World Aquaculture December 2019

WWW.WA S.ORG • WORLD AQUACULTURE • DECEMBER 2019 47 ( C O N T I N U E D O N P A G E 4 8 ) from the wild, supplying around 80 percent of the total seed used by the industry, with the remaining seed produced in hatcheries. Mussel spat must be transported from their place of origin (i.e. wild-sourced or hatchery-cultured) to grow-out farms (Fig. 4), a journey that may require as long as 72 hr. Current transport methods may result in high levels of stress for the mussel spat, which may have long-lasting consequences for performance of mussels during grow-out. Regardless of the spat source, retention of mussel spat is currently a bottleneck for the industry because substantial losses are observed after they are deployed to farms. It is unknown whether these losses are related to the natural behaviour of mussel spat, which can easily detach from ropes and drift, or related to spat mortality due to transport stress. Transport of live shellfish has always been implicated as a source of stress for the mussels, with scientists and aquaculturists trying to mitigate and reduce stress and the mortalities associated with it. In general, marine molluscs such as mussels are transported in “dry” conditions, with exposure to air for long periods of time that can affect their quality and survival. The Greenshell TM mussel is no exception and existing research has mainly focused on the time after harvest of adult mussels to keep the animals alive and in good condition until they reach their destination market. In this context, pre- conditioning of adult mussels has been evaluated to extend the shelf- life of mussels during transport (Zamora et al . 2019). However, a critical first step is to characterise the mechanisms that regulate the stress response in the animals and understand the effects that transport can have on survival and overall performance of mussels and other species in aquaculture. In a preliminary study, Greenshell TM spat attached to rope were subjected to simulated transport for 20 hr in a 1 m deep container. Samples of spat exposed to air at the top and deep within the biomass at bottom of the container were collected for oxidative stress analyses at the end of the simulated transport. We assayed the samples to understand the dynamics of a suite of markers related to the oxidation of proteins, lipids and DNA (i.e. protein carbonyls, lipid hydroperoxides and 8-hydroxydeoxyguanosine), and enzymatic antioxidants (SOD, CAT, GPx and GR). Mussel spat had an increase in oxidative damage and antioxidant response during simulated transport for 20 hr (Fig. 5). Temperature and relative humidity in transport containers did not vary with position inside the containers during the 20 hr of simulated transport (Top: 15.9 ± 0.4C and 95 ± 9 percent RH; Bottom: 15.6 ± 0.4 C and 95 ± 9 percent RH). Nevertheless, rope with mussel spat were clearly drier at the top of the container. It is therefore suspected that moisture in the transportation system, rather than relative air humidity may play a crucial role in reducing the stress levels experienced by Greenshell TM mussel spat during transport. In addition, in a recent trial we identified the effects of emersion and desiccation stress in Greenshell TM mussel spat, showing that after 20 hr of emersion spat can lose around 77 percent of its fresh weight (assumed to be water content) when exposed to emersion and low levels of relative humidity (30-80 percent RH), whereas at higher RH (98-100 percent) mussel spat only loses around 26 percent of fresh weight after 20 hr of emersion. This emersion and desiccation stress has a direct effect on survival of mussel spat after re-immersion in seawater. These findings have allowed transport practices to be refined and brought us closer to unravelling the complex causes of spat loss. FIGURE 4. Greenshell TM mussel farm in Marlborough Sounds, New Zealand. Photo: Sanford Ltd. FIGURE 5. Mean concentration of ROS biomarkers for oxidative damage (A) and antioxidants (B) in GreenshellTMmussel spat after 20 hr of simulated transport (± SD, n = 3). PC = protein carbonyls (nmol/mg protein); LPOX = lipid peroxides (nmol/g wet weight); 8-OHdG = 8-hydroxydeoxyguanosine (8-OHdG/106 dG); SOD = superoxide dismutase (Units/mg protein); CAT = catalase (µmol/min/ mg protein); GPOX = glutathione peroxidase and GR = glutathione reductase (nmol/min/mg protein). * means statistically different (P < 0.05) between top and bottom of transportation container.