|Progress and Results
Largemouth bass (Micropterus salmoides) is an important predator and a popular sportfish. However,
adult survival is often poor because of size-structured interactions in the first year of life. For
example, a link has been observed between poor first year survival and small size during the first summer.
Many fish grow faster when they consume fish prey instead of invertebrate prey. If age-0 largemouth bass
can switch to fish prey early during their first summer, they can grow faster, overwinter at a larger size,
and possibly survive better as adults. However, age-0 largemouth bass are gape-limited predators (i.e.,
the size of prey eaten is limited by mouth size). Consequently, naturally spawned age-0 largemouth bass
often are not large enough to consume young-of-year fish prey.
To test the role of size-structured interactions among age-0 largemouth bass, fish prey, invertebrate prey, fish competitors, and fish predators, we compared habitat-specific size through time, diet, stable isotope values, and distribution among three groups of age-0 largemouth bass [(1) naturally spawned or wild bass, (2) stocked phase 1 early-spawned bass, (3) stocked phase 2 early-spawned bass,] in Hillsdale Reservoir. The results of this whole-system manipulation will provide useful guidance for fisheries management and advance basic ecological knowledge about controls on first-year survival of this important predator.
During the initial sampling in 2014, we created effective, standardized, science-based sampling protocols that allowed us to sample the same sites with the same gear in the same way each other week (except when water levels or weather prevented standardized sampling). Sites within Hillsdale Reservoir were chosen to (a) provide a logistically-feasible but broad spatial coverage of the lake, (b) represent a range of habitat types (e.g., vegetation, beach, rock, wood, offshore - > 3 m deep), and (c) utilize sites with the highest catches from previous KDWPT largemouth bass sampling (Andy Jansen, personal communication). Although multiple habitats were present at some sites, all habitats were not present at any site. Backpack electrofishing and minnow traps did not catch many fish, but beach seine was very effective in catching largemouth bass such that seine was our primary collection gear.
We retained largemouth bass < 150 mm TL for laboratory analysis in 2014 and < 120 mm TL in 2015 because all fish < 150 mm in midsummer and fall were age-0. Numbers of small largemouth bass in each square meter of habitat were summarized as catch per unit effort (CPUE). Following sampling, fins from largemouth bass were sent to KDWPT for genetic sampling to identify stocking treatment (wild, phase 1, and phase 2). In the laboratory, 1-2 muscle fillet samples were dried for 24 h at 60oC, then ground into a fine powder for stable isotope analysis of carbon and nitrogen.
Stomach contents from young largemouth bass were analyzed using a standard diet protocol in which alimentary canals were removed and the contents were immediately fixed in 95% ethanol. Diet items were grouped into five functional groups: (1) benthic invertebrates, (2) zooplankton, (3) terrestrial invertebrates, (4) fish prey, and (5) unidentified prey. Diets were analyzed by three metrics: number of prey eaten, weight of prey eaten, and frequency of occurrence (i.e., number of individual largemouth bass within a sample that contained at least one individual of a prey type). For fish prey, we identified species by counting vertebrate of prey fish backbones. Potential prey and potential fish competitors [pelagic invertebrate prey (zooplankton), benthic invertebrate prey, potential fish prey (< 50% sampled largemouth bass length), and potential fish competitors (> 50% sampled largemouth bass)] were sampled monthly.
In 2014 at Hillsdale Reservoir, we captured 823 largemouth bass < 150 mm TL in 11 biweekly samples at 9 sample sites. In addition, we collected 190 CPUE samples, 657 largemouth bass isotope samples, 99 zooplankton prey samples, 99 benthic prey samples, and 190 prey fish samples. In 2015 at Hillsdale Reservoir, we captured 251 largemouth bass < 120 mm TL during 9 biweekly samples at 12 sample sites. In addition, we collected 130 CPUE samples, 216 largemouth bass isotope samples, 81 zooplankton prey samples, 81 benthic prey samples, and 130 prey fish samples.
Based on preliminary data analysis, in both years the highest mean catch per unit effort (largemouth bass per m2) (CPUE) for all three treatment groups (wild, phase 1, phase 2) occurred in vegetated (2014), beach (2014), and rock habitats (2015). No small largemouth bass were ever caught in wood or offshore habitats (2014, 2015). Wild largemouth bass were smaller than hatchery fish (phase 1, phase 2) throughout the field season for both years.
For all young largemouth bass (wild, phase 1, phase 2), benthic invertebrates were an important diet item by number, terrestrial invertebrates were consistently eaten, and, by weight, fish prey was the most important diet item. Fish prey increased in importance later in the summer and in fall. Diets for all young largemouth bass (wild, phase 1 hatchery, phase 2 hatchery) were complex and varied across years and habitats for all measures (numbers, weight, frequency of occurrence) and all taxonomic categories (benthic invertebrates, terrestrial invertebrates, zooplankton, fish).
Robert Mapes' MS thesis is entitled "How Multiple Approaches to Type, Size, and Arrangement of Habitat Patches Increase the Understanding of Space Use by Young Largemouth Bass: Using the Land Mosaic Concept to Inform Fisheries Management." The justification for this research synthesis is that although fisheries field research samples spatial variation within or across systems, fisheries biologists (especially those studying reservoirs) characterize space and its effect on reservoir fisheries in different ways. For example, researchers variably refer to different locations as replicates, patches, microhabitats, sites, and regions. This inconsistent use of space may impede progress for reservoir fisheries biology and management. Because different perspectives could provide alternative views of these systems, comparing, evaluating, and standardizing spatial approaches is important for fish ecology, fisheries biology, and fisheries management.
Mapes, R. M., and M. E. Mather. 2016. Location, location, location: Incorporating spatial context into fisheries research. 146th Annual Meeting of the American Fisheries Society, Kansas City, MO.
Mapes, R.L. and M.E. Mather. 2015. Habitat and resource use of age-0 largemouth bass in a Great Plains reservoir. Lake Erie Center Brown Bag Seminar. University of Toledo - Lake Erie Center.
Mapes. R. M., and M. E. Mather. 2015. Using the land mosaic concept to test how habitat heterogeneity alters the distribution of young-of-year largemouth bass in a Great Plains Reservoir. Midwest Fish and Wildlife Meeting, Indianapolis, IN.
Mapes, R., M.E. Mather, J.M. Smith, S.M. Hitchman and A. Earl. 2015. Is All Heterogeneity Created Equal? How Types of Habitat Heterogeneity Differentially Alter Distribution, Abundance, and Diets of Age-0 Largemouth Bass. Portland, Oregon.