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WOSHH & WOSH-eDNA-Sound Results
2024 spawning bed survey
A West of Scotland herring spawning ground near North Erradale, Wester Ross was surveyed (see the WOSHH update here and the WRFT update here) using Drop Down Videos (DDV) cameras and a Remotely Operated Vehicle (ROV) on the 12th and 20th of March, 2024. The survey was initiated after “milky” turquoise water was seen from shore, by Peter Cunningham WRFT, as well as from space (the Copernicus Satellite; see the BBC article here). The patch, likely caused by the milt (sperm) of spring-spawning herring was 1.5-2 km from shore.
During the two spawning ground surveys, herring eggs were found in an area measuring approximately 4 km by 700 m. A total of 32 videos were recorded, including near Longa and South Erradale (not pictured), with 16 showing eggs on the seabed (see map below).
Image: Map of the area where a herring spawning ground was surveyed using drop-down video cameras. Presence and absence of eggs at the location of camera drops (WOSHH & Wester Ross Fisheries Trust) was mapped alongside Priority Marine Features (Geodatabase of Marine features adjacent to Scotland).
Maerl beds
All spring-spawning herring eggs seen during the 2024 survey were on maerl with well-formed nodules (approximately 1.5-3 cm long). Herring eggs were particularly abundant on maerl ripples/megaripples and were prevalent on both live and dead maerl.
Maerl (pictured) is a type of coralline red algae that forms nodules, creating seabed “carpets” known as maerl beds. These beds are complex in structure, play a significant ecological role, and provide habitat for many marine species.
The results of the 2024 herring spawning ground surveys provide renewed evidence that maerl is an important spring-spawning herring spawning habitat (see life history). Thick carpets of herring eggs were also recorded on maerl beds nearby, off Red Point, in spring 2018/19, and historically in the Clyde. In addition, there is ample Local Ecological Knowledge among fishers that herring historically [and contemporarily] spawned on “coral” (a.k.a. maerl).
Maerl is extremely slow growing (0.5-1.5 mm per year) and fragile (Blake & Maggs, 2003). It is, therefore, a Priority Marine Feature and is recognized as an OSPAR threatened and declining habitat due to anthropogenic pressures (e.g., physical damage, sedimentation and climate change).
For herring, which rely on specific seabed habitat (e.g., maerl) for the survival of their eggs, limited availability of suitable spawning habitat could restrict recolonisation and rebuilding plans if spawning grounds are not considered in marine management strategies (Frost and Diele, 2022).
Environmental (e)DNA
A shore-based eDNA sampling programme was conceived by WOSHH and trialled in 2022 in Wester Ross (Applecross, South Erradale and Melvaig), in collaboration with the Wester Ross Fisheries Trust and local community members. The method proved successful and demonstrated that shore-based sampling, rather than the traditional boat-based method, effectively detects herring DNA. This is a promising development, as it lowers both the risks and costs of sampling.
In 2022, sampling was conducted every 3rd day between 1 Mar-12 April. At all three study locations, herring DNA was present in most samples during that year.
In 2023, sampling was conducted between 17 Jan-18 April at Arran (new site sampled by the Community of Arran Seabed Trust), Skye (new site sampled by the Skye and Lochalsh Rivers Trust), Applecross (2022 site) and Melvaig (2022 site). The low levels of herring DNA detected at Melvaig in January 2023, compared to the high levels found in March 2023, suggest that the sudden increase was likely due to herring shoals moving inshore rather than resident fish. At Arran, no herring DNA was found (except very low amounts on the last sampling date), and likewise at Applecross, where herring DNA was abundant in 2022. Repeated eDNA sampling revealed significant year-to-year variations in herring presence at Applecross, while herring were consistently detected at Melvaig in both 2022 and 2023.
2024 Results coming soon.
Image: Presence/Absence of Herring eDNA sampling conducted by WOSHH and its partners at the Scottish west coast in 2022 and 2023.
Plankton sampling for herring larvae
In 2023, marine plankton samples were collected for WOSHH by the Wester Ross Fisheries Trust, near Gairloch, and the Community of Arran Seabed Trust (COAST) and by other community members from the Isle of Arran. No herring larvae were found by the Arran team, whereas a few confirmed herring larvae and several clupeids (unable to determine if sprat or herring) were found again off Melvaig (pictured).
The repeated finding of herring larvae in Wester Ross in both 2022/23, in conjunction with gathered local ecological knowledge and eDNA (and sound recordings – see below) indicates that the Minch/Inner Sound is a spring-spawning herring hotspot.
2024 Results coming soon.
Image: Larva sampled off Melvaig, Wester Ross in 2023. © Dr Shraveena Venkatesh, Wester Ross Fisheries Trust.
Underwater sound recordings
To the best of our knowledge, no one has used underwater sound recorders during the herring spawning season to try to detect shoals. Discussing the topic with colleagues at the Scottish Association for Marine Science (SAMS), who discovered the FRTs, we agreed it was worth exploring.
In spring 2023, with the help of local fishers, we deployed small sound recorders, called HydroMoths, at four locations at Mull, the Inner Sound and the Minch. The devices were programmed to record 2 minutes every 5 minutes (i.e., 7-minute cycle) around the clock for approximately 41 days. Whilst no herring FRTs have been detected so far, the recorders captured hours of humpback whale calls near the mouth of Loch Gairloch (listen below), as well as dolphin whistles and clicks (listen below) on days when a large amount of herring eDNA was present in the water at the nearby Melvaig eDNA sampling site. The presence of a humpback whale, known to feed on schooling forage fish such as herring, is not common in the area this time of year and may have been linked to the presence of herring simultaneously observed nearby.
In 2024, we once again deployed underwater sound recorders at several locations in Loch Gairloch. Stay tuned to find out if the humpback was back!
Listen to humpback whale calls:
Listen to dolphin clicks and whistles:
Banner image: Herring eggs © Andy Jackson, SubSeaTV; reproduced with permission.