The gathering together of reef fish in aggregations for synchronised spawning is a phenomenal natural spectacle. Usually in coordination with the lunar cycle and with a clear purpose to reproduce, fish gather at specific locations, sometimes in groups of tens of thousands. Rising rapidly to the surface of the water, they release eggs and sperm in a spawning rush, before retreating to the relative safety of the reef.
Reef fish probably breed in this way to increase the chances of successful fertilisation, although it is possible that it is also a consequence of a lack of males large enough for paired mating. But it’s not only reef fish that do it. Other non-reef-dwelling species such as Atlantic cod also rely on this method of reproduction. Northwest Atlantic cod fisheries famously collapsed during the 1990s, and it is from this species that we must learn our cautionary tale.
Once an aggregation site is discovered, fishermen may seasonally re-visit the site, applying a high fishing pressure. Spawning aggregations are in fact hugely vulnerable to overexploitation, particularly due to the predictability in where and when they form and especially if they contain commercially valuable constituents. Destructive fishing practices have been implicated in the decline of or disappearance of spawning aggregations and this can be detrimental to the species.
Often catch data from aggregation sites is the only data available for population estimates. At aggregations, fisheries continue to achieve large catch volumes whether or not a population is teetering on the brink of collapse. This is termed the ‘illusion of plenty’ and highlights the importance of adopting a broader assessment of stocks if these commercial species are to be managed effectively. Responsible fishing also includes making sure the natural size, age and sex structures are maintained so that the spawning aggregations can continue to function and may involve the enforcement of marine protected areas and no-take zones.
Coral reefs cover less than 1% of the ocean floor yet they support a quarter of marine fisheries on which millions of people depend worldwide for their protein intake and livelihoods. By adopting a responsible management approach, we will foster the protection and management of these unique species and ensure we are not taken by surprise when populations disappear with little warning. This is precisely the approach that lacked in North Atlantic cod assessments and their populations shrivelled as a consequence.
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