Micropogonias undulatus - Atlantic croaker

The Atlantic croaker is native to the east coast of the United States and the Gulf of Mexico (northwestern and southwestern Atlantic Ocean). Here, the species is one of the most common bottom dwellers. There are also several populations on the Atlantic coasts of south Brazil and north Argentina ^[3].

First observation in Belgium

The Atlantic croaker was first observed in Belgium on the 17th of August 1998. A shrimp fisher caught a juvenile near the Ostend coast but did not know at first which species it was ^[2].  

Spreading in Belgium

So far, the species was observed only three times in Belgian waters ^[3]. After a first observation in 1998 in Ostend ^[4], the fish was found a second time in October 2011 in the Sea Scheldt in Antwerp ^[2]. Four years later – on the 24^th of August 2005 – another shrimp fisher caught an Atlantic croaker off our coast. This was the third – and currently the last – observation of the species in Belgian waters ^[3].

Whether or not the Atlantic croaker has established itself in Belgian waters is still a subject of discussion. In Dutch waters, the species is considered established. The ICES (International Council for the Exploration of the Sea) characterises the permanent settlement of this species as ‘uncertain’ ^[5]. On the other hand, some scientists say that it is plausible that the species permanently occurs in some Belgian ports or estuaries. To elaborate on this, scientists investigated the otoliths of young fish. These have – just like trees – growth rings that can be used to determine the age of the fish. If the fish live in unfavourable conditions, e.g. during transport in the ballast water of a ship or in the case of a scarce food supply, their growth slows down. This reduction in the growth rate is reflected in the growth rings of the otoliths formed at the time. In the individuals found in Belgian waters, this was not the case. Therefore, scientists believe that these individuals were born in our region ^{[2, 6]}. Based on these findings, the species is considered to be established in Belgium.

Spreading in neighbouring countries

In the Netherlands, after the first sightings of juvenile fish (October 2003: one individual in the eastern Wadden Sea; and October 2004: two individuals in the North Sea Canal) ^[2], there have been regular sightings of juvenile and adult individuals in the North Sea Canal and the Europoort area.

Several scientists argue that ‘all’ Atlantic croakers observed in western Europe got introduced during the (post-)larval stage or as juveniles in the ballast water of ships ^[7]. It is not easy for a coastal bottom-dwelling fish species to cross the Atlantic Ocean on its own. Others believe that the species is already reproducing here, e.g. in the North Sea Canal (the canal between Amsterdam and IJmuiden). Therefore, young animals found here are considered to have been born there ^[2].  

Since 2005, the Atlantic croaker has not been observed in Belgium, but it has been observed in the Netherlands. However, this does not prevent this species of fish from becoming more common in the (near) future. The Atlantic croaker is a coastal benthic species ^[4] with a wide tolerance for salinity and temperature and can, therefore, thrive in many places (eurytopic) ^[8]. The Scheldt estuary, with its diversity of habitats, is very suitable for this alien fish species.

In its area of origin, the larvae can reach nearby estuaries – up to 100 kilometres away – via the ocean currents ^[8]. Besides shipping (primary introduction), natural processes may also contribute to the further (secondary) spread of this species in Europe.

Furthermore, the Atlantic croaker is resistant to low oxygen levels (hypoxia). This resistance gives the species a better chance of survival than other native species ^[9].

Despite the few observations, the Atlantic croaker is classified in our area as an established non-indigenous species of the Scheldt estuary. The species does not have a significant impact on biodiversity and ecosystem functions ^[10]. Therefore, control measures have not yet been taken.

In America, the Atlantic croaker is known as a delicacy and is caught frequently. Because of this, the fish is threatened by overfishing in these regions ^{[4, 11]}.

The Atlantic croaker belongs to the family of the croakers or drums (Sciaenidae), which typically occur in subtropical and tropical waters. Seven species of drums are indigenous to the northeast Atlantic (including the Mediterranean Sea and the Black Sea) ^[12]. Only Argyrosomus regius– apart from the Atlantic croaker – is rarely found in the southern North Sea ^[4]. The Atlantic croaker looks very similar to the brown meagre Sciaena umbra, another species in the family of the drums. Based on the similarities, the Dutch name ‘Amerikaanse zeeraaf’ (translates into ‘American sea raven’) got suggested for this non-native species ^[4].

The Atlantic croaker is characterised by an elongated body, with an arched dorsal side and a straight ventral side. The underside of the mouth has three to five pairs of barbels. The body is silver-grey to bronze, with distinct diagonal stripes. Adult fish can grow up to 55 cm in length and weigh around 2.5 kg. The dorsal fin has two parts: the first has ten hard spines, and the second has one hard and 27 soft fin rays. The anal fin has two soft and eight hard rays ^{[2, 4]}.

The Atlantic croaker invades estuaries during its post-larval stage. In winter, adults migrate seawards to reproduce (down to 100 meters) ^[13]. In spring, they swim upstream to the estuaries to spend the summer. The species are typical of sandy and muddy soils and mainly feeds on benthos like worms, small crustaceans or fish ^{[2, 4, 9]}.

[1]          World Register of Marine Species (WoRMS) (2020). Micropogonias undulatus (Linnaeus, 1766). [http://www.marinespecies.org/aphia.php?p=taxdetails&id=151158] (2020-11-17).

[2]          Dekker, W.; Daan, N.; Heesen, H.; Van der Heij, W. (2005). De knorrepos Micropogonias undulatus (L.), een nieuwe vissoort in Nederland. De Levende Natuur 106(2): 66-67. [http://www.vliz.be/en/imis?module=ref&refid=78328]

[3]          Kerckhof, F. (2006). National report Belgium, 2005, in: ICES Advisory Committee on the Marine Environment. Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 16-17 March 2006 Oostende, Belgium - ICES. CM 2006(ACME:05). ICES: Copenhagen: pp. 43-45. [http://www.vliz.be/en/imis?module=ref&refid=111238]

[4]          Rappé, G. (2002). Eerste vangst van Micropogonias undulatus (Linnaeus, 1766), een Amerikaanse vis, in Belgische en Europese wateren. De Strandvlo 22(3-4): 119-121. [www.vliz.be/en/imis?module=ref&refid=31434]

[5]          Gollasch, S.; Haydar, D.; Minchin, D.; Wolff, W.J.; Reise, K. (2006). WGITMO input to REGNS - Introduced aquatic species of the North Sea coasts and adjacent brackish waters, in: ICES Advisory Committee on the Marine Environment. Report of the Working Group on Introductions and Transfers of Marine Organisms (WGITMO) 16-17 March 2006 Oostende, Belgium. CM Documents - ICES. CM 2006(ACME:05). ICES: Copenhagen: pp. 121-132. [http://www.vliz.be/en/imis?module=ref&refid=208974]

[6]          Kerckhof, F.; Haelters, J.; Gollasch, S. (2007). Alien species in the marine and brackish ecosystem: the situation in Belgian waters. Aquat. Invasions 2(3): 243-257. [http://www.vliz.be/en/imis?module=ref&refid=114365]

[7]          Stevens, M.; Rappé, G.; Maes, J.; Van Asten, B.; Ollevier, F. (2004). Micropogonias undulatus (L.), another exotic arrival in European waters. J. Fish Biol. 64(4): 1143-1146. [http://www.vliz.be/en/imis?module=ref&refid=59760]

[8]          Schaffler, J.J.; Reiss, C.S.; Jones, C.M. (2009). Spatial variation in otolith chemistry of Atlantic croaker larvae in the Mid-Atlantic Bight. Mar. Ecol. Prog. Ser. 382: 185-195. [http://www.vliz.be/en/imis?module=ref&refid=135071]

[9]          Mohan, J.; Walther, B. (2016). Out of breath and hungry: natural tags reveal trophic resilience of Atlantic croaker to hypoxia exposure. Mar.  Ecol. Prog. Ser. 560: 207-221. [http://www.vliz.be/nl/catalogus?module=ref&refid=296649]

[10]        VLIZ (2011). Indicatoren voor het Schelde-estuarium. VLIZ Special Publication, 50. Vlaams Instituut voor de Zee (VLIZ): Oostende. 165 pp. [http://www.vliz.be/imis?module=ref&refid=206086]

[11]        Barbieri, L.R.; Chittenden, M.E.; Jones, C.M. (1997). Yield-per-recruit analysis and management strategies for Atlantic croaker, Micropogonias undulatus, in the Middle Atlantic Bight. Fish. Bull. 95(4): 637-645. [http://www.vliz.be/en/imis?module=ref&refid=120955]

[12]        Chao, N.L. (1986). Sciaenidae, in: Whitehead, P.J.P. et al. Poissons de l'Atlantique du Nord-est et de la Méditerranée, Volume II = Fishes of the North-eastern Atlantic and the Mediterranean, Volume II. UNESCO: Paris: pp. 865-874. [http://www.vliz.be/en/imis?module=ref&refid=129246]

[13]        Forward, R.B.; Reinsel, K.A.; Peters, D.S.; Tankersley, R.A.; Churchill, J.H.; Crowder, L.B.; Hettler, W.F.; Warlen, S.M.; Green, M.D. (1999). Transport of fish larvae through a tidal inlet. Fish. Oceanogr. 8: 153-172. [http://www.vliz.be/en/imis?module=ref&refid=119083]

VLIZ Alien Species Consortium (2020). Micropogonias undulatus – Atlantic croaker. Non-indigenous species in the Belgian part of the North Sea and adjacent estuaries anno 2020. Flanders Marine Institute (VLIZ). 5 pp.