Jellyfish; aquaculture friend or foe More than just a blooming nuisance?
by Oliver Moore, Marine Biology Student, University of Plymouth
Jellyfish are perhaps some of the misunderstood forms of life on the planet, with their distinctive morphology being more reminiscent of a science fiction alien than an actual animal. Despite their unique looks, they are some of the most successful organisms in the world and may be one of the few winners from climate change.
Subsequently, they have begun to dominate our oceans over the last 50 years, gathering in colossal numbers and resulting in significant damage to marine open aquaculture operations around the world.
Whilst this was originally universally considered to be a huge issue, there has been increased interest in the scientific community in how we can utilise these blooms to our advantage, specifically in the use of aquaculture feed.
Three different classes of gelatinous zooplankton
The word 'jellyfish' actually encompasses a varied range of organisms as it refers to three different classes of gelatinous zooplankton within the phylum 'Cnidaria'. These include the Cubozoa (Box jellyfish), Hydrozoa and Scyphazoa (True jellyfish). However, 'jellyfish' typically refers to a member of the scyphozoa, with key species such as Rhizostoma pulmo (barrel jellyfish) and Aurelia aurita (moon jellyfish).
Scyphozoan jellyfish are characterised by their distinctive morphology, having a bell-shaped hood enclosing an internal structure (where tentacles are suspended) as well as the presence of nematocysts (capsules containing coiled filaments attached to barbs). Alongside their anatomy, scyphozoans are known for their unique multi-stage life cycle.
The high fecundity of jellyfish within its life history allows for the very rapid development of an incredibly large number of individuals, resulting in a jellyfish bloom. These blooms occur naturally in the marine environment, with earliest reports dating back 4,000 years to ancient Cretan pottery.
Nevertheless, it has been widely speculated by the scientific community that anthropogenic activity has resulted in an increased frequency of the blooms with them occurring every year as opposed to every decade (Lamb, 2017).
Whilst there are a multitude of factors that may be responsible for these increased blooms, Lucas Brotz at the University of British Columbia believes that overfishing may be the primary reason why they are happening, 'humans remove on the order of 100 million tonnes of wild animals from the oceans every year.
This corresponds to an absolutely staggering number of organisms, many of which are predators or competitors of jellyfish. It is very likely that jellyfish benefit from this fishing and overfishing in many regions'.
Other theories include eutrophication, ocean acidification and climate change, largely as a result of the natural resilience of jellyfish.
Jellyfish blooms are detrimental to aquaculture
Irrespective of the reason why these blooms are occurring, it is apparent that they are detrimental to aquaculture. There are many reports on jellyfish decimating Open Pen Sea Cage aquaculture, a recent example of which includes blooms of Muggiaea atlantica and Pelagia noctiluca destroying 80 percent of farmed salmon stocks off the west coast of Ireland in 2017, causing millions of pounds worth of damages.
There are a number of mechanisms in which gelatinous zooplankton can cause significant damage like this to aquaculture. Firstly, they can surround the nets and prevent the flow of oxygen into the pens, essentially causing a localised hypoxic zone; giving rise to the 'suffocation' of individuals trapped inside.
Nets may degrade jellyfish, resulting in loose nematocysts or even small individuals being inhaled by the fish. This nematocyst discharge can sting their gills giving rise to the loss of epithelial cells, focal hemorrhages and onset of necrosis. Jellyfish may also act as a vector for the bacterium Tenacibaculum maritimum which has the potential to exacerbate gill injury (via gill abrasion) and further damage to fish stocks.
Even a small exposure period of two hours is detrimental to the health of individuals, with the damages significantly worsening with a longer exposure (Bosch Belmar & M'Rabet. 2016). Although it originally appears that these blooms are overwhelmingly negative to the aquaculture industry, there may be the opportunity to utilise these blooms to our advantage.
Jellyfish being used in a commercial environment is not a new revelation, with a multitude of industries already benefiting from them and their products. Examples of this include the cosmetics sector, that has been extracting collagen from jellyfish for uses in anti-aging products.
There is also a significant market for the direct consumption of jellyfish as they are considered a delicacy across several southeast Asian countries, subsequently resulting in over 321,000 metric tons of jellyfish caught for food in 2001 (Omori, Makoto; Nakano, Eiji 2001). This evidence suggests that the idea of fishing jellyfish is already an established concept and may be able to be scaled up for use in farmed fisheries. Anthropogenically caused jellyfish blooms could therefore be exploited by fisheries, utilising techniques already established in the capture of wild jellyfish and then sold to the aquaculture sector.
Jellyfish as a potential marine ingredient
Previous perspectives on the implementation of jellyfish into aquaculture have involved its conversion into a marine ingredient as a potential protein source for farmed fish. However, the body composition of jellyfish may mean that this is not economically feasible and instead may be more appropriate as a functional feed additive.
Jellyfish are 95 percent water and 3-4 percent protein (wet weight) with a very low overall dry biomass, and this process of conversion into dry biomass for fish feed is also prohibitively time consuming.
This is done via soaking the jellyfish in pure ethanol or alum for 20-40 days, taking a significantly long period of time to produce (especially in comparison to other fishery protein sources). Because of this, it would be unrealistic to assume that the use of jellyfish fish feed products would be a viable primary protein alternative for use in aquaculture.
It is also worth noting that jellyfish are a major constituent in the wild diets of several high FIFO species that we currently farm, such as the bluefin tuna. It could be an option to directly add the jellyfish into their diets as we would see in the wild, however the downside of this is that it is impossible to control all aspects of nutrition and may inadvertently cause deficiencies in diets.
Despite their impracticality as primary fish feed, jellyfish may be incredibly beneficial in the production of functional feed additives as they contain several compounds that may promote overall fish health, leading to higher quality of fish produced. Jellyfish are osmo-conformers in their environment having large quantities of salt to be isotonic with the surrounding seawater.
This results in a substantial amount of sodium in the overall composition of jellyfish which can have the very specific use of conditioning anadromous (moving from freshwater to seawater), species such as Atlantic salmon to changes in salinity. This may therefore help simulate processes which would occur in their natural life history and reduce overall stress of individuals.
This jellyfish feed additive could be added relatively early in the smolt life stages of these fish to stimulate the gill and intestinal epithelial bound Na+K+/ATPase enzymes and other biochemical pathways which occur naturally in the physiological regulation of water and salt balance. This serves to encourage homeostasis and lessen osmotic shock when salmon smolts are transferred to full salinity seawater for grow-out.
The abundance of antioxidants in jellyfish
A major advantage of jellyfish in functional feed additives is their high abundance of antioxidants. Proteins isolated from the jellyfish species Rhopilema esculentum have been proven to have significant antioxidant abilities, both showing strong superoxide anion and hydroxyl radical-scavenging capabilities (Hua-hua et al. 2006). This may be hugely beneficial as a supplement to fish feed as these antioxidants may specifically help prevent oxidative loss in susceptible key vitamins such as E and A.
Both these lipid soluble vitamins play significant roles in the overall health and quality of all farmed fish and shrimp. Therefore, it is incredibly important to utilise ways to maximise their yield with jellyfish products, thus providing a potential solution for many issues in the management of aquatic animal health.
Whilst the use of jellyfish in dietary supplements for fish may sound ideal, there are still large barriers to overcome in achieving this, especially in regard to their capture. As Mark Gibbons et al demonstrated in their landmark 2016 paper, there are still massive unknowns in jellyfish ecology. It would be incredibly hard to sustainably capture jellyfish without knowing their overall population size, which is difficult to calculate given their huge seasonal variation.
The large-scale removal of jellyfish from an ecosystem may have unintended consequences to the marine environment but it is still undetermined to what extent this could impact due to the lack of literature on the topic. It's already been proven that jellyfish have a significant role in ecosystem dynamics acting as a keystone species by reducing dominance of zooplankton species (via predation) thus evening populations and subsequently promoting diversity.
An absence of jellyfish in these ecosystems could significantly shift the dynamics and have detrimental impacts of the local region, far outweighing the positives produced by their use as fish feed supplements. These negative aspects could be mitigated through the sustainable management of fisheries, but it would be a significant challenge to enforce quotas globally.
Developing a cautious and sustainable approach
In conclusion, the key to utilising jellyfish blooms to our advantage effectively would be to establish a cautious and sustainable approach. It is difficult to deny the huge potential benefits that jellyfish may have for aquaculture, but it is important to note that this is still a relatively understudied area of science and that haphazard modes of action may cause massive unintended damage to the marine environment.
It is evident that significant research is still required to fully understand the secondary effects of capturing jellyfish blooms, before we can begin to exploit them on a scale required to supply a growing aquaculture industry.