There are millions of purple sea urchins (Heliocidaris erythrogramma) in Port Phillip Bay chomping their way through kelp forests, which are important habitats for many creatures and critters that live within the bay. In their wake, urchins leave unsightly and biologically unproductive barrens, therefore causing reduced local biodiversity. Barrens can be flipped back into kelp forests if urchins are either removed or destroyed – but both options are extremely expensive. Fortuitously, the gonads (or roe) of urchins are a Japanese delicacy, with high quality roe fetching up to $450 kg-1. However, the roe of urchins harvested from barrens are typically unappealing and inedible, and therefore fishermen tend harvest them elsewhere.
However, in order to collect urchins from barrens to bring them to aquaculture facilities to fatten them up, it’s important to determine the best harvest method that does not affect their survival and external condition. Dr Fletcher Warren-Myers and researchers from the University of Melbourne and Deakin University tested if divers using a 3-pronged hook or careful hand-collection to harvest urchins affected their mortality and external condition both in the short- and long-term. Overall, they found that survival and condition was similar regardless of collection method. As divers using the hook method collected urchins twice as fast as by hand, this will become the method to harvest urchins in the future.
Ballan wrasse are crafty fish that eat sea lice off swimming salmon. While we know they are useful cleaner fish, we don’t know much about how well they can swim in different currents and at different temperatures. If we did, we could establish better deployment strategies and predict when their welfare may be at risk. Jeffrey Yuen, an honours student from the University of Melbourne, tested the standard and maximum metabolic rate, aerobic scope, and critical swimming speed of ballan wrasse acclimated at 5, 10, 15, 20, and 25°C, to see how performance differed. The wrasse were generally inactive and had low metabolic rates at lower temperatures. They also did not swim continuously between 5 and 20°C. Only at 25°C did they swim continuously, with an average critical swimming speed of 27 cm s-1. This is a fraction of the critical swimming speed of salmon. The much weaker swimming capacity of ballan wrasse means they won’t cope well in salmon farms with moderate to strong current speeds. Further, their low metabolic rates and inactivity at 5-10°C suggests that they won’t do their lice eating job well at these temperatures, limiting them to warmer places and times. Jeffrey’s results can be used by farmers to make sure they are stocking ballan wrasse in the right times and places in order to make sure cleaner fish are doing their jobs.
Submerged cages are an exciting new method being tested by the salmon aquaculture industry to avoid salmon lice infestations within farms. However, salmon do not cope with long-term submergence as they need to refill their swim-bladders regularly to maintain buoyancy. Assoc. Prof. Tim Dempster together with researchers from the Institute of Marine Research, Norway tested how salmon coped in submerged cages that were lifted to the surface weekly to allow surface access. Three submerged cages were positioned at 10 m depth were deployed for 8 weeks, and measures of welfare and behaviour were compared against three standard cages. They found that submerged fish swam 1.4 to 3.4 times faster, schooled tighter, and their swim bladder volumes declined gradually from the beginning to the end of each week. When cages were lifted and the surface became available, negatively buoyant fish immediately exhibited jumping and rolling behaviour. However, they found no evidence of acute buoyancy problems during submergence, and growth rates and welfare scores were similar to standard cages. Their research suggests farming salmon in submerged cages with weekly surface access is a viable method to prevent lice infestations.
Biofouling in bivalve aquaculture can cause economic losses for the industry. Therefore, strategies such as avoidance, prevention and treatment to minimise biofouling are key. The type of rope used to collect spat or grow bivalves can prevent or reduce fouling by harmful species, but this method largely remains untested. Additionally, a range of eco-friendly treatment measures also exist, but their effect on common biofoulers are unknown. Researchers from the University of Melbourne tested biofouling accumulation and spat collection for seven commercially used ropes with ambient and heated seawater, acetic and citric acid, and combinations of both applied across a range of exposure times to two commercially grown shellfish and three biofouling species. They found that rope type and treatment type were successful on some biofouling species without adversely affecting shellfish.
Many Norwegian Atlantic salmon farms are using warm water thermal delousing to control salmon lice in farms. However, treatments can lead to poor welfare outcomes for fish. Kathy Overton alongside researchers from the University of Melbourne and Institute of Marine Research tested if reverse thermal delousing by rapidly reducing water temperatures to very low treatment temperatures could reduce salmon lice without any negative side effects for fish. To do this, they tested the effects of transferring salmon from 15°C to cold water at different temperatures and durations. They found that treatments of −1°C water for 10 min and 1°C for 240 min treatments reduced mobile lice loads, but created more skin and eye damage than controls. While cold water treatment reduce mobile lice numbers, the cold shock reaction in salmon (illustrated in the video below) was identified as a major hurdle in industry-scale application.
Dissolved oxygen is fundamental to the fitness and survival of fish. When there is not enough oxygen available in the water, hypoxic conditions occur which can have significant implications for the growth, feed intake and survival of fish. Monitoring dissolved oxygen saturation is one of the most important environmental factors analysed in Atlantic salmon aquaculture. To mimic the reduced oxygen levels experienced by fish in crowded streams and in commercial salmon aquaculture farms, researchers from the Institute of Marine Research, including SALTT-lab alumni Dr Tina Oldham, researched how fish of different sizes coped with hypoxic conditions. They tested how metabolic rate and swimming performance of Atlantic salmon in three size classes (0.2, 1.0 and 3.5 kg) were affected by exposure to 45-55% dissolved oxygen saturation. They found that while hypoxia did not affect standard metabolic rate, it caused a significant decrease in maximum metabolic rate and resulted in reduced aerobic scope. Further, swimming speed for small (0.2 kg) salmon was reduced by 23%, whereas large (3.5 kg) salmon were able to have slightly lower or similar swimming speeds compared to standard conditions. Their research illustrated that moderate hypoxia reduces the capacity for activity and movement in Atlantic salmon, with smaller salmon most vulnerable to hypoxic conditions.
Parasite and disease outbreaks are a common issue for many aquaculture industries around the world, and efficient strategies to control the spread of them are scarce. The Atlantic salmon aquaculture industry is growing globally, with Norway producing the most salmon worldwide. However, salmon lice infestations hinder the growth of the industry and can have negative welfare outcomes. Salmon lice larvae are released from and transported among salmon farms by ocean currents, which create inter-farm networks of louse dispersal. Dr Francisca Samsing along with researchers from the University of Melbourne, Deakin University, and Institute of Marine Research investigated if introducing no-farming areas or ‘firebreaks’ could disconnect dispersal networks of salmon lice. Using a model to predict louse movement along the Norwegian coastline and analysis to identify potential firebreaks to dispersal, she identified one firebreak that split the network into two large unconnected groups of farms. She also found farms that should be removed during spring to prevent wild salmon migrating out into the ocean from getting bombarded with high infestation pressures. If applied to the industry, her model should help lower infestation pressure both at farms and in wild salmon populations.
Sea cage fish aquaculture attracts large aggregations of wild fish that opportunistically feed on farm waste. Over time, these fish can undergo physiological changes, and captive feeding trials indicate possible negative effects on their reproductive fitness. However, not much is known about the significance of this phenomenon for reproduction in wild fish over larger spatial scales. Dr. Luke Barrett with researchers from the University of Melbourne and the Institute of Marine Research investigated if coastal areas with intensive aquaculture impacts the fitness of wild fish. They collected Atlantic cod in southwestern Norway from two neighbouring areas with either a high or low density of Atlantic salmon farms, and compared a range of reproductive fitness metrics via a captive spawning trial. They found evidence that cod from the area with a high density of salmon farming produced smaller eggs which led to smaller larvae, indicating a possible reduction in reproductive investment among cod from the intensive salmon farming area.
With aquaculture industries expanding around the world, there are growing concerns about their environmental impacts and effects on wildlife. Aquaculture farms are thought to either repel, act as a population source, or act as an attractive population sink (or ecological trap) for a variety of species. To assess the state of knowledge on the impacts of aquaculture on wildlife worldwide, researchers from the University of Melbourne led by Dr. Luke Barrett conducted a review and meta-analysis of empirical studies to better understand the outcomes of interactions between aquaculture operations and wildlife. Effects of aquaculture on wild populations depended on the wild taxa and farming system. Overall, farms were associated with a higher local abundance and diversity of wildlife, but this effect was mostly driven by aggregations of wild fish around sea cages and shellfish farms. Birds were also more diverse at farms, but other taxa, such as marine mammals, showed variable and comparatively small effects. While they identified evidence for widespread aggregation ‘hotspots’ in several systems, the authors also found that very few studies collect the data needed to assess impacts of aquaculture on the survival and reproduction of farm-associated wildlife. Such data will be crucial for determining whether the behaviour of aggregating around farms results in higher or lower population growth for farm-associated wildlife.