 |
||||
|
||||
|
|
 |
||
|
||||
      |
Fish Diet and Condition: Responses to Low Oxygen Events in Central Lake Erie
Primary Investigator:
Steven Pothoven - NOAA/GLERL
Co-Investigators:
Henry Vanderploeg - NOAA/GLERL
Tomas Hook* -
Perdue University
James Roberts - University of Michigan*
Stuart Ludsin* -
Ohio State University*
NOAA Research Area:
Advancing understanding of ecosystems to improve resource management.
Performance Objective:
Increase number of regional coastal and marine ecosystems delineated with approved indicators of ecological health and socioeconomic benefits that are monitored and understood.
Research Milestones:
Meet annual targets for the number of coastal, marine, and Great Lakes ecological characterizations that meet management needs.
Executive Summary of Rationale
A prominent feature of Lake Erie’s central basin is the area of severe hypoxia/anoxia (aka the “dead zone”) that recurs annually during late summer. GLERL and collaborators have collected data from the central basin since 2005. This proposal is part of the multi-investigator group (see Mason proposal) attempting to understand the impacts of hypoxia on ecosystem function in Lake Erie, specifically fish condition, growth and diets. Work in 2008 will focus on data analysis and manuscript preparation.
Proposed Work
Current/Ongoing: YR2008
During 2008, work has focused on analysis and processing of samples collected in 2007. To date, all benthos samples have been picked (n=84), and are awaiting measurements to convert counts to biomass measures. For diet analysis and energy content determinations, a total of 228 emerald shiners, 260 rainbow smelt, and 406 yellow perch have been examined. We have also analyzed all 167 yellow perch muscle samples for nucleic acid analysis. Other fish are available in frozen storage and will be analyzed as needed.
Scientific rationale
Objectives:
a. Conduct diet analyses and process benthic invertebrate samples to quantify the effects of hypoxia on fish diet composition and selectivity
b. Use bomb calorimetry (and dry:wet ratios) to quantify the effects of hypoxia on energy density (caloric content) of fish
c. Use RNA:DNA analyses to quantify the effects of hypoxia on short-term growth of fish
Objective a.
Data collected during 2005 indicate that hypoxia can influence the vertical and
horizontal distribution of fish. We documented a shift in both yellow perch and
rainbow smelt from feeding more on benthos than zooplankton before hypoxia to feeding
more on zooplankton than benthos during hypoxia, with a return to benthic-feeding
after hypoxia. Additionally, we documented a reduction in total stomach biomass
(g/g dry weight) during the peak of hypoxia for both yellow perch and rainbow smelt
relative to the pre- and post-hypoxic period. During 2007, we collected fish
(including yellow perch, rainbow smelt, and emerald shiners) and their potential
benthic invertebrate prey in areas in and out of the hypoxic zone on two occasions.
During 2008, we hope to quantify diet patterns for these species and determine how
hypoxia and available prey influence diet composition and selectivity. Additional
fish species were also collected, including white perch, round gobies, gizzard shad,
and white bass, and these may also be examined for diet composition.
Objective b.
Determining energy densities in food web components is an important tool for
quantifying trophic dynamics and assessing how ecosystem changes such as hypoxia
affect lake resources such as fish production. Rand et al. (1994) suggested that
energy content of a fish is the most direct measure of fish condition. Energy
density, which is directly related to lipid content, provides information on the
ability of fish to grow, reproduce, and overwinter (Rottiers & Tucker 1982). Energy
content of a fish is also a measure of prey quality and quantity, a useful tool to
evaluate ecosystem changes such as the decline of Diporeia, and a necessary input
for bioenergetic models (Madenjian et al. 2000). Energy density can easily be
determined from dry:wet weight ratios for most fish (Hartman and Brandt 1995).
For 2005, we determined that there was a strong relationship between energy density
and dry:wet weight for both rainbow smelt (r2=0.97, n=51) and emerald shiner
(r2=0.90, n=40). Results from 2005 data also indicate that condition (from dry:wet
weight percentages) increased through September for rainbow smelt, but decreased
in October. The decrease in October was somewhat unexpected based on seasonal
trends for energy density of rainbow smelt from other systems (Vondracek et al.
1996), and may be a consequence of low food availability or marginal thermal
conditions for rainbow smelt. In contrast, condition of emerald shiners increased
from August through October. Emerald shiners are mainly found in the epilimnion,
and hypoxia might not negatively affect this speces, or may even improve food
availability if zooplankton were forced into the epilimnion by hypoxia.
Objective c.
The anoxic zone in Lake Erie likely influences the growth and nutritional status
of individual organisms in Lake Erie either directly (via physiological responses
due to low ambient oxygen concentrations) or indirectly (by affecting the availability
of potential prey or through occupation of inferior thermal habitats). However,
documenting effects on growth and nutritional status with traditional measures of
individual growth and condition (e.g., weight-length measures, energy densities)
is difficult, because although the anoxic zone in Lake Erie exists for a relatively
short time period (1.5 months), traditional measures of growth and condition
integrate feeding history and energetic utilization over the whole life-time of
an organism. Thus during 2005, we collected a suite of organisms in Lake Erie
in order to analyze ratios of RNA:DNA. The measurement of nucleic acid ratios
allows for indexing recent growth and condition of individual animals, and is
based on the notion that DNA concentrations within individual cells remain fairly
constant while RNA concentrations increase as protein synthesis increases. Thus,
a recently well-fed, active, growing individual should have a relatively high RNA
to DNA ratio compared to a starving, inactive individual.
Our 2005 sampling plan was designed to compare RNA:DNA ratios between organisms collected in control (continuously oxic) and treatment (seasonally hypoxic/anoxic) regions of Lake Erie’s central basin. During 2006, we followed up on these measures of wild organisms and used rearing experiments in an attempt to tease apart the actual mechanisms (direct physiological effect of low oxygen vs. changes in prey availability vs. occupation of inferior thermal habitat) through which hypolimnetic hypoxia may influence short-term growth and condition. We reared representative Lake Erie zooplankton (Daphnia), benthic macroinvertebrates (Chironomus), and fish (yellow perch, Perca flavescens) under different ambient conditions (treatment-specific prey densities, temperature, and oxygen concentrations) and subsequently measured RNA:DNA ratios of individual animals to index resulting short-term growth and condition. During 2007, we collected yellow perch and rainbow smelt within and outside of Lake Erie’s hypoxic zone for subsequent RNA:DNA analysis. In addition, we further explored the mechanisms underlying variation in RNA:DNA ratios of yellow perch by rearing perch under different conditions to elucidate the interacting effects of oxygen concentration, temperature, and food availability on short-term growth.
Fish trawl from Lake Michigan. Photo by J. Roberts, September 2007
Governmental/Societal Relevance
Our work will help resource managers, policy makers, and user groups (e.g. commercial, charter, and recreational fishermen) understand whether hypoxia poses a major or limited concern for managing Lake Erie fisheries. Little data is available whether seasonal hypoxia in Lake Erie ultimately affect fisheries, and if the affects are universal across important fish species. The project would help managers understand how hypoxia affects fishes ability to grow, overwinter, and reproduce in the central basin of Lake Erie.
Relevance to Ecosystem Forecasting
The basic biology that is learned from this research would feed directly into synthetic modeling and forecasting efforts being conducted as part of the IFYLE and the University of Michigan/GLERL ECOFORE programs. For example, the fish foraging and caloric content information in response to hypoxia would be used to calibrate and validate spatially-explicit bioenergetics-based growth-rate potential modeling efforts being conducted as part of ECOFORE. Likewise, the information on trophic dynamics could be use to support predictive CASM food web models being developed as part of IFYLE and ECOFORE. In the end, merging of these efforts would allow for development of ecosystem forecasting models that would assess how hypoxia interacts with other stressors (e.g., invasive species, nutrient inputs, climate) to influence fish distributions, behavior (feeding, migration), condition, and growth (production potential).
Products
Presentations:
Roberts, J, Hook, T, Ludsin, S., Pothoven, S. Vanderploeg, H. Response of yellow perch to hypoxia in Lake Erie’s central basin: spatial patterns. 138th Annual meeting of the American Fisheries Society, Ottawa, Canada, Aug 17-21, 2008.
Pothoven, SA, Ludsin, S, Vanderploeg, H., Hook, T. Effects of Lake Erie hypoxia on pelagic fish feeding ecology. (poster). 50th Conference on Great Lakes Research, IAGLR, May 28-June 1, 2007; University Park, PA.
Submitted Manuscripts:
Roberts, J, Hook, T, Ludsin, S, Pothoven, S, Vanderploeg, H, and Brandt, S. Effects of hypolimnetic hypoxia in Lake Erie’s central basin on distributions and diets of yellow perch. Marine Ecology Progress Series (in review)
Pothoven, S, Vanderploeg, H, Ludsin, S, Hook, T, and Brandt, S. Feeding ecology of emerald shiners and rainbow smelt in the central basin of Lake Erie. Journal of Great Lakes Research (in review)
Additional Information
The diet results show that yellow perch in the normoxic nearshore areas consumed mostly pelagic zooplankton similar to some of the hypoxic offshore sites (Figure 1). However, we also found that yellow perch foraging of benthic items in hypolimnetic areas with low dissolved oxygen concentrations was common in 2007 (Figure 1). From our trawl results we know most of the yellow perch at hypoxic sites were found suspended in the water column, suggesting they are dipping into the hypoxic hypolimnion, foraging on benthic items, and then returning to more oxygenated meta- and epilimnetic habitats (Figure 2). Our nucleic acid results show that yellow perch short-term growth and conditions was slightly better at the offshore hypoxic sites than at the shallower normoxic sites (Figure 3). These results suggest that the normoxic habitats found in nearshore areas during offshore hypolimnetic hypoxic conditions do not serve as a refuge for yellow perch.
Figure 1. Yellow perch diets from Lake Erie’s central basin during August and September 2007 by site. The depth and hypolimnetic dissolved oxygen concentration is shown for each site. Hypoxic sites indicated by red and normoxic site by black.
Figure 2. Relationship of hypolimnetic dissolved oxygen to trawl catch of yellow perch from Lake Erie’s central basin during August and September 2007. Results are shown for all ten sites sampled during 2007. Simple linear regression results are shown for bottom trawl data but not the mid-water trawl data due the large amount of zero catches.
Figure 3. Ratio of RNA to DNA for yellow perch muscle tissue collected during 2007 at hypoxic and normoxic sites in Lake Erie’s central basin.
*Link leads off GLERL's website