Report – ZL2101 – Success of Gadidae

ZL2101, 2000-04-25

Success of the Family Gadidae.

This essay tempts to discuss the success of the Family Gadidae.

What is success of a species?

– Is it the length of period the species lives?

– Is it the spatial or geographical distribution?

– How big the population is of that species, genera or family?

For this essay success is defined as geographical ranges, the diversity of diet, number of viable offspring and the number of taxa.

Economic value.

The cods belongs to the order Gadiformes (cods) and class Actinopterygii (ray-finned fishes).

The family Gadidae is, by far, the most important commercial fish species of the world, since the discovery of the New World. The cod had such big impact to the early economy of New England that it was called the “Newfoundland currency” and they even have stamps with cods on (MINET, 1999; Bond 1996).

Historical, Geographical and Climatical Ranges.

Jawless fish could be found about 480 Million years before “age of the fishes” that started in the late Silurian (400 Million years ago), the common characteristics of the fish was, larger armoured that were active predators (Knox et al. 1999).

The evolutionary history of the fifty-eight species of the family Gadidae is relatively poorly understood. The oldest fossil found is little controversy though (Fishbase, 1999) states it dates back to middle Tertiary e.g. 30-35 Million years ago while Bond (1996) and Nikol’skii (1961) states both that the fossil record is known from the beginning of the Paleocene e.g. 65 Million years ago.

According to the figure it indicates that some 18 Million years ago genus Trisopterus (green lines in figure), which is considered to be the first known gadid species, was endemic to the north-east Atlantic. About 6 Million years later the gadids started to swim west and south to occupy western Atlantic and the Arctic Ocean (blue lines) due to the drop in temperature (MUN, 1999, Nikol’skii, 1961). The Pacific Ocean got it first gadids about 3-4 Million years ago, this is simultaneously as the Bering Strait opened up (red lines). Genetical evidence support the hypothesis that the gadids swam through the Bering Strait though the pacific tomcod, Microgadus proximus, have almost identical mtDNA sequences as the Greenland cod, Gadus ogac (MUN, 1999). The differences between the two are less than found between different Atlantic cod.

Figure from Carr et al. 1999

Gadids can be found in all kinds of waters, everything from fresh, brackish to marine (MUN, 1999, Nikol’skii, 1961). They are also found in both temperate as in cold waters in both the southern and northern hemispheres (Bond, 1996; Nikol’skii, 1961).

Atlantic cod occurs from shallow waters (about 5 meters) to the edge of the continental shelf where depths of up to 600 meters are found (Carr, 1999).

An Atlantic cod becomes sexually mature at about 6 years of age, however it can vary between five to eight years. Males tend to mature at an earlier age than the females.

An 80cm long female cod produce about 2 million buoyant 1-2mm large eggs. The mode of dispersal leads to a great waste of sex-cells, but when both sperm and eggs are equally buoyant and will float in the same direction this waste is reduced to some extent (Norman, 1975) Larger females at about 120cm produce about 11 million eggs. According to Norman (1975) only one egg in every million becomes an adult fish. The about 5 mm long newly hatched larvae is dependent on food from the yolk sac for one to two weeks before settling down at the bottom (MINET, 1999).

Morphological diversity.

The family Gadidae is one out of families in the super-family Paracanthopterygii, but there is still some controversy regarding the relationships of the fishes (Bond, 1996, Nikol’skii, 1961 & Norman, 1975). There are three sub-families in eight genera, with total of 59 species (Nikol’skii, 1961). The Gadidae includes fishes with two or three dorsal fins. There are four or five actinosts and the gills are placed normally. With mostly dorsal and anal elements the caudal fin is of a “pseudocaudal” type (Bond, 1996)

Many cods have evolved, what is to be believed to be hearing though they have, a diverticula of the gas bladder in that is connected to the inner ear (Bond, 1996)

The periodic freshening during the Quaternary history of the Arctic marine waters was caused by the thawing of the glacier and it has been determine that all high-arctic Gadidae are euryhaline forms which can endure freshening of the water.

The burbot is perhaps the most under-appreciated species of the codfishes, but it is also the only exclusively freshwater cod in North America. The burbot feeds on literally anything edible under the surface (Blann, 1997). Its body looks like a crossing between a snake and an eel. The burbot population has declined in the Great Lakes due to the invasion of sea lamprey, but after extensive lamprey control it has started to recover and its numbers seems to be stable at present (Blann, 1997).

Diet.

Mattson, (1990) states that the cods test potential prey by vision, smell and taste to avoid swallowing low nutrient prey and they rarely eat artificial objects. But the cod (Gadus) is a mixed feeder, and among the strange objects taken from the stomach may be mentioned, keys, a hare, a partridge and a long piece of tallow candle (Norman, 1975). The reason why all those odd things are found in the stomach according to MINET (1999) is to be able to digest sea anemones, hydroids and other organisms that is growing on these strange objects.

The juveniles feed on zooplankton and phytoplankton, when they get older they feed on larger planktonic organisms and fry (Nikol’skii, 1961).

Most species feed on benthic invertebrates like molluscs, polychaetes, echinoderms and crustaceans (Nikol’skii, 1961). Adult cod feed mainly on capelin, herring, and launce, flounders, young Greenland turbot, crabs, shrimp, brittle stars and a host of other species of fish and shellfish (MINET, 1999). The Arctic cod ceases to feeding during spawning, and other fish feed less due to decrease the risk of eating their own offspring (Nikol’skii, 1961).

References

Blann, Kristen (1997) Experiment for the Ficheries, URL:

http://h2o.seagrant.wisc.edu/commuications/Publications/Fish/Burbot~1.htm

Bond, C.E. (1996) Biology of Fishes 2nd edition. Saunders Collage Publishing, USA.

Carr, S.M., Kivlichan, D.S., Pepin, P. and Crutcher, D.C. (1999) Canadian Journal of

Zoology 77: 19-26

Fishbase URL: www.fishbase.org/manual/fishbase/The_Families_Table.htm

Knox, B., Ladiges, P. and Evans, B. (1999) Biology. McGraw-Hill Book Company Australia

Pty Limited, Roseville.

Nikol’skii, G.V. (1961) Special ichthyology 2 nd edition. (ed. PST staff) Isreal Program for

Scientific Translations Ltd.

Marine Institute: Network of Educational Topics (1999)

URL: www.ifmt.nf.ca/mi-net/fishdeve/cod.htm

Memorial University of Newfoundland (1999)

URL: www.mun.ca/biology/scarr/Gadine.systematics.htm

Norman, J.R. (1975) The History of Fishes 3 rd edition. Ernest Benn Limited, London.

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