Fishes in Lagoons and Estuaries in the Mediterranean 3A: Migratory Fish

By Mohamed Hichem Kara and Jean-Pierre Quignard

Beyond their size (over 600,000 hectares), their wide geographical distribution and their geomorphological, hydro-climatic and eco-biological diversity, Mediterranean lagoons play an important role in regional economies. In addition, they are among the most vulnerable ecosystems to anthropization and climate change.

• Language: English
• Publisher: Wiley
• Release date: Mar 1, 2019
• ISBN: 9781119597384

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Preface

The conservation of the natural and economic heritage represented by Mediterranean lagoons and estuaries and the associated adjacent areas (wetlands, reed beds, sansouires and salt marshes) calls for an in-depth scientific knowledge of the past and present state and the functioning of these environments, and particularly of their plant and animal components. It is on this basis that appropriate management policies can be formulated.

Classed as transition zones between land and sea, these special ecosystems are matters of concern for both scientists and managers. The former group has accumulated significant knowledge of their abiotic characteristics and their functioning. They are now investigating the individuality of the resident populations, their interactions with the adjoining ecosystems and their future in the context of climate change. The latter group is seeking scientific and technical tools that will enable them to use these environments to their full potential, taking into account the increasing anthropic pressures.

In this series of books, divided into three stand-alone, complementary volumes, we have brought together scientific knowledge amassed over nearly two centuries on the fishes of the Mediterranean lagoons and estuaries. This summary has been compiled from documents published in local and international reviews and in general or specialized bioecological works on pure and applied ichthyology. We are, however, conscious that an entire fringe of works concerning lagoon and estuarine fishes has been omitted, this being the gray literature consisting of expert reports, academic projects and theses, etc.

The first volume, entitled Diversity, Bioecology and Exploitation, is a non-exhaustive approach to the characteristics of lagoons and estuaries, from a geo-geographical, hydrological and general bioecological viewpoint, and also looking at the ecophysiology and behavior of the fishes that live there. The general features of the exploitation and management of fish resources are also considered.

The second volume, Sedentary Fish, is devoted to the fishes that are so named because, being very euryvalent, they live out their entire lifecycle inside lagoons and estuaries. These fishes are not all specific to these environments; some have their counterparts in the sea or in fresh water.

The third volume, Migratory Fish, is concerned with fishes that, after spending time in lagoons, are obliged to return to their native marine or river environment to complete their lifecycle (genesic migrations), the physicochemical conditions in lagoons and estuaries (temperature, salinity, turbidity, etc.) being incompatible with the water properties required for their reproduction. Strictly hydroclimatic events can also be at the origin of migratory journeys.

The data provided in volumes two and three of this series of books are at three taxonomic levels: family, genus and species. Those concerning family and genus are relatively brief and general, while those relating to species are exhaustive and very detailed, for every aspect dealt with: systematics, genetics, phylogenesis, ecology, biology, behavior, etc.

This summary has been designed to permit rapid and comprehensive access to the body of scientific knowledge on lagoon and estuarine fishes and their sources. These data are indispensable in order to develop projects of research, infrastructure, management and conservation concerning these environments and their populations.

Mohamed Hichem KARA

Jean-Pierre QUIGNARD

September 2018

Foreword

Lagoons, deltas and estuaries are by definition transition zones and represent a distinctive element of the Mediterranean shoreline. In days of old, people used to come here to catch an abundance of fish, and this coastal fishing – practiced behind the shoreline in the channels of the salt marshes and in the estuary mouths – was at that time more highly prized than fishing in the open sea. Nowadays, although lagoon fishing represents only a small fraction of annual fish catches in the Mediterranean, estuarine and lagoon habitats continue to play a major role, be it as nurseries or in supporting an often-intensive mariculture, such as in Egypt, Italy and Greece.

This academic publication, patiently compiled by two eminent ichthyologists who are familiar with both shores, covers in three volumes the ichthyofauna of 303 lagoons and estuaries in the Mediterranean region, from the coastline of the Alboran Sea to Anatolia. Volume 1 outlines the vast geographical, geomorphological, hydrological, physicochemical and also historic diversity of Mediterranean lagoons, a diversity that has led to marked differences in the biology, reproduction, genetics, feeding and behavior of lagoon fishes.

Furthermore, the reader will find illustrated descriptions of 47 lagoon and estuarine species that have been studied, with a detailed discussion of systematics and of issues relating to biogeography, reproductive and feeding strategies, genetics and biodiversity. Throughout this work, a distinction is drawn between sedentary and migratory species – those that come and go each year between the lagoons where they find refuge, and the sea where they reproduce. However, the dividing line between these two worlds can sometimes be tenuous, and the authors introduce many central issues that remain unresolved, relating to, for instance, the genetic differentiation and adaptation (or preadaptation) between migratory and sedentary stocks, or the respective contributions made to the local fisheries by the lagoon nurseries and the marine shore area. The ichthyofauna of the studied sites is remarkably discrete: of the 249 species inventoried in 45 representative estuaries and lagoons, it will be noted that only 15 are found in 50% or more of the studied sites.

In the face of increasing anthropic pressures on the Mediterranean coast, already weakened by concrete urban development and its pathogenic wastes, by erosion, climate change, industrial and agricultural discharges into the sea, irresponsible mass tourism and the arrival of invasive Indo-Pacific species, the conservation and sustainable management of these areas and of lagoon fishing take on a certain urgency. The authors consider these topics at some length; their views are invaluable, drawn from their long experience in the field; I hope that many practitioners will find inspiration in them.

Because of the variety and expert knowledge of the themes covered, to its extensive bibliography and illustrations, this work is sure to become indispensable to the technicians and managers involved in fisheries and Mediterranean aquaculture. On a wider level, it will interest the many students and researchers working in ichthyology.

Frédéric BRIAND

Director General CIESM Mediterranean Science Commission

Introduction

Unable to carry out their entire lifecycle in lagoons and estuaries, as sedentary species do, so-called migratory species make regular movements, at relatively stable dates, which are predictable from one year to the next, between the sea and the lagoon, and vice versa, or between lagoons and freshwater (Hervé and Bruslé, 1979; Lasserre, 1989; Quignard and Zaouali, 1980, 1981; Quignard, 1984, 1994). Egg laying takes place either in the sea or in freshwater, but never in lagoons or estuaries. In this introduction, we will sketch the main lines that characterize the ecology, biology and the exploitation of this guild, before providing a detailed description, species by species. The list of species retained in this context is justified by their frequency in these environments and/or due to their confirmed abundance in the few sites they occupy.

It is important to know that many erratic occupants of lagoons and estuaries, still referred to as casual migrants, generally make short-lived incursions into these environments. These episodic visitors represent about 70% of the species we have mentioned (sedentary, migratory and occasional). Their frequency in the environments studied does not exceed 9% and is generally poorly represented (sometimes only one individual) (see Volume 1). Often, they are carnivorous ichthyophagous, as the Belone belone garfish, the Scombrus mackerel, etc., which chase the schools of small clupeidae Sardina pilchardus, of engraulids Engraulis sp., but also the atherine Atherina hepsetus, the red mullet Mullus sp., etc. In addition to these species, there are some exotic lessepsiens and herculeans whose presence is linked to more or less important geoecological manipulations (modifications of the channels connecting the sea to the lagoons, immersion of artificial reefs, opening of the Suez Canal, etc.) and general hydroclimatic upheaval.

From 97 fish censuses carried out in 45 Mediterranean lagoons (see Volume 1), the best-represented families of migrating fish are listed in descending order according to their occurrence in the censuses: Anguillidae, Moronidae, Mugilidae, Sparidae, Soleidae:

– eight species are present in at least 75% of the environments considered (Anguilla anguilla, Dicentrarchus labrax, Liza aurata, Sparus aurata, Mugil cephalus, Solea solea, Chelon labrosus, Liza ramada);

– seven species in at least 50% (Liza saliens, Diplodus annularis, D. sargus, Engraulis encrasicolus, Sarpa salpa, Belone belone, Mullus surmuletus);

– eight species in at least 25% (Pomatoschistus minutus, Platichthys flesus, Diplodus vulgaris, Lithognathus mormyrus, Diplodus puntazzo, Mullus barbatus, Boops boops, Scomber scombrus).

It is generally accepted that migration from the sea to the lagoon is induced by trophic and anti-predator needs, insofar as shallow lagoons are not favorable for the penetration of large predators, and intralagoonal algal and phanerogamic vegetation make sight hunting difficult. Lagoon-sea outmigration is, in turn, induced by reproductive needs and/or thermal constraints, and probably by other unidentified causes.

The specific richness of lagoon-migrating fish varies from one environment to another and depends on:

1) the species richness of the adjacent littoral zone and their morphological and behavioral type;

2) the structure of the channels connecting the lagoon to the sea;

3) the physical, chemical and other characteristics of lagoons;

4) the hydrodynamics of sea–lagoon and lagoon–sea exchanges.

Occupation of lagoons and migration phenology

In shallow lagoons (approximately 1 m), migrants are generally 0+ young individuals, larvae and juveniles depending on the species, whereas in deep lagoons, individuals of all ages make such displacements, but 0+ are still usually the most abundant age group. Apart from intrinsic lagoon factors, the success of the recruitment of migratory species depends on the success of breeding at sea, hydrographic conditions (survival and dispersal of larvae), the topographic and architectural qualities of the communication channels between the sea and the lagoon and the extent of the volume of water coming out from the lagoons to the sea.

Only a fraction of the marine population migrates to the lagoons (Quignard, 1984; Quignard and Zaouali, 1980, 1981; Lasserre, 1989; Mercier et al., 2012), but we have no estimate of the relative importance of this migratory phase in relation to the original marine stock, nor do we have knowledge about the determinism and the laws governing fish migrations between sea and lagoons. For example, what is the share that depends on the fish and which part is related to the conditions at sea and/or the lagoon? In other words, within the same species, why do some individuals move, while others remain at their original marine territory? Within a marine or a freshwater population, are there any sedentary or nomadic genetic lineages that regularly extend their distribution area to lagoons? Over the past 15 years, genetic, molecular and mineralogical studies (microchemistry of otoliths) have begun to provide interesting information about Mediterranean lagoon-thalassic migrants: the visit of different nurseries by the gilt-head sea bream S. aurata (Mercier et al., 2012; Tournois et al., 2013, 2017) and by the eel A. anguilla (Panfili et al., 2012); the return periodicity to the lagoon for the same gilt-head sea bream population (Mercier et al., 2012); the independence of lagoon recruitment from nesting sites by the sole S. solea (Morat et al., 2009); differentiation (the existence of exclusive alleles in lagoon migrants) and the genetic adaptation of the gilt-head sea bream (Chaoui et al., 2012; Guinand et al., 2016) and the sea bass (Lemaire et al., 2000, 2004–2005; Guinand et al., 2015) to local lagoon conditions; etc. At the genetic level, the question is whether these divergences are premigratory or whether they result from ongoing natural selection processes acting on the new recruits that colonize the lagoons, causing an increase in the frequency of the alleles that allows migrants to become adapted to lagoon systems.

The return to sea (outmigration) of marine migrants is made against the current during the entry of marine waters by the inlet. These trips are made by fish of all ages having stayed for a few weeks, a few months or a few years (eel) in the lagoons. For each species, they occur in several waves of groups of individuals that are often of the same size and sometimes the same sex, during a fairly constant period, from one year to another. However, we should emphasize that returning to the sea does not always have a reproductive purpose, since it involves juveniles that are far from sexual maturity. Perhaps, this could be induced by autumn lagoon hydroclimatic conditions being more unfavorable than those at sea (Hotos et al., 2000; Katselis et al., 2007) or by other yet unidentified factors.

Lifespan and growth

The lifespan and size of migratory species are much higher than those of sedentaries, most of whom are annual or subannual. For example, 12-year-old gilt-head sea breams visit the Mirna estuary in Croatia (Kraljević and Dulčić 1997), 11-year-old eels are found in the Commacchio lagoon in Italy (Rossi and Colombo, 1976) and 10-year-old sea basses live on the Tunisian coasts (Bouain, 1977). Migrants with a relatively long lagoon stay in comparison to their lifespan, such as the A. anguilla, the common goby P. minutus, two semelparous species, reach their maximum size in lagoons, before moving to lay at sea. As for other migratory species, likely to carry out several relatively short lagoon stays throughout their life, the comparative approach regarding the share of their growth at sea and in the lagoon, for the 0+ and especially for adults, is harder to pin down. Microchemical techniques applied to otoliths could teach us more.

The growth performance of migratory species is often judged to be better in lagoons than at sea, an opinion which is not shared by all researchers, some of whom describe these environments as deadly traps (Boutière, 1974) and even as places of death (Chauvet, 1986). Differences in growth also exist between neighboring lagoons (Bruslé and Cambrony 1992, Cambrony 1983, Quignard et al., 1984; Mosconi and Chauvet, 1990; Isnard, 2015), and even within the same lagoon among different biogeographical sectors (Escalas et al., 2015). Taking into account the adaptive strategies deployed by the species occupying lagoon ecosystems (Amanieu and Lasserre, 1982), the chances for better survival and for better growth vary considerably depending on lagoons, intra-lagoonal sites (marine sectors and continental sectors) and on the years (Amanieu, 1973). As a result, no uniform scenario can be drawn and the lagoon advantage regarding individual growth cannot always be retained. Comparative data between sea-lagoons, carried out at the same time (same period, same year), at marine and lagoon sites close to each other, are very little documented or entirely missing. Nevertheless, we can observe that certain lagoons or intralagoonal zones, especially those rich in continental supplies, can help postlarvae and juveniles to have a somatic condition beneficial to their survival and to their subsequent development at sea (Isnard et al., 2015). However, it is still difficult to assess the impact of the benefits gained by these juveniles on the course of their adult lives. However, Lasserre and Labourg (1974) admitted that the sizes reached after a lagoon stay had repercussions on the dynamics of marine stock.

Shallow lagoons, generally enriched with continental supplies, provide fish larvae and juveniles with better growth conditions than marine lagoons, especially deep ones. These shallow lagoons are often very rich in food and welcome yearly juveniles returning to sea the following year. They, therefore, constitute hot spots of fish larvae and juveniles of very good quality, which will move on and populate the sea, but also the surrounding deep lagoons that also welcome individuals older than 1 year. Shallow lagoons thus contribute to the supply of deep lagoons with good juveniles that, in turn, indirectly provide them with recruitment of new good larvae and juveniles born of good male and female begetters which reproduce at sea. The interactive loop, sea–shallow lagoon–deep lagoon–sea, is thus closed. Knowledge of the size and geographical area concerned by this type of interaction is essential for the reasoned management of fishery.

Other biological and behavioral traits

Migrants are gonochoric, with the exception of sparidae, and all spawn at sea. Nevertheless, lagoon egg-laying has been observed in gilt-head sea bream kept in cages in the Messolonghi–Etoliko lagoon (Dimitriou et al., 2007). Unlike sedentary species, their first sexual maturity is usually late (later than 1 year of age). Their eggs are laid in open water; are pelagic, small, very numerous (thousands or even millions) and are not subject to parental care. With the exception of the eel and the P. minutus goby, which only participate in one spawning season in their life (semelparous spawners), all migrants are iteroparous spawners that participate in several spawning seasons during their lifetime. The emission of gametes takes place in large promiscuity, within groups or schools of fish. For the P. minutus, the only migratory nesting species, a female lays successively in the nest of several males (polyandry); thus, there is a formation of ephemeral couples, limited to the duration of the act of laying eggs. While the migratory circuit and spawning sites of eels are becoming better and better known (Amilhat et al., 2016), their spawning behavior still remains mysterious. With the exception of P. minutus, a nesting fish which cares for its eggs, all migrants have a much higher oocyte fecundity than lagoon-sedentary species. For example, the absolute fecundity of the flounder is 325,800–1,450,000 (25–45 cm TL) (Vianet, 1985), whereas it is 50,000–272,000 (36–56 cm TL) for the sea bass (Kara, 1997). For the breeding gilt-head sea bream, the relative fecundity is 1,000,000–2,000,000/kg (Zohar et al., 1984).

If the lagoon-sedentary species, characterized by their small maximum size, exploit the small planktonic, nectonic and benthic (endogenous and epigeic) preys, the migrants whose individuals have a very wide range of sizes have a dietary behavior which differs largely from one to the other, ranging from micro- to macrophagy of invertebrates and vertebrates. Whether in deep or in laminar lagoons, or in estuaries, they exploit all levels, from the substrate’s bacterial film (some mullets) to molluscs (sea breams), crustaceans, fish (sea bass) and plants for the S. salpa porgy, the only lagoon-migratory herbivore. They are also interested in all sizes and all forms of prey (planktonic, endo- and epibenthic, sessile or vagile, nectonic), according to their size and needs. Less subject to predation than sedentary species, they use and export lagoon energetic resources to the sea and to freshwater. The rivalry for the access to prey between individuals in the migratory guild is relatively small, given the relatively high degree of specialization, especially among large specimens, be they juveniles or adults. On the other hand, there is significant trophic competition between sedentary and migratory species at the larval, postlarval and juvenile stages (Gisbert et al., 1996; Shaiek et al., 2015).

A strong contribution to fishery resources

Migratory species represent the main fish richness of lagoons whose production amounts to 156,000 tons/year (Cataudella et al., 2015), that is to say, 17–20% of the total fish caught in the Mediterranean. The yield per hectare is estimated at 118 kg/year (Pérez-Ruzafa and Conceptión, 2012). The main species fished are the eel, the mullet (Liza sp., C. labrosus, M. cephalus), the gilt-head sea bream and the sea bass, but in different proportions depending on climatic and geographical zones. In the lagoons of the eastern Mediterranean, the eel is small and the catches are dominated by various sparidae, whereas in western lagoons, and especially to the north, these can reach 80% of production. The seasonal rhythmicity of the coming and going of migratory fish is used for capturing them at the weirs, using fyke nets, eel baskets and secondarily globes, but it is essentially the individuals leaving the lagoons that are fished, particularly in Italy (De Angelis, 1960; Ravagnan, 1978; Kapesky and Lasserre, 1984), Tunisia (Chauvet, 1984, 1988), Greece (Pearce and Crivelli, 1994; Rosecchi and Charpentier, 1995) and Algeria (Chaoui et al., 2006). For several years, a decrease in lagoon halieutical production has been observed (Skinner and Zalewski 1995; Crespi 2002; Chaoui et al., 2006; Djabou et al., 2012; Zoulias et al., 2014). In the face of this depletion, it has sometimes been sought to strengthen the stock of certain species with fish farming products (the sea bass at Bages-Sigean and at Thau in France, the gilt-head sea bream at Bardawil in Egypt). The second attitude that has often been adopted has aimed at conservation, thus prohibiting fishing during a period of the year (North Tunis, Mauguio and other lagoons). With regard to the eel, a plan to ensure that at least 40% of adult individuals leave the lagoons in order to lay their eggs at the Sargasso Sea has been implemented not only to maintain, but to re-establish stocks.

We should bear in mind that other fishery products are important resources for fishermen in Mediterranean lagoons. This is the case of the salted and dried eggs obtained from the M. cephalus flathead grey mullet, commonly known as the striped mullet or common mullet. This product sells for around 200 euros/kg regardless of the country (Cataudella et al., 2015).

The fishery management of lagoonal ichthyological resources is complicated due to the existence of specific intra-lagoonal stocks of sedentary fish (silversides, gobies, blennies, etc.), which are relatively independent from marine stocks, and stocks of migratory species (mullets, gilt-head sea breams, sea basses, etc.) exploited in lagoons and at sea (shared fisheries). In the latter case, marine fisheries have an impact, not only on the marine part of the stock, but also on the lagoon part, since the lagoon stock comes from the sea (recruitment from the marine stock). Thus, excessive fishing in the lagoon can only aggravate the situation of an overfished marine stock. It is therefore at interactive fisheries where the mutual impact can be very strong, especially in areas where the lagoon system is highly developed and can, therefore, temporarily accommodate a significant part of the fundamental marine stock of fish of one or more species (recruitment of migrants). Under these conditions, a concerted management of the two substocks, marine (fundamental) and lagoonal, is recommended because, until now, the manager has not been able to access reliable information regarding the proportion of migratory individuals in relation to the stock of sedentary marine individuals. Nevertheless, using the microchemistry of otoliths, Tournois et al. (2017) have shown that lagoon nurseries contribute more to the local fishery of the Gulf of Lion than the inshore marine area does. In addition, the results of recent works on population genetics tend to show some divergence, from this perspective, between marine and lagoonal stocks of sea bass (Lemaire et al., 2000), gilt-head sea breams (Chaoui et al., 2012), common sole (Murten et al., 2009) and anchovies, among which two species can be recognized (Borsa et al., 2004). Let us observe that the eel, a long-term thalassotoc migrant fish found in lagoons and in freshwater, is a special case. This species is only subject to intralagoonal fishing. Therefore, its management is essentially lagoonal (without forgetting the fresh water) and should seek to ensure the return to sea of a number of spawners compatible with the renewal of the Atlanto-Mediterranean stock.

In the world’s lagoons, aquaculture produces about 3.4 million tons (5.7% of global aquaculture production). In the Mediterranean, most of the fish farm production in lagoons is made up of typical migratory lagoonal species (sea bass, gilt-head sea bream). In 2008, it reached 66,738 tons for the sea bass and 133,026 tons for the sea bream (FAO, 2010). In Egypt, the breeding of mugilids (M. cephalus, L. ramada, L. seheli, L. saliens, L. aurata, Creni mugil sp.) is important, with a production of 986,820 tons (GAFRD, 2012). Most of this activity is based on the fishing of fish larvae in lagoons in order to feed livestock farms. However, the impact of this practice on the future of lagoon and marine stocks has not been evaluated yet. As a result, the precautionary principle was adopted by this country as a protective measure prohibiting this practice.

Status and threats

Finally, among the lagoon-estuarine species, some are threatened, according to the red lists of Mediterranean fish species (Abdul-Malak et al., 2011). The A. anguilla is in critical danger of extinction, the P. minitus is vulnerable and the sea bass D. labrax is near threatened. However, these threats concern fewer species than the sedentary guild (of which there are nine in total).

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