Fisheries Research Report No. 1706, 1965

Michigan Department of Conservation
Research and Development Report No. 42
Institute for Fisheries Research Report No. 1706, 1965
The Population Dynamics of the Crayfish, Orconectes virilis In Relation to Predation by the Brook Trout, Salvelinus fontinalis

Walter M. Momot

      Introduction.-Results of studies of the quantitative aspects of trophic ecology are expressed in terms of the productivity of the component food groups involved. Important in the trophic dynamics of any ecosystem are the myriad of organisms, mainly invertebrates, that contribute to the secondary level of production. These are the primary consumers in the food chain dependent on the primary producers for energy and in turn serve as food organisms for higher levels in the chain.
      Arthropods are one of the principal components of the secondary level of productivity in aquatic ecosystems. Previous work on the productivity of arthropod populations in natural freshwater ecosystems has in general neglected the crustaceans, e.g., Borutsky (1939), Anderson and Hooper (1956), Miller (1941) and Lundbeck (1926). Recently studies of secondary productivity among crustacea have been made on cladocerans (Hall, 1964) and amphipods (Cooper, 1964). These crustacea are relatively short-lived, small organisms.
      Among the freshwater crustacea of North America crayfish are a somewhat special type of consumer, i.e., they are larger and have a longer life span than other members of the class. Consideration of the comparative population dynamics of members of this taxonomic class raises questions such as: (1) how does size and length of life span influence the rate of energy flow and the rate of turnover, (2) are population regulating mechanisms similar in the small, short-lived forms, and in larger long-lived crayfish?
      Crayfish are certainly among the most conspicuous and abundant invertebrates of the marl lakes of northern Michigan. Typically marl lakes are lakes of high transparency, have little or no higher aquatic vegetation and have light colored sediments that are low in organic matter but contain a high percentage of marl (Hale, 1903). Marl incrustations produced by algae usually cover rocks and other fixed objects within the epilimnion. Benthic algae probably account for a major portion of the energy fixed by photosynthesis within these lakes (Hooper and Ball, 1964), and in lakes of this type of production by phytoplankton is low compared to non-marl lakes (Raymond, 1937). Production of invertebrate bottom fauna other than crayfish in marl lakes is also low and is probably in part limited by the sparsity of aquatic plants (Wohlschlag, 1950). Since crayfish frequently abound in these waters and seemingly far exceed other bottom fauna in terms of standing crop, it is of considerable ecological interest to examine their production rates and their trophic position.
      Earlier studies of North American crayfish have dealt chiefly with life histories or have provided estimates of the standing crops in shallow artificial pond (e.g., Tack, 1941; Goellner, 1943; Van Deventer, 1937). None of these studies have focused attention upon productivity and the resilience to predation of a natural population within a lake. The objective of this study, viz. to define the role of the crayfish as a consumer in the trophic dynamics of a marl lake ecosystem, required detailed knowledge of the growth, reproduction, mortality, and density of the crayfish population of this habitat. To meet the above objective it was also necessary to measure the annual net production of this crustacean, to establish some insight into the factors controlling its production, and to compare its production with that of other members of the benthos.
      The principal mortality factor examined was the effect of trout predation upon the crayfish population. This predation study made possible measurement of the efficiency of energy transfer from one trophic level to another and the assessment of the relative importance of this interspecies interaction. Only when the pattern of population interaction is understood can populations be manipulated so as to produce an increase in efficiency of energy transfer from one trophic unit to another. By means of such studies in the efficiency, with which an economically important predator utilizes the net annual production of its prey, can be increased. The ecological relationship between trout and crayfish presented in this paper can serve as an example of this avenue of approach; an approach that can be used to direct and to channelize the potential productivity of an aquatic ecosystem for maximum human benefit.