The vast majority of published papers concerning seagrass meadows contain statements to the effect that seagrass beds serve as important nurseries for many species. We reviewed more than 200 papers that were relevant to the nursery role hypothesis. We used both vote counting and meta-analytic techniques to evaluate whether the body of previous studies that report seagrass meadows to be nursery grounds actually contain data that support this proposition. We restricted our analyses to papers that compared seagrass beds to other habitats, and examined data on a variety of well-studied species concerning their density, growth, survival and migration to adult habitat. Within this group of papers, we considered potential factors that could influence the nursery function (e.g. location, or laboratory vs field studies). We also evaluated case histories of well-documented large-scale seagrass losses on the nursery function. Major results were consistent with the expectations that abundance, growth and survival were greater in seagrass than in unstructured habitats. Abundance data also suggested that seagrass beds in the Northern Hemisphere might be more important as nursery areas than those in the Southern Hemisphere. Surprisingly, few significant differences existed in abundance, growth or survival when seagrass meadows were compared to other structured habitats, such as oyster or cobble reefs, or macroalgal beds. Nor were there decreases in harvests of commercially important species that could clearly be attributed to significant seagrass declines in 3 well-studied areas. However, there were decreased abundances of juveniles of commercially important species in these areas, suggesting a strong link between seagrass abundance and those of juvenile finfish and shellfish. One important implication of these results is that structure per se, rather than the type of structure, appears to be an important determinant of nursery value. Clearly, more rigorous studies that test all aspects of the nursery role hypothesis are clearly needed for seagrass meadows as well as other structured habitats. The results of such studies will allow better decisions to be made concerning the conservation and restoration of marine habitats.

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Critical evaluation of the nursery role hypothesis for seagrass meadows

Book reviewFishes of the North-eastern Atlantic and the Mediterranean: Volume II. P.J.P. Whitehead, M.-L. Bauchot, J.-C. Hureau, J. Nielsen, E. Tortonese (Editors). UNESCO Press, Paris, 1986, pp. 510–1007, FF 250, Dfl 105.00, ISBN 92-3-002308-6

The Arctic charr Salvelinus alpinus exhibits 1–4 sympatric morphs in postglacial lakes, of which one or two are epibenthic zoobenthos feeders, one is a limnetic planktivore and one is a piscivorous form. In addition, northern rivers support partly migratory populations with anadromous and freshwater resident fish. The morphs vary in their coloration, morphology, life history, behaviour and genetic characteristics. The morphs usually differentiate according to their ontogenetic stage at maturity, which parallels paedomorphism in amphibians. The young usually start as epibenthic zoobenthivores, but may become pelagic at a certain size according to the predation risk experienced at that time. From a length of > 20–25 cm, charr start to become piscivorous. The proportion of piscivorous fish increases with increasing body size. In partly anadromous populations, fish that mature before smolting become freshwater resident, the others anadromous. In some rivers, the morphs occupy separate niches (epibenthic and limnetic), from emergence onwards. The morphs exhibit different degrees of reproductive isolation that vary from a high degree of interbreeding to complete isolation. Usually, they spawn within morph (assortative mating), but alternative male mating behaviour (sneaking, fighting) may occur in stream-spawning populations and at great depths in lakes. Morphologically specialized morphs appear to feed more effectively than intermediate forms, and selection according to feeding mode, site fidelity and associated assortative mating are prerequisites for the evolution of the different morphs. Charr morphs develop into stable feeding niches under conditions of intense intraspecific competition when there is little competition with other species. Sympatric morphs exhibit different degrees of speciation, but similar morphs in different systems are not individual species because of (1) their polyphyletic origin, (2) the supporting systems are often young, transient environments making the future situation for the populations uncertain, and (3) the genetic differentiation among morphs is low. Sympatric morphs may interbreed and produce fertile hybrids. Nevertheless, sympatric charr morphs should be managed as separate species. Changes in the natural conditions or human impacts to which the morphs are adapted will have a strong influence on the persistence and survival of each different morph.

Polymorphism and speciation in Arctic charr

We examined the nursery role of salt marshes for transient nekton by searching the lit- erature for data on density, growth, and survival of juvenile fishes and decapod crustaceans in marshes and using meta-analyses to test hypotheses. We analyzed density data from 32 studies con- ducted throughout the world. Based on fish density, habitat types could be ranked from highest to lowest as: seagrass > vegetated marsh edge, nonvegetated marsh, open water, macroalgae, oyster reefs > vegetated inner marsh. However, patterns of habitat use varied among the 29 fish species rep- resented. For decapod crustaceans (seven species), habitat types were ranked: seagrass > vegetated marsh edge > nonvegetated marsh, vegetated inner marsh, open water, macroalgae > oyster reef. We identified only 5 comparative studies on transient nekton growth in salt marshes. Fish growth in non- vegetated salt marsh was not significantly different from growth in open water or in macroalgae beds but was significantly lower than in seagrass. Growth of decapod crustaceans was higher in vegetated marsh than in nonvegetated marsh. Nekton survival in salt marsh (11 studies analyzed) was higher than in open water, lower than in oyster reef/cobble and not significantly different from survival in seagrass. When density, growth and survival are all considered, the relative nursery value of salt marshes for nekton appears higher than open water but lower than seagrass. Vegetated marsh appears to have a higher nursery value than nonvegetated marsh; however, tidal dynamics and nek- ton movement among marsh components complicates these comparisons. The available data have a strong geographical bias; most studies originated in the northern Gulf of Mexico or on the Atlantic coast of the United States. This bias may be significant because there is some evidence that salt marsh nursery value is dependent on geography, salinity regimes and tidal amplitude.

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Salt marshes as nurseries for nekton: testing hypotheses on density, growth and survival through meta-analysis

The sagittal otolith of 348 species, belonging to 99 families and 22 orders of marine Teleostean fishes from the north and central eastern Atlantic and western Mediterranean were described using morphological and morphometric characters. The morphological descriptions were based on the otolith shape, outline and sulcus acusticus features. The morphometric parameters determined were otolith length (OL, mm), height (OH, mm), perimeter (P; mm) and area (A; mm 2 ) and were expressed in terms of shape indices as circularity (P 2 /A), rectangularity (A/(OL×OH)), aspect ratio (OH/OL; %) and OL/fish size. The present Atlas provides information that complements the characterization of some ichthyologic taxa. In addition, it constitutes an important instrument for species identification using sagittal otoliths collected in fossiliferous layers, in archaeological sites or in feeding remains of bony fish predators.

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Otolith atlas for the western Mediterranean, north and central eastern Atlantic

The estuarine food habits of the European eel, Anguilla anguilla (L.), in the upper zone of the Tagus estuary have been studied through the analysis of 210 specimens collected during 1988. Eels, measuring from 136 to 627 mm in total length, fed on prey organisms ranging from 2.0 to 76.0 mm in length. Amphipods and the shore crab, Carcinus maenas (L.), were the most important food items found. In more saline muddy bottom areas polychaetes, bivalves and shrimp increased their importance as prey, replacing the amphipods as preferential food. Fish were always a secondary, although significant, prey item. The occurrence of seasonal variations in eels' diet could be attributed to fluctuations in prey densities. There was evidence of a close relationship between the quantity of prey organisms in the benthos and the composition of eels' stomach contents, but a certain degree of food selection based on size, concealment capacity and motility of prey, was found.

On the food of the European eel, Anguilla anguilla (L.), in the upper zone of the Tagus estuary, Portugal

El presente trabajo describe la morfologia general de los otolitos utriculares, lapilli, de los peces teleosteos, propone una terminologia para sus partes, identifica sus dos principales tipos morfologicos, aporta algunos ejemplos de su utilizacion en la identificacion de especies, y discute su utilidad en estudios de filogenia y sistematica de peces.

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The utricular otoliths, lapilli , of teleosts: their morphology and relevance for species identification and systematics studies

Do eel grass beds and salt marsh borders act as preferential nurseries and spawning grounds for fish? An example of the Mira estuary in Portugal

The general morphology of the asterisci from recent European teleosts is described, terminology for their parts is proposed, their three major morphological types are identified and the relevance of the morphology of these otoliths in species identification, and as a source of information for studies on fish phylogeny and systematics is discussed.

Carlos A. Assis

Papers

Otolith atlas for the western Mediterranean, north and central eastern Atlantic

On the food of the European eel, Anguilla anguilla (L.), in the upper zone of the Tagus estuary, Portugal

The utricular otoliths, lapilli , of teleosts: their morphology and relevance for species identification and systematics studies

Do eel grass beds and salt marsh borders act as preferential nurseries and spawning grounds for fish? An example of the Mira estuary in Portugal

The lagenar otoliths of teleosts: their morphology and its application in species identification, phylogeny and systematics