How big are your fish pellets?

Samantha Andrews
05 September 2016, at 1:00am

In aquaculture, feed is amongst the most important factors that can impact fish growth rates. Most commonly found in pellet form, feed must be able to provide all of the required nutrients, offer the highest possible conversion rates, and of course be economically viable for commercial use. There are many aspects to creating the optimal pellet - including the pellet size.

As a general rule of thumb, pellet are generally considered optimal at sizes 25% – 50% of the fish’s mouth width. However, as with anything, the relationship between pellet size and growth rate is not entirely straight forward.

Pellet size is clearly important from one key physiological perspective – the individual’s mouth. Give a fish a pellet larger than its gape, and not only may it have trouble eating that pellet, but it could even choke. The way a fish eats is also an important factor in considering what size pellet to offer. Gilthead sea bream (Sparus aurata), for example, essentially “chew” large pellets before swallowing them. During chewing, some feed is lost into the water, and with it precious nutrients that could otherwise be used for growth. Recent work from scientist Mateo Ballester Moltó (Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario, Spain) demonstrated that feeding pellets smaller in size than manufacturer recommendations to gilthead sea bream reduced the need for chewing, reducing the loss of up to 42 grams of feed per kilogram of fish.

Smaller pellets may seem the obvious answer, but they also have their issues. Work by Professor Mostafa Hossain, who is now based at the Bangladesh Agricultural University, demonstrated fingerling African catfish (Clarias gariepinus) evacuate small feed particles from their body more rapidly than larger particles, reducing nutrient assimilation in the gut and in turn affecting growth rates. 1 millimetre pellets produced lower growth than those of 1.5 millimetre and 2 millimetre sized pellets. Other issues with small pellets lie in the fact that they contain relatively lower levels of nutrients compared to larger ones. Give a fish too small-a-sized pellets, and it has to spend more time finding and consuming enough pellets to meet its energy requirements. Inefficient pellet size can increase energy expenditure during feeding, diverting it away from energy that would otherwise be used to increase growth.

Larger pellets are not always better in terms of growth. Professor Hossain’s catfish growth rates did not continue to improve as pellets increased beyond 2.5 millimetres, with 3 millimetre pellets producing lower growth rates than the 1.5 & 2 millimetre pellets.

One of the reasons why this is may lay in the simple fact that larger pellets are… well… larger. Larger pellets have a lower surface to volume ratio than smaller pellets, and as such nutrients from larger pellets may be assimilated more slowly than small pellets.

Given the findings of Professor Hossain’s catfish study, one can see that there can be an optimal pellet size for maximizing nutrient extraction before excretion.

Some farmed species, such as Atlantic salmon (Salmo salar) can adapt to varying aquaculture conditions including, work from Dr Jason Bailey during his time at the Swedish University of Agricultural Sciences suggests, pellet size.

Split into different ‘pellet-size’ groups, salmon were fed pellets ranging from ¼ to 4 times the recommended size. To account for delivery rates, groups were further split to receive their food at different delivery rates. Perhaps surprisingly Dr Bailey and colleagues found no statistically significant alterations in growth rates after 4 – 5 months of trials, either from pellet size, delivery rates, or a combination of the two.

The implications for salmon aquaculturalists are simple – use the most pellet size and delivery rate that is most efficient for your operation and produces the least waste.

Other studies have also failed to find a direct relationship between pellet size and growth. Focusing on largemouth bass (Micropterus salmoides), Kentucky State University’s Professor James Tidwell found no significant difference in growth rates between those fed 5.5 millimetre sized floating pellets, and those fed 13 millimetre sized floating pellets for 5 months.

It is worth noting that Professor Tidwell also tested for differences in growth rates between 5.5 millimetres sized floating pellets and 5.5 millimetre sinking pellets, which did produce a significant result.

Bass fed on sinking pellets were larger at harvest than those fed on floating pellets. For those looking to develop largemouth bass farms, this may be good news. Floating pellets are typically more expensive than their sinking counterparts. Whether larger sinking pellets would result in improved growth rates is yet to be seen.

Certainly the development of optimal feed remains an area of significant research and industry interest. We can expect to see a continuation of the already considerable advancements made in feed technology, including further optimisation of pellet size and nutrient delivery.

Ultimately, however, the optimal size for a given farm will likely come down to the knowledge and expertise of each aquaculturalists, and their adjustments to feeding strategies that will produce the maximum results for their fish and their farms.