AERF Special Report

"Evaluation of the Ecological Assessment of Grass Carp Impacts on Aquatic Communities"

Eric D. Dibble
Department of Wildlife and Fisheries
Mississippi State University

Aquatic Ecosystem Restoration Foundation 97-4. Draft Report

Executive Summary

An assessment of the historical scientific literature on ecological impacts associated with the use of grass carp for controlling aquatic vegetation was conducted. Comprehensive computer data bases and selected hardcopy reference sources on fisheries research were accessed for grass carp literature. These searches yielded 2,486 citations, of which 153 scientific articles were pertinent to the development of this report.

Major findings of this assessment include:

    1. Current data are not available to adequately answer important questions required to accurately assess grass carp as an ecologically-sound aquatic plant management strategy.
    2. Previous research has emphasized measurement of direct impacts of grass carp on the aquatic community, such as removal of plant biomass and feeding selectivity.
    3. Very few data are available on the indirect impacts grass carp have on the aquatic community, such as alteration of structural habitat, water quality, phytoplankton, filamentous algae, and effects on predator-prey interactions, or non-piscine vertebrates (amphibians, reptiles, water fowl, mammals).
    4. Direct impacts on habitat heterogeneity are not adequately quantified.
    5. Early life stages of grass carp and direct and indirect impacts on aquatic communities remain unstudied.
    6. Studies to address how grass carp feeding and selectivity alters plant species composition and architecture and direct and indirect implications to habitat heterogeneity are lacking.

Research recommendations:

Understanding ecology is paramount to appropriate management of aquatic communities, including the using of grass carp as a plant control technique. Therefore, future research recommendations based on this assessment are:

    1. Investigation should quantify both direct and indirect ecological impacts of grass carp within and across vegetated habitat to confirm these impacts on habitat heterogeneity.
    2. Additional studies should be devoted to determining how grass carp impact multi-species interactions, such as intra- and interspecific competition and trophic predator-prey relationships.
    3. Future investigation should quantify individual responses by organisms to grass carp habitat alterations to better quantify indirect impacts on population-dynamics, growth, and animal distribution.
    4. In addition, designs of investigation should address ecological impact of all life stages of grass carp.
    5. Lastly, studies should combine both small- and large-scale experiments, where results of mesocosm and pond evaluations are extrapolated and verified at natural community levels (e.g. lakes, rivers, and reservoirs).

Abstract

The goal of this report was, 1) to evaluate the availability of ecological data in previous research on grass carp that attempts to validate community impacts, 2) to identify strengths and limitations in the ecological assessment of this research, and 3) to provide potential design and trajectory of future research needed to obtain adequate ecological data. A comprehensive review of literature on grass carp research was conducted, and current ecological theory synthesized to assess the ability of the available data to validate ecology of grass carp impacts on freshwater communities. An attempt to uncover all studies published in both primary and grey literature, and studies unpublished, i.e., government reports, technical bulletins, and conferences was made. Approximately 25 years of literature were searched, from the early to mid 70's to the most current. All citations that included ecological discussion relative to the use of grass carp was delineated by topical emphasis, and topics were organized into tabular format and references included for efficient access. The research in this literature was further evaluated by delineating different levels of assessment attempted and identify the response variables measured. Most (95%) of the research emphasized structural response variables, rather than dynamic processes within the community; no investigations directly measured impacts on multi-species interactions or behavior of individual organisms. The two most commonly measured response variables to grass carp impacts were aquatic plant abundance and water quality parameters, yet no attempt was made to measure for impacts on different levels of habitat heterogeneity. Most investigations measured the direct impacts of grass carp, and very few data were available to quantify indirect impacts grass carps have on the community, <2% of all studies reviewed attempted to validate causal mechanisms for these indirect impacts. Large system scaled investigation (>10 h) was most prevalent, representing >35% of the studies, and >35% of the studies were conducted within 2-3 yrs. Previous studies emphasized impacts by adult grass carp and neglecting younger life stages; < 3% of these investigations mentioned post-larval stages, and no data were available on potential impacts of early life stages. This evaluation concludes that ecological data gathered from previous grass carp studies are limited, and to thoroughly evaluate the environmental impacts of grass carp on freshwater communities, future emphasis is needed on investigating hypotheses deduced from ecological theory.