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To inform, inspire and motivate the world about wildlife and why its conservation is critical to the future of humanity.

Joe Brennan

Fish like the Sound of Healthy Coral Reefs

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Using underwater speakers to broadcast recordings of healthy coral reef ecosystems encourages fish to find and settle on damaged and man-made reefs.

Lizard Island Reef, Queensland

The results of an experiment undertaken at Lizard Island, Queensland, show that playing the sounds of a healthy reef ecosystem is associated with higher rates of colonisation of man-made reefs by fish species, as compared to the rates seen at reefs where the sounds weren’t broadcast. 

The study examined thirty-three man-made coral-rubble piles designed to approximate a reef structure. Eleven reefs included a broadcast speaker. The remaining ones were used as control groups, half with a dummy speaker and half without. 

Damselfish (Pomacentrus nagasakiensis) Image from Wikipedia

Over a forty day survey period the reefs with acoustic enrichment showed the highest rates of colonisation by damselfish (a common and abundant family of reef fishes), which remained in high abundance throughout the survey. After forty days damselfish were twice as abundant at enriched reefs as compared to the control group reefs.

At the end of the survey period, more juvenile fish were present at acoustically enriched reefs. The overall number of species was higher, and the developing fish communities more diverse. 

This means that playing healthy reef sounds at a location attracted higher numbers and more varied species mixes of herbivorous, omnivorous, planktivorous, invertivorous and piscivorous fish.

The two control groups – reefs with dummy speakers and reefs without – showed no significant differences between each other for any of the survey measures. 

It seems that new reefs are not colonised equally, and the appearance and added habitat diversity provided by a speaker doesn’t improve a reef’s attractiveness to fish. 

Climate change and human-caused disturbance is triggering large-scale change to coral reefs around the world. The broad scale impact of coral bleaching events is particularly concerning to those who are tasked with managing and conserving the ecosystems.

Coral with fish by lpittman from Pixabay

Diverse fish assemblages help to maintain the overall health of coral reefs because they perform a variety of important functional processes. Having healthy fish populations gives a reef a better chance of staying healthy and recovering from damage. 

The problem is that a damaged reef sounds less appealing to young fish looking for a place to live. 

While smell is another important factor leading fish to new homes, it’s hard to efficiently replicate. Broadcasting the sounds of a healthy reef is much more practical for reef managers, and it’s a known attractant for a wide variety of juvenile fish.

It’s not known exactly how the broadcasted sounds affected the fish communities – whether they made the reefs easier to find or more attractive to settle on once the fish arrived; or a mix of both. 

Regardless of the exact mechanism, it looks like broadcast speakers might be a useful tool in bringing fish back to damaged coral reefs.

This is likely to benefit wider areas than those immediately surrounding the loudspeakers, with fish forming small-scale communities that in turn create more niches for different species to fill; increased activity (sound and smell) helping to direct more fish to the area; and natural behaviours causing the residents and their offspring to migrate away from the original settlement area as they progress through their lifecycles.

Sound as a tool for ecosystem management is only a recent development. It has been used to affect the behaviour of some birds and mammals, and fish living in various fresh and marine environments show habitat selection influenced by sound. 

Using sound to help manage coral reefs might guide new techniques for conserving various other aquatic systems where restoration projects are underway.

Joe Brennan

Keystone Species Re-introductions: An analysis of the River Otter Beaver Trial

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Wildlife re-introduction projects are becoming more prevalent. Good monitoring goes a long way towards allying community concerns when it comes to ecologically important – but socially questioned – wildlife management.

The European beaver (Castor fiber) was once found in watercourses throughout Britain. Over-hunting led to its extinction, which – although not precisely recorded – likely occurred in the late 18th Century. 

Beaver Caster Fiber Eurasian(Image by RalfS from Pixabay)

The beaver is recognised as a keystone species – having a disproportionately large effect on its environment in relation to its own abundance. Their well-known waterway engineering creates habitats and entire ecosystems – and the prehistoric British landscape, and its riparian-dwelling plants and animals – evolved within such environments. 

Removal of the beaver and subsequent major alterations to British hydrology led to the development of a highly productive agricultural landscape – at the expense of riparian wildlife.

In 2009, the Scottish Beaver Trial was the first such enterprise to re-introduce beavers to the British landscape. Following this, several such trials have been implemented.

In February 2015, the Devon Wildlife Trust issued a licence for the River Otter Beaver Trial (ROBT) to release beavers into the wild. This followed the discovery of a wild breeding group of beavers living on the River Otter in East Devon. 

As part of a campaign supporting the retention of these wild beavers, the re-introduction project aimed to assess the impacts of beavers on the Otter catchment; its wildlife, local economy and people. Scientific assessment of these factors was hoped to provide a sound evidence base in order to determine the future of the population.

A robust monitoring programme was designed to assess the positive and negative impacts of beavers in the River Otter Catchment, and the five-year study was implemented. 

Monitoring Programme Results

With the set term ending in March 2020, the results offer future guidance on beaver re-introductions in Devon – with outcomes pertinent to the rest of the United Kingdom.  

The study has shown that the beaver population in the River Otter Catchment is increasing, and 13 separate territories were identified by 2019; stemming from the two family groups that founded the project in 2015.

The population has dispersed through the River Otter and smaller River Tale, as well as lesser tributaries – and shows that the Catchment can support a self-sustaining beaver population. 

Beaver dam (Image by Daina Krumins from Pixabay)

This population expansion has led to 28 beaver dams recorded in 2019 – predominantly in smaller tributary streams as the main stem of the Otter River is too large. The Tale experiences temporary, dynamic dams which are washed out in seasonal high flows.  

Of the 594 kilometres of watercourses within the Catchment, 1.9 kilometres (representing 0.3% of waterways) were impounded by beavers when assessed in October 2019.

This activity has of course impacted surrounding land use to some extent. Management of several spillways, culverts, agricultural land and waterways close to roads and access tracks has been required to keep water levels at acceptable levels. 

In one instance, riverside orchards were impacted by the feeding behaviour of beavers, as well as maize crops – which could be managed with minor visual disturbance to the areas (from such devices as tree guards).

Several trees were also felled over public footpaths and subsequently cleared.

As for benefits gained, beaver activity has delivered significant ecological benefit – with new wetland habitat created and managed. This in turn has benefited amphibians, wildfowl and water voles.

Fish abundance was found to be 37% higher within a beaver dam pool on the River Tale, as compared to upstream and downstream sites. Increases in minnow and lamprey populations were observed. Comparatively, the habitat and flow dynamics formed by washed-out dams created favourable habitat for juvenile trout. 

It remains unknown as to the impact of dams on migrating salmonoids in the Catchment; however trout were observed navigating past dams during high flow events. 

One village at risk of flooding saw a reduction in peak flows due to a series of dams constructed upstream, which spread water over the floodplain.

The results of the five year monitoring program show that the costs of beaver activity – while present and requiring acknowledgement and management – were outweighed by the benefits provided to ecosystems and communities. 

It’s recognised that, at a catchment scale, the benefits incurred can accrue at the same locations as the cost (in such examples as biodiversity gains due to wetland creation) but also in other locations (such as flood threat reduction downstream of dams). This is of importance as those who benefit most from a factor mightn’t be the ones bearing the cost. 

Increasing public acceptance of the beaver re-introduction project was also shown through the monitoring. This is a positive outcome showing that attitudes can and do change in regards to wildlife management – as long as information is accurately collected and demonstrated to those concerned; and appropriate management is put in place.  

Joe Brennan