Wakame in Port Phillip Bay is not all bad

Photo by Luke Barrett

Introduced species always seem to get a bad rap – they can outcompete native species, or allow other introduced species to prosper, which can ultimately reduce native biodiversity. In coastal environments around the world, native seaweed populations have declined, with a range of non-native species taking over. With urchin barrens growing throughout Port Phillip Bay, there is less available habitat for native fishes. Dr Luke Barrett tested if fish utilised habitats formed by wakame, an introduced seaweed, as much as habitats containing local seaweeds. He found that native fishes did not distinguish between native and wakame habitats, and that fishes had similar fitness metrics in both habitat types. Compared to natural reefs that had been urchin-grazed, wakame canopies resulted in higher fish abundance and biodiversity. Therefore, despite being considered a pest species, wakame canopies can provide important habitats for endemic fish and may play a role in sustaining native fauna populations in this degraded ecosystem.

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Jumping through oil to remove lice – does it work?

Chemical use in controlling parasites in salmon aquaculture can have a variety of negative impacts, including poor fish welfare and the release of chemicals into the environment after treatment, which can affect ecosystems and the sea life living in them. With this in mind, researchers from the University of Melbourne and Institute of Marine Research teamed up to test if a floating oil layer containing a dissolved chemical could be combined with an innate salmon behaviour to remove lice. They found that the presence of an oil layer did not deter salmon from jumping through it. Also, they found that as the concentration of the chemical dissolved in the oil layer increased, lice removal also increased. This alternative chemical treatment administration method would require minimal fish handling and also allow for simple recollection after treatment, and is therefore more environmentally friendly. It’s a win-win for everyone!

 

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What’s the best method to collect sea urchins for aquaculture?

 

A diver surfacing with a bag full of urchins. Photo by Fletcher Warren-Myers.

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.

Sea urchin aquaculture is an emerging industry in Australia, with several universities working on cultivating a variety of urchin species across Australia. The SALTT lab previously identified that harvesting adult urchins directly from barrens and feeding them with quality feeds to produce marketable roe is a nifty solution that can reduce the number of urchins in barrens while simultaneously profiting from the increasing demand for roe.

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.

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Can ballan wrasse keep up with salmon?

Image by Geir Friestad via Flickr

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.

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How submerged cages affect salmon welfare and behaviour

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.

 

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Methods to prevent and treat biofouling in shellfish aquaculture

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.

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Can cold water be used to remove lice?

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.

Read the full article here.


Struggling to breathe: small salmon struggle with hypoxia more than larger salmon

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.

Read the full article here.


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