Giant Fishes of the Rivers

How much longer will we have them?

The vertebrate class of Pisces contains members of two major groups of fishes: the cartilaginous species and the bony fishes. The most massive (if not the longest as well) of all fish are the Whale Shark (Rhincodon typus) and the Basking Shark (Cetorhinus maximus). These two species are peaceful giants, feeding on plankton that they filter out of the ocean water with their comb-like gill rakers. However, in this blog I want to write about giant freshwater and not marine fish. Yes, river fish, for several of them are bigger than the marine bony species (with one exception: the oarfish Regalecus glesne, of which during the 1967 “Walther Herwig” South Atlantic Expedition I caught a glimpse of one 7.30 m long specimen. This midwater species is said to reach a maximal length of 13 m).

What prompted me to write this blog was the sad news that the giant Yangtze River paddlefish Psephurus gladius was officially declared extinct. It thus follows the Yangtze freshwater dolphin Lipotes vexillifer, affectionately called ‘the baiji’ in Chinese but declared extinct in 2006. In 1970 there were still 25 tons of Yangtze paddlefish caught, but the last individual was probably killed in 2007. Specimens of at least 5 m in length existed. Another Chinese giant with a length of approx. 4 m is the Chinese sturgeon Acipenser sinensis, but it may also soon be gone forever for its population decreased by 98% from 1973-2010. As of this year (2021) all commercial fishing in the Yangtze will be banned for 10 years, but as welcome a measure as this is, it cannot bring back what is lost forever.

Probably the largest with a maximum length of around 7 m of all freshwater giants is the Beluga sturgeon Huso huso of the Caspian Sea and the Sea of Azov. Because of the value of its roe (the famous “Caspian caviar”) it is farmed, but the eggs of its also farmed but only 5.5 m long relative, the Huso dauricus, are considered even tastier. In northern Europe adults of the 3 – 4 m long sturgeon Acipenser sturio used to enter rivers to reach their breeding grounds upstream, but one of the few streams where these sturgeons now still breed is the Gironde in France. Recolonisation efforts are in progress elsewhere, but so far with minimal success. A huge surprise for our professor and us students of the Zoology Class at the University of Kiel in 1968 was when an almost 2 m long catfish of the species Silurus glanis was delivered. It had been caught the day before in the small North German freshwater stream known as the Trave. The East Asian Mekong River Pangasianodon gigas catfish and South American species of river catfish are said to reach maximum lengths of 4 m and can become a danger to bathers. Of the other South American freshwater giants the Arapaima gigas with a reported maximum length of 3.4 m and the electric eel Electrophorus electricus with a length of 2.5 m need to be mentioned.

Africa’s largest freshwater fish is the Nile perch, but it hardly reaches 2 m. On the other hand there are some quite big bony fishes in the North American Mississippi-Missouri river system. There is the North American paddlefish Polyodon spathula and the fierce-looking alligator gar that can both reach a length of 3 m. Although not in imminent danger of becoming extinct, these two species are considered vulnerable and an analysis of what led to the extinction of the Yangtze River paddlefish may help save the North American species and other freshwater fish giants too. It is believed that the biggest threats to the vulnerable freshwater fish giants (but even smaller species as well) are dams that block the fish’s access to upstream spawning grounds, and furthermore overfishing, the use of illegal methods (like electro-fishing, nets with unacceptably small mesh widths, and explosives). Pollution of the river water by chemicals like insecticides, by effluents and other wastes from urban areas, and by fertilizers used in agriculture must not be forgotten. But what is certainly interesting is that rivers compared with the oceanic habitat contained more giant bony fish species than the sea. Any suggestions why?  

lication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

You Don’t See Them, but They Sure Are There

The Ears that Fish Have

That fish can hear was documented nearly 100 years ago by Nobel Prize winner Karl v. Frisch, famous for his work on honeybee communication. Freshwater fish of the carp family, like most bony fishes, not only possess swim bladders that allow them to stay buoyant at different depths, but unlike others also possess small bony connections between the fish’s inner ear and the swim bladder. The gas in the latter gets compressed in response to sound pressure in the water, starts vibrating, and then transmits the signal via the aforementioned small bones, known as Weberian ossicles, to sensitive hair cells of the inner ear. Sound amplitude (loudness) and frequency (pitch) are important, but even in fish with the best hearing, sounds above 6,000 Hz would be ultrasound to them and in swim bladder-lessfish like the fast mackerel and the tuna but even in the more sluggish Antarctic icefishes only lower frequencies can be detected.

Sounds are longitudinally transmitted waves, whose frequencies and amplitudes may vary. Because of the water’s greater density than that of air, sounds are propagated 4.8 times faster in water than in air. In a broad sense sounds are generated by movements or vibrations and in water can be the results of an animal’s vocalizations or activities, of sounds created by ice-floes or logs rubbing or bumping against each other, breaking waves, anthropogenically-produced noise like explosions and disturbances created by ships. In order to sense the sounds, fish use ear stones, i.e. so-called otoliths. Bathed in endolymphatic fluid and resting on a pad of receptor cells with sensory hairs, the two major otoliths are located just below the brain in two bony sacs of the inner ear known as the saccule and the utricle. The otoliths are part of the bony vestibule’s two regions, i.e. the cochlear portion (for hearing) and the vestibular portion with its semicircular canals (for balance and angular change). Thus, otoliths can be said to be involved in the detection of gravity and linear accelerations and serve as a structure of hearing in fish, so well explained in a recent review by Dr. Tanja Schulz-Mirbach of Munich.

Otoliths are hard, durable structures that consist primarily of calcium carbonate (CaCO3) in the form of aragonite.They remain largely unchanged during the digestion in the stomach and gut of a predator. They are thus an excellent structure to estimate a fish’s age, because their size increases by periodically laid down alternating opaque and translucent bands that consist of CaCO3 and collagen fibres. As daily increments are regularly added, researchers can correlate the number of layers with the fish’s body length and use the tabulated data to identify the fish’s age. What makes the study complicated is that the otoliths, not being translucent enough to count the layers, need to be sectioned. Furthermore, although the shapes of the otoliths are species-specific, they can vary in individuals of the same species, depending on the fish’s developmental stage and if the fish was actively swimming or passively drifting.

In Antarctic icefish my Polish colleague Ryszard Traczyk and I have recently concluded that the more spherical otoliths of larval specimens and the longish otolith shapes of the adults are the results of the inertia and friction experienced by the otoliths in their endolymphatic fluid when the fish swim: larvae swim less than adult icefish and the latter swim less than mackerel (which possess the most elongate otoliths). It is entirely possible that oscillations of the dense otoliths generate shearing forces that deflect the sensory hairs of the cells they are resting on, when the oscillations are due to disturbances in the water made by nearby prey or the approach of a predator. Responses to such disturbances in the “acoustico-lateralis” vicinity of the fish would then not only be sensed by the lateral line system, but picked up by the fish’s otoliths too and sent to the brain via the 8th cranial nerve, often referred to as the vestibulocochlear nerve. So, do fish make some noise and can they hear? Actually only a few produce sounds, but all bony fish can hear. However, there’s certainly no need to whisper when you sit in front of your aquarium and watch your colourful aquatic beauties in their 3-dimensional world.

© Dr V.B. Meyer-Rochow and, 2021. Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.

How did Ludwig Von Bertalanffy Prevent Fish Species being Over-Exploited?

As more and more consumers turn away from meat, especially that of mammals, they do, however, turn to fish. Consequently, there is increasing pressure on fish stocks in the wild, but a growing opportunity for fish culturists to improve fish rearing facilities. When I was still a student of Fisheries Science in 1967 and lectures I attended dealt with fish stock assessments, catch per unit effort, fish populations, age structures, longevities and survival rates of fishes, time and again Ludwig Von Bertalanffy was mentioned and equations he had developed were quoted and written with chalk on the blackboard (yes, chalk and blackboard in those days). But why were Von Bertalanffy’s calculations so useful and why are they still part of the backbone of fisheries assessments of fish stocks today?

Ludwig von Bertalanffy was born near Vienna in 1901 and although his parents got divorced when he was ten, he did enjoy a good home education until then, when he became a grammar school student. He had the famous anti-Darwinist Paul Kammerer as his neighbour and soon began to apply his mathematical interests to biology and the living world. He is now often regarded as the founder of General Systems Theory, which has inputs from thermodynamics, cybernetics and biology. At the University of Vienna his fields of expertise could be called Theoretical Biology and Philosophy and in 1937 he got a Rockefeller scholarship to work in the USA. When he failed to secure immigrant status, he returned to Vienna in 1938 and joined the Nazi-party. After the war he found living in Austria difficult, moved to the University of London in 1948 and from there two years later to appointments at various American and Canadian universities. He died in 1972 in Buffalo, New York.

In his biological research, Von Bertalanffy was interested in psychology, psychiatry, development and growth phenomena and concluded that thermodynamic principles worked well in closed systems, but not in open systems like those comprising living organisms. He came up with a simple growth equation for biological organisms that models mean length of animals in relation to age:  L(a) = L [1 – exp (-k(a – a0))],  where a is age, k is the growth coefficient, a0 is the value used to calculate size when age is zero and Lis asymptotic size (which means the rate of growth continually decreases as an individual ages but never completely stops). The equation above is the solution of the linear differential equation:  dL/da = k(L– L) and applicable to organisms that do not cease to grow when adult (unlike, for instance humans, which actually shrink when reaching old age), but keep growing albeit at increasingly slower and smaller rates as they age. Fish are some of these animals and since it is important for fisheries biologists to know at what age (or body length) individuals of a species become reproductive and therefore should not be ‘harvested’ until old enough to have reproduced at least once, it’s obvious that much emphasis has been placed on Von Bertalanffy’s growth curve that relates age to body lengths.

To make this relationship ‘work’, it is crucial to know how old a fish is at any given length. Helping fisheries scientists in this matter are age-rings on the scales of fish (not unlike those that one uses to age trees). The problem is that not all fishes live in climatic zones in which there are distinct seasonal changes that result in age rings on the scales and secondly not all fish species even have scales. In my research with the Polish scientist R. Traczyk, we worked with Antarctic icefish that have no scales and live in constantly ice-cold water. In such cases one uses daily increments of extremely narrow CaCO3 layers, visible in sectioned ear-stones of the fish examined under the microscope. The layers provide an accurate estimation of the fish’s age that can be correlated with the fish’s total body length. What is then still left to discover is at what age and body length these fish spawn. For that to find out, fish have to be trawled near spawning grounds and females must be measured and examined as to whether they still have mature eggs in their ovaries or had already spawned. Once all the essential data are in, one can use the Von Bertalanffy growth curve to make recommendations to the fishing industry at what size it is ‘safe’ to harvest and market a species without depleting the population of younger and still immature specimens. Although Von Bertalanffy’s work doesn’t save all fish from being ‘fished’, it does help to ascertain that there are still enough youngsters around to maintain the population.

© Dr V.B. Meyer-Rochow and, 2021.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and with appropriate and specific direction to the original content.