Ichthyoplankton - AbsoluteAstronomy.com

Ichthyoplankton are the eggs and larvae

Larvae

In Roman mythology, lemures were shades or spirits of the restless or malignant dead, and are probably cognate with an extended sense of larvae as disturbing or frightening...

of fish. They are usually found in the sunlit zone of the water column, less than 200 metres deep, which is sometimes called the epipelagic or photic zone

Photic zone

The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

. Ichthyoplankton are planktonic, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larva swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the zooplankton

Zooplankton

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larva are themselves eaten by larger animals.

Ichthyoplankton can be a useful indicator of the state and health of an aquatic ecosystem

Aquatic ecosystem

An aquatic ecosystem is an ecosystem in a body of water. Communities of organisms that are dependent on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems....

. For instance, most late stage larvae in ichthyoplankton have usually been predated, so ichthyoplankton tends to be dominated by eggs and early stage larvae. This means that when fish, such as anchovies

Anchovy

Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...

and sardine

Sardine

Sardines, or pilchards, are several types of small, oily fish related to herrings, family Clupeidae. Sardines are named after the Mediterranean island of Sardinia, around which they were once abundant....

s, are spawning, ichthyoplankton samples can reflect their spawning output and provide an
index of relative population size for the fish. Increases or decreases in the number of adult fish stock

Fish stock

Fish stocks are subpopulations of a particular species of fish, for which intrinsic parameters are the only significant factors in determining population dynamics, while extrinsic factors are considered to be insignificant.-The stock concept:All species have geographic limits to their...

s can be detected more rapidly and sensitively by monitoring the ichthyoplankton associated with them, compared to monitoring the adults themselves. It is also usually easier and more cost effective to sample trends in egg and larva populations than to sample trends in adult fish populations.

History

Interest in plankton originated in Britain and Germany in the nineteenth century when researchers discovered there were microorganism

Microorganism

A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s in the sea, and that they could trap them with fine-mesh nets. They started describing these microorganisms and testing different net configurations. Ichthyoplankton research started in 1864 when the Norwegian government commissioned the marine biologist G. O. Sars to investigate fisheries around the Norwegian coast. Sars found fish eggs, particularly cod

Cod

Cod is the common name for genus Gadus, belonging to the family Gadidae, and is also used in the common name for various other fishes. Cod is a popular food with a mild flavor, low fat content and a dense, flaky white flesh. Cod livers are processed to make cod liver oil, an important source of...

eggs, drifting in the water. This established that fish eggs could be pelagic, living in the open water column like other plankton. Around the beginning of the twentieth century, research interest in ichthyoplankton became more general when it emerged that, if ichthyoplankton was sampled quantitatively

Quantitative research

In the social sciences, quantitative research refers to the systematic empirical investigation of social phenomena via statistical, mathematical or computational techniques. The objective of quantitative research is to develop and employ mathematical models, theories and/or hypotheses pertaining to...

, then the samples could indicate the relative size or abundance

Abundance (ecology)

Abundance is an ecological concept referring to the relative representation of a species in a particular ecosystem. It is usually measured as the large number of individuals found per sample...

of spawning fish stock

Fish stock

Sampling methods

Research vessel

A research vessel is a ship designed and equipped to carry out research at sea. Research vessels carry out a number of roles. Some of these roles can be combined into a single vessel, others require a dedicated vessel...

s collect ichthyoplankton from the ocean using fine mesh nets. The vessels either tow the nets through the sea or pump sea water onboard and then pass it through the net.

Plankton tows: Three main types are:
The PairoVET tow, used for collecting fish eggs, drops a net about 70 metres into the sea from a stationary research vessel and then drags it back to the vessel.
The bongo tow drags nets shaped like bongo drum
Bongo drum
Bongo or bongos are a Cuban percussion instrument consisting of a pair of single-headed, open-ended drums attached to each other. The drums are of different size: the larger drum is called in Spanish the hembra and the smaller the macho...

s from a moving vessel. The net is lowered to about 200 metres and then allowed to rise to the surface as it is towed. In this way, a sample can be collected across the whole photic zone
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

where most ichthyoplankton is found.
The manta trawl
Manta trawl
A Manta Trawl is a net system for sampling the surface of the ocean. It resembles a manta ray, with metal wings and a broad mouth. The net it pulls is made of thin mesh, and the whole trawl is towed behind a scientific research vessel...

tows a net from a moving vessel along the surface of the water, collecting larvae, such as grunion
Grunion
Grunion are two fish species of the genus Leuresthes: the California grunion, L. tenuis, and the Gulf grunion L. sardinas...

, mahi-mahi
Mahi-mahi
The mahi-mahi or common dolphinfish is a surface-dwelling ray-finned fish found in off-shore temperate, tropical and subtropical waters worldwide. It is one of only two members of the Coryphaenidae family, the other being the pompano dolphinfish...

, and flying fish which live at the surface.

After the tow the plankton is flushed with a hose to the cod end (bottom) of the net for collection. The sample is then placed in preservative fluid prior to being sorted and identified in a laboratory.

Plankton pumps: Another method of collecting ichthyoplankton is to use a Continuous Underway Fish Egg Sampler (CUFES). Water from a depth of about three metres is pumped onto the vessel and filtered with a net. This method can be used while the vessel is underway.

Developmental stages

Ichthyoplankton researchers generally use the terminology and development stages introduced in 1984 by Kendall and others. This consists of three main developmental stages and two transitional stages.

Developmental stages according to Kendall et al. 1984
Main stages	Egg stage	Spawning to hatching. This stage is used instead of using an embryonic Embryo An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination... stage because there are aspects, such as those to do with the egg envelope, that are not just embryonic aspects.
	Larval stage	From hatching till all fin rays are present and the growth of fish scale Fish scale Fish scale may refer to:* The rigid plates on the skin of a fish* Fishscale, an album by Ghostface Killah* Fishscale cocaine* Fish Scales, a rapper... s has started (squamation). A key event is when the notochord Notochord The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes... associated with the tail fin on the ventral side of the spinal cord develops flexion Flexion In anatomy, flexion is a position that is made possible by the joint angle decreasing. The skeletal and muscular systems work together to move the joint into a "flexed" position. For example the elbow is flexed when the hand is brought closer to the shoulder... (becomes flexible).	The larval stage can be further subdivided into preflexion, flexion, and postflexion stages. In many species, the body shape and fin rays, as well as the ability to move and feed, develops most rapidly during the flexion stage.
	Juvenile stage	Starts with all the fin rays being present and scale growth underway, and completes when the juvenile becomes sexually mature or starts interacting with other adults.
Transitional stages	Yolk-sac larval stage	From hatching to absorption of the yolk-sac
	Transformation stage	From larva to juvenile. This metamorphosis Metamorphosis Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation... is complete when the larva develops the features of a juvenile.

Ecology

Fish eggs and larvae are eaten by many marine organisms. In particular, they form a significant part in the diet of marine invertebrates, such as copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, arrow worms, jellyfish

Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

, amphipods, marine snails and krill

Krill

Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...

. Because they are so abundant, marine invertebrates inflict high overall mortality rates. Adult fish also predate fish eggs and larvae. For example, haddock

Haddock

The haddock , also known as the offshore hake, is a marine fish distributed on both sides of the North Atlantic. Haddock is a popular food fish and is widely fished commercially....

were observed satiating themselves with herring

Herring

Herring is an oily fish of the genus Clupea, found in the shallow, temperate waters of the North Pacific and the North Atlantic oceans, including the Baltic Sea. Three species of Clupea are recognized. The main taxa, the Atlantic herring and the Pacific herring may each be divided into subspecies...

eggs back in 1922. Another study found cod

Cod

in a herring spawning area with 20,000 herring eggs in their stomachs, and concluded that they could predate half of the total egg production. Fish also cannibalise their own eggs. For example, separate studies found northern anchovy (Engraulis mordax

Engraulis

Engraulis is a genus of anchovies. It currently contains nine species.-Species:* Engraulis albidus Borsa, Collet & J. D. Durand, 2004* Engraulis anchoita Carl Hubbs & Marini, 1935 * Engraulis australis Engraulis is a genus of anchovies. It currently contains nine species.-Species:* Engraulis...

) were responsible for 28% of the mortality in their own egg population, while Peruvian anchoveta

Peruvian anchoveta

The Peruvian anchoveta is a fish of the anchovy family, Engraulidae.Anchoveta are pelagic fish in the southeastern Pacific Ocean, and are regularly caught on the coasts of Peru, and Chile. They live for up to 4 years, reaching 20 cm, with recruitment occurring after only about 6 months when...

were responsible for 10% and South African anchovy (Engraulis encrasicolus

Engraulis

) 70%.

The most effective predators are about ten times as long as the larvae they predate on. This is true regardless of whether the predator is a crustacean, a jellyfish, or a fish.

Recent research

In 2010, a group of scientists reported that fish larvae can drift on ocean currents and reseed fish stock

Fish stock

s at a distant location. This finding demonstrates, for the first time, what scientists have long suspected but have never proven, that fish populations can be connected to distant populations through the process of larval drift.

The fish they chose to investigate was the yellow tang

Yellow tang

The yellow tang is a saltwater fish species of the family Acanthuridae. It is one of the most popular aquarium fish.-Appearance:...

, because when a larva of this fish find a suitable reef it stays in the general area for the rest of its life. Thus, it is only as drifting larvae that the fish can migrate significant distances from where they are born. The tropical yellow tang is much sought after by the aquarium

Aquarium

An aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...

trade. By the late 1990s, their stocks were collapsing, so in an attempt to save them nine marine protected areas (MPAs) were established off the coast of Hawaii. Now, through the process of larval drift, fish from the MPAs are establishing themselves in different locations, and the fishery is recovering. "We've clearly shown that fish larvae that were spawned inside marine reserves can drift with currents and replenish fished areas long distances away," said one of the authors, the marine biologist Mark Hixon. "This is a direct observation, not just a model, that successful marine reserves can sustain fisheries beyond their borders."

External links

Ichthyoplankton Survey Methodology Presentation by Yoshinobu Konishi, SEAFDEC-MFRDMD.
Salmon Eggs Hatching at the Seymour Hatchery Youtube video.

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

History

Sampling methods

Developmental stages

Ecology

Recent research

See also

External links