To lay their eggs, herring require a seabed of gravel, and there has long been scientific evidence that disturbance of herring spawning grounds, for example by gravel dredging, has an effect on the spawning success rates of herring. Further studies have pointed out that while herring may leave spawning grounds, an absence of spawning in any particular year does not mean that the spawning ground will not be used again and thus not required to maintain a resilient herring population. The advice from ICES Herring Assessment Working Group (HAWG) has always been firm: in areas where spawning occurs, extraction should not.
Gravel is, however, a commercially important product, with an estimated 93.5 million cubic metres of sand and gravel removed from European waters in 2012 (ICES WGEXT), and the advice from ICES HAWG has repeatedly been challenged. As spawning bed choices and spawning times vary, what is the extent of the herring's gravel needs in terms of time and space?
How long does spawning last?
Spawning of the main North herring population begins in the north of the North Sea in September and then progresses southwards with time, ceasing in January in the eastern English Channel. Smaller coastal populations tend to spawn in spring, whereas anecdotal accounts suggest that small populations may spawn from July in the north of the North Sea.
Leave those kids alone
As it takes about 10–15 days for the eggs to develop in the North Sea and the duration of the yolk-sac stage is 10–15 days, the total time for a spawning bed to remain undisturbed in order for allowing a successful survival of spawn will on average be around 30 days. The potential herring spawning habitat takes up quite a large area of the North Sea and the need for the undisturbed area appeared somewhat challenging for a productive gravel extracting industry. Could this be mitigated?
First stop MARS
The opportunity arose for a collaborative case study between ICES HAWG and the Pelagic Advisory Council (PAC) to investigate the possibility of the gravel industry extracting gravel in known spawning areas without disturbing herring spawning. During the annual meeting of ICES Working Group on Marine Systems (WGMARS) in late 2014, the Pelagic Advisory Council (PAC) and the HAWG chairs (Lotte Worsøe Clausen and Niels Hintzen) organised a workshop to exchange expertise in relation to herring spawning grounds in the North Sea and the advice related to the protection of these.
A mapping exercise with
researchers, commercial industry, and Government advisors and other
stakeholders (Mark Russell, British Marine
Aggregate Producers Association, Ian Reach, MarineSpace Ltd, Alex Wiseman, Scottish Pelagic Fishermens
Association, Verena Ohms, Pelagic Advisory Council, Claus Reedtz
Sparrevohn, Danish Pelagic Producer Organisation,
Martin Pastoors, Pelagic Freezer-Trawler
Association) compiled the current knowledge of herring spawning sites to produce
a common map of the perceived herring spawning grounds in the North Sea.
Representatives from science, fishery, and gravel extraction companies then
presented their state-of-the-art knowledge of the potential herring spawning
grounds, as well as the actual use of these spawning grounds by both fish and
humans.
Extracting data
The workshop provided an opportunity for the marine aggregate industry to share a new methodology that has been developed to map areas of potential herring spawning habitat at a regional sea scale drawing on a wide range of data sources as part of the environmental impact assessment process for licensing sand and gravel extraction in the UK. Mark Russell took the opportunity to differentiate between the widespread areas of potential herring spawning habitat that are present in the Southern North Sea and Eastern English Channel and the discrete areas of commercially viable sand and gravel resource, based on the geological origins of the two deposits and their spatial distribution.
Russell explained, "This enables a better understanding of the level of potential interaction between sand and gravel extraction operations and potential spawning habitat. Through better understanding, scientific advice can be framed in a more proportionate way that more accurately reflects not only the potential vulnerability of an interaction (in this case herring spawning habitat and sand and gravel extraction), but also the potential exposure given the relative spatial scales involved and the range options that are available to further mitigate and manage any risks."
Better together
The outcome of the workshop were detailed maps which are the foundation for further work; the PAC is currently looking into funding opportunities for a dedicated project on mapping of herring spawning grounds that would include an even wider field of stakeholders (fishers from relevant nations, wind farms companies, tidal turbine companies, dredging companies, statistical and mapping experts).
Ohms noted, "This is only one example where science and stakeholders work together to provide better management advice for pelagic fisheries. However, what makes the current collaboration on mapping herring spawning grounds especially valuable is the involvement of the dredging industry. They have a lot of relevant information to contribute. In the future, when moving to an ecosystem approach to fisheries management, it will become even more necessary to liaise with other sectors and the current work is at the forefront of such developments."
ICES advice
For HAWG, the discussions in the workshop included reflections on the advice. Advice should reflect the depth of information and understanding of the collaborative knowledge base. While the current advice formulation leaves no room for manoeuvring as it is rather categorical and specific, if the advice was expanded with words like 'unless properly mitigated/regulated…' or '…likely significant effect of the activity must be assessed…' the advice would reflect the current uncertainties around the effects of the use of herring spawning ground/bed. These reflections are now mirrored in the HAWG advice sheets.
Further data groundwork combined with potential collaborative surveys would mean an improved ability to give relevant and timely advice on the spatial and temporal uptake of the marine environment by herring spawning and highlight where compromises or trade-offs could be made.
