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Case studies

White & black Anglerfish

European hake

European pilchard

Atlantic Mackerel

White and black anglerfish

The white anglerfish (Lophius piscatorius) and black anglerfish (L. budegassa) are amongst the most valuable commercial fishes in Europe with a total landing volume of around 30K tons corresponding to a value of around 142 million euro.

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Both species are assessed as three stocks in the Northeast Atlantic (the Northern Shelf, the Northern and the Southern stocks), whereas in the Mediterranean stocks have not yet been defined. Both species are relevant target of multispecies trawl fishery of multiple EU countries, but there are important uncertainties about species composition (in some areas white and black anglerfish are not distinguished for evaluation), stock structure and, most of all, their status in terms of population size, exploitation rate. Therefore, this implies an important difficulty for giving a good advice of how to best manage this resource.

Recent research by members of the research group have shown that the white anglerfish is composed by a single genetically homogeneous population within the Northeast Atlantic (Aguirre-Sarabia et al. submitted), calling for stock merging, but so far, no studies on the population connectivity of the black anglerfish have been performed. Importantly, recent research has shown hybridization between the two species, reaching up to 20% of hybrids in the white anglerfish population (Aguirre-Sarabia et al. submitted), pointing that further studies are required to evaluate the effect of hybrids in the current and future assessment of white and black anglerfish.

European Hake

The European hake (Merluccius merluccius) has traditionally been considered as one of the most important species in the Northeast Atlantic and the Mediterranean. 

In the Northeast Atlantic, hake is widely distributed from Mauritanian to Norway, but only two management units are considered (the northern and southern stocks). Despite extensive knowledge of the biology of the species and strict managing frame, hake in the Mediterranean is considered overexploited and the status of the Southern stock is suspected to be at high risk of overexploitation (FAO 2018; ICES 2020). In contrast, through the implementation of a recovery plan (EC No811/2004), the Northern stock has recovered in the last ten years, after more than two decades of overexploitation, although recruitment remains highly variable.

The wrong stock delimitation (the two Northeast Atlantic stocks are separated by the Capbreton Canyon without any clear biological evidence) could explain the poor performance of the stock assessment models fitted to the Southern stock data and to the assessment not being done on the right biological unit. Indeed, previous research by the research team (Leone et al. 2019) has shown that there is a mismatch between the biological and management units in hake, pointing to the need of additional analyses to redefine the stock structure in the North East Atlantic.

Additionally, in the northern stock, the main spawning season ranges from February to April, whereas in the southern stock, spawning occurs all year-round with the presence of several other peaks in summer and autumn (Korta et al. 2015), not being clear whether these represent two genetically isolated spawning components. Moreover, during the last decade a northward expansion of European hake has been observed (Baudron et al. 2020) which could be due to an increase in biomass in the Bay of Biscay and Celtic Sea, or to reasons related with environmental variation (e.g. climate change).

European Pilchard

The European pilchard or sardine (Sardina pilchardus) is one of the most important pelagic fish resources in Atlantic waters of the Iberian Peninsula and represents an important source of income for local economies.

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Nowadays sardine in the Northeast Atlantic is separated in three stocks: the Iberian stock (South Bay of Biscay and South Galicia, Portuguese waters and Gulf of Cadiz), the Bay of Biscay stock (North and Central Bay of Biscay and Celtic Sea-English Channel stock).

Genetic studies suggest strong differentiation between Mediterranean and Atlantic populations and, within the latter, studies suggest several sardine sub-populations based on regional differences in body morphology, maturity, growth, spawning seasonality and demography (Silva et al. 2006) and on density-dependent source-sink movements between adjacent areas (Silva et al. 2019). Moreover, the genetic basis of the spawning seasonality (spring and autumn spawners) which follows the cycle of water temperature has not been resolved.

Thus, genetic analyses are not conclusive, and more research is needed to elucidate connectivity in European pilchard in the Northeast Atlantic. On the other hand, long-term changes in sardine maturation (associated to reproductive potential and productivity) mediated by temperature oscillation have been described, whose understanding and potential association with genetic adaptation is essential to disentangle the adaptation capacity of the species.

Atlantic Mackerel

The Atlantic mackerel (Scomber scombrus) is a commercially important migratory pelagic fish that forms large shoals that can reach millions of individuals.

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This species has traditionally been grouped in five spawning components (two in the Northwest and three in the Northeast Atlantic) that migrate north during the feeding season. Within the Northeast Atlantic, mackerel is assessed as a single stock since 1996, although total allowable catches (TAC) levels in the EU are still set at component level (southern, western and North Sea).

Studies on the western component, report extensively mixing with a potential homing tendency (Jansen et al. 2013; Jansen & Gislason 2013), and work by the research team (Rodríguez-Ezpeleta et al. 2016) has shown genetic differentiation between Northwest Atlantic, Northeast Atlantic and Mediterranean samples. However, so far, no conclusive study on the population connectivity of Northeast Atlantic mackerel exists. Yet, Inferring the degree of mixing between spawning components of Atlantic mackerel is crucial to establish effective management actions in the current situation of no agreement on a management strategy covering all parties fishing the species exists.

On the other hand, in the mid-2000s, the species started expanding into new areas in Greenland, Iceland and the Nordic Seas during the summer (Olafsdottir et al. 2019) reaching up to a three-fold increase in 2014 and implying a shift of 400 km northward and 1650 km westward of the centre of gravity. However, in 2020, a large decline in Icelandic waters, no presence in Greenland waters, and increased biomasses in the central and northern part of the Norwegian Sea have been reported (Nøttestad et al. 2020).

Although, spatial distribution of mackerel seems to be related to environmental conditions (Olafsdottir et al. 2019), the origin of the Greenland and Icelandic mackerel is not clear, and neither is whether this species is genetically adapting to changing environmental conditions.