THE IMPORTANCE OF AQUACULTURE FEED SIZE IN THE SUSTAINABILITY OF AQUACULTURE

By Dr Mustapha ABA. Scientific Aquaculture Expert. Fish Nutrition. Rabat. Morocco.

The rapid development of intensive aquaculture poses a number of problems in terms of its sustainability, given that feed is the major source of pollution in aquatic environments. In aquaculture, little attention has been paid to feed size, which is a crucial factor affecting growth, survival, yield, fish welfare, environmental impact and the cost of aquaculture production.

This paper highlights the crucial importance of aquaculture feed sizes for finfish and shellfish in aquaculture nutrition research, and suggests improvements for better feed use in aquaculture farms in line with sustainability, considering that feed particle size can affect aquaculture productivity, and that feed waste is the main source of pollution in aquaculture systems.

In order to optimise the development of aquaculture, in addition to obtaining information to enable the formulation of good diets that meet the requirements of fish, greater importance should be given to feed processing techniques, in order to ensure better use of the nutrients contained in diets, reduce production costs and have less impact on the environment (Aba, 2020), and the expansion of aquaculture depends on the development of intensive farming techniques, feed formulation, characteristics such as pellet size and feeding practices, to optimise feed efficiency (Ramos-Júdeza and Duncana, 2022).

During the last two decades, concerns about the quality of fish feeds and their effects on the aquaculture environment have reached a high level (Fernandes et al, 2007; Yildiz et al, 2017), and the environmental effect of fish farms is considered a critical element hindering the development of sustainable aquaculture (Akhan, S. and Gedik, 2011), whose activities not only stimulate the growth of algae but also affect phytoplankton communities, as well as deteriorating the quality of the water in which fish live (Anderson et al, 2002; Bouwman et al, 2013).

Feed is the main source of waste in aquaculture systems (Akinwole et al, 2016; Martins et al., 2010). Waste production from fish feed depends on many factors, including its nutrient composition, production method (extruded vs. pelleted), feed to fish size ratio, amount of feed per unit time, feeding method and storage time (Miller and Semmens, 2002). White et al. (2018) indicate the relationship between feed management and the economic efficiency of aquaculture farmers and other actors in the chain to select cost-effective feeds, and that poor feed management during feeding can lead to economic losses (Welker et al., 2018).

Waste produced by aquaculture is classified into four forms: gas (H2S), liquid (effluent), semi-solid and solid (particulate fraction), the last two being referred to as sediment or sludge (Dauda et al, 2019), while solid waste is divided into two categories: suspended solids and settleable solids (Schumann, and Brinker, 2020).

The level of pollutants discharged into the environment is a direct function of the quantity of feed consumed and its digestibility (Chiquito-Contreras et al, 2022), which in turn depends on the size of the feed and the development stage of the fish, hence the need to feed fish with feed of the right size.

The grinding area is one of the most important in a feed mill, as it is responsible for adjusting the particle size of the ingredients (Couto, 2008). A fine degree of grinding is a prerequisite for good stability of fish feeds in water. High water stability is defined as the ability of feed to retain water for a certain time with minimal leaching before consumption, yet the stability of pellets in water is strongly influenced by the method used in the pelleting process, the particle size of the ingredients, the raw materials, and the type of binders used (Obaldo et al., 2002, Onada and Ogunola, 2020).  In addition, pellet size is another factor that can potentially affect feed stability in water, as larger pellets are generally considered to be more stable in water (Obaldo and Tacon, 2001). The increase in the surface/volume ratio of smaller pellets is considered to be attributed to their lower stability in water (Obaldo and Tacon, 2001).

In aquaculture, it is of the utmost importance that fish are provided with food of an appropriate size and adapted to the size of the fish so that they can develop as expected (Ljungqvist et al, 2011). Pellet size is the size (diameter and length) of the feed pellets (expressed in millimetres) and should be one of the first aspects observed in the feed purchasing process.

 The optimum pellet size has been suggested to be around 2% of the length of the fish (Linner and Brannas, 1994).

 Others have suggested that for a range of fish species, the optimal feed size appears to be 25-50% of the width of the mouth (Wankowski, 1979; Tabachek, 1988; Azaza et al., 2010); Zakes et al. 2013) and (Mattila and Koskela, 2017).

It is well known that there is a direct relationship between fish mouth opening and pellet size; pellets too large for a fish’s mouth opening are often not used effectively and, as fish grow, pellet size must also increase (Goldan et al., 1997). Larger pellets may have the advantage of being more easily detectable by the animals (Stradmeyer et al., 1988) and of reducing the energy expenditure of the animals in searching for food. However, the size of the feed, which can be rapidly detected by the animals, may not be easily ingested if the diameter of the pellet does not correspond to the diameter of the mouth, as in the case of crustaceans for example (Smith et al., 2009).

Feed is formulated according to the nutritional requirements of each developmental stage of the fish, which vary throughout the rearing stages, and for feed to be apprehended and ingested, the size of the pellet must be compatible with the opening of the fish’s mouth (approximately ¼ of the mouth open), otherwise the use of feed with inadequate pellet size can significantly impair feeding, as the pellet size is adapted to the size of the fish so that it can grow as expected (Ljungqvist et al, 2011), moreover, it has been shown that fish growth rate is closely related to feed pellet size (Azaza et al,2010). However, the size of feed distributed can affect the amount of food a fish can ingest during the rearing period, as undesirable pellet size can lead to feed wastage, as fish may be unable to ingest the required amount of feed (Bailey et al. 2003).

Feed particles should be of a size that can be easily ingested, which is favourable for increasing fish growth. Large fish can ingest small particles but require more energy to capture an adequate quantity, which reduces feeding efficiency (Alava, 2002). The effects of food particle size on fish growth and food conversion are generally dependent on fish size. Particle size is therefore likely to have an impact on growth heterogeneity, which is generally undesirable in fish farming ( Azaza et al, 2010 :

The size of the pellets supplied to aquatic animals could also have a significant effect on feed ingestion and wastage, as they fragment into small fragments during the ingestion process and can become so small that they can no longer be used by the animals (Smith et al., 2009).

The efficiency of feed digestion can be influenced by, among other factors, the surface area of feed exposed to digestive secretions and the time taken for feed to pass through the gastrointestinal tract (NRC, 2011). Grinding food reduces particle size, exposing a larger area of action to digestive enzymes (Laurinen et al., 2000). However, this can reduce the transit speed of feed through the gastrointestinal tract (Hayashi et al., 1999; Sveier et al., 1999).

Optimising feed in aquaculture has long been a major concern. The composition of the feed must not only meet the nutritional requirements of the fish, but it must also produce the highest feed conversion efficiency, the fastest growth and the least waste possible, by means of an optimally sized feed for the sizes of the fish and crustaceans in order to contribute to the sustainability of aquaculture, which is causing increasing concern, particularly with regard to the waste produced by poor feed management.