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ASK A SCIENTIST: ANSWERS TO QUESTIONS General Fish Biology Index to Questions FISH BIOLOGY
FISH PHYSIOLOGY FISHERIES
FISH BIOLOGY Q: A question from my 8 year old grandson: How do fish get into lakes? (Not by stocking, but originally!) A: You have a sharp and inquisitive grandson! There are a few explanations as to how freshwater fish came to exist and live in lakes, and there is not necessarily just one answer. One could be that there may have been a connection, river or stream, to the lake at one time and the fish travelled up this river to the lake and the access was later cut-off. A recent example is a type of Pacific salmon called Kokanee. Kokanee were sockeye salmon that were landlocked at some point in their life history and can no longer migrate to the ocean. Another way is that in the past the continents were covered in ice. When the ice melted back, it left lakes of freshwater and since these newly formed lakes may have been connected to oceans they could migrate into these new habitats. Some fish, such as lungfish, possess the ability to be out of water for short periods of time and could potentially move into nearby inland lakes. These are just a few explanations, but if you have heard other ones I would love to hear them!
A. Yes, fish can get sick. There are several viruses known to infect marine fish, including Infectious Pancreatic Necrosis Virus (IPNV) and many others. Fish can also get bacterial infections, which may prove to be fatal. External and internal parasites can infect fish, and if there are too many of these, the fish may not grow properly or may die. Freshwater and saltwater aquarium fish are susceptible to a variety of diseases. Check under "General Questions" in the Answer File section of the OceanLink site for more information about marine disease in general.
Q: I am looking for a resource to help me identify a fish that was caught off of coastal Long Island in about 70 ft of water. It is brown, about 0.75-1.0 m long, has a mouth large enough to swallow a basketball, and a bony plate on its head with two horns. Any ideas? Thank you. A: Unfortunately I am not familiar enough with fishes of the east coast to be able to help with identification without more detail. The following information would be useful: does the fish have scales? what was the fish's body shape (i.e. long and slender or short and rounded)? how deep was the water column where it was caught (i.e.: is it a benthic or pelagic fish)? was the colouration uniformly brown or was the ventral or dorsal side lighter in colour? how many fins did it have? where were the fins situated on the body and what were they shaped like? how long were the "horns" or spines on its head? did it have any more bony plates on its body? what were the teeth shaped like? Do you still have the fish or a picture of it? I suggest borrowing a taxonomic key from your local library to identify your specimen, although they often require a large amount of morphological detail.
A. The blood of marine teleost fish (the "typical" bony fish such as a tuna or a herring) is not as salty as the sea water that they are swimming in. It is about 1.4 % salt, as opposed to the 3.5% salt found in most sea water. If a marine teleost fish did not have any mechanisms for dealing with this situation, water would leave their bodies via osmosis, and they would dehydrate! (Can you imagine a fish dehydrating in the middle of the ocean!) Marine teleost fish solve this problem by drinking the salty water, retaining the water, and excreting the salts. They have special chloride excreting cells called ionocytes in the gills which actively pump salts out of the fishes body. Since the marine teleost fish do need water, they would probably get thirsty if they neglected to drink. It's hard to conceive of a situation where a these fish could not find some water to drink, however!! Interestingly, sharks have solved the problem of living in salty water in a very different fashion, and they never have to drink water. Check out the answer to the question on freshwater sharks for more on this. Fish do not sleep in the same way that mammals do (they don't have eyelids!), but they do enter into a resting phase for part of the day. Some fish are active in the daytime and rest at night (diurnal), while others are active at night and rest in the daytime (nocturnal). In their resting state, the fish are generally very still, and do not move about or feed. Some fish may remain in burrows, dens, or are otherwise hidden while they are resting. Parrotfish secrete mucus from their skin, creating a "cocoon" to wrap themselves in during the night. This cocoon serves to hide their scent from the nocturnal predators that are patrolling the coral reefs looking for a meal.
Q: How do fish sleep? A: Fish don't exactly sleep the same way you or I sleep. Rather, many fish exhibit resting behaviour where they are in an inactive state for part of the day. For example, in a coral reef area there are some fish which are active during the day and rest in crevices or caves at night, while other fish are active at night and rest during the day. When these fish are inactive, they are usually resting quietly on the bottom and are hopefully out of the sight of any predators. Some fish, like tuna, must constantly swim to keep water moving over their gills, and are therefore active at all times (although they might be less active and swim slower during the night). Their eyes stay open because fish don't have eyelids. Some sharks have nictitating membranes, which cover their eyes but are believed to function more for protection of the eye than to stop light from coming in.
A. As far as what kind of marine fish there are - literally tens
of thousands of kinds, from huge whale sharks to tiny gobies, and
they eat many different kinds of food, from algae to other fish
to marine mammals.
Q: WHAT FISH ARE POISONOUS.BECAUSE I AM DOING A PROJECT ON POISONOUS FISH I NEED HELP CAN YOU GIVE ME SOME INFO THANKS SO MUCH!! A: There are quite few fish that are poisonous. Some fish have only mild toxins, whereas others have deadly ones. The Pacific Northwest rockfish have spines on their dorsal fins (back) that contain a mild toxin, that if you are jabbed by the spines they may induce a throbbing and burning pain, swelling, and even a fever. More lethal cousins of the rockfish are the lionfish and the stonefish. The lionfish has beautiful long fins that contain deadly venom and the stonefish (who looks exactly like a rock) contains a lethal venom, that if a person is accidentally steps on or touches this fish they are injected with a poison that will kill the person if it is not treated with an anti-venom. Another group of fish, the pufferfish, have a lethal toxin called tetrododoxin in their spines. Tetrodotoxin is also an extremely deadly venom to all animals, including humans!
Q: What is the distinction between elongated fish (such as the hag fish) and eels? A: This is a good question. There are many (approximately 60)
families of fish that have elongate bodies. The body type is thought
to have been a convergent trait in evolution, as it facilitates
forward and backward movement into and out of tight places and
sediments. There are 15 families of anguilliform fish, which are
the "true" eels. They are distinguished from other families
of eel-like fishes by the loss of a pelvic girdle and by a modified
upper jaw that is formed by fusion of the premaxilla, vomer, and
ethmoid bones.
Wolf eel (Anarrhichthys ocellatus)
Hagfish (Anarrhichthys ocellatus) Hagfish are very different from the anguilliforms. They are in a separate class than the bony fish (class osteichthyes), or the cartilagenous fish (class chondrichthyes). Hagfish are members of class Agnatha - they have a cartilagenous skeleton and lack a jaw.
Q: I don't know if you can answer this, since it is about freshwater fish instead of marine fish, but my question is: What is the biggest freshwater fish that lives it's whole life in freshwater. The white sturgeon grows to over 600 cm, but lives most of it's life in brackish or saltwater, so I was wondering what the biggest fish in just freshwater was. Sorry if this in an inconvenience. A: The Beluga sturgeon (Huso huso) is considered to be the largest freshwater fish in the world. It lives in eastern Europe and Asia, and can attain a length of 8.6 meters (28 feet) and weigh up to 1300 kg (590 lbs)! As you say, they are anadromous (spending part of their life in salt water), but apparently landlocked populations can develop. There are some strictly freshwater species: North American lake sturgeon (Acipenser fulvescens), and three river sturgeons of the genus Scapphirhynchus.
A: From what I have found, bait balls are a schooling behaviour
of bait fish (herring, anchovies, etc.) when they are round up by
a group of predators. Each fish wants to get away from the edge,
so they end up spinning together into a tight ball to get away.
They may even come shooting out of the water surface Check here for video clips of bait balls!
Q: Is there a scientific term for when a fish changes colour? If so what is that term? I understand about the chromatophores, expanding etc. but want to know what the process is called? A: The change in colour of some fish and cephalopods is caused
by expansion and contraction of chromatophores. But in crustaceans
(lobsters and crabs) it is caused by the variable distribution of
pigments within the chromatophores. The migration of pigment granules
is under hormonal control and agitation, fear ect. can release hormones
that will cause a colour change.
Q: Which is the smallest fish -marine and fresh water? Details - length, weight etc. A: The smallest marine fish is Schindleria praematurus, found in Samoa in the South Pacific: 12-19 mm in length, weight 2 mg. The smallest freshwater fish is the dwarf pgymy goby, Pandaka Pygmaea, at 9mm long. I could not find the weight for this guy but i'm sure its not much!
Q: What is the effect of deeper water/less light on the color(s) of fish? A: We see the colour of objects because this is the colour they reflect from the white light spectrum. At the surface, where a lot of light is present, fish come in many colours - especially in the tropics. Colour may be useful for attracting mates, camoflauge, communication etc. In the bathypelagic zone (200-1000m) fish tend to be black or red. However, because red light is quickly filtered out of the water column (blues and greens penetrate the deepest), and the light given off by bioluminescence is blue/green - there is no red light reflected and even red fish look black at this depth! Even deeper in the ocean, in the abyssopelagic zone (1000-4000m), organisms are often transparent or black. At such deep depths many animals can bioluminesce. Small organs called photophores give off light and are often used to attract prey.
A:The worlds fastest fish is the Sailfish (Istiophorus platypterus)
which can attain speeds of 68 miles per hour over distances of 100
meters (320 feet). In a comparison the sailfish can swin faster
than most people drive their cars! The sailfish is from the Family
Istiophoridae which includes other fish such as Marlins and Spearfishes
which can also attain high rates of speed.
FISH PHYSIOLOGY A. The fact that some fish have blood that will not freeze even at temperatures slightly below zero was first discovered in 1969 in Antarctic fishes by DeVries and Wohlschlag. This property of fish blood was caused by an antifreeze molecule in the fish's blood. The molecule turned out to be a glycopeptide. The antifreeze has a repeating structure of disaccharides on a polypeptide backbone. Since then, many other fish have been discovered, both in the Arctic and Antarctic that have antifreeze molecules in their blood. There is a lot of similarity in the structure of the antifreeze molecules in these different fish. Humans in the arctic don't have this molecule in their blood for the same reason that SCUBA diving humans don't develop gills, or that you don't develop wings when you fly in a plane. It takes a very long time for a species to develop a special adaptation by evolution. If you are asking why we don't inject this antifreeze into humans in the Arctic, the reason is that humans, like other mammals, are homeothermic. We maintain a body temperature that is higher than the ambient temperature. An antifreeze molecule in our blood would be useless, since we would be dead long, long, before our blood ever reached anywhere near the freezing point.
A: There are actually several different species of fish that can breathe atmospheric oxygen. Most of these fish have special adaptations that allow them to acquire oxygen from the atmosphere. Mudskippers (Periopthalamus spp.) are in the family Gobioidei, and these fish are usually found inhabiting mangrove swamps and mudflats of the tropical Indian Ocean. These fish can climb out of the water using their pectoral fins, and feed on terrestrial insects and other invertebrates. Instead of using gills to obtain oxygen from the water, these fish breathe air which they trap in high vascularized opercular cavities. Another fish in the Gobioidei family that breathes air is the longjaw mudsucker, Gillichthys mirabilis. This fish absorbs oxygen from the air in its large and highly vascularized mouth cavity. Another kind of fish that can breathe air is the bowfin (Amia calva) which is in the Holostei. This fish can obtain oxygen from the water using its gills and it can also breathe air through its lung. When the temperature of the water gets too warm, this fish will supplement oxygen absorbed from the water with atmospheric oxygen. Gars are another type of fish that can breathe air. Like the bowfin, gars are not obligate air breathers. There is also a group of animals that are called the Lungfishes. The lungfishes are a subclass of the Osteichthyes (bony fishes). These fish have lungs and internal nostrils, too! At one point these fish were believed to be ancestral to the tetrapods, the animals that left the oceans to begin the invasion of the terrestrial environment. So as you can see, there are quite a few different species of fish that can breathe air outside of the water. Many of these species are adapted to living in warm waters with relatively low concentrations of dissolved oxygen. The ability of fish to breathe air was one of the important steps along the evolutionary pathway that led to the establishment of vertebrate life on land.
Q: How do fishes' gills work? A: Fish gills are a pretty complex structure, and are very well adapted to getting oxygen out of water. Gills are made up of filaments (the feathery red things) attached to a rigid gill arch. The arches are hollow and have arteries inside them that contain blood low in oxygen. These arteries branch into smaller arterioles that run inside the filaments. Each flat filament has many tiny folds on it (called lamellae) to increase surface area. In fast moving fish, the surface area of the gills may be ten times that of the actual animal. Tiny capillaries branch off of the arterioles and carry the blood close to the inner surface of the lamellae. Because the oxygen concentration is less in the blood than in the water flowing over the gills, the oxygen from the water naturally diffuses into the blood. There is an adaptation that fish have to maximize the flow of oxygen into the blood called countercurrent exchange. This is when the water flowing over the lamellae is in the opposite direction as the blood flowing through the capillaries. In this way, the concentration of oxygen in the blood as it moves through the capillaries is always lower than the water, and oxygen will diffuse over the whole length of the lamellae. Once the blood is fully oxygenated from it's trip through the gills,
it is pumped back into the body and used by the fish for energy,
filling the float bladder, and for nearly all of the metabolic processes
in the fishes' body.
Q: Could you please explain why fish need salt water and why the salt helps fish to survive? A: Actually, not all fish need salt water. There are some fish
that only live in fresh water, some that live only in salt water,
and some that can adapt to both environments!
Q: Do fish pee? A. Fish do actually pee. Marine fish excrete very low volume of concentrated urine that is formed in the kidneys, as in other vertebrates. For the most part though, their nitrogenous wastes are excreted through their gills during respiration. Pretty cool!
Q: I would like to know how fish digestive system works. Most
fish eat other fish, prawns and crabs with hard shells. How do they
digest the hard shells, bones, etc or However, many species of fish that feed on crustaceans and insects
have been found to have a relatively high chitinase activity in
their stomachs. The chitinase is an enzyme that breaks down chitin,
the material that makes up a large proportion of the arthropod exoskeleton.
The presence of these enzymes allows the fish to obtain some nutrition
from the chitinous shells of prey items such as crabs, squid, and
insects.
Q: Are fish colour blind? A: To explain how colour vision operates in fish, it is important to understand how light is transferred through the water column. Radiant energy from the sun comes in varying wavelengths. Visible colours are in the electromagnetic spectrum between 400 to 700 nanometers (1 nanometer = 10-9 m). This spectrum includes all visible colours from red (400 nm) to violet (700 nm). Water absorbs light as it penetrates the water column and different wavelengths of light are absorbed more readily than others. Red and violet light components are absorbed rapidly, and penetrate only shallow waters. Green (530 nm) and blue (460 nm) light are absorbed more slowly, and therefore penetrate the seawater more deeply. Red and violet light are completely absorbed within the first few metres, whereas blue light can penetrate up to 100 metres in the same conditions. Fish have eyes that that are similar to all vertebrates, including humans. There are two types of sensory cells in fish retina, cones and rods. Rods are sensitive to low light levels. Fish species that are active during dawn and dusk have more rods than cones, and nocturnal and deep-sea species have only rods. Rods are sensitive to deep-penetrating short-wavelength light which allows vision in the water column where little light is available. Some cartilaginous fish and most bony fish have cones, which are responsible for photoreception in bright light. There four different types of cone pigments found in the eyes of fish, and each is sensitive to different wavelengths of light. Different types of fish may have two to three different types of cones depending on where they live in the water column. Shallow water fish species usually have three types of cones (red, blue, green) which allows them to see the wide spectrum range that available in shallow waters. Marine fish living at moderate depths have cone pigments that are sensitive to blue and green light. Finally, deep-sea fish have only rod pigments which are sensitive to the short wavelength light that is able to penetrate great depths. To make a long story short, yes some fish see in colour. It is the shallow water fish species that see the greatest range of colour because it is only at shallower depths that the entire range of visible colour wavelengths are not yet absorbed by the water.
Q: Can fish change their buoyancy? If so, how? A: Yes, many species of fish can change their buoyancy. There are four different strategies that fish use: 1) incorporation of large quantities
of low-density compounds in the body (i.e. oils and fats) Many species of sharks use oils to regulate their buoyancy. The hydrocarbon squalene is a low density oil that is found in sharks and is believed to function in buoyancy control. Swimbladders allow for precise control of the fish's depth because the volume of gas they contain can be regulated quite easily. The idea behind the swimbladder is relatively simple. Air is less dense than water, therefore, air floats on top of water. In the same way, if you think of an oil spill, the oil floats on top of the water because oil is less dense. The swimbladder of the fish holds air, which allows the fish to be lighter in the water and making it more buoyant. If the fish lets air out of the swim bladder it will get more dense and begin sinking. Why do fish care about buoyancy?
Why can some fish osmoregulate, and how can they live in both water: salty and not?
Marine fish face the opposite problem - they are usually in an
environment that is more concentrated in salt than them. Without
any regulation the high salt concentration of the ocean would draw
all the water out of them. To prevent dehydration, marine osmoregulators
drink salt water and actively transport salts to the environment
with chloride cells on its gills. Salts are also excreted in the
urine.
Q: I just listened to the Orca Sounds on your webpage and was wondering if other species made sounds like this? A: smaller fish do make sounds but not like the Orca whale sounds. Sometimes these smaller fish sounds can be a thumping sound from their swim bladders. Some species are able to make sounds we can hear. however the animals that make the most sounds are the whales and the dolphins. These sounds from the whales and dolphins are used for communication and echo-location between each other. The species that makes one of the most beautiful sounds in the ocean is the humpback whale. **update:
Q: What is the pH level for most ocean fish? A: Most fish live in ocean waters with a PH level between 6-8. If the levels of Carbon dioxide are higher the fish will have a lower ph level. If the PH is high the carbon dioxide levels will be higher as well. Most fish rely on ions to regulate their internal ph levels.
FISHERIES
Q: Recently in the Sydney Morning Herald there has been discussion about the depletion of blue-fin tuna stocks in Australian waters due to overfishing by Japanese "experimental" fishing. Could you please send me more details about this case. A. I am aware of the confrontation between Japan and Australia regarding the fisheries of the bluefin tuna. The population of tuna in and around southern Australia has been known to be in decline for the past 20 years. The decline in numbers is largely due to human over-exploitation, similar to the situation with cod on the east coast of Canada. Since fish are so mobile and elusive, it is difficult get a true measure of the numbers of animals in a population. Fisheries managers rely on models to estimate the numbers of fish in a population, and produce quotas based on population estimates made from the previous years' catch. Based on the Australian government's maximum harvest model, if the tuna continue to be fished at the current rate, there is only a one in three probablity that the population will return to an acceptable (pre 1985) size. If the population is kept below that size (I'm not sure what the numbers are) the optimum catch (or maximum harvest yield) cannot be attained. The recovery of the stock is in everyone's interest, because the larger the stock, the more fish can be harvested without a decrease in population size. Australia is taking a conservationist stand (very wise), saying that a reduction in quotas is necessary, to maintain any fishery at all. However, Japan's position is one of ignorance and defiance of the scientific and biological reality of the situation. Their "experimental fishery" is thought to be a joke, because increasing any quota will be detrimental to the viability of this species (how scientific is that???). Basically, Japan does not think it is getting a big enough piece of the pie. My interpretation of their "experimental" catch is that it is a manoeuvre to circumvent measures in the fisheries treaty that was signed with Australia and New Zealand several years ago. The Japanese apparently are not concerned about the future of the bluefin in Australian waters. I don't know if that has helped you any. I have several papers on fisheries modelling and the formulation of quotas. If you would like to see them, let me know. Also, for further info, you may want to contact the Australian Institute of Marine Science. There should be someone at AIMS that can supply you with more of the scientific basis for the Australian government's argument (current stock size, projected stock size, quota models).
Q: As an angler, reduced fish population is of great concern but all the legislation (and peer group pressure) requires returning small fish that are not even at reproductive age rather than maintaining fish old and large enough to reproduce. My gut feeling is that keeping the big ones for the pot and returning the small (which taste better) is counter productive. Any computer modelling being done to check this? A. That is a very interesting question. The theories and models behind fisheries management are a bit of a mess because they are a mix of biology and economics. The resulting legislation is often moulded to optimize the economic and biological tradeoffs for any one given species of fish.
There are several reasons why I might agree with your logic. First of all, removing the largest fish (the usual scenario) effectively removes valuable genes (i.e. the genes for larger size) from a population. Secondly, as you have said, the larger fish are usually the mature, gravid adults. As a result of fishing for a larger size class, more of the existing reproductive population is consumed.
One other reason against fishing the smaller size classes is that it is not very cost effective. Because of their smaller size, many more of the small fish need to be caught to match the economic/nutritional value of a single larger fish. This means that more time and energy is required per kilo, and subsequently higher prices in the supermarket. In terms of economics, anglers and sports fishing make up a relatively small proportion of dollar value of the annual catch. The big bucks (and political weight) lie in commercial fisheries, so much of the fisheries legislation, models, quotas and regulations are reflective of the commercial fishing climate. Unfortunately, anglers are often forced to bear the burdens of regulations and restrictions that arise from commercial overfishing and bad fisheries management. Currently, there are several different approaches to fisheries modelling, and there are numerous papers being published in the scientific literature that discuss the principles and problems of these tools. Computer modelling is routinely used in efforts to estimate population sizes and to generate quotas. Here are a few good references: 1) Roughgarden, J. and F. Smith. 1996. Why fisheries collapse and what to do about it. Proceedings of the National Academy of Sciences 93: 5078-5083. (very good paper) 2) Munro, G.R. and A.D. Scott. 1985. in Handbook of Natural Resource and Energy Economics, ed. Kneese, A.V. and Sweeney, J.L. (North-Holland, Amsterdam). Vol.2, pp.623-677. 3) Chichilnisky, G. and G. Heal. 1993. Journal of Economic Perspectives 7: 65-86. 4) Cushing, D.H. 1981. Fisheries Biology, 2nd edition. University of Wisconsin Press, Madison, WI.
Q: How do the fishing industries
impact or affect the oceans. What i'am trying to say is, what are
some negative and positive impacts, and how can they be improved? A: The impacts of fishing
are an especially contraversial and important topic. Particularily
in the Maritimes. I suggest you try and get information locally.
Nova Scotia is Canada's #1 fish exporter, so look in the phone book
or ask your parents/grandparents about organizations (environmental
groups, fisherman groups) that you could talk to. For general fisheries
information, check out:
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How
do fish get into lakes?
BACK TO 
A:
You’re right! Most fish are osmoregulators, which means that
they regulate their internal environment within a range that is
suitable for proper cellular function. There are two types of osmoregulators:
those that can tolerate only a small change in the solute concentration
of their external environment, called stenohaline, and those that
can deal with large changes in the solute concentration of their
external environment, called eurohaline.
However,
the main argument for restricting the catch of the smaller size
classes is to conserve for the future. The smaller size classes
will grow to the adult stage and begin reproducing. By not
fishing the smaller size fish, the future generation and the future
of the stock is effectively secured.
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