Fisheries

The released egg is protected by a fairly tough chorion or egg case. Within this the cytoplasm and yolk are contained by a vitelline membrane. Often one or more oil globules are present. Fertilization occurs by a spermatozoon passing through a funnel-shaped micropyle leading to a fusion of the pronuclei …

It is not at all clear why some species have evolved a strategy of producing many small eggs, and others the opposite strategy of fewer, larger ones. The relationships between these characters and the implication for the species can be summarized (Blaxter, 1988) as follow: Fecundity and egg size are …

Vertebrates of both sexes possess a germinal epithelium consisting of germ and somatic cells. The gonads are mesodermal in origin and develop in close association with the nephric system. In elasmobranchs a lateral cortex gives rise to the ovary, and a more medial area becomes the testis. One of these …

According to Karel Liem (1980) most of the enormous variety and range of foods eaten by fishes is obtained through only three basic feeding styles: ram feeding, suction feeding, and manipulation or biting. Virtually all species use one, and, because they are not mutually exclusive, most species use two of …

Like other animals, fish possess an array of digestive enymes by which large macromolecular nutrients are broken down into smaller molecules that can be assimilated. Most fish possess seven main digestive enzymes – trypsin, maltase, amylase, two +aminopepsidases (carboxypepsidase a, carboxypepsidase b), lipase, and alkaline phosphatase. Almost all the major …

The vast majority of living fishes are predatory. Of some 1100 species of extant elasmobranchs, only 13 (1.2%) – the basking shark (Cetorhinus maximus), megamouth shark (Megachasma pelagios), whale shark (Rhincodon typus), manta ray (Manta birostris), and about nine species of devil rays (genus Mobula) – have forsaken the actively …

The digestive tract of fishes is divided into four regions: the foregut (esophagus and stomach, if present), mid-gut, hindgut, and rectum. The foregut begins at the posterior boundary of the gill cavity or pharynx and includes the esophagus, the stomach, when present, and the pylorus. Typically, the esophagus is a …

Living Agnathan fishes possess conical, rasping tooth-like structures made of keratin, the same structural protein found in human hair and nails, on their tongue, rather than enamel covered, bony teeth typical of other vertebrates. The lamprey also has similar teeth around its mouth. They also have no identifiable esophagus or …

Blood ionic composition in marine elasmobranchs and holocephalans is some what higher than in teleosts, but its osmolarity is very much higher, close to that of seawater. This is because the blood contains, in addition to the usual ions, large amounts of low-molecular weight nitrogenous solutes, chiefly urea. First found …

But optimal foraging strategies may not remain identical throughout an organism’s lifespan and many species exhibit a trophic ontogeny or shift  in food types and sizes, or feeding styles at different stages in their life cycle. These are necessitated by the fishes own increase in size, by morphological developments or …