Relationships Between Aquatic Plants and Reproduction of FishesBy Jim Pitts
reprinted from American Currents, Jan.-Feb. 1976
Aquatic plants have long been associated as probably the most influential factors in fish ecology. The direct and indirect influences plants have on the reproduction of fishes was long overlooked, however, in discussion of the ecological contributors towards the success of fishes.
PLANTS AS SPAWN INDUCERS
Another area of spawn induction by plants is chemical in nature. This is more important in the still water fishes than in stream dwellers. Decaying aquatic vegetation is responsible for the release of many organic substances in the water some of which influence the gonadal development of fishes. These are gonatropins, and certain fish reared in water devoid of these hormones never develop mature sex organs (Scheel).
Oxygen content of the water may influence the breeding of soma species such those in
the genus Corydoras. The relationship between plants and oxygen in a body of standing
water is self evident.
AQUATIC PLANTS AS SPAWN RECEPTACLES
Those plants that remain unchanged are usually receptacles for egg scattering species of fishes. These fish, (cyprinids for example) swim into or over the clumps of plants such as Elodea or Vallisneria and randomly scatter the eggs in furious bursts. This often takes place in such shallow water that the backs of the fishes break the surface. The closer the spawning occurs to the edge of the water, the less chance that other species will venture to that depth to devour the eggs. The plants in which the eggs settles serve several purposes. The most obvious is the role of hiding the eggs from the predation of other fishes. Fine leafy plants such as Myriophyllum serve this end well. Another form of protection is when the plants prevent the eggs from drifting out into the depths of the body of water, by diffusing the action of the waves and current near the shoreline. Those species of fish that lay semi or adhesive eggs are well benefited by plants in that they anchor their eggs onto the leaves. This prevents the eggs from falling to the bottom where they are more subject to be eaten by small invertebrates. The eggs of fishes in dense growths of plants are further benefited by a more favorable hydrochemical environment (Nikolsky 1963). Both live and dead plants help in this way, The release of nitrates has a definite effect on the pH of a body of water. When decaying plants release their nitrogenous organic compounds the water around them becomes soft and acidic. In moderation this condition is necessary for the reproduction of many tropical and sub-tropical fishes. There is evidence that C02 has influence on the enzyme hatching mechanism of the killifishes which lay eggs directly on the plant.The other common utilization of plants by nondestructive methods occurs when some fishes deposit their eggs in nests that are hidden in thick growths of plants.
Under another category we find plants that are modified by the fish in some way in
order to facilitate a spawning site or nest. Some fishes such as the Cichlids and the
Centrarchids remove plant material from the spawning site in order to produce a more
sterile environment for their broods. Not only decaying vegetation, but also large rooted
plants are removed from the area in meticulous fashion by these intelligent fish. Other
nest builders assume a more positive relationship with aquatic plants. Probably the basic
documented case is that of the sticklebacks (Gasterateoidea). These relatives of the
seahorses carefully select and dislodge the appropriate plants and weave them into a
cylindrical nest in which spawning and early rearing of the fry occurs. The family of
Anabantids or labyrinth fishes also use bits and pieces of plants to weave into a thick
mat of bubbles that the male forms at the surface of the water. Amia calva, the bowfin, is
reported to build some type of nest utilizing plant materials (Nikolsky 1963).
ADVERSE AFFECTS OF AQUATIC PLANTS ON FISH REPRODUCTION
According to Bennett, "There is evidence that dense stands of submersed rooted aquatic plants may bind up nutrient materials throughout the growing season so that they are not available for production of phytoplankton and the organisms that feed upon phytoplankton," This means that although fish hatches would be large, the chances for larval survival would be greatly diminished. The table at the end of this paper shows the data supporting this, Potomageton foliosus and P. nodosus were the plants involved in this case.In relation to fishing lakes and ponds, over-production of plants is usually frowned upon by management and fishermen. The survival rate of fishes beyond forage size is inversely proportional to the size of the fishes that reach maturity.
A few aquatic plants are predacious on the fry of fishes. The most common of these is probably the bladderworts (Utricularia) (Podinova 1959). The plant Aldrovandia also utilizes the fish larvae as a source of nutrients. "The leaves of Aldrovandia fold together to form a chamber, the interior of which is covered with hairs. When these hairs touch an animal the leaf claps together and the animal is caught." (Nikolsky). These plants also reduce the amount of planktonic crustaceans in the water around them and thus decrease the chances for larval survival by removing one of its major food sources.
Nikolsky, G.V. (1963). The Ecology of Fishes. New York. Academic Press.
Rodinova, L.A. (1959) The feeding of bladderwort on animals. Papers for Higher Schools, ser. Biol. Sci. no, 3.
Scheel. J. (1968) Rivulins of the Old World. Jersey City, New Jersey. T.F.H. Publications Inc.
Other Information Sources
Burroughes, R.D. (1973). The Sticklebacks, Tropical Fish Hobbyist, Vol. XXI, no. 9.
Gerking, S.D. (editor) (1966) The Biological Basis of Freshwater Fish Production, New York, John Wiley & Sons, Inc.
Hall, G.E. (editor) (1971) Resevoir Fisheries and Limnology. Washington D.C., The American Fisheries Society.
Table From Bennett
Used with permission. Article copyright retained by author.