Utilising copepods as live feeds to prevent Vibrio spp. infections in marine fish larvae
According to FAO’s latest (2022) edition of The State of World Fisheries and Aquaculture marine aquaculture is growing but still produces only half the tonnage of freshwater aquaculture. When looking at fish from capture fisheries, marine species contribute by far with the largest tonnage, and there is robust demand for high value marine fishes.
One of the major bottlenecks within marine aquaculture is the supply of managed hatchery-bred fish larvae. Within freshwater aquaculture, most freshwater fish larvae are large when they emerge from the egg and can easily be fed with formulated feed pellets. For marine fish larvae, most larvae must be fed with live feeds from day one, and for some species, for up to a month after birth. Hence, in marine aquaculture hatchery production, live feeds like rotifers, Artemia and copepods are often the only options to produce marine fish larvae of high value species. Copepods, in particular, are important, as they are a natural live feed for fish larvae. In contrast to rotifers and Artemia, copepods are biochemically superior and do not need enrichment before being fed to the fish larvae. Further, copepods exhibit behaviour that triggers an attack response from many marine fish larvae.
Preventing microbial contamination of fish larvae is critical
One of the obstacles to produce and use live feeds is microbial control, preventing pathogenic bacteria from being introduced to the fish larvae through the live feeds. The management of bacterial communities in live feeds has been widely studied in rotifers and Artemia. For example, with regard to Artemia research a model system has been developed to understand, control, and protect against pathogenic bacteria in Artemia cultures. Whereas with copepod cultivation, and its use in marine aquaculture, very few researchers are working with pathogenic bacteria control. It is an understudied but very promising area of research in marine aquaculture.
Vibrio spp. are known fish pathogenic bacteria found in both fresh- and marine aquaculture and leading to severe production and economic losses for the aquaculture sector. The Vibrio genera are among the most common and widespread disease-causing agents, and Vibrio infections play a leading role in constraining the growth of the aquaculture sector worldwide. Especially during the early larval stages of development, Vibrio species are a common cause of high mortality rates in reared fish. For juveniles and adult fish, vaccines are used to prevent Vibrio, but this is not an option in the larval stages since fish larvae are too fragile to vaccinate. Moreover, these early life stages are also where the fish larvae are highly susceptible to diseases such as Vibrio.
A copepod variety that produces omega-3s
At Roskilde University (Denmark) researchers have been cultivating a new promising tropical copepod species for use as live feed in marine aquaculture. The cyclopoid copepod (Apocyclops royi) is one of a few known copepod species that can biosynthesise fatty acids. They can be fed with, for example, yeast that only contains short-chained non-essential fatty acids and will then biosynthesise and prolong the fatty acids into long chained essential omega 3 and 6 fatty acids. Omega 3 and 6 fatty acids are just as essential in the diet for fish larvae as well as they are in the diets for humans.
Almost all other copepods need to obtain the essential fatty acids in their diets, hence the only option is to feed them with the correct microalgae strains. This complicates copepod production and around 70% of copepod cultivation costs are solely related to the production of microalgae. So, removing this link in the production chain makes copepod cultivation easier and cheaper. Another huge advantage is that, when they are not restricted to microalgae, it is possible to experiment with other feed types that have pro- or prebiotic properties as well. Feed companies producing pelleted feeds or derivates have developed different feed products with pre- and probiotic properties, that both target potentials within the gut of the fed species, and also the surrounding aquatic cultivation medium, as a bioremediation effect.
Generating a carry over protection from copepod to larva is the goal
Now the job is “just” to find the correct feed, or the combination of feeds, and/or derivatives to try to prevent Vibrio infections in live feeds and secure a potential carry over effect to the marine fish larvae in hand. This “treatment” with different feed types, preventing Vibrio in live feed, could reveal a preventive treatment, and thus save marine aquaculture hatcheries from many losses.
A recent master’s thesis from Roskilde University investigated how different feed types affected A. royi when infected with Vibrio anguillarum, a so-called challenge experiment. To ensure that A. royi could be cultivated on the different feed types, multiple cultures were established and fed to A. royi for a minimum of a month before conducting the challenge experiment. This revealed that A. royi could be fed on microalgae, inert feed pellets and different derivates, such as yeast. This outcome was not a surprise, since another recent study from this research group showed that A. royi is an ambush feeder. If it senses a prey or particle, it attacks and attempts to eat it1. So, a prerequisite for a feed for A. royi is that it needs to have the correct size and be able to stay in suspension in the culture water. After finding appropriate feed items the researchers challenged A. royi with different concentrations of V. anguillarum for three days. During this period V. anguillarum was tagged with a luminescent which was revealed when exposed to the correct wavelength of light (see black and white images).
Some of the other foods used in the study have, according to the producer, strong pathogen binding properties and will activate several immune receptors. Yeast derivatives are examples of foods that in some cases show strong pathogen bindings or simply physically occupy the sites inside the gut of a species where a pathogenic bacteria would normally bind.
Microalgae the most effective at preventing infection
A reduced effect of V. anguillarium colonies over time was observed on A. royi when fed some of these products. Surprisingly it was discovered that some microalgae diets were effective in preventing V. anguillarum infections. So, aquafeeds known to prevent Vibrio in shrimps did not have the same significant preventive effect in copepods, even though they are both crustaceans. On the other hand, microalgae worked very well in prevention of Vibrio for the three days of the experiment. This could either be due to the presence of another Vibrio strain or that the shrimps are a more advanced crustaceans than copepods. A possible explanation for less V. anguillarum infection in A. royi fed on microalgae could be because a diet of microalgae contains all fatty acids, and A. royi do not need to utilize energy to biosynthesis fatty acids, which is the case when A. royi are fed, for example, on yeast derivatives.
Nevertheless, interesting food candidates were found, and the next step is to carry out experiments over a longer time span. Another important step is to investigate possible carry-over effects to marine fish larvae. In other words, if a “Vibrio free” copepod is fed to marine fish larvae, does one weaken or boost the fish larvae so they will have less mortality even when challenged with Vibrio? The team claims to have good reasons to believe that they will see enhanced effects when they feed A. royi with pro- or prebiotic properties to marine fish larvae.
Associate Professor Per M. Jepsen; Naja Bech, MSc; Rasmus B. Sandvig, MSc, Roskilde University, Denmark