Abstract
Integrated multi-trophic aquaculture (IMTA) is the co-culturing of multiple species over trophic levels from primary producers to higher trophic level consumers in order to create a more sustainable and productive ecological system. The practice has been suggested to be a potential solution to mitigate the negative environmental impacts that may result from intensive monoculture systems, such as intensified fin fish aquaculture and high-density culture of bivalves. Specifically, culturing macroalgae with Perna canaliculus (green-lipped mussel) in New Zealand has the potential to greatly improve the country’s aquaculture industry. It can particularly help in terms of productivity, minimizing the environmental footprint of aquaculture farms and extending the carrying capacity of mussels in semi-enclosed inlets and sounds. Macroalgae can act as a biofilter improving the water quality by absorbing and assimilating inorganic nutrients that are released from the mussels. In turn, the mussels can uptake the suspended particulate organic matter (SPOM) that is produced by the macroalgae. Semi-enclosed bodies of water with high local retention of coastal neritic water masses, such as the fjords in Fiordland NZ and the Otago Harbor, provide ideal study systems in which to observe and analyze the interactions between species at different trophic levels and gather information on how basal organic matter and nutrients are exchanged.
The aim of the current project was to determine which species of macroalgae provided the most suitable particulate organic matter for green-lipped mussel feeding and growth. The objective was accomplished by first utilizing environmental data from 5 individual fjords in Fiordland, NZ – Bligh Sound, Breaksea Sound, Chalky Inlet, Charles Sound and Nancy Sound – to determine how environmental gradients influenced δ 13C and δ 15N values of the macroalgae and organic matter in the ecosystem. These data then provided a tracer for organic matter derived from macroalgae and phytoplankton that would likely be taken up by mussels. Then, multiple samples of Ecklonia radiata collected within individuals and among sites were used to identify the individual and site level variability in isotopic values in order to understand how these species produce particulates available to different mussel species. Samples of Macrocystis pyrifera were analyzed for δ 13C and δ 15N values to determine the variability within one individual. In order to experimentally test differences in organic matter uptake at the site level, green-lipped mussels were out planted into four sites along the length of the Otago Harbor and analyzed for their δ 13C and δ 15N after approximately 2 months to determine which source pool of organic matter they were preferentially feeding on in areas of the Harbor with different abundances and compositions of macroalgae.
It was concluded that there are two distinct source pools of basal organic matter directly available to mussels in Fiordland, identified by the δ 13C values. The first particulate organic matter source pool, which was largely made up of phytoplankton, was more depleted in the heavy isotope 13C than the particulate organic matter derived from macroalgae. Correspondingly, there were site- level differences in the δ 13C values of both macroalgae and SPOM between 5 individual fjords in Fiordland, NZ. In addition, δ 13C and δ 15N values of organic matter derived from E. radiata and M. pyrifera differed significantly within individuals. Three different mussel species from Fiordland were also analyzed for their δ 13C and δ 15N values and the data were used to model uptake of the alternate sources of organic matter, phytoplankton and macroalgae. It was concluded that a large component of their diet consisted of suspended particulate organic matter derived from macroalgae.
Green-lipped mussels out planted in the inner sites of the Otago Harbor during the summer months (December-February) were observed to grow more and correspondingly have a higher condition index compared to the sites at the outer harbor. This is suspected to be due to the high-water retention that is approximately double the rate observed on the outer coast. Green-lipped mussels that were out planted in Waitati Inlet, in Blueskin Bay, also followed the same trend. This location is exposed and experiences high flushing times and resulted in mussels that had a lower growth rate and condition index. The isotope samples were collected to perform a mass balance model for the experimental manipulation.
Perna canaliculus are an important aquaculture species in New Zealand and have the potential to be cultured alongside macroalgae to increase the growth rates of both species groups while also providing an economic and environmental benefit for the aquaculture industry. It was found that P. canaliculus grow better at sites with high coastal retention, which allows for the abundance of a high-quality food source and likely entrainment of kelp-derived particulate organic matter.