP can be better utilized so that more effective utilization of formulated SBM-diet can be achieved.