Tarpons

Biology, Ecology, Fisheries

Stephen Spotte

• Editore: Wiley
• Anno: 2016
• ISBN:
  9781119185703
• Edizione: 1
• Argomento: Technology & Engineering
• Categoria: Fisheries & Aquaculture

Stephen Spotte, Mote Marine Laboratory, Sarasota, Florida, USA

Tarpons arose from an ancient lineage, and just two species exist today, confined to the tropics and subtropics: Megalops atlanticus in the western and eastern Atlantic and Megalops cyprinoides distributed widely across the Indo-West Pacific. The Atlantic tarpon is considered king of the saltwater sport fishes and supports a multi-billion dollar recreational fishery in the U.S. alone. The Pacific tarpon, which is much smaller, is less valued by anglers. Both have limited commercial value but offer considerable potential for future aquaculture because of their hardiness, rapid growth, and ease of adaptation to captivity.

This book is the latest and most thorough text on the biology, ecology, and fisheries (sport and commercial) of tarpons. The chapters comprise clear, intricate discourses on such subjects as early development and metamorphosis, population genetics, anatomical and physiological features and adaptations, migrations, reproductive biology, and culminate with a concise overview of the world's tarpon fisheries. A comprehensive appendix includes Spotte's original translations of important papers published previously by others in Spanish and Portuguese and unavailable until now to English readers.

Tarpons: Biology, Ecology, Fisheries will be of considerable interest and use to fishery and research biologists, marine conservationists, aquaculturists, and informed anglers

CHAPTER 1
Development

1.1 Introduction

The developmental biology of tarpons is so unusual that it seems a fitting subject for the opening chapter. I had originally intended to present the ontogeny of Atlantic and Pacific tarpons side by side, with salient features and timing emphasized at least partly in tabular form, but inconsistencies in the literature made it impossible. Specimens in the various reports were often measured at different lengths and unknown ages. Although captive rearing eliminates the age problem, it introduces confounding factors that can compromise normal rates of growth and development. Then, too, descriptions ranged in quality from detailed to superficial. Taxonomists sometimes favored particular characters, relegating others to lesser status or ignoring them. In short, I could not get comparative descriptions to line up without generalizing, which would have diluted the entire effort. What I present is therefore detailed, but in narrative form, with the two species treated separately.

Nonetheless, the pattern of their ontogeny is similar. The descriptions presented follow staging systems devised in the 1960s and 1970s by Brazilian and US scientists for Atlantic tarpons. Those for the Pacific species are less detailed and cohesive. The objective is to offer a detailed summary of tarpon ontogeny book-ended by separate sections devoted to just the leptocephalus larva.

1.2 The tarpon leptocephalus

There was a time when nobody knew what young tarpons looked like, but some still claimed to have seen them. Among the many tall tales is this whopper recorded in a letter from Charles H. Townsend to Mr. Grant and reproduced by Beebe (1928: 230). Townsend was traveling to the Galápagos Islands to capture giant tortoises, probably for the New York Zoological Society’s Bronx Zoo, when he penned this:

“In conversation with Mr. S. A. Venable of the Zone Police Force [Panamá Canal Zone Police], an experienced [Atlantic] tarpon fisherman, I was informed that the fish is viviparous. He has repeatedly observed the females seeking shallow water, generally less than 4 feet deep, where a continuous stream of young fish was poured from her vent, the young being apparently little more than ¼-inch long. The young immediately seek refuge in groups, under the large scales of the mother, each scale standing outward at an angle of probably 30°. The young clustered in these scale shelters as thickly as they could. Mr. Venable’s many observations lead him to believe that the young shelter under the scales ten days or more, when they are ¾-inch long. The mother soon rids herself of the young by shaking herself and by leaping.”

Probably because the smallest tarpons he ever saw were juveniles, taxidermist and sportsman Victor Brown of Everglades City, Florida thought they hatch fully formed. In a letter to Kaplan (1937: 91), Brown wrote: “The newly spawned tarpons, 1 to 3 inches long, immediately commence to work their way entirely out of salt water into fresh water streams, into the multitude of small creeks and canals, some going as far inland as 25 miles from the Gulf [of Mexico].”

Contrary to these kinds of statements, baby tarpons do not emerge as miniature adults. They hatch from fertilized eggs as yolk-sac larvae before morphing into leptocephali, larval forms unique to relatively few species of fishes (Hulet and Robins 1989; Inoue et al. 2004; Wang et al. 2003). Greenwood et al. (1966) established the superorder Elopomorpha based on representatives of all its subgroups having leptocephalus larvae (Fig. 1.1). A leptocephalus is a bizarre shape-shifting creature, laterally compressed, transparent with a mucinous pouch, and described variously as ribbon-, band-, or leaf-shaped. Elopomorpha is a monophyletic group, the leptocephalus an elopomorph synapomorphy. Order Elopiformes (tarpons and ladyfishes) occupies the most basal place in elopomorph phylogeny, Albuliformes and a clade consisting of Anguilliformes and Saccopharyngiformes making up a sister group (Fig. 1.2). Smith (1989: 961–962) provided an abbreviated key to elopiform leptocephali occurring in the western North Atlantic.

Fig. 1.1 Higher-level classification of orders in the Elopomorpha along with numbers of taxa presently included. Representative larval and adult body forms are illustrated for each group. The Elopiformes, to which the two extant species of tarpons (Megalops atlanticus and M. cyprinoides) belong, is represented by a ladyfish, of which six species exist (Elops spp.).

Source: Inoue et al. (2004: 275 Fig. 1).

Fig. 1.2 A modern phylogenetic hypothesis about the monophyly of Elopomorpha. See source publication for history and details.

Source: Inoue et al. (2004: 276 Fig. 2B).

What constitutes a “larval fish” has been standardized to some extent (e.g. Richards 2006). The traditional definition is the interval between hatching and absorption of the yolk sac, the post-larval stage extending from termination of the larval stage to appearance of juvenile characters. In Gopinath’s (1946: 8) opinion, certain groups fail to conform with this progression. He listed specifically the bonefishes, ladyfishes, tarpons, left-eye flounders, and tonguefishes, “even though they are post-larvae according to the [accepted] definition”, and termed them larvae instead, “since these [fishes] undergo a complete metamorphosis before the assumption of adolescent characters.” In other words, by Gopinath’s definition, a tarpon leptocephalus remains a larva to the moment it commences metamorphosis. Wade (1962: 548) considered the leptocephalus to the start of its metamorphosis as a post-larva, the larval period evidently restricted to the interval between hatching and appearance of the leptocephalus (Stage 1, see below); that is, synonymous with the yolk-sac larva. So did Alikunhi and Rao (1951), although their terminology is less clear.

A more modern treatment of how a larval fish is defined (presumably a tarpon or any other) partitions the concept into four post-hatch stages in which flexion refers to when the notochord becomes flexible. These are: (1) yolk-sac; (2) pre-flexion (complete yolk-sac absorption and beginning of notochord flexion); (3) flexion (start of notochord flexion to its completion); and (4) post-flexion (end of notochord flexion and start of metamorphosis). ¹ The last initiates post-larval transformation, or the metamorphic stage (start of metamorphosis to completion of fin-ray development and beginning of squamation), after which juvenile traits appear and development proceeds seamlessly to the adult form with eventual attainment of sexual maturity.

1.3 Staging tarpon ontogeny

To my knowledge, eggs and yolk-sac larvae of either species of tarpon have not been described. Anyanwu et al. (2010) of the Nigerian Institute for Oceanography and Marine Research purportedly obtained fertilized eggs collected in the wild, then hatched and reared them to the fry stage in laboratory aquariums (Chapter 8.6). Surviving fry were transferred to earthen ponds and grown to juveniles. Specific information was not provided. The ultimate goal was to develop these procedures so that a reliable source of fry could be available consistently to fish farmers.

The few details in this report are tantalizing and apparently unpublished formally, but if backed by adequate data would indicate that knowledge of early tarpon biology has advanced more quickly in western Africa than in the Western Hemisphere. For example, Anyanwu et al. (2010: 6) wrote, “The fertilized eggs are available in the coastal waters of Ondo State [Nigeria] which can be collected and hatched in the laboratory.” They implied that fertilized eggs are recognized, collected, and cultured routinely by fish farmers (Chapter 8.6). The fertilized ova hatched after 24 hours, and early larvae measured 5.3–6.8 mm TL (5.0–5.7 mm SL). Plate 4 (p. 8) in their report is described as a photograph of the anterior half of a yolk-sac larva. Hatchlings experienced heavy mortality after 5 days, which Anyanwu and colleagues suggested could have resulted from a lack of appropriate food. This is doubtful, considering that evidence of feeding has been found only after metamorphosis (Section 1.6, Chapter 7.7, Appendix B).

In discussing subsequent larval stages of Atlantic tarpons, I rely mainly on descriptions of Jones et al. (1978: 53–62), which evidently were compiled from other sources, notably Mercado Silgado and Ciardelli (1972) and Wade (1962). Also see Mercado Silgado (1969, 1971) and Moffett and Randall (1957).

The protoc...