Ecological Impact of Introduced Crayfish on Benthic Fish in Strawberry Creek Frank Yoon Abstract The introduction of the non native crayfish Pacifastacus leniusculus in Strawberry Creek may be expected to have negative impacts on the stream ecosystem Recent studies have shown that the declining fish population in Strawberry Creek may be due to the presence and establishment of a large crayfish population The purpose of this experiment is to investigate the interactions between the crayfish and the prickly sculpin Cottus asper a native benthic fish in Strawberry Creek The hypothesis that crayfish prey on benthic fish was tested using eight isolation cages placed within the creek four served as controls and four were experimental A separate laboratory experiment was also conducted to facilitate direct observations of the interactions using a twelve gallon aquarium separated by a wire mesh screen One side served as the control and the other side was the experimental Two fish were placed in every cage and the experimental cages each harbored two crayfish in addition to the sculpins After ten days the remaining fish and crayfish were counted All of the fish within the control cages survived while only two survivors were found in the experimental cages Two of the experimental cages only had one survivor each while the rest of the experimental cages resulted in zero fish The results of the experiment showed that fish mortalities were significantly higher when crayfish were present The loss of fish could be partly due to predation which was directly observed in the laboratory observation tank The crayfish aggressively attacked the fish yet did not actively pursue them The results of this experiment may show the potential for crayfish to reduce fish abundance if not local extinctions in Strawberry Creek Introduction The introduction of non native species to an area can result in a wide range of ecological changes Schoener 1993 the most dramatic of which is the replacement of native species by invaders In addition introduced species can cause habitat shifts after competitive interactions and can affect the trophic structure of their communities Schoener 1993 In streams species interactions such as predation herbivory and interference competition are direct interactions that influence the community ecosystem Allan 1995 The structure of these communities are directly affected by shifting species abundance and species composition Direct effects of one species on another in ecological communities may determine the number and kinds of species that can successfully coexist Schoener 1993 These chains of direct interactions in food webs can lead to important consequences for stream communities Power 1992 The crayfish offers an example of an important component of the interactions between trophic levels in aquatic ecosystems Crayfish are abundant omnivores in many aquatic habitats and may be expected to have important direct effects on benthic communities Momot 1984 Studies examining natural crayfish populations have indicated that crayfish are abundant consumers in many ecosystems and consume invertebrates filamentous green algae and detritus Creed 1994 It is also documented that crayfish can be expected to prey significantly on the eggs of fish reduce macroinvertebrate abundance eliminate native crayfish and often exert a considerable influence on their surrounding habitats Holdich 1988 Researchers believe that crayfish act as a keystone predator and interact at all levels in the trophic structure and are high enough in it to influence the nature and extent of interactions among a whole network of species Momot 1978 Because of their ability to modify aquatic macrophyte and macroinvertebrate communities the introduction of crayfish could have a major effect on the structure and composition of the littoral zone which in turn could possibly affect the fish community Chambers 1990 Hobs et al 1989 concluded that once an exotic crayfish is introduced into a lake or stream it may impose considerable environmental stress on the system and in all too many instances irreparable shifts in species diversity occur Given the important role of crayfish in aquatic ecosystems it is not surprising that this large bodied species has had a number of effects on local systems Because of these effects the introduction of the non native crayfish Pacifastacus leniusculus might be expected to have negative impacts in stream ecosystems such as Strawberry Creek Although the origins of crayfish establishment in this body of water is unknown their presence may have affected the creek ecosystem According to a recent study the declining fish population in Strawberry Creek can possibly be attributed to the presence and establishment of a large crayfish population Gold 1998 The purpose of this experiment is to further investigate the aspects of the competing behavior of crayfish and their ecological interactions especially the impact of the introduced species on the native fish community in Strawberry Creek One study based on one population in the River Great Ouse England determined that crayfish may affect the abundance of benthic or bottom feeding fish through predation Guan et al 1997 Because there have been no prior studies concerning crayfish ecology in Strawberry Creek this experiment will be a preliminary study to investigate predation by the introduced crayfish on the native benthic fish prickly sculpin Cottus asper Study Organisms Pacifastacus leniusculus is distributed throughout regions of the western United States It is native to Washington Oregon and Idaho and was introduced to parts of Colorado Nevada and California Fuerst 1978 The distribution of prickly sculpin Cottus asper lies on the western coast of the United States extending from the Pacific slope drainages in Washington down to Ventura River in California Page 1991 These benthic fish thrive in a variety of habitats ranging from freshwater brackish and marine environments They feed primarily on aquatic insect larvae and bottom invertebrates and share similar habitat requirements as the crayfish Page 1991 Study Area The upper Strawberry Creek watershed located east of Oxford Street in Berkeley CA is composed of two major branches the North and South Forks According to the Strawberry Creek Management Plan the total watershed area is 1 82 mi2 Storm water routing as well as stream culverting and channel confinement have significantly altered the natural
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